D2.1Theme D — Continuity and change
Cell and nuclear division — IB Biology SL Revision Notes & Topical Questions
5 min read 13 parts Lesson 5 of 14 2 quizzes
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
D2.1 Cell and nuclear division — IB BIOLOGY SL notes: Describe mitosis, meiosis, cytokinesis and chromosome behaviour, and explain how meiosis causes variation and non-disjunction.Cell theory — new cells are produced from pre-existing cellsA parent cell gives two daughter cells.A…
- Two types of division
- Nuclear division comes first
- Animal cytokinesis
- Plant cytokinesis
- Equal division
- DNA replication
Part 1 of 11Cell division and cytokinesis
1Cell theory
- Cell theory new cells are produced from pre-existing cells
- A parent cell gives two daughter cells.
- A multicellular body results from many divisions starting from one zygote.
- Early embryonic stem cells later specialise into tissues and organs.
2Purposes
- Purposes growth, replacement, tissue repair, and asexual and sexual reproduction
3Two types of division
- Two types of division one gives daughter cells genetically identical to the parent and the other gives genetically different cells
4Nuclear division comes first
- Nuclear division comes first it prevents anucleate cells, and then cytokinesis divides the cytoplasm
5Animal cytokinesis
- Animal cytokinesis a cleavage furrow forms
- A contractile ring of actin and myosin forms under the plasma membrane at the equator.
- It contracts and pulls the membrane inwards to separate the two cells.
6Plant cytokinesis
- Plant cytokinesis a cell plate forms at the equator
- Vesicles carrying carbohydrates, lipids and proteins fuse to make the two membranes.
- Other vesicles deposit pectin and cellulose by exocytosis to make new cell walls.
| Feature | Animal cell | Plant cell |
|---|---|---|
| Structure formed | Cleavage furrow | Cell plate |
| Materials | Actin-myosin contractile ring | Golgi vesicles with lipids, proteins and carbohydrates |
| Direction | Membrane pulled inwards from the edge | Plate grows outwards from the centre |
| Final boundary | Two plasma membranes | Two membranes plus a new wall of cellulose and pectin |
Part 2 of 11Equal and unequal cytokinesis
Equal division
- Equal division each daughter cell must receive organelles
- Each needs at least one mitochondrion, and plant cells also need a chloroplast.
- Organelles can only be made from pre-existing ones.
Unequal division
- Unequal division oogenesis in humans and budding in yeast
Oogenesis
1
Before birth germinal epithelial cells in the foetal ovary form primary oocytes.
2
From puberty a primary oocyte divides into a secondary oocyte and a small polar body.
3
The secondary oocyte divides into an ovum and another polar body.
4
The polar bodies degenerate.
- Purpose
- the ovum keeps most of the cytoplasm and food reserves for the embryo
Part 3 of 11Roles of mitosis and meiosis
Mitosis
- Mitosis gives genetically identical, usually diploid, cells
- It is used for growth, tissue repair, cell replacement, embryonic development and asexual reproduction.
- It maintains the chromosome number and genome.
Meiosis
- Meiosis gives genetically different haploid cells used as gametes
- It occurs only in the testes and ovaries in humans.
- It generates genetic diversity.
DNA replication
- DNA replication replication in interphase comes before any nuclear division
- Each chromosome then has two identical sister chromatids joined at the centromere.
- At the start of anaphase the centromere splits and each chromatid is again counted as a chromosome.
Part 4 of 11Condensing and moving chromosomes
Packaging
- Packaging human DNA can exceed 50 000 µm but must fit in a nucleus under 5 µm across
- In interphase the DNA is loosely wound on histones as chromatin.
- Each nucleosome is DNA coiled around eight histone proteins.
- In prophase, supercoiling aided by histones and enzymes condenses it into chromosomes.
Microtubules
- Microtubules tubulin fibres of the cytoskeleton that move the chromosomes
- α- and β-tubulin form dimers that are added to or removed from the ends to lengthen or shorten the tubule.
- Motor proteins use ATP to carry chromosomes along the microtubules.
Centrosomes
- Centrosomes organise the spindle and contain two centrioles in animal cells
Part 5 of 11Phases of mitosis
| Feature | Prophase | Metaphase | Anaphase | Telophase |
|---|---|---|---|---|
| Chromosomes | Condense, visible as two chromatids | Lined up at the equator | Chromatids separated and moving to the poles | Reach the poles and decondense |
| Nuclear membrane | Breaks down into vesicles | Absent | Absent | Re-forms |
| Spindle | Forms from the poles | Attached to centromeres by kinetochores | Microtubules shorten | Breaks down |
| In a micrograph | Thread-like and tangled | Straight central line | V-shaped chromatids | Two clusters at opposite ends |
1Prophase
- Prophase the nucleolus disappears and the centrosomes, replicated in G2, move to opposite poles
2Metaphase
- Metaphase each sister chromatid attaches to a spindle fibre from the opposite pole
3Anaphase
- Anaphase the centromeres split and the spindle fibres pull the chromatids to the poles
4Telophase
- Telophase nucleoli re-form in each new nucleus
Part 6 of 11Identifying phases
1Interphase
- Interphase a dark nucleus with chromatin but no distinct chromosomes
- Most cells are in interphase because it takes most of the cell cycle.
2Prophase against telophase
- Prophase against telophase prophase shows one chromosome mass and telophase shows two masses at opposite poles
3Cytokinesis
- Cytokinesis a furrow in animal cells and a cell plate in plant cells
4Mitotic index
Mitotic index cells in mitosis divided by the total cells counted, showing the rate of division
- Root tips are a common tissue for seeing mitosis.
Part 7 of 11Meiosis
1Meiosis I (reduction division)
- Meiosis I (reduction division) homologous chromosomes pair and then separate
- The chromosome number halves from 2n to n here and not in meiosis II.
- Each chromosome still has two chromatids.
2Meiosis II
- Meiosis II sister chromatids separate, as in mitosis, to give four haploid nuclei
- There is no replication between meiosis I and II.
1
DNA replicates in interphase.
2
Homologous chromosomes pair as bivalents and cross over.
3
The pairs line up randomly at the equator.
4
Homologous chromosomes move to opposite poles.
5
In the two new cells the chromosomes line up singly.
6
Sister chromatids separate.
7
Four haploid cells form.
Part 8 of 11Need for meiosis
1Sexual life cycle
- Sexual life cycle fertilisation fuses gamete nuclei and doubles the chromosome number
- Meiosis halves the number, so it does not double every generation.
- Offspring are genetically distinct from each other and from the parents.
2Asexual life cycle
- Asexual life cycle offspring are genetically identical to the parent
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter nuclei | Two | Four |
| Chromosome number | Kept (2n to 2n) | Halved (2n to n) |
| Genetic make-up | Identical | Varied |
| Homologues pair? | No | Yes, in prophase I |
| Crossing over | No | Yes |
| Role | Growth, repair and asexual reproduction | Producing gametes |
Part 9 of 11Variation from meiosis
Crossing over
- Crossing over in prophase I homologous chromosomes pair by synapsis to form a bivalent of four DNA molecules
- Non-sister chromatids break and rejoin at chiasmata at the same positions.
- Genes exchange between maternal and paternal chromatids.
- This makes recombinant chromatids with new allele combinations.
Random orientation
- Random orientation bivalents line up at the equator in a random orientation
- Each homologue attaches to a different pole.
- Different bivalents assort independently.
- The number of combinations is 2n where n is the haploid number, so humans give 223 = 8 388 608.
Random fertilisation
- Random fertilisation any gamete may fuse with any other, adding more variation
Significance
- Significance variation helps a species adapt and evolve
Part 10 of 11Non-disjunction
1Non-disjunction
Non-disjunction chromosomes fail to separate in anaphase I or anaphase II of meiosis
- Gametes have n + 1 or n - 1 chromosomes.
- A zygote from such a gamete has the wrong chromosome number.
2Down syndrome (trisomy 21)
- Down syndrome (trisomy 21) three copies of chromosome 21 give 47 chromosomes
- Features include growth delay and reduced intellectual ability, with possible sight or hearing problems.
- The risk rises with maternal age because non-disjunction is more likely in older ova.
3Other trisomies
- Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18) are very serious
4Detection
- Detection karyotyping of foetal cells from amniocentesis or chorionic villus sampling
Part 11 of 11More from the specification
1Mitosis vs meiosis
- Mitosis produces two genetically identical diploid daughter cells for growth and tissue repair;
- meiosis produces four genetically diverse haploid gametes.
2Cell cycle regulation
- Progression through interphase (G1, S, G2) and M-phase is governed by internal biochemical checkpoints and cyclin-dependent kinases.
3Meiotic reduction division
- Homologous chromosome separation during meiosis I halves the somatic chromosome complement from diploid (2n) to haploid (n).
4Cytokinesis
- The physical cleavage of cytoplasm following nuclear division, forming a cleavage furrow in animal cells and a cell plate in plants.
Key terms
| Term | Meaning |
|---|---|
| Cytokinesis | The division of the cytoplasm after nuclear division. |
| Sister chromatids | The two identical DNA copies of a replicated chromosome joined at the centromere. |
| Bivalent | A pair of homologous chromosomes paired in prophase I. |
| Crossing over | The exchange of alleles between non-sister chromatids of a bivalent. |
| Mitotic index | The number of cells in mitosis divided by the total number of cells counted. |
| Non-disjunction | The failure of chromosomes to separate correctly in meiosis. |
| Haploid | Having one set of chromosomes (n). |
| Chiasma | The point where non-sister chromatids cross over. |
Avoid these mistakes
1
Saying meiosis halves the chromosome number in meiosis II — the halving is in meiosis I
2
Saying that the cell divides before the nucleus — nuclear division comes first and then cytokinesis
3
Saying sister chromatids are homologous chromosomes — sister chromatids are identical copies and homologous chromosomes are a maternal and paternal pair
4
Saying crossing over happens between sister chromatids — it happens between non-sister chromatids
Marked practiceQuizzes for this lesson
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.