The Cell Cycle

The cell cycle is the ordered sequence of events by which a cell grows and divides. It has two main phases: interphase (the cell grows and copies its DNA) and mitotic phase (cell division occurs).

G1 Phase

Cell grows, produces proteins, and prepares for DNA replication. Checkpoints ensure the cell is healthy enough to divide.

S Phase (Synthesis)

DNA is replicated. Each chromosome is duplicated to form two identical sister chromatids joined at the centromere.

G2 Phase

Cell continues to grow, checks DNA for errors, and prepares the machinery for mitosis.

Cancer and the cell cycle: Cancer occurs when cell cycle checkpoints fail, allowing damaged or mutated cells to divide uncontrollably. Understanding the cell cycle is fundamental to developing cancer treatments.

Mitosis

Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent cell. It is used for growth, repair, and asexual reproduction. Human body cells have 46 chromosomes (diploid, 2n); mitosis maintains this number.

PROPHASE

Chromosomes condense and become visible. The mitotic spindle forms. The nuclear envelope breaks down.

METAPHASE

Chromosomes line up along the equator (middle) of the cell. Spindle fibres attach to the centromeres.

ANAPHASE

Sister chromatids are pulled apart to opposite poles of the cell by the spindle fibres. Each pole now has a complete set of chromosomes.

TELOPHASE

Nuclear envelopes reform around each set of chromosomes. Chromosomes decondense. The cell then divides (cytokinesis), producing two identical daughter cells.

Meiosis

Meiosis produces four genetically different daughter cells, each with half the chromosome number (haploid, n = 23 in humans). It occurs in the gonads (testes and ovaries) to produce gametes (sperm and eggs). Fertilisation restores the diploid number.

Meiosis I (Reductive division)

Homologous chromosome pairs separate. Crossing over (exchange of chromosome segments between homologues) occurs in prophase I, creating new allele combinations. Result: 2 haploid cells.

Meiosis II (Like mitosis)

Sister chromatids separate (similar to mitosis). Each of the 2 cells from Meiosis I divides. Result: 4 haploid cells, each genetically unique.

Feature	Mitosis	Meiosis
Daughter cells	2	4
Chromosome number	Same (2n → 2n)	Halved (2n → n)
Genetic variation	None (identical)	Yes (crossing over, independent assortment)
Purpose	Growth, repair	Sexual reproduction (gametes)

Cell Differentiation and Stem Cells

All cells in the human body contain the same DNA, yet we have over 200 different cell types (neurons, muscle cells, red blood cells, etc.). Cell differentiation is the process by which cells become specialised by activating only a subset of their genes.

Stem cells

Stem cells are undifferentiated cells that can divide and differentiate into specialised cell types. Embryonic stem cells (totipotent/pluripotent) can become any cell type. Adult stem cells are more limited but maintain and repair specific tissues (e.g., bone marrow stem cells produce blood cells).

How differentiation works

Chemical signals (from neighbouring cells and the environment) switch specific genes on or off. A cell that will become a neuron activates neuron-specific genes and silences genes for other cell types. Once differentiated, most cells cannot revert to a stem cell state.

Medical applications

Stem cell research has enormous potential for treating diseases like Parkinson's disease, diabetes, and spinal cord injuries. Australian researchers are active in this field. The ethical debate around embryonic stem cells has led to research into induced pluripotent stem cells (iPSCs), where adult cells are reprogrammed to a stem cell state.

Key Vocabulary

Term	Definition
Diploid (2n)	Having two sets of chromosomes (one from each parent). Human body cells are diploid (2n = 46).
Haploid (n)	Having one set of chromosomes. Human gametes are haploid (n = 23).
Crossing over	The exchange of segments between homologous chromosomes during meiosis I, creating new allele combinations and genetic variation.
Stem cell	An undifferentiated cell that can self-renew and differentiate into specialised cell types.

Worked Examples

1

A human cell with 46 chromosomes undergoes mitosis. How many chromosomes does each daughter cell have?

Step 1: Recall that mitosis produces genetically identical daughter cells with the same chromosome number as the parent.

Step 2: Parent cell = 46 chromosomes (2n). Mitosis maintains the chromosome number.

Answer: Each daughter cell has 46 chromosomes.

2

A cell with 46 chromosomes undergoes meiosis. How many chromosomes do the gametes have, and how many gametes are produced?

Step 1: Meiosis halves the chromosome number.

Step 2: 46 ÷ 2 = 23 chromosomes per gamete (haploid).

Step 3: Meiosis produces 4 daughter cells.

Answer: 4 gametes, each with 23 chromosomes. When two gametes fuse at fertilisation, the resulting zygote has 46 chromosomes.

3

Explain why gametes produced by meiosis are genetically unique, even from the same individual.

Reason 1 — Crossing over: During prophase I, homologous chromosomes exchange segments. This creates chromosomes with new combinations of alleles that did not exist in either parent chromosome.

Reason 2 — Independent assortment: At metaphase I, homologous pairs line up randomly at the equator. For 23 pairs, there are 2²³ (over 8 million) possible chromosome combinations in the gametes.

Reason 3 — Random fertilisation: Any sperm can fertilise any egg, multiplying genetic diversity further.

Answer: These three mechanisms together ensure that virtually every gamete is genetically unique, producing genetic diversity in offspring.

Knowledge Check

Select the correct answer for each question.

Question 1

During which phase of the cell cycle does DNA replication occur?

S phase G1 phase G2 phase Mitosis (M phase)

Question 2

During which phase of mitosis do chromosomes align along the equator of the cell?

Prophase Metaphase Anaphase Telophase

Question 3

Meiosis produces gametes that are haploid. What does haploid mean?

Having two complete sets of chromosomes Having four sets of chromosomes Having one set of chromosomes (half the normal number) Having no chromosomes

Question 4

Crossing over during meiosis increases genetic variation. When does it occur?

During metaphase II During the S phase During cytokinesis During prophase I

Question 5

All cells in the human body contain the same DNA, yet liver cells look and function differently from neurons. This is because:

Cell differentiation causes different genes to be switched on or off in different cell types Different cells have different DNA sequences due to mutations during development Neurons have more chromosomes than liver cells Liver cells undergo meiosis while neurons undergo mitosis

Cell Biology