Cell Division — A-Level Biology Revision Notes

The Cell Cycle

Interphase — G₁, S and G₂

Interphase is not a resting phase — it is the most metabolically active period of the cell cycle. It is divided into three sub-stages:

G₁ (Gap 1): The cell grows in size, produces new organelles and proteins, and carries out its normal metabolic functions. Mitochondria and chloroplasts replicate. ATP is produced by respiration. Centrioles replicate to form two pairs.

S phase (Synthesis): DNA replication occurs — each chromosome is copied to produce two identical sister chromatids joined at the centromere. The DNA content of the nucleus doubles from 2n to 4n (in arbitrary units, e.g. from 4 AU to 8 AU).

G₂ (Gap 2): The cell continues to grow and synthesises proteins needed for mitosis (e.g. tubulin for spindle fibres). The cell checks the replicated DNA for errors before committing to division.

When a cell is not actively dividing, it enters a quiescent state called G₀. Some cells (like nerve cells) remain in G₀ permanently; others can re-enter the cell cycle when stimulated.

Understanding Chromosomes

Human cells contain 46 chromosomes organised in 23 homologous pairs. One chromosome in each pair comes from the mother (maternal) and one from the father (paternal). Homologous chromosomes are identical in size and carry the same genes (though they may carry different alleles of those genes). When chromosomes are arranged in their homologous pairs, the cell is described as diploid (2n).

Replicated vs Non-Replicated Chromosomes

Before DNA replication (during G₁), each chromosome is a single linear molecule of DNA — a non-replicated chromosome. It appears as a simple rod-like structure.

After DNA replication (during S phase), each chromosome consists of two identical copies called sister chromatids, joined at a region called the centromere. The chromosome now has the characteristic X-shape. Although it looks like two chromosomes, it is still counted as one chromosome because the sister chromatids are joined.

The Stages of Mitosis

Mitosis is the division of the nucleus that produces two genetically identical daughter nuclei. It is followed by cytokinesis — the division of the cytoplasm — to produce two complete daughter cells. The stages of mitosis are remembered with the mnemonic PMAT.

Prophase

Prophase is the first visible stage of mitosis. The chromatin threads (which were spread throughout the nucleus during interphase) shorten and condense to form visible X-shaped chromosomes, each consisting of two sister chromatids joined at the centromere. The two pairs of centrioles migrate to opposite poles of the cell and begin to form the spindle fibres between them. An aster (star-shaped arrangement of spindle fibres) forms around each pair of centrioles. The nucleolus disappears and the nuclear envelope breaks down, releasing the chromosomes into the cytoplasm.

Metaphase

The chromosomes align in a single row along the equator (middle) of the cell. Each chromosome is attached to spindle fibres via its centromere. The spindle now spans the entire cell from pole to pole. This is the stage where chromosomes are most visible and condensed — it is the best stage for producing a karyotype (an image of all the chromosomes arranged in their homologous pairs).

Anaphase

The centromeres divide and the spindle fibres shorten, pulling the sister chromatids to opposite poles of the cell. The centromere leads the way, giving the chromatids a characteristic V-shaped or arrowhead appearance as they are pulled through the cytoplasm. Once separated, each chromatid is now called a chromosome. This is the stage where the chromosome number effectively doubles momentarily — there are now 92 chromosomes (in a human cell) briefly present before the cell divides.

Telophase and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense back into chromatin threads. A new nuclear envelope reforms around each set of chromosomes, and the nucleolus reappears. Cytokinesis then divides the cytoplasm. In animal cells, a cleavage furrow forms as the cell membrane pinches inward. In plant cells, a cell plate forms across the equator, which develops into a new cell wall. The result is two genetically identical diploid daughter cells, each entering G₁ of interphase.

The Mitotic Index, Growth and Cancer

Calculating Time in Each Stage

The number of cells observed at each stage of the cell cycle is directly proportional to the time spent in that stage. If the total cell cycle lasts 24 hours and 82% of cells are in interphase, then time in interphase = (82 ÷ 100) × 24 = 19.68 hours. The same calculation applies to each stage of mitosis.

Cancer and the Mitotic Index

Cancer is the uncontrolled division of cells by mitosis, leading to a mass of cells called a tumour. Cancer cells spend proportionally less time in interphase and more time in the stages of mitosis compared to normal cells. This means the mitotic index is higher in cancerous tissue.

Calculating Population Growth

Organisms that reproduce by mitosis (such as bacteria) double their population with each generation. The formula is: N = N₀ × 2ⁿ, where N is the final number of organisms, N₀ is the starting number, and n is the number of generations. For example, 2 bacteria dividing every 20 minutes for 3 hours (9 generations) gives: N = 2 × 2⁹ = 1,024 bacteria.

The Squashed Root Tip Practical

The root tip is used to observe cells at different stages of mitosis because the apical meristem (at the very tip of the root) is a region of rapid cell division. Above the apical meristem is the mitotic zone where cells in all stages of mitosis can be seen. Above this is the elongation zone where cells grow in length. The very tip is protected by the root cap, where no mitosis occurs.

Key Requirements for a Good Preparation

The root tip must be fully squashed so that cells form a single layer with no overlap — overlapping cells make it impossible to identify individual stages. A stain (such as aceto-orcein or toluidine blue) must be applied to make the chromosomes visible under the microscope. A well-prepared squash shows clearly separated cells, each with visible chromosome arrangements that can be identified as interphase, prophase, metaphase, anaphase or telophase.

The mitotic index can then be calculated by counting the number of cells in each stage across a field of view and applying the formula.

The Significance of Mitosis

Mitosis produces genetically identical daughter cells with the same diploid chromosome number as the parent cell. This is essential for three key biological processes:

Growth: Mitosis produces new cells, allowing multicellular organisms to increase in size. Growth occurs by a combination of cell division and cell enlargement.

Repair and replacement: Damaged tissues are repaired and worn-out cells are replaced by mitosis. For example, skin cells are constantly replaced, and wound healing depends on mitotic division of cells at the wound edge.

Asexual reproduction: Some organisms reproduce asexually by mitosis — for example, bacteria (binary fission), yeast (budding), and plants (vegetative propagation). The offspring are genetically identical clones of the parent.

In animals, mitosis occurs in all body cells (somatic cells). In plants, mitosis occurs in the meristems — regions of actively dividing cells found at root tips, shoot tips, and in the cambium.

Meiosis — Overview

Meiosis is a type of cell division that produces four genetically different haploid daughter cells (gametes) from one diploid parent cell. Unlike mitosis, meiosis involves two consecutive divisions: meiosis I (the reduction division, where homologous chromosomes are separated) and meiosis II (similar to mitosis, where sister chromatids are separated).

Where Meiosis Occurs

In animals, meiosis produces gametes: sperm in the testes (males) and eggs in the ovaries (females). In flowering plants, pollen is produced by meiosis in the pollen sacs of the anther, and the egg cell is produced by meiosis in the ovary.

DNA Content Through Meiosis

Starting with a diploid cell with a DNA content of 4 AU (arbitrary units) and 4 chromosomes in 2 homologous pairs:

The Stages of Meiosis in Detail

Meiosis I — The Reduction Division

Prophase I

Chromatin condenses to form visible X-shaped chromosomes. Homologous chromosomes pair up in a process called synapsis, forming structures called bivalents. While paired, crossing over occurs — non-sister chromatids (one from the maternal chromosome, one from the paternal) exchange segments of DNA at points called chiasmata. This creates new combinations of alleles on each chromatid, generating genetic variation. The nuclear envelope breaks down and spindle fibres begin to form.

Metaphase I

The bivalents (homologous pairs) align along the equator. Each pair of homologous chromosomes is attached to spindle fibres from opposite poles via the centromere. Critically, the orientation of each bivalent is random — the maternal chromosome could face either pole. This is called random assortment (or independent assortment) and creates additional genetic variation. In humans, with 23 pairs of chromosomes, there are 2²³ = 8,388,608 possible combinations of maternal and paternal chromosomes in the gametes.

Anaphase I

The spindle fibres shorten and pull whole homologous chromosomes to opposite poles. Unlike anaphase of mitosis, the centromeres do NOT divide — each chromosome still consists of two sister chromatids. This is the step that separates homologous pairs and reduces the chromosome number.

Telophase I and Cytokinesis

The homologous chromosomes arrive at opposite poles and the cell divides into two haploid daughter cells. Each cell now has only one chromosome from each homologous pair (though each chromosome still consists of two sister chromatids).

Meiosis II — Separating Sister Chromatids

Prophase II

In each of the two haploid cells, the chromosomes (still X-shaped) become visible. New spindle fibres form, orientated at right angles to those in meiosis I.

Metaphase II

Chromosomes align along the equator individually (not as bivalents). They attach to spindle fibres via the centromere. Random assortment can occur again at this stage, adding further variation.

Anaphase II

The centromeres divide and the spindle fibres shorten, pulling sister chromatids to opposite poles. This is the same process as anaphase of mitosis.

Telophase II and Cytokinesis

The chromatids (now called chromosomes) arrive at opposite poles. Nuclear envelopes reform and both cells divide by cytokinesis. The final result is four genetically different haploid gametes.

How Meiosis Creates Genetic Variation

Meiosis is the major source of genetic variation in sexually reproducing organisms. Three mechanisms contribute:

1. Crossing Over (Prophase I)

During prophase I, non-sister chromatids of homologous chromosomes exchange segments of DNA at chiasmata. This recombines alleles that were originally on different chromosomes, producing chromatids with new allele combinations that did not exist in either parent chromosome. For example, if a maternal chromatid carried alleles for brown eyes and blood group A, and a paternal chromatid carried alleles for blue eyes and blood group B, crossing over could produce a chromatid with brown eyes and blood group B — a combination found in neither parent.

2. Independent Assortment (Metaphase I)

The random orientation of bivalents at the equator means that maternal and paternal chromosomes are distributed randomly into daughter cells. With 23 pairs in humans, there are 2²³ (over 8.3 million) possible combinations. This means any two gametes from the same parent are almost certainly genetically unique.

3. Random Fusion of Gametes at Fertilisation

Any sperm can fertilise any egg. Combined with the variation already present in each gamete, this produces an astronomically large number of possible genetic combinations in the offspring.

Additionally, mutations (random changes to the DNA sequence) can occur during meiosis, introducing entirely new alleles into the population.

Mitosis vs Meiosis — Complete Comparison

Where Cell Division Appears on Your Specification

Related Resources on This Site