IB Biology · Theme D Continuity and change · Cells
D2.1 Cell and nuclear division
Every new cell comes from a parent cell dividing in two, nucleus first and then cytoplasm. Mitosis keeps the chromosome number; meiosis halves it and shuffles alleles into new combinations. At HL, cyclins time the cell cycle, and mutations in its control genes cause tumours.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D2.1.1 New cells only come from cells dividing
A parent cell (mother cell) divides to produce exactly two daughter cells.
The parent does not survive alongside them; its whole contents are shared out.
The huge cell numbers in a body come from many repeated divisions.
Students often think the parent survives beside its daughters, giving three cells. In fact it ceases to exist; everything it had is divided between the two.
Students often think a cell splits into many small cells at once. In fact each division gives two.
D2.1.2 Cytokinesis splits the cytoplasm, differently in animals and plants
Cytokinesis divides the cytoplasm after the nucleus has divided.
In animal cells a contractile ring of actin and myosin pinches the membrane inwards.
In plant cells vesicles fuse at the equator into a cell plate of membrane and wall.
The plate grows outwards until it joins the existing wall.
Students often think plant cells pinch in from the outside like animal cells. In fact a new wall is built from the middle outwards.
Students often think the contractile ring is made of spindle microtubules. In fact it is actin and myosin, just inside the membrane.
D2.1.3 Cytokinesis is usually equal, but not always
Most divisions give two daughter cells of similar size.
Each daughter needs at least one mitochondrion, and one of any organelle made only by division.
In human oogenesis the egg keeps nearly all the cytoplasm; polar bodies get almost none.
In budding yeast a small bud receives one nucleus and some organelles, then separates.
Chromosomes are still shared equally in unequal cytokinesis; only the cytoplasm is divided unevenly.
Students often think cells build new mitochondria from scratch. In fact they arise only by division of existing ones, so each daughter needs at least one.
Students often think the bigger daughter also gets more chromosomes. In fact nuclear division shares chromosomes equally whatever the cytoplasm does.
D2.1.4 Mitosis keeps the genome; meiosis halves it and adds variety
Mitosis gives two nuclei with the same chromosome number and the same genome.
It serves growth, cell replacement, tissue repair and asexual reproduction.
Meiosis halves the chromosome number, giving four haploid nuclei, and generates genetic diversity.
The nucleus must divide before the cell does, or one daughter is anucleate.
Students often think cytokinesis is the last phase of mitosis. In fact mitosis is nuclear division; cytokinesis is a separate step that follows.
Students often think mitosis separates homologous pairs. In fact it separates sister chromatids; homologues do not pair in mitosis.
D2.1.5 DNA is replicated once before either division
During interphase every DNA molecule is copied, before mitosis or meiosis starts.
Each chromosome then consists of two identical sister chromatids, joined at the centromere.
The chromosome number does not change; only the amount of DNA doubles.
Chromatids stay together until anaphase; before meiosis there is no second replication.
Students often think replication doubles the chromosome number to 92. In fact it doubles the DNA; 46 chromosomes now each have two chromatids.
Students often think the two chromatids are the maternal and paternal homologues. In fact they are identical copies of one molecule.
D2.1.6 Chromosomes condense and are moved by microtubules in both divisions
DNA winds around histone proteins and the fibre is supercoiled, making chromosomes short and thick.
Condensation happens only for division; in interphase the DNA is extended.
Spindle microtubules attach at centromeres and, with motor proteins, move chromosomes.
Microtubules do not contract; they shorten by losing subunits while motors walk along them.
Students often think condensation is the helix twisting tighter on its own. In fact DNA is wound on histones and the fibre is supercoiled.
Students often think spindle fibres contract like muscle. In fact motor proteins move the chromosome and the microtubule shortens.
D2.1.7 The four phases of mitosis
Prophase: chromosomes condense, the nuclear envelope breaks down, the spindle forms.
Metaphase: chromosomes line up on one plane at the equator, chromatids facing opposite poles.
Anaphase: centromeres divide and sister chromatids are pulled to opposite poles.
Telophase: nuclear envelopes re-form and chromosomes decondense; two identical nuclei exist.
Students often think the nuclear envelope stays intact throughout. In fact it breaks down in prophase and re-forms in telophase.
Students often think mitosis halves the DNA between daughters. In fact each gets one chromatid of every chromosome: a whole genome.
D2.1.8 Reading the phase from a diagram or micrograph
Intact nucleus, no distinct chromosomes: interphase, not part of mitosis.
Condensed chromosomes visible but scattered, not on one plane: prophase.
Chromosomes on one plane across the middle: metaphase.
Two sets moving apart: anaphase; two groups at the poles with envelopes: telophase.
Students often call a cell with an intact nucleus prophase, or a resting stage of mitosis. In fact it is in interphase, outside mitosis.
Students often call any cell with chromosomes in the middle metaphase. In fact scattered chromosomes are prophase; only a single aligned plane is metaphase.
D2.1.9 Meiosis: two divisions, one diploid nucleus to four haploid
Diploid (2n) means two sets of chromosomes, one from each parent; haploid (n) means one.
Meiosis I separates the homologous chromosomes of each pair: the reduction.
Meiosis II separates the sister chromatids; DNA is not replicated between the divisions.
Without halving, fertilisation would double the chromosome number every generation.
Students often think meiosis I separates chromatids and meiosis II separates homologues. In fact it is the other way round.
Students often think a haploid cell has half the genes. In fact it has one full set: one copy of every gene.
D2.1.10 Non-disjunction and Down syndrome
Non-disjunction is a pair failing to separate in anaphase I, or chromatids in anaphase II.
One gamete gets an extra chromosome; another lacks that chromosome.
A gamete with two copies of chromosome 21, plus a normal gamete, gives trisomy 21.
The zygote has 47 chromosomes; this is Down syndrome, an error of number, not sequence.
Students often think Down syndrome is a base-sequence mutation on chromosome 21. In fact it is an extra whole chromosome from non-disjunction.
Students often think the extra chromosome appears at fertilisation. In fact it arises in meiosis in one parent.
D2.1.11 Meiosis makes variation: random orientation and crossing over
A bivalent is a homologous pair lying side by side in prophase I.
Random orientation: each bivalent faces the poles independently, giving 2ⁿ combinations.
In humans that is 2²³, about 8 million; each gamete still gets one of every pair.
Crossing over swaps DNA between non-sister chromatids in prophase I, recombining existing alleles.
Students often think crossing over is between sister chromatids. In fact it is between non-sister chromatids of the two homologues.
Students often think crossing over creates new alleles. In fact it makes new combinations of alleles already present.
D2.1.12 Proliferation for growth, replacement and repair HL
Cell proliferation is repeated cell cycles increasing cell number.
Plants grow by division in meristems at root and shoot tips; early embryos divide throughout.
Skin cells beneath the surface divide and replace those shed from the surface.
In wound healing, cells at the edges proliferate to cover the gap.
Students often think plant cells divide all over the plant. In fact division for growth is confined to meristems.
Students often think cells round a wound stretch to fill it. In fact new cells are produced by proliferation at the edges.
D2.1.13 The cell cycle: G1, S, G2, mitosis, cytokinesis HL
Interphase is G1, then S, then G2, in that order.
G1: growth and protein synthesis. S: DNA replication. G2: further growth and preparation.
Mitosis and cytokinesis follow; daughter cells enter G1 of the next cycle.
Interphase takes most of the cycle; mitosis is short.
Students often think mitosis takes most of the cycle. In fact interphase does; mitosis is brief.
Students often think DNA is replicated in G2. In fact it is replicated in S phase, between G1 and G2.
D2.1.14 Interphase is busy, not resting HL
The cell grows by biosynthesis of proteins throughout interphase and DNA in S phase.
Mitochondria and chloroplasts increase in number by growth and division of existing ones.
After S phase the cell holds twice the DNA, though the chromosome number is unchanged.
Students often think interphase is a resting phase. In fact it is metabolically very active: synthesis, replication and organelle growth.
Students often think DNA content stays constant through interphase. In fact it doubles at S phase.
D2.1.15 Cyclins let the cell pass checkpoints HL
Cyclins are proteins whose concentrations rise and fall during the cycle.
Different cyclins peak at different points in the cycle.
A specific cyclin must reach a threshold for the cell to pass a checkpoint.
Its concentration then falls, so the checkpoint cannot be passed again until it rebuilds.
Students often think one cyclin rises steadily through the cycle. In fact each rises to a peak and falls, and different ones peak at different times.
Students often think cyclins are a brake. In fact reaching the threshold is what allows the cell to move on.
D2.1.16 Mutations in cell-cycle genes drive uncontrolled division HL
A proto-oncogene is a normal gene whose product promotes the cell cycle.
A mutation can turn it into an oncogene: over-active, driving division without control.
A tumour suppressor gene restrains the cycle; a mutation that disables it removes the brake.
Either kind of mutation leads to uncontrolled division and a tumour.
Students often think oncogenes are foreign genes brought in by viruses. In fact every cell has proto-oncogenes; mutation converts them.
Students often think tumour suppressor mutations make the gene over-active. In fact they stop its product working.
D2.1.17 Benign versus malignant, and measuring the mitotic index HL
A benign tumour stays in one place, does not invade, and causes no cancer.
A malignant tumour invades neighbouring tissue and can spread by metastasis.
The primary tumour is where division began; secondary tumours are formed by cells that spread.
Mitotic index = cells in mitosis ÷ total cells observed.
A high index means rapid proliferation, which is normal in a meristem or an embryo; malignancy is judged by invasion, not by rate alone.
Students often think benign means not really a tumour. In fact it is uncontrolled division, just without invasion or spread.
Students often think a secondary tumour arises independently. In fact it grows from cells that detached from a malignant primary.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement correctly describes how new cells are generated in a living organism?
Answer and reasoning
A parent cell divides to produce two daughter cells, and everything the parent contained is shared out between them. — In all living organisms new cells arise by division of a parent (mother) cell into two daughter cells. The parent cell does not persist; its whole contents are divided between the two daughters.
A parent cell produces two daughter cells and then goes on living alongside them as a third cell. — A student who reads 'mother' and 'daughter' as a human family picks this. The parent cell is the material from which the two daughter cells are made; no third cell remains.
A parent cell splits at one moment into many small daughter cells, which then grow to full size in the tissue. — A student who pictures one cell fragmenting into many picks this. Each division produces exactly two daughter cells; large numbers come from repeated divisions.
A parent cell splits its cytoplasm into two halves, each of which then assembles a nucleus of its own from the cytoplasm. — A student who thinks a nucleus can be built from scratch picks this. The nucleus divides first by mitosis or meiosis, and each daughter cell receives one of the two nuclei produced.
2 How does cytokinesis in an animal cell differ from cytokinesis in a plant cell?
Answer and reasoning
Both cells are pinched in two by a contractile ring, but in a plant cell the ring acts more slowly because it must squeeze the wall as well. — A student who applies the GCSE 'the cytoplasm divides' model to every cell picks this. A plant cell cannot be pinched through its rigid wall; a cell plate is built from vesicles instead.
An animal cell is pinched in two by a ring of actin and myosin; a plant cell builds new membrane and wall from vesicles at its equator. — In an animal cell a ring of contractile actin and myosin proteins pinches the cell membrane together. In a plant cell vesicles assemble sections of membrane and cell wall at the equator, which grow outwards to divide the cell.
An animal cell is pinched in two by spindle microtubules; a plant cell builds new membrane and wall from vesicles at its equator. — A student who merges the spindle with the contractile ring picks this. Microtubules move chromosomes; the ring that pinches an animal cell is made of actin and myosin.
An animal cell is pinched in two by a ring of actin and myosin; in a plant cell the existing wall grows inwards from the edges until it meets. — A student who makes the old wall do the furrowing picks this. The new wall in a plant cell starts at the centre, as a cell plate formed from vesicles, and grows outwards to join the existing wall.
3 Which statement correctly compares the roles of mitosis and meiosis in eukaryotes?
Answer and reasoning
Mitosis keeps the full set of genes in each cell, whereas meiosis produces cells that carry only half of the organism's genes. — A student who hears 'halves the chromosome number' as 'halves the genes' picks this. A haploid nucleus has one chromosome of every pair and so one copy of every gene.
Mitosis keeps the chromosome number and genome of the cell the same, whereas meiosis halves the chromosome number and generates diversity. — Mitosis maintains the chromosome number and genome of cells, producing genetically identical nuclei. Meiosis halves the chromosome number and generates genetic diversity among the nuclei it produces.
Mitosis doubles the chromosome number, which meiosis then halves, so together the two processes keep the number constant. — A student who thinks replication or mitosis doubles the chromosome number picks this. Mitosis maintains the number; it is fertilization that doubles it and meiosis that halves it.
Mitosis separates the two chromosomes of each homologous pair into different cells, whereas meiosis separates the sister chromatids. — A student who transfers the separation of homologues from meiosis to mitosis picks this. Mitosis separates sister chromatids, so both members of every pair go to each daughter cell.
4 How is the DNA of a eukaryotic chromosome condensed before nuclear division?
Answer and reasoning
The double helix itself twists ever more tightly, with no proteins involved, until the molecule is short enough to be seen. — A student who hears 'supercoiling' as more of the helix's own coiling picks this. The naked helix cannot be packed into a chromosome; the coiling is of DNA wound around histone proteins.
The DNA is wound around histone proteins, and the fibre so formed is supercoiled into a much shorter, thicker structure. — Condensation depends on histones: DNA wound around histones forms nucleosomes, and the DNA–protein fibre is then coiled and supercoiled, shortening the chromosome so it can be moved without tangling.
The DNA is already packed as a condensed chromosome throughout the cycle; staining at division merely makes it visible. — A student who takes the karyotype image as the permanent state of DNA picks this. Between divisions the DNA is extended for transcription and replication; it condenses only for nuclear division.
The DNA is copied, and the two new copies pack tightly against each other to form the short, visible chromosome. — A student who confuses replication with condensation picks this. Replication happens in interphase and produces two chromatids; it is supercoiling around histones, not copying, that shortens them.
5 Which statement about the phases of mitosis in a human cell is correct?
Answer and reasoning
In anaphase, the two chromosomes of each homologous pair are separated and moved to opposite poles. — A student who transfers the separation of homologues from meiosis I to mitosis picks this. Mitosis separates chromatids, keeping both homologues of every pair in each daughter nucleus.
In metaphase, the chromosomes line up at the equator while still enclosed within the nuclear envelope. — A student who thinks the nuclear envelope persists picks this. The envelope breaks down in prophase so that spindle microtubules can attach to the chromosomes; it re-forms in telophase.
In prophase, each chromosome is replicated so that it consists of two chromatids by the end of the phase. — A student who thinks chromatids are made when chromosomes appear picks this. Replication takes place in interphase; in prophase the two existing chromatids become visible as the chromosome condenses.
In anaphase, the sister chromatids of each chromosome are separated and moved to opposite poles. — Anaphase is the phase in which the centromeres divide and the two identical chromatids of every chromosome are moved to opposite poles, so that each pole receives a complete, identical set.
6 Which statement correctly outlines the two rounds of segregation in meiosis?
Answer and reasoning
In meiosis I the two chromosomes of each homologous pair separate; in meiosis II the sister chromatids of each chromosome separate. — Meiosis I is the reduction division: the two chromosomes of each bivalent go to opposite poles, so each nucleus has one chromosome of every pair. Meiosis II separates the two chromatids of each of those chromosomes.
In meiosis I the sister chromatids of each chromosome are separated, and in meiosis II the homologous chromosomes of each pair are separated. — A student who assumes the first division is 'the same as mitosis' picks this. The order is the reverse: homologues separate first, chromatids second.
In meiosis I the homologous chromosomes are separated; the DNA is then replicated so that meiosis II can separate the new chromatids. — A student who applies 'replicate then divide' to each division picks this. No replication occurs between the divisions; the chromatids separated in meiosis II were made before meiosis I.
In meiosis I each chromosome is split into its maternal chromatid and its paternal chromatid, and meiosis II shares these between nuclei. — A student who thinks the two chromatids come from the two parents picks this. Sister chromatids are identical copies; the maternal and paternal chromosomes are the two members of a homologous pair.
Sister chromatids of one chromosome exchange lengths of DNA, so that the two chromatids come to differ from each other. — A student who remembers 'chromatids exchange segments' without 'non-sister' picks this. Sister chromatids are identical, so exchanging DNA between them could produce no new combination.
The exchange of DNA alters the base sequence at the exchanged region, creating new alleles of the genes found there. — A student who confuses new combinations with new alleles picks this. Crossing over exchanges existing alleles between homologues; new alleles arise only by mutation.
The exchange between chromatids decides which chromosome of each pair moves to each pole of the cell in anaphase I. — A student who merges crossing over with random orientation picks this. Which homologue goes to which pole is decided by the random orientation of the bivalent at metaphase I, a separate process.
Non-sister chromatids of homologous chromosomes exchange lengths of DNA, producing new combinations of alleles. — Crossing over occurs in prophase I, while homologous chromosomes are paired as a bivalent. Non-sister chromatids exchange corresponding lengths of DNA, so each chromatid carries a new combination of the alleles present on the two homologues.
8 Which pair of examples correctly matches cell proliferation to its role? HL
Answer and reasoning
Cells throughout the stem and leaves of a plant dividing: growth; cells in an early-stage animal embryo dividing: repair of damage to the embryo. — A student who models plant growth on an animal embryo picks this. Mature stem and leaf cells do not normally divide; proliferation for growth happens in meristems, and in an early embryo it is for growth, not repair.
Cells in the meristems of a plant dividing: growth; cells beneath the surface of the skin dividing: routine replacement of cells shed from the surface. — Proliferation for growth in plants is confined to meristems. In skin, cells lost from the surface are continually replaced by proliferation of cells beneath it, an example of routine cell replacement.
Cells at the surface of the skin dividing to replace themselves as they wear away: cell replacement; cells in the meristems of a plant dividing: growth. — A student who assumes replacement happens where loss occurs picks this. The cells shed from the skin surface are not the ones that divide; routine replacement depends on proliferation of cells beneath the surface, which move outwards to replace those lost.
Cells at the edge of a skin wound enlarging to fill the gap: wound healing; cells in an early-stage animal embryo dividing: growth. — A student who pictures wound closure as stretching picks this. Healing of a skin wound is an example of proliferation: cells at the wound edge divide repeatedly to produce the new cells that fill the gap.
9 The mass of DNA per nucleus was measured for many cells from a growing tissue. Most nuclei contained 6.0 pg, a smaller group contained 12.0 pg, and a few contained values between 6.0 and 12.0 pg. Which interpretation is correct? HL
Answer and reasoning
The 12.0 pg nuclei have twice as many chromosomes as the 6.0 pg nuclei, so they belong to cells that have just completed mitosis. — A student who equates DNA mass with chromosome number picks this. Replication doubles the DNA without changing the chromosome number, and cells that have just completed mitosis are back at 6.0 pg.
The intermediate values are measurement errors, because the DNA content of a nucleus does not change during the inactivity of interphase. — A student who thinks interphase is a resting phase picks this. Interphase includes S phase, in which DNA is actively synthesized, so intermediate values are expected in cells caught during replication.
The 12.0 pg nuclei are in prophase, when the DNA is replicated to form the chromatids, and every interphase nucleus contains 6.0 pg. — A student who thinks chromatids are made in prophase picks this. Replication is completed in S phase of interphase, so G2 nuclei already contain 12.0 pg before mitosis begins.
The 6.0 pg nuclei are in G1, the 12.0 pg nuclei have completed S phase and are in G2 or entering mitosis, and the intermediate values are nuclei in S phase. — DNA is synthesized during S phase of interphase, so a nucleus that has completed replication contains twice the DNA of a G1 nucleus, and a nucleus caught during replication contains an intermediate amount. The chromosome number is the same in all of them.
10 Which statement correctly describes how mutations in genes that control the cell cycle lead to uncontrolled cell division? HL
Answer and reasoning
Mutations make both proto-oncogenes and tumour suppressor genes over-active, so both kinds of gene then stimulate division more strongly. — A student who treats all cancer-related mutations alike picks this. A tumour suppressor mutation is a loss of function: the product that would halt division no longer works.
An oncogene is a gene that a healthy cell does not possess, so uncontrolled division can begin only when one is introduced from outside. — A student who reads 'oncogene' as a foreign cancer gene picks this. Every cell has proto-oncogenes; a mutation in one of the cell's own genes converts it into an oncogene.
A mutation can turn a proto-oncogene into an oncogene that drives division, or stop a tumour suppressor gene restraining division. — Proto-oncogenes are normal genes whose products promote the cell cycle; a mutation can make one an oncogene with an over-active product. Tumour suppressor genes restrain the cycle; mutations that disable them remove that restraint. Either change results in uncontrolled division.
Mutations in cell-cycle genes occur in gametes, so uncontrolled division appears only in the offspring that inherit the mutated gene. — A student who assumes all mutations are inherited picks this. Most such mutations arise in somatic cells and cause a tumour in the individual in which they occur; they are not passed on.
Read the ones marked not yet in Learn, then Verify.
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20 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A dividing cell is examined after its two new nuclei have formed. Many small vesicles are seen gathering and fusing in a line across the equator of the cell, midway between the nuclei. Which conclusion is justified?
Answer and reasoning
It is a plant cell: the vesicles mark the line along which the cell membrane will be pinched inwards to split the cytoplasm. — A student who expects every cell to pinch in two picks this. Pinching by a contractile ring is the animal mechanism; a plant cell is divided by a cell plate built from the vesicles themselves.
It is a plant cell: the vesicles carry enzymes that soften the existing wall so that it can grow inwards across the cell. — A student who thinks the old wall closes like a furrow picks this. The existing wall takes no active part; the vesicles at the equator build a completely new wall that grows outwards.
It is a plant cell: the vesicles are assembling sections of new membrane and cell wall to separate the daughter cells. — Vesicles fusing at the equator to form a cell plate is the mechanism of cytokinesis in plant cells. The fused vesicle membranes become the new cell membranes and their contents become the new cell wall.
It is an animal cell: the vesicles are new organelles that have been made from scratch so that each daughter cell has a full set. — A student who thinks organelles are assembled from raw materials at division picks this. Organelles such as mitochondria are made by division of existing ones, and vesicles fusing at the equator identify a plant cell forming a cell plate.
2 Which statement about the division of the cytoplasm between two daughter cells is correct?
Answer and reasoning
The cytoplasm is divided equally in every cell division, so that each daughter cell receives exactly half of each type of organelle. — A student who has only seen symmetrical diagrams picks this. Division of the cytoplasm is usually equal but not in all cases; oogenesis and yeast budding are unequal.
A daughter cell that receives no mitochondria can synthesize new ones from proteins made on its ribosomes, so none need be shared. — A student who thinks a cell can build any structure from raw materials picks this. Mitochondria arise only from existing mitochondria, which is exactly why each daughter must receive at least one.
When cytoplasm is divided unequally, the daughter cell that receives more cytoplasm also receives the larger share of the chromosomes. — A student who equates the size of a cell with its share of everything picks this. The chromosomes are shared equally by nuclear division; only the cytoplasm is divided unequally.
Each daughter cell must receive at least one mitochondrion, because mitochondria form only by division of existing ones. — Mitochondria, like chloroplasts, can only be made by division of a pre-existing organelle. A daughter cell that received none could not make any, so at least one must be passed to each cell.
3 In human oogenesis, the first meiotic division produces a large secondary oocyte and a very small polar body. What does this unequal cytokinesis achieve?
Answer and reasoning
The oocyte keeps almost all the cytoplasm and organelles for the future zygote, while both cells receive a complete haploid set of chromosomes. — Unequal cytokinesis concentrates the cytoplasm, with its organelles and nutrient stores, in the one cell that can become an egg. Nuclear division has already shared the chromosomes equally, so the polar body has a full haploid set but almost no cytoplasm.
The oocyte receives a larger share of the chromosomes along with the extra cytoplasm, so the polar body has fewer chromosomes to be discarded. — A student who thinks a bigger cell must hold more genetic material picks this. Meiosis I gives each product one chromosome of every pair; the inequality is only in the cytoplasm.
The polar body is the result of a failed equal division; in normal oogenesis the cytoplasm is divided equally between two egg cells of equal size. — A student who believes cytokinesis is invariably equal picks this. Unequal division is the normal course of human oogenesis and is one of the guide's two examples of unequal cytokinesis.
The polar body is a fragment of cytoplasm pinched off without a nucleus, so the oocyte keeps all of the chromosomes from the division. — A student who pictures a bud or fragment forming without nuclear division picks this. The polar body receives one of the two nuclei produced by meiosis I; it is small because it receives almost no cytoplasm.
4 Nuclear division normally occurs before the cytoplasm of a cell is divided. What would result if a cell divided its cytoplasm into two without first dividing its nucleus?
Answer and reasoning
Each daughter cell would build itself a new nucleus from the DNA and proteins present in its share of the cytoplasm. — A student who thinks a nucleus can be assembled like any other organelle picks this. Chromosomes exist only as copies of previous chromosomes, so a nucleus can arise only by division of an existing nucleus.
Both daughter cells would receive a nucleus, because DNA replication has already doubled the number of chromosomes to provide two sets. — A student who thinks replication produces two sets of separate chromosomes picks this. Replication produces two chromatids in each chromosome inside one nucleus; only nuclear division can separate them into two nuclei.
One daughter cell would be anucleate: with no chromosomes it could not transcribe genes to replace its proteins or divide again. — Nuclear division is needed before cell division to avoid the production of anucleate cells. A cell without a nucleus has no genome, so it can make no new mRNA and cannot divide.
This could not happen, because splitting the cytoplasm is the final phase of mitosis and so must follow the division of the nucleus. — A student who treats cytokinesis as a phase of mitosis picks this. Cytokinesis is a separate process from nuclear division, which is why the guide emphasizes that nuclear division is needed first.
5 What is the structure of a chromosome after DNA replication and before anaphase?
Answer and reasoning
Two DNA molecules, one inherited from each parent, joined together to form a homologous pair. — A student who reads the X-shaped diagram as a homologous pair picks this. The two chromatids are identical copies made by replication; homologous chromosomes are separate chromosomes.
A single elongated DNA molecule, which is not copied to form a second chromatid until prophase. — A student who thinks the second chromatid appears when chromosomes condense picks this. Replication is complete in interphase; prophase only makes the two existing chromatids visible.
Two separate chromosomes, each with one DNA molecule, so that the chromosome number has doubled. — A student who counts each chromatid as a chromosome picks this. Until anaphase the two chromatids are held together and count as one chromosome, so the chromosome number is unchanged.
Two identical elongated DNA molecules, the sister chromatids, held together at the centromere. — After replication each chromosome consists of two elongated DNA molecules (chromatids), exact copies of each other, which remain held together until anaphase.
6 A cell of an organism has 8 chromosomes before its DNA is replicated. How many chromosomes and how many DNA molecules are in its nucleus at metaphase of mitosis?
Answer and reasoning
8 chromosomes, 16 DNA molecules — Replication before mitosis gives each of the 8 chromosomes two chromatids, each a DNA molecule, so there are 16 DNA molecules. The chromatids stay joined until anaphase, so the chromosome number is still 8.
16 chromosomes, 16 DNA molecules — A student who counts each chromatid as a chromosome picks this. Two chromatids held together at the centromere are one chromosome; the number does not double until anaphase separates them.
8 chromosomes, 8 DNA molecules — A student who thinks replication has not yet happened at metaphase picks this. DNA replication is a prerequisite for mitosis and is complete in interphase, so each chromosome already has two chromatids.
16 chromosomes, 32 molecules of DNA — A student who doubles the chromosome number at replication and then gives each 'new' chromosome two chromatids picks this. Replication doubles the DNA once; the chromosome number stays at 8.
7 In both mitosis and meiosis, chromosomes are moved to the poles of the cell during anaphase. What causes this movement?
Answer and reasoning
The spindle microtubules contract in the same way as muscle fibres, pulling the chromosomes towards the poles. — A student who takes 'the fibres pull' literally picks this. Microtubules cannot contract; they shorten by losing subunits, and the movement is produced by motor proteins.
The separated chromatids repel one another and so drift apart to opposite ends of the cell without any attachment. — A student who has seen anaphase drawn without a spindle picks this. Chromatids do not move on their own; each is attached at its centromere to microtubules and moved by motor proteins.
Microtubule motor proteins move the chromosomes along spindle microtubules, which shorten as the chromosomes travel. — Chromosome movement uses microtubules and microtubule motors. Motor proteins at the centromere move the chromosome along the microtubule towards the pole, and the microtubule shortens as it does so.
A ring of actin and myosin at each pole contracts, dragging the chromosomes attached to it away from the equator. — A student who merges the contractile ring with the spindle picks this. Actin and myosin form the ring that pinches an animal cell in cytokinesis; chromosomes are moved by microtubules and their motors.
8 A parent cell with 46 chromosomes divides by mitosis. Why are the two daughter cells genetically identical to each other and to the parent cell?
Answer and reasoning
Every chromosome was replicated into two identical chromatids before division, and one chromatid of every chromosome goes to each daughter cell. — Replication makes an exact copy of each chromosome as its sister chromatid. Anaphase sends one chromatid of every chromosome to each pole, so both daughter nuclei receive 46 chromosomes carrying the same genome as the parent.
Each daughter cell receives half of the parent cell's 46 chromosomes and then replicates them to restore the full set of 46. — A student who thinks division halves the genetic material picks this. Each daughter cell receives all 46 chromosomes, one chromatid of each; replicating 23 chromosomes could not recreate the other 23.
Each daughter cell receives one chromosome from each of the 23 homologous pairs, and both chromosomes of a pair carry the same genes. — A student who applies meiosis I to mitosis picks this. Homologues carry the same genes but may carry different alleles, so cells receiving different homologues would not be identical; mitosis gives each daughter both homologues of every pair.
Mitosis first doubles the number of chromosomes to 92, and then 46 of them are moved into each of the two daughter cells. — A student who counts chromatids as chromosomes picks this. Replication doubles the DNA, not the chromosome number; there are 46 chromosomes of two chromatids each, until anaphase separates the chromatids.
9 Two cells are seen in a stained micrograph of a root tip. Cell 1 has an intact nucleus in which no separate chromosomes can be distinguished. Cell 2 has no nuclear envelope, and its condensed chromosomes lie in a single line across the middle of the cell. Which identification is correct?
Answer and reasoning
Cell 1 is in prophase; cell 2 is in metaphase. — A student who counts interphase as the first stage of mitosis picks this. Prophase is recognized by condensed, visible chromosomes; a cell with an intact nucleus and no distinct chromosomes is in interphase.
Cell 1 is in interphase; cell 2 is in metaphase. — An intact nucleus with no distinct chromosomes shows a cell between divisions, in interphase. Condensed chromosomes aligned on one plane at the equator, with the nuclear envelope gone, define metaphase.
Cell 1 is in interphase; cell 2 is in prophase. — A student who identifies prophase by 'condensed but not yet separating' picks this. Chromosomes aligned in a single plane at the equator are the defining feature of metaphase; in prophase they are scattered.
Cell 1 is in interphase; cell 2 is in early anaphase. — A student who has swapped the names of the two middle phases picks this. Anaphase begins when the chromatids separate and move apart; while the chromosomes are still aligned on the equator the cell is in metaphase.
10 A student counted 100 cells in a micrograph of a growing root tip. 85 cells had an intact nucleus with no visible chromosomes; 6 had condensed chromosomes scattered through the cell; 3 had chromosomes aligned across the equator; 4 had two groups of chromatids moving apart; 2 had two groups of chromosomes at the poles with nuclear envelopes forming. Which conclusion is correct?
Answer and reasoning
All 100 cells are in mitosis, because the 85 with an intact nucleus and no visible chromosomes are in prophase. — A student who takes interphase to be a stage of mitosis picks this. An intact nucleus with no distinct chromosomes is the appearance of interphase; only the 15 cells with condensed chromosomes are in mitosis.
Nine of the cells are in metaphase, because scattered and aligned condensed chromosomes both lie in the middle of the cell. — A student who identifies metaphase by position rather than alignment picks this. Only the 3 cells with chromosomes aligned across the equator are in metaphase; the 6 with scattered chromosomes are in prophase.
About 15% of the cells are in mitosis, and prophase is the most frequent phase among the dividing cells. — The 85 cells with an intact nucleus are in interphase. The other 15 are in mitosis: 6 prophase, 3 metaphase, 4 anaphase and 2 telophase, so 15% of the cells are dividing and prophase is the commonest phase.
The tissue has stopped dividing, because far fewer than half of the cells show any phase of mitosis. — A student who expects mitosis to occupy most of the cycle picks this. Interphase is much the longest part of the cell cycle, so 15% of cells in mitosis is typical of an actively growing root tip.
11 A diploid nucleus of an organism with 2n = 14 undergoes meiosis. What is produced?
Answer and reasoning
Four diploid nuclei, each containing 14 chromosomes — A student who thinks the DNA is replicated again between the two divisions picks this. There is one replication before meiosis I only, so the two divisions halve the chromosome number.
Four haploid nuclei, each with half of the organism's genes — A student who equates half the chromosomes with half the genes picks this. Each haploid nucleus has one chromosome of every pair and therefore one copy of every gene.
Two haploid nuclei, each containing 7 chromosomes — A student who thinks meiosis is a single halving division picks this. Meiosis I produces two nuclei and meiosis II divides each again, giving four haploid nuclei.
Four haploid nuclei, each containing 7 chromosomes — Meiosis is a reduction division: two successive divisions produce four haploid nuclei from one diploid nucleus. With 2n = 14, each haploid nucleus has n = 7 chromosomes, one of each homologous pair.
Gametes could be produced by mitosis instead, because when two diploid gametes fuse the zygote they form is diploid like its parents. — A student who does not track the chromosomes through fertilization picks this. Two diploid gametes would give a zygote with four sets, and the number would double in every generation.
Fertilization combines the chromosomes of two gametes, so the number must be halved beforehand to keep it constant across generations. — Fusion of two gametes adds their chromosome sets together. Halving the number in meiosis means that fertilization restores the diploid number instead of doubling it, so the number stays constant from one generation to the next.
Each gamete must carry only half of the parent's genes, so that the offspring gets one half from each parent and no gene is duplicated. — A student who thinks haploid means half the genes picks this. Every haploid gamete carries a full set of genes; the offspring inherits two complete sets, one from each parent.
Mitosis doubles the chromosome number each time a body cell divides, so meiosis is needed to bring the number back down again. — A student who thinks mitosis doubles the chromosome number picks this. Mitosis maintains the number; it is fertilization that would double it, and meiosis compensates for fertilization.
13 A child has Down syndrome. Both parents have 46 chromosomes in each of their body cells. Which statement about the origin of the condition in this child is correct?
Answer and reasoning
A mutation in the zygote altered the base sequence of one gene on chromosome 21, so the child carries a faulty allele of that gene. — A student who thinks every genetic condition is a gene mutation picks this. The genes on chromosome 21 are normal; the condition arises because a whole extra copy of the chromosome is present in every cell.
One parent produced a gamete with 24 chromosomes because chromosome 21 failed to separate in meiosis, so the zygote had 47 chromosomes. — Non-disjunction is an error of meiosis: the two chromosomes 21, or their chromatids, fail to separate and both go into one gamete, which has 24 chromosomes. Fertilization by a normal gamete with 23 gives a zygote with three copies of chromosome 21 and 47 chromosomes in total.
The extra chromosome 21 was produced at fertilization, when the nuclei of a normal sperm and a normal egg fused and their chromosomes combined. — A student who places the error at the moment the zygote forms picks this. Fusion of two normal gametes gives 46 chromosomes; the extra copy was already present in one gamete because of non-disjunction in meiosis.
One parent's meiosis gave a gamete two copies of chromosome 21, and every other gamete from that division had the normal number of 23. — A student who has only seen the trisomic outcome drawn picks this. When both copies go to one pole, the other pole receives none, so another gamete from the same division lacks chromosome 21 altogether.
14 A species has a diploid number of 8. Ignoring crossing over, how many different combinations of maternal and paternal chromosomes can random orientation of bivalents produce in its gametes?
Answer and reasoning
16, because each of the 4 bivalents can be oriented in 2 ways, independently of the others — There are 4 bivalents (2n = 8, so n = 4). Each can be oriented with its maternal or its paternal chromosome facing a given pole, and the orientations are independent, so the number of combinations is 2⁴ = 16.
8, because each of the 4 bivalents contributes 2 possible orientations to the total — A student who adds the possibilities (2 + 2 + 2 + 2) or multiplies 2 × 4 picks this. Independent orientations multiply, giving 2 × 2 × 2 × 2 = 16.
256, because each of the 8 chromosomes can be oriented in 2 ways, independently of the others — A student who uses the diploid number instead of the number of bivalents picks this. Chromosomes orient as pairs, so the exponent is the number of bivalents, 4, not 8: 2⁴, not 2⁸.
4, because a gamete receives one chromosome from each of the 4 bivalents, giving 4 combinations — A student who counts the bivalents instead of multiplying their orientations picks this. Each of the 4 bivalents has 2 orientations, and independent orientations multiply: 2⁴ = 16.
15 Which statement correctly gives the sequence of phases in the cell cycle? HL
Answer and reasoning
Interphase, made up of G1 and G2, is followed by S, in which the DNA is replicated, and then by mitosis and cytokinesis. — A student who places replication just before mitosis picks this. S phase lies between G1 and G2; by G2 the DNA has already been replicated and the cell is growing and preparing for mitosis.
Interphase, a resting period in which the cell is inactive, is followed by mitosis and then by cytokinesis. — A student who takes 'interphase' to mean a pause picks this. Interphase is metabolically active: the cell grows, synthesizes proteins and DNA and increases its organelles.
Interphase, made up of G1, S and G2 in that order, is followed by mitosis and then by cytokinesis. — The cell cycle by which cells proliferate runs G1, S and G2 (the stages of interphase), then mitosis, then cytokinesis; the daughter cells then enter G1 of the next cycle.
Mitosis, which occupies most of the cycle, is followed by cytokinesis and then by a short interphase of G1, S and G2. — A student who judges the length of a phase by how much is taught about it picks this. Interphase takes most of the cycle; mitosis is a short part of it, and the sequence is interphase first.
16 The concentration of one cyclin was measured in cells passing through the cell cycle. It was low in G1, rose through S and G2, peaked as mitosis began and then fell sharply. When a chemical stopped the concentration of this cyclin from rising beyond half its usual peak, the cells remained in G2. Which conclusion is best supported? HL
Answer and reasoning
A threshold concentration of this cyclin must be reached to pass the checkpoint that allows the cell to enter mitosis. — The cyclin rises towards mitosis, and cells that cannot accumulate enough of it are held in G2. This is what is expected if a threshold level of a specific cyclin is required to pass the checkpoint into mitosis.
This cyclin acts as a brake on the cycle, so the cells could not enter mitosis because its concentration had failed to fall. — A student who thinks cyclins hold the cell back picks this. The cyclin rises before mitosis and the cells that could not raise it enough were the ones held back, so it is a signal to proceed, not a brake.
The concentration of a cyclin should rise continuously through the whole cycle, so the sharp fall shows that these cells had died. — A student who pictures one cyclin rising steadily picks this. Cyclin concentrations rise and fall during the cycle; the fall after the peak is normal, not a sign of death.
Once a cyclin reaches its peak it stays at that level, so the sharp fall shows that the cells had left the cycle. — A student who treats a checkpoint as a switch thrown once picks this. A cyclin's concentration falls again after its checkpoint has been passed; the fall is part of the normal cycle.
17 Which statement about the control of the cell cycle by cyclins is correct? HL
Answer and reasoning
A single cyclin increases steadily in concentration throughout the cycle, and its level at any moment tells the cell which phase it is in. — A student who remembers one rising line picks this. There are several cyclins, and each rises to a peak and then falls at its own point in the cycle.
The concentrations of different cyclins rise and fall at different points in the cycle, and a threshold level of a specific cyclin is needed to pass each checkpoint. — Cyclins are a group of proteins whose concentrations increase and decrease during the cycle. Each checkpoint is passed only when the concentration of a particular cyclin has reached a threshold level.
Cyclins hold the cell at each checkpoint, and the cell passes through only when the concentration of the relevant cyclin falls below a threshold. — A student who reads 'control' as 'stop' picks this. A rise in cyclin concentration above the threshold is what allows the checkpoint to be passed.
Once the concentration of a cyclin has risen above the threshold for a checkpoint, it stays at that high level for the rest of the cycle. — A student who pictures a one-way switch picks this. After the checkpoint is passed the cyclin's concentration decreases again, which is why the cycle can repeat.
18 Which statement correctly distinguishes between types of tumour? HL
Answer and reasoning
A benign tumour is a harmless overgrowth of normal tissue, not the result of uncontrolled division, so it is not a true tumour. — A student who takes 'benign' to mean 'normal' picks this. A benign tumour is formed by uncontrolled division; it is benign because its cells do not invade or spread, not because they are normal.
A secondary tumour is a second tumour that arises independently in a different tissue, from a different set of mutations. — A student who hears 'secondary' as 'another one' picks this. A secondary tumour is formed by cells that spread from the primary tumour by metastasis; it is not an independent tumour.
A malignant tumour is one that grows faster and reaches a larger size than a benign tumour; size and rate of growth define which is which. — A student who equates cancer with speed of growth picks this. Tumours of both kinds vary in growth rate; malignancy is defined by invasion and the capacity for metastasis.
A malignant tumour invades neighbouring tissue and can spread to form secondary tumours; a benign tumour does neither and does not cause cancer. — Malignant tumours invade neighbouring tissue and have the capacity for metastasis, forming secondary tumours; they cause cancer. Benign tumours stay in one place, do not invade or spread and do not cause cancer.
19 A student examined 240 cells in a micrograph of a growing tissue. 204 cells had an intact nucleus with no visible chromosomes, and the remaining 36 cells had condensed chromosomes visible. What is the mitotic index of the sample? HL
Answer and reasoning
0.15 — Cells with condensed chromosomes are in mitosis; those with an intact nucleus and no visible chromosomes are in interphase. Mitotic index = number of cells in mitosis ÷ total number of cells observed = 36 ÷ 240 = 0.15 (15%).
0.18 — A student who divides the dividing cells by the non-dividing cells picks this: 36 ÷ 204 = 0.18. The denominator of the mitotic index is the total number of cells, 240.
0.85 — A student who expects most cells to be in mitosis assumes the larger group must be the dividing one and calculates 204 ÷ 240. An intact nucleus with no visible chromosomes is the appearance of interphase; only the 36 cells with condensed chromosomes are in mitosis.
1.00 — A student who counts a cell with an intact nucleus as being in prophase treats all 240 cells as dividing, giving 240 ÷ 240. Interphase is not a phase of mitosis, so the 204 cells with an intact nucleus are excluded.
20 The mitotic index was determined for three samples: cells from a root-tip meristem, 0.12; cells from a tumour removed from a patient, 0.10; cells from a mature leaf, 0.00. A student concluded that the tumour must be benign because its mitotic index is lower than that of a healthy meristem. Which evaluation of this conclusion is correct? HL
Answer and reasoning
It is not justified: a mitotic index as high as 0.12 shows that the meristem itself is cancerous, so it cannot serve as a healthy tissue for comparison. — A student who reads all rapid division as cancer picks this. A meristem is a normal tissue that proliferates for growth; a high mitotic index there is expected and is not evidence of a tumour.
It is not justified: the mitotic index measures how many cells are dividing, but malignancy depends on the capacity to invade tissue and metastasize. — A mitotic index shows the rate of proliferation. Benign and malignant tumours both divide uncontrollably and vary in rate; malignancy is defined by invasion of neighbouring tissue and metastasis, which a count of dividing cells cannot reveal.
It is justified: malignant tumours are defined by dividing faster than normal tissue, so a tumour dividing more slowly than a meristem is benign. — A student who defines malignancy by growth rate picks this. Rates of division vary among tumours of both kinds; the defining feature of a malignant tumour is invasion and the capacity for metastasis.
It is not justified: benign tumours have a mitotic index of zero because their cells have stopped dividing, so this tumour must be malignant. — A student who thinks 'benign' means the cells are not dividing picks this. A benign tumour is itself the product of uncontrolled division and has a mitotic index well above zero; the index cannot settle the question either way.
That was your twenty minutes. Real practice on D2.1 is past-paper questions marked against the mark scheme.
What the exam asks of D2.1
Paper 1A shows a micrograph or diagram and asks for the phase, or asks you to pick the true statement about meiosis I versus II. Paper 1B may give a cell population to count for mitotic index or a cyclin graph to interpret. Paper 2 uses *outline* for the phases and *explain* for how meiosis produces variation: name the mechanism, say when it happens, then state the effect on combinations. HL questions use *distinguish* for benign and malignant tumours and *explain* for how a single mutation can lead to uncontrolled division.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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