Passage of information

1. Overview

Body cells of most organisms are diploid (2n): they carry two complete sets of chromosomes arranged in homologous pairs, one set from each parent. Gametes, however, must be haploid (n) so that when two of them fuse at fertilisation the diploid number is restored rather than doubled each generation. Meiosis is the reduction division that achieves this, producing four genetically different haploid cells from one diploid cell through two successive divisions (meiosis I and meiosis II). The process also creates variation through crossing over, independent assortment and, later, the random fusion of gametes.

Key Definitions

• Haploid (n): containing one complete set of chromosomes, as found in a gamete.
• Diploid (2n): containing two complete sets of chromosomes, one set inherited from each parent, as found in most body cells.
• Homologous pair: a pair of chromosomes, one maternal and one paternal, that are the same length, have the centromere in the same position and carry the same genes (though possibly different alleles) in the same order.
• Meiosis: a type of nuclear division that produces four genetically different haploid cells from one diploid cell, involving two divisions.
• Reduction division: a division that halves the chromosome number, separating the homologous chromosomes of each pair so that each daughter cell receives only one chromosome from each pair.
• Crossing over: the exchange of equivalent lengths of chromatid, and so of alleles, between non-sister chromatids of a homologous pair during prophase I.
• Independent assortment: the random orientation of homologous pairs (and of sister chromatids in meiosis II) at the equator, so that either member can pass to either pole.
• Fertilisation: the fusion of the nuclei of a male and a female haploid gamete to form a diploid zygote.

Content

Haploid and diploid: chromosome sets

A diploid (2n) cell contains two complete sets of chromosomes, while a haploid (n) cell contains one complete set. These definitions are about sets of chromosomes, so they apply to every organism and should be stated in general terms. In humans the diploid number is 46 (23 pairs) and the haploid number is 23, but a definition that works only for humans is incomplete: the key idea is "one set" versus "two sets".

Homologous pairs of chromosomes

In a diploid cell the chromosomes occur as homologous pairs. The two members of a pair:

• are the same length;
• have the centromere in the same position;
• carry the same genes in the same order along their length.

One member of each pair is maternal (from the egg) and the other is paternal (from the sperm or pollen). Because they may carry different alleles of those genes, the two homologues are not identical copies: they match in genes but can differ in alleles.

Why a reduction division is needed

Gametes are produced by meiosis from diploid parent cells. If gametes were diploid, fertilisation (the fusion of two gametes) would double the chromosome number every generation. A reduction division prevents this: meiosis halves the chromosome number so each gamete is haploid. When a haploid male gamete fuses with a haploid female gamete, the diploid number is restored in the zygote ($n + n = 2n$) and stays constant from one generation to the next.

Behaviour of chromosomes during meiosis (plant and animal cells)

Before meiosis begins, the DNA replicates so each chromosome consists of two identical sister chromatids joined at a centromere. Meiosis then proceeds through two divisions and eight named main stages. The same chromosome behaviour occurs in plant and animal cells; the main difference is how the spindle is organised and how the cell finally splits.

The table below summarises the eight stages. Read it across to see what each cell structure is doing at each stage, and read it down to compare meiosis I with meiosis II.

Stage	Chromosome behaviour	Nuclear envelope	Spindle	Cell division
Prophase I	Chromosomes condense; homologues pair up as bivalents; crossing over between non-sister chromatids	Breaks down	Forms (centrioles in animals; no centrioles in plants)	—
Metaphase I	Bivalents line up at the equator; random orientation = independent assortment	Absent	Attaches to each homologue	—
Anaphase I	Whole homologous chromosomes pulled to opposite poles (each still 2 chromatids)	Absent	Shortens, pulling homologues apart	—
Telophase I	Homologues reach the poles; chromosome number now halved	May reform	Breaks down	Furrow (animal) or cell plate (plant) gives 2 haploid cells
Prophase II	Chromosomes condense again (each still 2 chromatids)	Breaks down (if reformed)	New spindle forms, at right angles to the first	—
Metaphase II	Single chromosomes line up at the equator; chromatids orient randomly	Absent	Attaches to each chromosome	—
Anaphase II	Centromeres divide; sister chromatids pulled to opposite poles	Absent	Shortens, separating chromatids	—
Telophase II	Chromatids (now chromosomes) reach the poles	Reforms	Breaks down	Furrow or cell plate gives 4 haploid cells in total

Two points to keep clear:

• Meiosis I is the reduction division. It separates whole homologous chromosomes, so it is here that the diploid number is halved to haploid.
• Meiosis II resembles mitosis. It separates sister chromatids, so the cells stay haploid but the chromatids become separate chromosomes.

A note on the cell type:

• In animal cells, centrioles move to opposite poles and organise the spindle, and the cell splits when the cell surface membrane furrows inward (cleavage furrow).
• In plant cells, there are no centrioles but a spindle still forms, and the cell splits when a cell plate forms to build a new cell wall between the two cells.

Identifying stages from photomicrographs and diagrams

You should be able to recognise the main stages from images, so it helps to have a fixed set of decision rules:

• Paired or single? Chromosomes lined up as pairs (bivalents) at the equator means metaphase I; chromosomes lined up singly means metaphase II.
• What is moving to the poles? Whole chromosomes (each still two chromatids) moving apart is anaphase I; single chromatids moving apart is anaphase II.
• Any chiasmata? Visible bivalents with crossing-over points (chiasmata) point to prophase I.

The quickest check is to count the chromosome content. Asking "are the chromosomes paired or single?" and "do they have two chromatids or one?" tells you whether you are in meiosis I or meiosis II.

Worked example

Exam-style question: A micrograph shows a cell in which individual chromosomes are arranged singly along the equator, and each chromosome is clearly made of two chromatids. There are no paired-up bivalents. Identify the exact stage of meiosis shown and explain how you decided. [3]

Model answer:

• The chromosomes are single, not in homologous pairs, so the cell is in meiosis II (homologues were already separated in meiosis I). [1]
• The chromosomes are lined up at the equator, which is a metaphase arrangement. [1]
• Therefore the stage is metaphase II. (Each chromosome still has two chromatids because the centromeres have not yet divided, which happens at anaphase II.) [1]

How meiosis generates genetic variation

Meiosis produces genetically different gametes by two mechanisms:

• Crossing over — during prophase I, non-sister chromatids of a homologous pair exchange equivalent lengths, swapping alleles and creating new combinations of alleles on a chromatid.
• Independent assortment — the random orientation of homologous pairs at metaphase I, and of sister chromatids at metaphase II, means each gamete receives a random mix of maternal and paternal genetic material. With $n$ homologous pairs, the number of possible combinations from independent assortment alone is $2^n$.

Random fertilisation then adds a further layer: any one of a vast number of genetically different male gametes can fuse with any one of a vast number of genetically different female gametes, so the random fusion of gametes produces genetically different individuals.

Worked example

Exam-style question: A plant species has a diploid number of $2n = 14$. Ignoring crossing over, calculate the number of genetically different gametes that can be produced from one parent by independent assortment, and state one further source of variation that occurs during meiosis. [3]

Model answer:

• The number of homologous pairs is $n = \tfrac{14}{2} = 7$. [1]
• The number of combinations from independent assortment is $2^n = 2^7 = 128$ genetically different gametes. [1]
• A further source of variation is crossing over, the exchange of alleles between non-sister chromatids of a homologous pair during prophase I. [1]

(For comparison, humans have $n = 23$ pairs, giving $2^{23}$ possible combinations from independent assortment alone.)

Key Equations

Number of gamete combinations from independent assortment, where $n$ is the number of homologous pairs: $$\text{combinations} = 2^{n}$$

Common Mistakes to Avoid

• Defining haploid and diploid using only human numbers (23 and 46). The marks are for the general idea — one set of chromosomes (haploid) versus two sets (diploid) — which applies to every organism. Quote human numbers only as an example, not as the definition.
• Confusing crossing over with independent assortment. Use crossing over for the exchange of alleles between non-sister chromatids in prophase I, and independent assortment for the random orientation of pairs at the equator. These are two separate sources of variation.
• Saying "genes" or "information" are exchanged in crossing over. Be precise: it is alleles that are exchanged, between non-sister chromatids of a homologous pair.
• Mixing up anaphase I and anaphase II. In anaphase I whole homologous chromosomes separate (the reduction step). In anaphase II the sister chromatids separate. Forgetting that chromosomes still have two chromatids after meiosis I leads to wrong stage identification.
• Forgetting that plant cells lack centrioles. A spindle still forms in plant cells; do not state that centrioles organise the spindle as if it were universal. Describe spindle formation generally and treat centrioles as the animal-cell case.
• Describing cell division the same way for both cell types. Animal cells divide by the cell surface membrane furrowing inward; plant cells form a cell plate to build a new wall. Match the description to the cell type asked about.

Exam Tips

• When defining haploid and diploid, use the phrase "sets of chromosomes" — that wording shows the definition is general.
• For a homologous pair, link same length, same centromere position, same genes in the same order but possibly different alleles — these are the comparison points to include.
• In stage-identification questions, decide first whether chromosomes are paired or single at the equator: paired means metaphase I, single means metaphase II. Apply the same logic to anaphase (whole chromosomes vs chromatids).
• When explaining variation, name all three sources where relevant: crossing over, independent assortment and random fertilisation — each is a separate marking point.
• For "describe the behaviour during meiosis" answers, mention the nuclear envelope breaking down/reforming, the spindle forming, and how the cell surface membrane (or cell plate in plants) divides the cell — these specific structures earn credit.
• Show your working in calculation questions: state $n$, then $2^n$, so method marks are earned even if the final number is slipped.