Biology  by Bradford
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D2.1 Cell and nuclear division: revision map

The subtopic in the middle and twelve tasks round it: label five diagrams, complete two tables, put a process in order and fill 63 gaps. Every standard level statement has a box; the four boxes for higher level only are dashed and badged HL, and drop off for an SL class. Type into the blanks and check them, or print it on A4, Letter or A3 with an answer key.

D2.1 practice questionsThe whole D2 map

D2.1

Cell and nuclear division

Cells · Continuity and changeName

D2.1.8Name the phase

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D2.1.11Sources of variation

Early in meiosis I, homologous chromosomes pair up as a . chromatids cross at a and swap matching sections, giving new combinations of alleles. Then each pair lines up facing either pole by chance, , so a cell with n pairs can give combinations of chromosomes in its gametes: over 8 million for the 23 pairs in a human.

D2.1.12, D2.1.13, D2.1.14The cell cycleHL

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Cells proliferate by this cycle: for growth, in plant and early embryos, and to replace cells lost from the and heal its wounds. Interphase is a busy time: the cell makes proteins and other components, and its grow and divide, as do chloroplasts in a plant.

D2.1.16, D2.1.17When division runs wildHL

A mutation can turn a gene that stimulates division into an , which keeps division switched on, or knock out a gene that normally holds it back. Cells then divide out of control, forming a tumour. A tumour is a cancer: it invades neighbouring tissue, and cells breaking away from this primary tumour can form tumours elsewhere, . A benign tumour grows more slowly and stays where it is.

D2.1.4, D2.1.9Mitosis and meiosis

MitosisMeiosis
Divisions of the nucleusone
From one diploid nucleustwo diploid nucleifour nuclei
Chromosome numberhalved
Daughter nuclei are genetically
Used forgrowth, repair, asexual reproduction, in a sexual life cycle

The nucleus divides before the cell does, so that no daughter cell is left , with no nucleus at all.

D2.1.5, D2.1.6Ready to divide

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Before mitosis or meiosis every DNA molecule is , and the two copies stay held together until . The DNA winds round and then coils on itself again and again, , making a short, thick chromosome that can be moved without tangling. Motor proteins do the moving.

D2.1Cell and nuclear division

D2.1.9, D2.1.10Two rounds of separation

Meiosis has two divisions. In the first, the two chromosomes of each pair go to opposite poles; in the second, the sister separate. If a pair fails to separate, , a gamete ends up with an extra chromosome. A child from such a gamete with three copies of chromosome has .

D2.1.15Timing the cycleHL

timeconcentration

Proteins called rise and fall in concentration through the cell cycle. A cell can pass each only when a particular one reaches a level; that one then again.

D2.1.1, D2.1.2, D2.1.3Splitting the cytoplasm

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Cytokinesis usually splits the cytoplasm , but not in human , where one cell keeps nearly all of it, nor in yeast, which reproduces by . Every daughter cell needs at least one , as these can only be made by the division of existing ones.

D2.1.7From one cell to two

Number the steps in order.

  1. The cytoplasm divides, giving two cells.
  2. Sister chromatids separate and are pulled to opposite poles.
  3. A nuclear membrane forms round each group, and the chromosomes uncoil.
  4. Chromosomes condense and become visible, and the nuclear membrane breaks down.
  5. Microtubules attach at the centromeres and line the chromosomes up across the middle of the cell.

Each new nucleus gets one chromatid of every chromosome, so the two are genetically .

D2.1.17Mitotic indexHL

SampleCells in mitosisCells countedMitotic index
Normal skin6200
Benign tumour2000.10
Malignant tumour40160

Mitotic index = number of cells in mitosis ÷ . Cells in mitosis are picked out by their visible, condensed .

D2.1 examSix stages of meiosis in a cell with 2n =

Six stages of meiosis in a cell with 2n =1234
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Word bank 0.030.2520212ⁿanaphaseanaphase Ianaphase IIanucleatebivalentbuddingcell platecell wallcentromerecheckpointchiasmachromatidschromosomescontractile ringcyclinscytokinesisdifferentDown syndromeequallyfallsG1G2gameteshaploidhistoneshomologousidenticalmaintainedmalignantmeristemsmetaphasemetaphase Imetaphase IImetastasismicrotubulemitochondriamitochondrionmitosisnon-disjunctionNon-sisteroncogeneoogenesisprophaserandom orientationreplicatedS phasesecondarysister chromatidskinsupercoilingtelophasethresholdtotal number of cellstumour suppressortwovesicle

Answer key

Every box on the sheet with its gaps filled in, as the subtopic reads in full.

  1. LabelSplitting the cytoplasm

    1. contractile ring
    2. vesicle
    3. cell plate
    4. cell wall

    Cytokinesis usually splits the cytoplasm equally, but not in human oogenesis, where one cell keeps nearly all of it, nor in yeast, which reproduces by budding. Every daughter cell needs at least one mitochondrion, as these can only be made by the division of existing ones.

  2. TableMitosis and meiosis

    MitosisMeiosis
    Divisions of the nucleusonetwo
    From one diploid nucleustwo diploid nucleifour haploid nuclei
    Chromosome numbermaintainedhalved
    Daughter nuclei are geneticallyidenticaldifferent
    Used forgrowth, repair, asexual reproductiongametes, in a sexual life cycle

    The nucleus divides before the cell does, so that no daughter cell is left anucleate, with no nucleus at all.

  3. LabelReady to divide

    1. sister chromatid
    2. centromere
    3. microtubule

    Before mitosis or meiosis every DNA molecule is replicated, and the two copies stay held together until anaphase. The DNA winds round histones and then coils on itself again and again, supercoiling, making a short, thick chromosome that can be moved without tangling. Motor proteins do the moving.

  4. OrderFrom one cell to two

    1. Chromosomes condense and become visible, and the nuclear membrane breaks down.
    2. Microtubules attach at the centromeres and line the chromosomes up across the middle of the cell.
    3. Sister chromatids separate and are pulled to opposite poles.
    4. A nuclear membrane forms round each group, and the chromosomes uncoil.
    5. The cytoplasm divides, giving two cells.

    Each new nucleus gets one chromatid of every chromosome, so the two are genetically identical.

  5. LabelName the phase

    1. prophase
    2. metaphase
    3. anaphase
    4. telophase

  6. GapsTwo rounds of separation

    Meiosis has two divisions. In the first, the two chromosomes of each homologous pair go to opposite poles; in the second, the sister chromatids separate. If a pair fails to separate, non-disjunction, a gamete ends up with an extra chromosome. A child from such a gamete with three copies of chromosome 21 has Down syndrome.

  7. GapsSources of variation

    Early in meiosis I, homologous chromosomes pair up as a bivalent. Non-sister chromatids cross at a chiasma and swap matching sections, giving new combinations of alleles. Then each pair lines up facing either pole by chance, random orientation, so a cell with n pairs can give 2ⁿ combinations of chromosomes in its gametes: over 8 million for the 23 pairs in a human.

  8. LabelThe cell cycle HL

    1. G1
    2. S phase
    3. G2
    4. mitosis
    5. cytokinesis

    Cells proliferate by this cycle: for growth, in plant meristems and early embryos, and to replace cells lost from the skin and heal its wounds. Interphase is a busy time: the cell makes proteins and other components, and its mitochondria grow and divide, as do chloroplasts in a plant.

  9. GapsTiming the cycle HL

    Proteins called cyclins rise and fall in concentration through the cell cycle. A cell can pass each checkpoint only when a particular one reaches a threshold level; that one then falls again.

  10. GapsWhen division runs wild HL

    A mutation can turn a gene that stimulates division into an oncogene, which keeps division switched on, or knock out a tumour suppressor gene that normally holds it back. Cells then divide out of control, forming a tumour. A malignant tumour is a cancer: it invades neighbouring tissue, and cells breaking away from this primary tumour can form secondary tumours elsewhere, metastasis. A benign tumour grows more slowly and stays where it is.

  11. TableMitotic index HL

    SampleCells in mitosisCells countedMitotic index
    Normal skin62000.03
    Benign tumour202000.10
    Malignant tumour401600.25

    Mitotic index = number of cells in mitosis ÷ total number of cells. Cells in mitosis are picked out by their visible, condensed chromosomes.

  12. Exam diagramSix stages of meiosis in a cell with 2n =

    1. anaphase I
    2. anaphase II
    3. metaphase I
    4. metaphase II