All cells arise from other cells
AQA
A-Level
Differentiation makes cells specialised, and many differentiated cells no longer divide. A mature mammalian red blood cell loses its nucleus, so it has no nuclear DNA to replicate and cannot divide. A phloem sieve tube element also loses its nucleus at maturity; it retains mitochondrial and plastid DNA but lacks nuclear DNA, so it cannot divide. Neurones and mature skeletal muscle fibres are terminally differentiated: they leave the cell cycle and enter a non-dividing state called G0. Xylem vessel elements are dead at maturity. Some differentiated cells, such as hepatocytes, can re-enter the cell cycle to repair tissue, but division is mostly restricted to specific populations: stem cells in bone marrow, intestinal crypts and the basal layer of the skin, and plant meristems at root and shoot tips.
56
Objectives
36
Exam Tips
65
Pitfalls
68
Key Terms
69
Mark Points
Subtopics in this area
Within multicellular organisms, not all cells retain the ability to divide.
Eukaryotic cells that do retain the ability to divide show a cell cycle.
DNA replication occurs during the interphase of the cell cycle.
Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication.
The behaviour of chromosomes during interphase, prophase, metaphase, anaphase and telophase of mitosis.
The role of spindle fibres attached to centromeres in the separation of chromatids.
Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells.
Meiosis is covered in section 3.4.3 Students should be able to: recognise the stages of the cell cycle: interphase, prophase, metaphase, anaphase and telophase (including cytokinesis) explain the appearance of cells in each stage of mitosis.
Mitosis is a controlled process.
Uncontrolled cell division can lead to the formation of tumours and of cancers.
Many cancer treatments are directed at controlling the rate of cell division.
Binary fission in prokaryotic cells involves: replication of the circular DNA and of plasmids division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids.
Being non-living, viruses do not undergo cell division. Following injection of their nucleic acid, the infected host cell replicates the virus particles.
Required practical 2: Preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index.
All cells arise from other cells Revision Guide
Learning Objectives
What you need to know and understand
- Name two specialised cells that cannot divide and give the structural reason in each case.
- Explain why a root tip is chosen over other plant tissue when preparing a slide to observe mitosis.
- Suggest why damage to cardiac muscle after a heart attack is not repaired in the way a skin wound is.
- Identify, from a description of a tissue, whether it is likely to contain cells that are still in the cell cycle.
- Name the stages of the cell cycle in order and state what happens to the DNA in each.
- Explain why interphase is the longest stage and why calling it a resting stage is wrong.
- Complete a table distinguishing a cell cycle involving mitosis from binary fission, using DNA shape, cell type and number of daughter cells.
- Predict how the proportion of cells seen in each stage on a slide relates to the length of that stage.
- State when in the cell cycle DNA is replicated.
- Explain the difference between DNA mass and chromosome number during interphase.
- Describe the structure of a chromosome after S phase.
- Explain the importance of DNA replication occurring before mitosis.
- Explain why the two cells produced by mitosis are genetically identical, referring to replication in interphase.
- State three roles of mitosis in a multicellular organism.
- Contrast mitosis with meiosis in terms of number of divisions, number of daughter cells and genetic variation.
- Complete a comparison of mitosis and binary fission using DNA shape, cell type and number of daughter cells.
- Describe what happens to the chromosomes in each named stage of mitosis, in order.
- Identify the stage of mitosis shown in a micrograph and justify the answer from what is visible.
- Explain why chromatids appear V shaped as they move towards the poles in anaphase.
- Explain why most cells on a stained root tip slide are in interphase rather than in a mitotic stage.
- Describe where spindle fibres attach to a chromosome and why attachment from both poles is needed.
- Explain how shortening spindle fibres separates sister chromatids in anaphase.
- Suggest the effect on the daughter cells of a drug that prevents spindle fibres forming.
- Explain why a cell whose mitochondria are inhibited cannot complete anaphase.
- Describe how the cytoplasm divides in an animal cell and in a plant cell, and explain the difference.
- Suggest what a cell would look like if mitosis occurred but cytokinesis was prevented.
- Explain why cytokinesis is described as usually occurring rather than always occurring.
- Interpret data from a drug that inhibits cytokinesis and state what happens to cell number and nuclei.
- Name the stage of mitosis shown in a micrograph and explain the appearance from the event taking place.
- Distinguish metaphase from anaphase in an image by the position and movement of the chromatids.
- Explain what a cell containing two nuclei and one cytoplasm shows about which stage was blocked.
- Recognise interphase cells on a slide and explain why they are the most numerous.
- Describe two checkpoints in the cell cycle and state what each one verifies before division continues.
- Explain how genes control whether the cell cycle continues or is halted.
- Explain how apoptosis prevents a cell with damaged DNA from producing damaged daughter cells.
- Predict the effect on a tissue of losing the function of a gene that normally halts the cell cycle.
- Explain how a single base change in DNA can lead to uncontrolled cell division, working through the protein produced.
- Distinguish a benign tumour from a malignant tumour using rate of growth, encapsulation and spread.
- Explain why the immune system can identify tumour cells as abnormal body cells.
- Suggest why a blood test may detect a cancer before a tumour can be seen or felt.
- Explain how a drug that prevents DNA replication stops a cancer cell completing the cell cycle.
- Explain how a spindle inhibitor blocks mitosis and state at which stage division halts.
- Suggest why hair loss and reduced immunity are common side effects of chemotherapy.
- Evaluate data showing the effect of increasing drug concentration on the number of cells in a culture.
- Describe binary fission in the order the specification gives, from replication of the circular DNA to division of the cytoplasm.
- Explain why the two daughter cells may contain different numbers of plasmids but the same circular DNA.
- Complete a table distinguishing binary fission from a cell cycle involving mitosis.
- Explain why bacterial numbers can rise exponentially under favourable conditions.
- Describe viral replication in sequence, from attachment through to release of new virus particles.
- Explain why viral replication is not cell division and cannot happen outside a host cell.
- Explain why a virus can infect only certain cell types, referring to attachment proteins and receptors.
- Suggest why a drug that inhibits reverse transcriptase reduces the replication of a retrovirus.
- Describe how to prepare a stained root tip squash and explain the purpose of the acid, the stain and the squash.
- Describe how to set up and use an optical microscope to identify stages of mitosis.
- Calculate a mitotic index from counts of cells in mitosis and total cells, and express it as a percentage.
- Suggest two reasons other than counting errors why students obtain different mitotic indices from the same method.
Marking Points
Key points examiners look for in your answers
- One mark for naming a cell that cannot divide and giving the structural reason, for example a mature red blood cell lacks a nucleus and so cannot replicate its DNA.
- One mark for explaining that terminally differentiated cells leave the cell cycle and enter a non-dividing state known as G0.
- One mark for naming a site where actively dividing cells are found, such as a plant meristem, bone marrow or an intestinal crypt.
- One mark for linking the retention of the ability to divide to the growth, repair or replacement of specific tissues.
- One mark for distinguishing differentiation (becoming specialised) from division (producing more cells), for example a neurone is specialised but does not divide.
- one mark for interphase being the longest stage and being made up of G₁, S and G₂
- one mark for the linear DNA being replicated during S phase so that each chromosome becomes two identical sister chromatids
- one mark for mitosis followed by cytokinesis producing two genetically identical daughter cells
- one mark for placing the cell cycle in eukaryotic cells, with replication of linear DNA, in contrast to binary fission and circular DNA in prokaryotes
- one mark for growth and protein synthesis during G₁ and G₂ preparing the cell for division
- State that DNA replication occurs during interphase, specifically the S (synthesis) phase, before mitosis.
- Explain that the mass or quantity of DNA doubles during interphase, but the chromosome number remains constant.
- Describe the product of replication as a chromosome consisting of two identical sister chromatids joined by a centromere.
- Explain that this process ensures each daughter cell receives genetically identical copies of DNA following cell division.
- one mark for two daughter cells being produced, each genetically identical to the parent cell and to each other
- one mark for the chromosome number being maintained because replicated sister chromatids are separated, one of each pair going to each cell
- one mark for placing mitosis in eukaryotic cells and linking it to replication of linear DNA
- one mark for a correct role: growth, repair, replacement of cells or asexual reproduction
- one mark for stating that the identical copies were made by the parent cell during DNA replication in interphase
- one mark for prophase: chromosomes condense and become visible, the nuclear envelope breaks down and the spindle forms
- one mark for metaphase: chromosomes line up along the equator, attached by their centromeres to spindle fibres
- one mark for anaphase: the centromeres divide and the chromatids are pulled to opposite poles of the cell
- one mark for telophase: chromatids reach the poles, uncoil, and a nuclear envelope reforms around each group
- one mark for correctly naming a stage from an image and explaining the appearance by the event causing it
- one mark for spindle fibres being protein microtubules extending from the poles of the cell
- one mark for spindle fibres attaching to the centromere of each chromosome, with attachment from both poles
- one mark for the centromere dividing and the fibres shortening in anaphase, pulling sister chromatids to opposite poles
- one mark for ATP being required for the spindle fibres to shorten
- one mark for the consequence that each pole receives one complete and identical set of chromatids
- one mark for cytokinesis being division of the cytoplasm, after telophase, producing two separate cells
- one mark for the animal cell mechanism: the cell-surface membrane constricts or pinches inwards to form a cleavage furrow
- one mark for the plant cell mechanism: vesicles fuse along the equator to form a cell plate, because the cellulose wall cannot pinch inwards
- one mark for recognising that if cytokinesis is prevented the cell is left with two nuclei, appearing to be stopped at telophase
- one mark for identifying that the block is on cytoplasm or new membrane formation, not on nuclear division
- one mark for naming the stage shown, using the mitotic name and not a meiotic one
- one mark for the visible evidence, for example chromosomes aligned along the equator, or two groups of chromatids at opposite poles
- one mark for explaining the appearance by the underlying event, such as spindle fibres shortening and pulling chromatids apart
- one mark for identifying a cell containing two nuclei as one in which cytokinesis has been prevented after telophase
- one mark for recognising interphase from an intact nucleus with no visible individual chromosomes
- One mark for stating that the cell cycle is regulated at checkpoints, such as before DNA replication, before mitosis, or before chromatids separate.
- One mark for giving a specific checkpoint function: checking cell size, checking DNA has replicated completely, checking DNA is undamaged, or checking spindle attachment.
- One mark for explaining that some genes code for proteins that stimulate division while others code for proteins that halt division or trigger apoptosis.
- One mark for stating that the rate of cell division is balanced against the rate of cell death so that tissue size is maintained.
- One mark for explaining that damaged cells are destroyed by apoptosis rather than being allowed to divide.
- One mark for a change in the DNA base sequence or triplet caused by mutation.
- One mark for a resulting change in the sequence of amino acids, and so in the primary and tertiary structure of the protein.
- One mark for rapid or uncontrollable cell division, or for cell division that can no longer be regulated.
- One mark for the mass of cells forming a tumour, with malignant tumours invading surrounding tissue or spreading to form secondary tumours.
- One mark for the mutation affecting a gene that controls the cell cycle.
- A drug preventing DNA replication stops the cell completing interphase and dividing.
- A drug preventing spindle formation stops chromatids separating during anaphase.
- Treatments affect rapidly dividing cells, damaging both tumour cells and normal rapidly dividing tissues.
- A named side effect explained by damage to a normal rapidly dividing tissue, such as hair loss or reduced white blood cell production.
- The cell may undergo apoptosis once the cycle is arrested at a checkpoint.
- Replication of the circular DNA, with plasmids replicated separately and independently of it.
- Division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA.
- Each daughter cell receives a variable number of plasmid copies, because plasmid segregation is not perfectly equal.
- Binary fission occurs in prokaryotes and contrasts with the eukaryotic cell cycle, in which DNA replication happens in interphase (S phase) and mitosis is the nuclear division that separates the replicated linear DNA.
- No spindle forms and no nuclear envelope breaks down, because prokaryotes have no nucleus; the circular DNA is not wound around histones.
- Attachment proteins on the virus bind to complementary receptors on the host cell.
- Viral nucleic acid enters the cell by injection or engulfment.
- The nucleic acid is replicated inside the cell, or reverse transcriptase makes DNA from viral RNA.
- The host cell produces viral proteins, such as capsid proteins or enzymes.
- New virus particles are assembled and released by lysis or budding.
- Use the root tip because it contains the meristem where dividing cells are located
- Hydrolyse in hot hydrochloric acid to separate the cells
- Squash to a single layer so that light can pass through the specimen
- Stain the sample so that chromosomes become visible for identifying stages of mitosis
- Calculate mitotic index as the number of cells in mitosis divided by the total number of cells counted
Examiner Tips
Expert advice for maximising your marks
- 💡When asked why a specific tissue does not repair, name the cell, state it is terminally differentiated or lacks a nucleus, and give the consequence.
- 💡Learn one animal and one plant site of active division (for example bone marrow and root tip meristem) to use as concrete examples.
- 💡If a table asks you to tick features against mitosis and binary fission, remember replication of linear DNA and eukaryotic cells belong to the cell cycle, replication of circular DNA and prokaryotic cells to binary fission, and two daughter cells to both.
- 💡Quote the sub-stages G₁, S and G₂ by name; interphase alone is often not enough for a describe mark.
- 💡Use the proportions of the cycle to explain why most cells in a stained root tip are in interphase.
- 💡When interpreting graphs of DNA mass during the cell cycle, look for the doubling of DNA mass to identify the S phase of interphase.
- 💡Always distinguish clearly between 'chromosomes' and 'chromatids' in your answers to avoid losing marks on cell cycle questions.
- 💡Use the words genetically identical; similar and the same are both too weak for the mark.
- 💡If a table contrasts mitosis with binary fission, tick two daughter cells for both and reserve linear DNA and eukaryotic cells for mitosis.
- 💡Mention interphase replication whenever you explain why the daughter cells are identical; it is the reason, not an aside.
- 💡Learn one distinguishing visible feature per stage so you can identify a micrograph in seconds: scattered threads, a single line, two moving groups, two dense groups.
- 💡Chromatids is the safer word from anaphase onwards; chromosomes is accepted but homologous chromosomes is rejected.
- 💡In a describe the behaviour question, track the chromosomes themselves and do not drift into describing the spindle only.
- 💡Name the centromere explicitly; answers that only say the spindle pulls the chromosomes apart usually lose the attachment mark.
- 💡Add the ATP requirement when the question mentions mitochondria near the spindle or asks why respiration inhibitors stop mitosis.
- 💡If asked to predict the effect of a spindle inhibitor, say chromatids cannot separate so daughter cells receive an unequal number of chromosomes.
- 💡When a stem says a substance stimulates cytokinesis, the predicted effect of inhibiting it is binucleate cells, not fewer mitoses.
- 💡Use the cellulose cell wall as the reason for the difference between plant and animal cytokinesis; it is the explanation that carries the mark.
- 💡Say cytoplasm divides, not the cell splits, so it is clear which part of division you mean.
- 💡Answer in two parts, stage then cause, because the two halves usually carry a mark each.
- 💡Check the stem for the word mitosis before writing any stage name, and never add a Roman numeral.
- 💡Use the number of visible groups of chromosomes as your first clue: none, scattered, one line, two moving groups, two settled groups.
- 💡Give a checkpoint a job: it is not enough to say the cycle is checked, you must say what is checked.
- 💡Link control back to genes whenever a question mentions mutation; the mutation must affect a gene controlling the cycle.
- 💡Build the chain in order, DNA base sequence to amino acid sequence to tertiary structure to loss of function to uncontrolled division; each link can carry a mark.
- 💡Use the word division rather than growth when describing the process, and when comparing benign and malignant tumours give the same feature for both sides, for example rate of growth, capsule or invasion.
- 💡Name the specific stage of the cell cycle the treatment acts on, such as S phase or anaphase, to provide a complete explanation.
- 💡For side effects, explicitly name the normal tissue, state that it divides rapidly, and link it to the symptom.
- 💡If data show cell number falling with dose, describe the trend first and then explain it using cell cycle mechanisms.
- 💡In a comparison table, replication of circular DNA and no spindle belong to binary fission; replication of linear DNA in interphase and spindle formation in mitosis belong to the eukaryotic cell cycle; two daughter cells applies to both.
- 💡Use the word variable about plasmid number; it is the detail the specification singles out.
- 💡Do not describe stages by name; binary fission has none, so a staged answer signals you have imported mitosis.
- 💡Describe viral replication as a clear, chronological sequence of events from attachment to release.
- 💡Use the term 'nucleic acid' rather than 'DNA' when the virus is unspecified, as it could contain either DNA or RNA.
- 💡Take a mean across several fields of view and state this; a single field is not representative.
- 💡Show both counts used in the mitotic index calculation, not just the final figure.
Common Mistakes
Pitfalls to avoid in your exam answers
- Stating red blood cells do not divide without giving the structural reason, which is that they lack a nucleus. Correction: state that the nucleus is absent, so nuclear DNA cannot be replicated.
- Saying a phloem sieve tube element has no DNA at all. Correction: it lacks a nucleus and nuclear DNA but retains mitochondrial and plastid DNA; the absence of nuclear DNA is why it cannot divide.
- Assuming every cell in a multicellular organism is actively progressing through the cell cycle, whereas many enter G0. Correction: state that many differentiated cells have left the cycle.
- Sampling a mature region of a plant root rather than the meristematic tip when a dividing cell population is required. Correction: use the root tip, where meristematic cells are dividing.
- Confusing differentiation, which is becoming specialised, with division, which is producing more cells. Correction: keep the two processes distinct.
- calling interphase a resting stage, or saying nothing happens during it
- drawing the cycle with mitosis taking up most of the time
- listing interphase as one of the stages of mitosis rather than of the cell cycle
- attributing a cell cycle to bacteria, which divide by binary fission instead
- saying the DNA replicates during mitosis, when replication occurs in S phase of interphase
- Stating that the chromosome number doubles during interphase; Correction: The mass of DNA doubles, but the chromosome number remains the same until the chromatids separate in anaphase.
- Placing DNA replication in prophase; Correction: Replication occurs strictly during interphase, before the stages of mitosis begin.
- Confusing chromatids with homologous chromosomes; Correction: Sister chromatids are identical copies joined at a centromere, whereas homologous chromosomes are pairs of chromosomes (one maternal, one paternal) that are not identical.
- saying mitosis produces four daughter cells or haploid cells, importing meiosis into the answer
- claiming mitosis halves or doubles the chromosome number
- treating mitosis as the whole cell cycle rather than the nuclear division within it
- writing that mitosis creates genetic variation
- saying the daughter cells are similar rather than genetically identical
- naming a stage anaphase I when the question is about mitosis; the mark scheme rejects the meiotic label outright
- saying homologous chromosomes are separated in anaphase, when it is sister chromatids, with homologous chromosomes explicitly rejected
- describing chromosomes lining up in pairs at the equator in metaphase, which is metaphase I of meiosis
- saying individual chromosomes are visible during interphase
- leaving out the breakdown and reformation of the nuclear envelope, which carries marks in prophase and telophase
- saying spindle fibres attach to the ends or the arms of a chromosome rather than to the centromere
- writing that the spindle pushes the chromatids apart instead of shortening and pulling them
- calling the separating structures homologous chromosomes rather than sister chromatids
- forgetting that a chromatid is dragged centromere first, which is why it looks V shaped
- assuming plant cells cannot form a spindle because they lack centrioles
- listing cytokinesis as a fifth stage of mitosis instead of a separate division of the cytoplasm
- describing a cell plate forming in an animal cell, or a cleavage furrow in a plant cell
- saying a drug that blocks cytokinesis stops DNA replication or prevents mitosis, when nuclear division has already finished
- assuming organelles are divided in exactly equal numbers between the daughter cells
- omitting the observable evidence, two nuclei in one cell, when asked what a cytokinesis inhibitor would produce
- giving a meiotic name such as anaphase I for a mitotic figure, which mark schemes reject
- explaining the appearance by restating it, for example anaphase because it looks like anaphase
- calling the separating structures homologous chromosomes instead of chromatids
- confusing metaphase with anaphase when the chromatids have only just begun to move apart
- describing the whole of mitosis when the question asks only for the stage shown and why
- Writing that mitosis is controlled by the nucleus without naming a checkpoint, a gene or apoptosis. Correction: name a specific checkpoint, gene or process.
- Confusing control of the cell cycle with a controlled variable in an experiment because the question uses the word control. Correction: in this context control means regulation of the cycle.
- Swapping the roles of genes that stimulate division and genes that halt it. Correction: state clearly which type stimulates and which type halts the cycle.
- Describing apoptosis as accidental cell damage and death rather than programmed cell death. Correction: apoptosis is a controlled, programmed process.
- Saying cells divide when the organism needs them to, with no mechanism given. Correction: give the mechanism, such as checkpoint control or gene regulation.
- Writing that the protein is not formed, or that different amino acids are formed. Correction: the credited idea is a change in the sequence of amino acids.
- Writing growth or cell reproduction where the question requires cell division or cell replication. Correction: use the term cell division.
- Treating every tumour as a cancer, when only malignant tumours are cancerous. Correction: benign tumours are not cancerous.
- Saying the cell wants to divide or decides to divide instead of saying regulation has been lost. Correction: state that regulation has been lost.
- Describing metastasis as the tumour growing larger rather than as cells spreading to a new site. Correction: metastasis is the spread of cells to form secondary tumours.
- Saying drugs kill only cancer cells, when they act on any rapidly dividing cell.
- Saying a spindle inhibitor prevents DNA replication, or a replication inhibitor prevents spindle formation.
- Explaining side effects by the drug being poisonous rather than its effect on normal dividing tissue.
- Claiming treatment repairs or removes the mutation from the DNA.
- Writing that treatment stops growth rather than stopping cell division.
- Writing that binary fission involves mitosis, a spindle, or chromosomes lining up on an equator. Correction: binary fission has none of these; mitosis is nuclear division in eukaryotes.
- Saying each daughter cell receives an identical number of plasmids. Correction: plasmid number is variable because segregation is not perfectly equal.
- Confusing binary fission with meiosis and claiming four daughter cells. Correction: binary fission produces two daughter cells.
- Describing the circular DNA as a chromosome without qualification. Correction: the specification refers to circular DNA, so use that term.
- Stating that DNA replication occurs during mitosis. Correction: replication occurs in interphase (S phase); mitosis separates the already-replicated DNA.
- Describing the virus as dividing, growing, or undergoing mitosis.
- Writing that a receptor protein on the virus binds the cell; the virus carries the attachment protein and the host carries the receptor.
- Claiming the virus makes its own proteins, when the host cell's ribosomes do.
- Listing steps out of order, such as stating proteins are made before the nucleic acid enters the cell.
- Counting only the dividing cells and forgetting that the denominator is every cell in the field, including those in interphase
- Explaining different mitotic indices as counting errors, rather than biological or sampling reasons such as different fields of view
- Twisting the coverslip during the squash, which tears cells instead of spreading them into one layer