DP Biology HL D2.1 Cell and Nuclear Division

DP Biology HL D2.1 Cell and Nuclear Division - Full notes

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DP Biology HL | D2.1 Cell and Nuclear Division

DP BIOLOGY · THEME D: CONTINUITY AND CHANGE

D2.1

Cell and Nuclear Division

Full notes for HL · IB-style

Contents

1	Why cells divide
2	Chromosomes and their condensation
3	Mitosis
4	Cytokinesis and the mitotic index
5	Meiosis
6	Meiosis and genetic variation
7	Non-disjunction
8	The cell cycle and its control (HL only)
9	Cancer: oncogenes and tumour suppressor genes (HL only)
10	Why cells divide
11	Cytokinesis in animal and plant cells
12	Equal and unequal cytokinesis
13	Chromosome condensation
14	The stages of mitosis
15	Identifying stages in micrographs
16	Mitotic index
17	Meiosis overview
18	Sources of variation in meiosis
19	Comparing mitosis and meiosis
20	Non-disjunction
21	The cell cycle (HL)
22	Mixed review: worked problems
23	Additional exam-style practice
24	Cancer and uncontrolled division
25	Karyograms and meiosis errors
26	Common misconceptions and links
27	Exam-style question with markscheme
28	Check your understanding
29	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain the roles of cell division in growth, repair and reproduction
explain chromosome condensation and the structure of a replicated chromosome
describe the phases of mitosis
explain cytokinesis in plants and animals and calculate a mitotic index
describe the stages of meiosis and explain the halving of chromosome number
explain how crossing over and independent assortment generate variation
explain non-disjunction and its consequences
explain the phases of the cell cycle and the role of cyclins
explain how mutations in oncogenes and tumour suppressor genes cause cancer

Sections marked HL only (Sections 8 and 9) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How do cells divide to produce identical cells for growth, or gametes with half the chromosomes for reproduction? How does meiosis create genetic variation?

How this topic is examined

D2.1 is examined through micrographs of dividing cells (identify the phase), mitotic index calculations, diagrams of meiosis and questions on variation and non-disjunction. HL questions include cyclin graphs and cancer.

Describe what the chromosomes are doing in each phase, and compare mitosis and meiosis in a table.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Why cells divide

Two kinds of division for two purposes.

Think about it

A skin cell and a sperm cell are both produced by cell division. Why must the sperm cell have only half as many chromosomes?

Cells divide to produce new cells. Mitosis produces two genetically identical daughter cells, each with the same number of chromosomes as the parent. It is used for growth of multicellular organisms, repair and replacement of tissues, and asexual reproduction.

Meiosis produces four daughter cells, each with half the number of chromosomes (haploid) and genetically different from each other. It is used to produce gametes for sexual reproduction. Halving the number is essential: when two haploid gametes fuse at fertilization, the diploid number is restored.

Before either type of division, the DNA is replicated (D1.1), so each chromosome consists of two identical sister chromatids.

2 Chromosomes and their condensation

Long, thin DNA molecules must be packed for division.

For most of the cell cycle, chromosomes are long, thin and uncoiled, so genes can be transcribed. Before division, they condense (supercoil), becoming short and thick so they can be moved without tangling. In eukaryotes, DNA wraps around histones to form nucleosomes, which are then coiled further.

After replication, each chromosome consists of two sister chromatids, identical DNA molecules joined at the centromere. In mitosis, the sister chromatids are separated, so each daughter cell receives one copy of every chromosome.

3 Mitosis

Prophase, metaphase, anaphase, telophase.

Prophase: chromosomes condense and become visible as two sister chromatids. The nuclear membrane breaks down. Spindle fibres (microtubules) begin to form from the centrosomes (with centrioles in animal cells), which move to opposite poles.

Metaphase: spindle fibres attach to the centromere of each chromosome (at the kinetochore). Chromosomes line up along the equator (metaphase plate) of the cell.

Anaphase: the centromeres divide, and spindle fibres shorten, pulling the sister chromatids apart to opposite poles. Each chromatid is now a separate chromosome.

Telophase: chromosomes reach the poles and uncoil. A nuclear membrane re-forms around each set. Two genetically identical nuclei have formed.

Phase	Key events
Prophase	chromosomes condense; nuclear membrane breaks down; spindle forms
Metaphase	chromosomes line up at the equator; spindle fibres attached to centromeres
Anaphase	centromeres divide; sister chromatids pulled to opposite poles
Telophase	chromosomes decondense; nuclear membranes re-form

Figure 1. The sequence of mitosis.

Common mistake

Saying chromosomes split in prophase. Sister chromatids separate in anaphase.

Quick check

1. What happens in metaphase?

2. When do sister chromatids separate?

Answers: 1. chromosomes line up at the equator 2. anaphase

4 Cytokinesis and the mitotic index

Dividing the cytoplasm; measuring how fast cells divide.

Cytokinesis is the division of the cytoplasm after mitosis, producing two cells. In animal cells, a ring of actin and myosin contracts around the equator, pinching the membrane inward to form a cleavage furrow until the cell divides. In plant cells, which have a rigid wall, vesicles from the Golgi gather at the equator and fuse to form a cell plate, which develops into new plasma membranes and a new cell wall.

Cytokinesis can be equal, or unequal: in oogenesis, one large egg cell and small polar bodies are produced (D3.1); in budding yeast, a small bud forms.

The mitotic index is the proportion of cells in a tissue that are undergoing mitosis. It indicates how rapidly the tissue is dividing; high values are found in growing tissues (root tips) and in tumours.

Mitotic index = number of cells in mitosis ÷ total number of cells

WORKED EXAMPLE 1 Mitotic index

In a root tip squash, 300 cells were counted: 240 in interphase, 30 in prophase, 12 in metaphase, 8 in anaphase and 10 in telophase. Calculate the mitotic index.

Solution

Cells in mitosis = 30 + 12 + 8 + 10 = 60M1

Mitotic index = 60 ÷ 300 = 0.20A1

Quick check

1. How does cytokinesis differ in plant and animal cells?

2. Why would a tumour have a high mitotic index?

Answers: 1. plants: cell plate; animals: cleavage furrow 2. cells divide rapidly and uncontrollably

5 Meiosis

Two divisions halve the chromosome number.

Meiosis involves two divisions after one round of DNA replication, producing four haploid cells.

Meiosis I (the reduction division): in prophase I, chromosomes condense and homologous chromosomes pair up to form bivalents; crossing over occurs between non-sister chromatids. In metaphase I, bivalents line up at the equator, with the orientation of each pair random. In anaphase I, homologous chromosomes are pulled to opposite poles (sister chromatids stay together). In telophase I, two haploid cells form, each chromosome still with two chromatids.

Meiosis II is like mitosis: in each of the two cells, chromosomes line up at the equator (metaphase II), sister chromatids separate (anaphase II), and four haploid nuclei form (telophase II).

Meiosis I halves the chromosome number because homologous pairs are separated: each daughter cell gets one chromosome from each pair.

	Mitosis	Meiosis
Number of divisions	one	two
Daughter cells	two	four
Chromosome number	same as parent (diploid)	half (haploid)
Genetic identity	identical to parent	genetically different
Homologous pairing	no	yes (prophase I)
Crossing over	no	yes
Use	growth, repair, asexual reproduction	gamete production

Quick check

1. What is separated in anaphase I?

2. Why does meiosis halve the chromosome number?

Answers: 1. homologous chromosomes 2. homologous pairs are separated in meiosis I

6 Meiosis and genetic variation

Two processes make every gamete unique.

Crossing over: in prophase I, non-sister chromatids of homologous chromosomes exchange segments at points called chiasmata. This produces chromatids with new combinations of alleles (recombinant chromatids).

Random orientation (independent assortment): in metaphase I, each homologous pair lines up independently of the others, so the maternal and paternal chromosomes of each pair can go to either pole. With n pairs of chromosomes, there are 2ⁿ possible combinations. In humans (n = 23), that is 2²³, over 8 million combinations, before crossing over is even considered.

Random fertilization: any one of the millions of possible sperm can fuse with any one of the possible eggs, multiplying variation further. Together, these processes ensure that offspring of sexual reproduction are genetically unique (except identical twins).

WORKED EXAMPLE 2 Combinations from independent assortment

An organism has a diploid number of 8. How many genetically different gametes can be produced by random orientation alone?

Solution

n = 4 pairsM1

Combinations = 2⁴ = 16A1

Quick check

1. What are chiasmata?

2. How many combinations does random orientation give in humans?

Answers: 1. points where crossing over occurs between non-sister chromatids 2. 2²³, about 8.4 million

7 Non-disjunction

When chromosomes fail to separate.

Non-disjunction is the failure of homologous chromosomes (in anaphase I) or sister chromatids (in anaphase II) to separate properly during meiosis. The result is gametes with one extra or one missing chromosome. If such a gamete is fertilized, the zygote has an abnormal chromosome number (aneuploidy).

Down syndrome (trisomy 21) results from a gamete with two copies of chromosome 21 fusing with a normal gamete, giving three copies. The risk of non-disjunction increases with maternal age. Other examples include Turner syndrome (XO) and Klinefelter syndrome (XXY).

Figure 2. The incidence of Down syndrome rises with maternal age.

Quick check

1. Define non-disjunction.

2. What causes Down syndrome?

Answers: 1. failure of chromosomes or chromatids to separate in meiosis 2. an extra copy of chromosome 21 (trisomy 21)

8 The cell cycle and its control (HL only)

Cyclins drive cells through the cycle.

HL only

This section is Additional Higher Level content. SL students do not need it.

The cell cycle is the sequence of events between one division and the next. Interphase is the longest phase and is very active metabolically: in G₁, the cell grows and makes proteins and organelles; in the S phase, DNA is replicated; in G₂, the cell grows further and prepares for division. Mitosis and cytokinesis follow. Cells that stop dividing enter G₀.

Progress through the cycle is controlled by proteins called cyclins. Cyclins bind to and activate enzymes called cyclin-dependent kinases (CDKs). Each cyclin-CDK complex phosphorylates target proteins that trigger the next stage of the cycle. Different cyclins rise and fall at different stages, ensuring that the stages occur in the right order. Checkpoints (for example at the end of G₁ and G₂) check that conditions are right, such as that DNA is undamaged and fully replicated, before the cell proceeds.

Figure 3. Different cyclins peak at different stages of the cell cycle.

9 Cancer: oncogenes and tumour suppressor genes (HL only)

When control fails.

HL only

This section is Additional Higher Level content. SL students do not need it.

Cancer results from uncontrolled cell division. Proto-oncogenes are normal genes that promote cell division (for example, genes for growth factor receptors). When mutated, they become oncogenes, which are permanently active, stimulating division even without signals. Tumour suppressor genes normally halt the cell cycle or trigger cell death when DNA is damaged; the best-known, p53, is mutated in about half of all cancers. Loss of these genes removes the 'brakes'.

A primary tumour forms where the cancer starts. Metastasis occurs when cancer cells break away, travel in blood or lymph, and form secondary tumours elsewhere. Several mutations are usually needed for a cell to become cancerous, which is why cancer risk increases with age and with exposure to mutagens (D1.3).

Quick check

1. What happens in the S phase?

2. What do cyclins do?

3. Distinguish oncogenes and tumour suppressor genes.

Answers: 1. DNA replication 2. activate CDKs, which trigger progression to the next stage 3. oncogenes: mutated, permanently stimulate division; tumour suppressors: normally stop division, lost by mutation

10 Why cells divide

Growth, repair, reproduction.

Cell division produces new cells for growth, tissue repair and replacement, asexual reproduction (mitosis) and gamete production (meiosis). Each division involves nuclear division followed by cytokinesis.

WORKED EXAMPLE 3 Purposes (basic)

State three reasons for mitosis.

Solution

Growth; repair; asexual reproductionA2

11 Cytokinesis in animal and plant cells

Dividing the cytoplasm.

In animal cells, a ring of actin and myosin forms a cleavage furrow that pinches the cell in two. In plant cells, vesicles from the Golgi form a cell plate at the equator, which develops into a new cell wall.

WORKED EXAMPLE 4 Comparing (standard)

Compare cytokinesis in animal and plant cells.

Solution

Animal: cleavage furrow (actin ring) pinches inwardsR1

Plant: cell plate forms from vesicles; new wallR1

12 Equal and unequal cytokinesis

Oogenesis and budding.

Usually cytoplasm divides equally, but some divisions are unequal: in oogenesis, polar bodies receive little cytoplasm so the egg keeps most; in yeast budding, a small bud forms.

Quick check

1. Why is unequal cytokinesis useful in oogenesis?

Answers: 1. the egg retains most cytoplasm and nutrients for the embryo

13 Chromosome condensation

Supercoiling.

Before division, chromosomes condense by supercoiling around histones, becoming short and thick. This prevents tangling and breakage, allowing chromosomes to be moved and separated accurately.

Quick check

1. Why must chromosomes condense before mitosis?

Answers: 1. to be moved efficiently without tangling or breaking

14 The stages of mitosis

PMAT.

Stage	Key events
prophase	chromosomes condense; spindle forms; nuclear envelope breaks down
metaphase	chromosomes line up at the equator; spindle fibres attach to centromeres
anaphase	centromeres divide; sister chromatids pulled to opposite poles
telophase	chromosomes decondense; nuclear envelopes reform

WORKED EXAMPLE 5 Anaphase (standard)

Describe what happens during anaphase.

Solution

Centromeres split; spindle fibres shortenR1

Sister chromatids move to opposite polesR1

15 Identifying stages in micrographs

Root tip squashes.

In micrographs of root tips, interphase cells have a visible nucleus; prophase cells have condensed chromosomes; metaphase shows a line at the equator; anaphase shows two groups moving apart; telophase shows two nuclei forming.

WORKED EXAMPLE 6 Identification (standard)

A cell shows chromosomes in a line across the middle. Identify the stage.

Solution

MetaphaseA1

Try this

Prepare a root-tip squash of garlic or onion: hydrolyse in warm HCl, stain with toluidine blue, squash under a coverslip, and identify stages.

16 Mitotic index

Measuring division activity.

Mitotic index = number of cells in mitosis/total number of cells observed. A high index indicates rapid division (root tips, tumours).

WORKED EXAMPLE 7 Calculation (standard)

In a field of 120 cells, 18 are in mitosis. Calculate the mitotic index.

Solution

18/120M1

= 0.15A1

Quick check

1. In a tumour sample, 45 of 150 cells are dividing. Calculate the mitotic index and comment.

Answers: 1. 0.30: high; rapid division typical of cancer

17 Meiosis overview

Halving the chromosome number.

Meiosis consists of two divisions producing four haploid cells, genetically different from each other and from the parent. Meiosis I separates homologous chromosomes (reduction division); meiosis II separates sister chromatids.

WORKED EXAMPLE 8 Haploid (standard)

A cell with 2n = 8 undergoes meiosis. How many chromosomes are in each daughter cell?

Solution

4A1

18 Sources of variation in meiosis

Crossing over and independent assortment.

Crossing over in prophase I exchanges segments between non-sister chromatids of homologous pairs, producing new allele combinations. Independent assortment of homologous pairs in metaphase I gives 2n combinations (n = haploid number). Random fertilization further increases variation.

WORKED EXAMPLE 9 Combinations (exam level)

How many chromosome combinations can a human produce by independent assortment alone?

Solution

223M1

= 8 388 608A1

19 Comparing mitosis and meiosis

A summary.

Feature	Mitosis	Meiosis
number of divisions	1	2
daughter cells	2, diploid	4, haploid
genetic identity	identical	different
homologous pairing	no	yes (prophase I)
crossing over	no	yes
purpose	growth, repair	gametes

20 Non-disjunction

Errors in chromosome separation.

If homologous chromosomes or sister chromatids fail to separate, gametes have one too many or one too few chromosomes. Fertilization can produce trisomy (Down syndrome, trisomy 21) or monosomy (Turner syndrome, XO). Risk increases with maternal age.

WORKED EXAMPLE 10 Down syndrome (standard)

Explain how Down syndrome arises.

Solution

Non-disjunction of chromosome 21 during meiosisR1

Gamete with two copies fertilized: three copies (trisomy 21)R1

21 The cell cycle (HL)

Phases and control.

The cell cycle: G1 (growth), S (DNA replication), G2 (preparation), then mitosis and cytokinesis. Cyclins and cyclin-dependent kinases control progression through checkpoints. Mutations in these controls can lead to cancer.

WORKED EXAMPLE 11 Checkpoints (exam level)

Explain the role of cyclins.

Solution

Cyclin concentrations rise and fall in the cycleR1

Bind and activate CDKs that trigger the next phaseR1

22 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 12 Mixed problem (exam level)

Explain how meiosis and fertilization maintain chromosome number across generations while producing variation.

Solution

Meiosis halves chromosome number; fertilization restores diploid numberR1

Crossing over and independent assortment produce varied gametesR1

Random fertilization combines themR1

23 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 13 Paper 1 question (standard)

In which stage do homologous chromosomes separate? A. anaphase of mitosis B. anaphase I C. anaphase II D. prophase I

Solution

BA1

WORKED EXAMPLE 14 Paper 2 question (exam level)

Outline the events of prophase of mitosis.

Solution

Chromosomes condense; become visibleR1

Spindle forms; nuclear envelope breaks downR1

24 Cancer and uncontrolled division

Tumours.

When cell cycle control fails, cells divide uncontrollably, forming tumours. Benign tumours remain localized; malignant tumours invade and metastasize. Chemotherapy targets rapidly dividing cells, which is why it affects hair follicles and the gut lining.

Quick check

1. Why does chemotherapy cause hair loss?

Answers: 1. it targets rapidly dividing cells, including hair follicle cells

25 Karyograms and meiosis errors

Detecting abnormalities.

Karyograms from fetal cells (amniocentesis or chorionic villus sampling) detect non-disjunction. Non-invasive prenatal testing analyses fetal DNA fragments in the mother's blood.

Quick check

1. Give one risk of amniocentesis.

Answers: 1. small risk of miscarriage

26 Common misconceptions and links

Students often think…	Actually…
chromosomes split in prophase	chromatids separate in anaphase
meiosis produces two cells	it produces four haploid cells
crossing over happens between sister chromatids	it happens between non-sister chromatids of homologues
cancer is caused by a single mutation	usually several mutations are needed

How D2.1 connects to the rest of the course

Topic	Connection
D1.1 DNA replication	S phase
D3.1 Reproduction	gametogenesis
D3.2 Inheritance	independent assortment and alleles
A3.1 Diversity	karyotypes and chromosome number
D1.3 Mutations	cancer

27 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 4. Cells in each phase in a root tip squash.

(a) Calculate the mitotic index. [2]

(b) Describe what happens in anaphase of mitosis. [2]

(d) Explain how meiosis produces genetic variation. [3]

(e) Explain the role of cyclins in the cell cycle. [3]

Markscheme

Part	Answer	Marks
(a)	60 ÷ 300	M1
	= 0.20	A1
(b)	centromeres divide	A1
	sister chromatids pulled to opposite poles	A1
(c)	one vs two divisions	A1
	two vs four cells	A1
	diploid vs haploid	A1
	identical vs different; crossing over only in meiosis	A1
(d)	crossing over in prophase I	R1
	random orientation of bivalents in metaphase I	R1
	2ⁿ combinations; new allele combinations	R1
(e)	cyclins bind and activate CDKs	R1
	complexes phosphorylate target proteins	R1
	trigger progression; concentrations rise and fall in sequence	R1

Examiner's comments. In (c), comparisons must be paired; separate lists scored fewer marks.

28 Check your understanding

Section A: multiple choice

1. Mitosis produces

A. two genetically identical cells B. four haploid cells C. gametes D. genetically different cells

2. Homologous chromosomes pair in

A. prophase of mitosis B. anaphase II C. prophase I D. metaphase II

3. The mitotic index is

A. cells in interphase ÷ total B. cells in mitosis ÷ total cells C. total ÷ dividing cells D. number of chromosomes

4. In plant cytokinesis

A. a cleavage furrow forms B. the wall dissolves C. no new wall forms D. a cell plate forms

5. With n = 3, random orientation gives

A. 3 B. 8 combinations C. 6 D. 9

6. Down syndrome results from

A. non-disjunction of chromosome 21 B. crossing over C. a frameshift D. a point mutation

7. DNA is replicated in

A. G₁ B. G₂ C. mitosis D. S phase

8. p53 is

A. an oncogene B. a cyclin C. a tumour suppressor gene D. a CDK

Section B: structured

9. Describe the stages of mitosis. [4]

10. Explain how meiosis halves the chromosome number. [3]

11. Explain non-disjunction. [3]

12. Explain how cancer can develop from mutations. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C

9. Prophase (condense, membrane breaks, spindle), metaphase (equator), anaphase (chromatids separate), telophase (membranes re-form).

10. One replication then two divisions; homologues separated in meiosis I; one of each pair per cell.

11. Failure of homologues or chromatids to separate; gametes with extra or missing chromosome; trisomy.

12. Proto-oncogenes mutate to oncogenes; tumour suppressors lost; uncontrolled division; tumour; metastasis.

29 Summary and topic map

Idea	Remember
Mitosis	two identical diploid cells; growth, repair
Phases	prophase, metaphase, anaphase, telophase
Cytokinesis	cleavage furrow (animals), cell plate (plants)
Mitotic index	cells in mitosis ÷ total
Meiosis	two divisions; four haploid cells
Variation	crossing over, random orientation (2ⁿ), fertilization
Non-disjunction	trisomy 21
HL	cell cycle; cyclins and CDKs; oncogenes; tumour suppressors

Before the exam, make sure you…

•identify phases by chromosome behaviour
•compare mitosis and meiosis point by point
•calculate mitotic index
•explain both sources of variation in meiosis

Cell and Nuclear Division on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 5. Topic map for Cell and Nuclear Division.

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