GCSE Biology revision

Meiosis

Reproduction

AQA 4.1.1.1

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AQA student objectives

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ALL 🎯

Identify and describe What Are Gametes?, Chromosomes: A Quick Recap and Step 1: DNA Replication in the context of meiosis.

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Explain the relationships between What Are Gametes?, Chromosomes: A Quick Recap and Step 1: DNA Replication and how they contribute to meiosis.

SOME 🎯🎯🎯

Apply and analyse knowledge of What Are Gametes?, Chromosomes: A Quick Recap and Step 1: DNA Replication to interpret unfamiliar information about meiosis.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What Are Gametes?

Gametes are sex cells (e.g. sperm and egg cells) that are produced during sexual reproduction and contain only half the genetic material of a normal body cell.
Gametes are described as haploid cells because they carry only one set of chromosomes, whereas normal body cells are diploid, containing two sets.
When two haploid gametes fuse during fertilisation, they form a diploid cell with a full set of genetic information — one set from each parent.

1. What Are Gametes?AQA

Notes

Gametes are sex cells (e.g. sperm and egg cells) that are produced during sexual reproduction and contain only half the genetic material of a normal body cell.
Gametes are described as haploid cells because they carry only one set of chromosomes, whereas normal body cells are diploid, containing two sets.
When two haploid gametes fuse during fertilisation, they form a diploid cell with a full set of genetic information — one set from each parent.

Chromosomes: A Quick Recap

Almost every human body cell contains 23 different types of chromosomes inside its nucleus, each carrying genetic information.
For each of the 23 chromosome types, there are two copies — one maternal chromosome (from the mother) and one paternal chromosome (from the father).
Because there are 23 types with 2 copies each, human body cells contain a total of 46 individual chromosomes.

2. Chromosomes: A Quick RecapAQA

Notes

Almost every human body cell contains 23 different types of chromosomes inside its nucleus, each carrying genetic information.
For each of the 23 chromosome types, there are two copies — one maternal chromosome (from the mother) and one paternal chromosome (from the father).
Because there are 23 types with 2 copies each, human body cells contain a total of 46 individual chromosomes.

Step 1: DNA Replication

Before meiosis begins, the cell replicates all of its DNA so that every chromosome is copied.
Replication does not create entirely new chromosomes; instead, each chromosome gains an extra arm, giving it an X shape where one arm is the original and the other is the copy.

Step 2: First Division (Meiosis I)

The replicated chromosome pairs line up along the centre of the cell, with maternal and paternal versions of each chromosome sitting side by side.
The left-to-right positioning of each chromosome pair is completely random, meaning the arrangement is different every time meiosis occurs.
The chromosome pairs are then pulled apart and the cell splits in two, randomly distributing maternal and paternal chromosomes between the two new cells.
Because of this random distribution, the two resulting cells each contain a different combination of maternal and paternal chromosomes, making their DNA genetically different from one another.

4. Step 2: First Division (Meiosis I)AQA

Notes

The replicated chromosome pairs line up along the centre of the cell, with maternal and paternal versions of each chromosome sitting side by side.
The left-to-right positioning of each chromosome pair is completely random, meaning the arrangement is different every time meiosis occurs.
The chromosome pairs are then pulled apart and the cell splits in two, randomly distributing maternal and paternal chromosomes between the two new cells.
Because of this random distribution, the two resulting cells each contain a different combination of maternal and paternal chromosomes, making their DNA genetically different from one another.

Step 3: Second Division (Meiosis II)

In the second division, the chromosomes line up along the centre of each cell again, but this time the two arms of each X-shaped chromosome are pulled to opposite sides.
Both cells then divide in half, producing a total of four cells from the original single cell.

Genetic Uniqueness of Gametes

All four cells produced by meiosis are genetically unique due to the random distribution of chromosomes during the first division and small changes that occur throughout the process.
Each of the four gametes contains 23 chromosomes (in humans), which is half the number found in a normal diploid body cell.

6. Genetic Uniqueness of GametesAQA

Notes

All four cells produced by meiosis are genetically unique due to the random distribution of chromosomes during the first division and small changes that occur throughout the process.
Each of the four gametes contains 23 chromosomes (in humans), which is half the number found in a normal diploid body cell.

Gametes in Male and Female Animals

In male animals, the four haploid cells produced by meiosis go on to develop into sperm cells.
In female animals, the haploid cells produced by meiosis develop into egg cells.

From Fertilisation to a New Organism

When a sperm cell and an egg cell fuse during fertilisation, they form a diploid cell containing a full set of 46 chromosomes — 23 from each parent.
This diploid cell then divides repeatedly by mitosis, eventually developing into an embryo, then a foetus, and finally a fully grown organism.

8. From Fertilisation to a New OrganismAQA

Notes

When a sperm cell and an egg cell fuse during fertilisation, they form a diploid cell containing a full set of 46 chromosomes — 23 from each parent.
This diploid cell then divides repeatedly by mitosis, eventually developing into an embryo, then a foetus, and finally a fully grown organism.