The cell cycle, its control, and haploid and diploid cells – Cell and Nuclear Division (Senior 5 Biology) | Ganzaa

Cell and Nuclear Division

Lessons

Senior 5 · Biology

Cell and Nuclear Division

4 lessons

Lesson 1 of 4

The cell cycle, its control, and haploid and diploid cells

25 min

Why some cells never stop dividing

The cells lining your small intestine are worn away and replaced every few days, so the stem cells at the base of the gut lining divide over and over again. Most nerve cells in your brain, by contrast, stopped dividing before you were born and will never divide again. Your liver cells sit somewhere in between: they rarely divide, but if part of the liver is removed they start again until the organ has regrown. Each cell decides whether and when to divide, and the decision is checked at several points. In Senior 3 you met the stages of the cell cycle; this lesson explains how the cycle is controlled, and how biologists describe the number of chromosome sets in a cell.

Definition

Cell cycle: the regular sequence of events between one cell division and the next: interphase (G1, S and G2) followed by mitosis and cytokinesis (M phase).

G0: a resting state outside the cycle, entered from G1, in which a cell carries out its functions but does not divide.

Checkpoint: a control point in the cell cycle where the cell checks that conditions are right before it moves into the next phase.

Mitotic index: the percentage of cells in a sample that are in mitosis: $\text{mitotic index} = \frac{\text{number of cells in mitosis}}{\text{total number of cells}} \times 100$ .

The phases of the cell cycle

G1 (first gap): the cell grows, makes proteins and organelles, and carries out its normal functions. It is the most variable phase: hours in a fast-dividing cell, years in a slow one.
S (synthesis): the DNA is replicated, as you saw in Unit 3, and new histones are made. Each chromosome now consists of two sister chromatids.
G2 (second gap): the cell grows further, checks the copied DNA, and makes the proteins needed for division, such as tubulin for the spindle.
M phase: mitosis divides the nucleus, and cytokinesis divides the cytoplasm.

How the cycle is controlled

Each checkpoint works like a guard at a gate:

G1 checkpoint (the restriction point): is the cell big enough, are there enough nutrients, have growth factors signalled "divide", and is the DNA undamaged? If not, the cell pauses, enters G0, or repairs its DNA. The protein p53 stops the cycle here when DNA is damaged and, if the damage cannot be repaired, makes the cell destroy itself (apoptosis).
G2 checkpoint: has all the DNA been replicated, and correctly?
Spindle checkpoint (in metaphase): is every chromosome attached to spindle fibres from both poles? Anaphase cannot start until it is.

The signals that move a cell past each checkpoint are proteins called cyclins and cyclin-dependent kinases (CDKs). A CDK is an enzyme that adds phosphate groups to other proteins, switching them on or off, but it is active only when bound to a cyclin. Different cyclins build up and are destroyed at different stages of the cycle, so different CDKs are switched on in turn and drive the cell from one phase to the next. Leland Hartwell, Tim Hunt and Paul Nurse shared the 2001 Nobel Prize in Physiology or Medicine for discovering these controls. When the controls fail, cells can divide without stopping, which is the start of cancer (Lesson 2).

Worked example 1: mitotic index in a root

A student made squashes from two regions of a bean root and counted 1 000 cells in each (illustrative data): 82 cells were in mitosis in the region just behind the root tip, and 3 cells in the region 10 mm further back. (a) Calculate the mitotic index of each region. (b) Explain the difference.

(a) Tip region: $\frac{82}{1\,000} \times 100 = 8.2\,\%$ . Region further back: $\frac{3}{1\,000} \times 100 = 0.3\,\%$ .
(b) Just behind the tip is the meristem, where cells divide rapidly to make new cells. Further back, cells have stopped dividing; they are elongating and becoming specialised (many are in G0).

Use: pathologists use the same measurement on tissue samples: a tumour with a high mitotic index is growing fast and is usually more dangerous.

Definition

Homologous chromosomes: a pair of chromosomes, one from each parent, with the same length, the same centromere position and the same genes at the same loci, though possibly different alleles.

Diploid (2n): having two complete sets of chromosomes, one set from each parent, as in body cells. Haploid (n): having one complete set, as in gametes.

Polyploid: having more than two complete sets of chromosomes, such as triploid (3n) or hexaploid (6n).

Haploid and diploid cells

Two different counts change during the cycle, and A-level questions test whether you can keep them apart:

The number of chromosomes (count the centromeres) stays the same through interphase: a diploid human cell has 46 in G1 and still 46 in G2.
The amount of DNA doubles in S phase. Biologists call the DNA in one haploid set "1C": a diploid cell has 2C in G1 and 4C in G2, and a gamete has 1C.

Organism	Body cells (2n)	Gametes (n)
Human	46	23
Cattle	60	30
Common bean (Phaseolus vulgaris)	22	11
Cassava	36	18
Onion	16	8

Many crop plants are polyploid. Bread wheat is hexaploid, with six sets of 7 chromosomes (42 in all). The cooking and dessert bananas grown in Rwanda are triploid, with three sets of 11 chromosomes (33 in all); Lesson 4 explains why this makes them seedless.

Worked example 2: counting through one cell cycle

Cattle have $2 n = 60$ . For one cell, give the number of chromosomes, the number of chromatids and the DNA content (in C) in (a) G1, (b) G2, (c) anaphase of mitosis, (d) each daughter cell.

Stage	Chromosomes	Chromatids	DNA
(a) G1	60	60 (one per chromosome)	2C
(b) G2	60	120	4C
(c) Anaphase	120 (each separated chromatid now counts as a chromosome)	120	4C
(d) Daughter cell	60	60	2C

Khan Academy: "Cell cycle control". Watch for the checkpoints and for how cyclins and CDKs act as the cell's signals to move from one phase to the next.

Warning

Common mistakes

"The chromosome number doubles in S phase." The DNA doubles; the chromosome number does not. Each chromosome simply gains a second chromatid.
"Interphase is a resting phase." It is the busiest phase: growth, protein synthesis and DNA replication. The resting state is G0.
Calling sister chromatids "homologous chromosomes". Sister chromatids are identical copies; homologous chromosomes come from different parents.

Tip

Counting rule: count chromosomes by counting centromeres. Count DNA by counting chromatids, or use C-values.

✦

Practice: the cell cycle and ploidy

Describe what happens in G1, S and G2.
State what is checked at (a) the G1 checkpoint, (b) the spindle checkpoint.
Explain how cyclins control the activity of CDKs.
Explain why the protein p53 is called a "guardian of the genome".
Distinguish between homologous chromosomes and sister chromatids.
In a sample of 600 cells, 27 are in mitosis. Calculate the mitotic index.
A cassava cell ( $2 n = 36$ ) is in G2. How many chromosomes and chromatids does it have, and what is its DNA content in C?
A gamete of a plant has 2.5 pg of DNA. How much DNA would a body cell of the plant have in G1 and in G2?

Show answersHide answers

G1: growth, protein and organelle synthesis, normal functions. S: DNA replication and histone synthesis. G2: further growth, checking of replicated DNA, synthesis of proteins for division such as tubulin.
(a) Cell size, nutrients, growth signals and DNA damage. (b) That every chromosome is attached to spindle fibres from both poles.
A CDK is active only when bound to a cyclin; cyclin levels rise and fall during the cycle, so each CDK is switched on at a particular stage, where it phosphorylates proteins that drive the next phase.
It stops the cycle at the G1 checkpoint when DNA is damaged, allowing repair, and triggers apoptosis if the damage cannot be repaired, so damaged DNA is not passed on.
Homologous chromosomes are a pair, one from each parent, with the same genes but possibly different alleles; sister chromatids are two identical copies of one chromosome made in S phase, joined at the centromere.
$\frac{27}{600} \times 100 = 4.5\,\%$ .
36 chromosomes, 72 chromatids, 4C.
The gamete is 1C = 2.5 pg; G1 body cell = 2C = 5.0 pg; G2 = 4C = 10 pg.

Key points

Cell cycle: G1 → S → G2 → M; cells may leave G1 for G0.
Checkpoints at G1, G2 and the spindle; cyclins switch on CDKs; p53 halts the cycle when DNA is damaged.
Diploid (2n) body cells, haploid (n) gametes; polyploid crops such as wheat (6n) and bananas (3n).
DNA content doubles in S phase (2C → 4C); chromosome number does not.

Sources and credits

Written by Ganzaa. Phase lengths follow the typical values for human cells in culture given in B. Alberts and colleagues, Molecular Biology of the Cell (Garland Science). No text was copied.
Video: Cell cycle control, Khan Academy, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples and the practice questions are written by Ganzaa. The cell counts in Worked example 1 are illustrative.

1Why some cells never stop dividing

2The phases of the cell cycle

3How the cycle is controlled

4Haploid and diploid cells

Why some cells never stop dividing

The phases of the cell cycle

How the cycle is controlled

Haploid and diploid cells