Meiosis
Biology: AQA A Level, topic 4.3.2
Role of Meiosis
Meiosis is the process that produces haploid gamete cells from diploid cells. Meiosis is necessary for sexual reproduction to take place.
Sexual reproduction
- Gametes are the sex cells (e.g. sperm cells and egg cells) that fuse together during sexual reproduction.
- Gametes are haploid (have half the number of chromosomes as diploid cells).
- When the gametes fuse together, a new diploid organism (an organism with two copies of every chromosome) is created.
- An individual inherits one copy of a chromosome from each parent.
-min,w_800.png)
Producing gametes
- Haploid gametes are produced from diploid cells in an organism.
- The gametes are produced in a process called meiosis.
Genetic variation
- Meiosis produces four genetically different daughter cells.
- Each daughter cell receives a different combination of chromosomes.
- Meiosis increases genetic diversity in this way.
Random fertilisation
- Sexual reproduction involves the random fusion between gametes.
- Any two gametes could fuse together to produce a random combination of chromosomes.
- Meiosis and random fertilisation further increase genetic diversity in this way.
Meiosis
Meiosis is the production of haploid gametes from diploid cells. The steps involved in this process are:
1) Producing sister chromatids
- The chromosomes in the cells replicate to produce an identical copy of each other.
- The replicated chromosomes are called chromatids.
- The chromatids condense to form a double-armed chromosome. Each arm is made from one chromatid and is joined by a centromere.
- Each arm is called a sister chromatid.
2) Arranging the chromosomes
- Chromosomes in diploid organisms are found in homologous pairs (chromosomes with identical genes but different alleles).
- The chromosomes in a pair are arranged in the centre of the cell.
- The chromosomes are moved by fibres called spindle fibres.
3) Separating the chromosomes
- The homologous chromosomes are separated and pulled to opposite ends of the cell by the spindle fibres.
- The separation of the chromosomes from each pair is random.
- Random combinations of chromosomes are pulled to each side of the cell.
4) Cell division (meiosis I)
- As the homologous chromosomes are separated, the cell divides into two cells.
- One chromosome from a pair is separated into each cell.
- This division is called meiosis I.
- Meiosis I produces two genetically different cells.
5) Separating sister chromatids
- The sister chromatids are separated by the spindle fibres and pulled to opposite ends of the cell.
- The separation of the sister chromatids is random.
6) Cell division (meiosis II)
- As the sister chromatids are separated, the cell divides into two cells.
- One chromatid from a chromosome is separated into each cell.
- This division is called meiosis II.
- Meiosis II produces four genetically different haploid cells.
Genetic Variation
Meiosis produces four genetically different daughter cells. Genetic variation is generated in each daughter cell in two ways. These are:
Crossing over
- The chromosomes of a homologous pair are arranged down the middle of the cell before the first cell division in meiosis I.
- Sections of DNA are swapped between the chromosomes in a pair in a process called crossing over.
- Crossing over produces new combinations of alleles. This process is a key part of genetic recombination.
- This increases genetic variation.
Independent segregation
- The chromosomes in a homologous pair are separated in meiosis I in a random formation to produce two genetically different daughter cells.
- The combination of chromosomes in the two daughter cells is random.
- The process of separating chromosomes into random combinations is called independent segregation.
- Independent segregation increases genetic variation.
Test yourself on Meiosis
Which of the following statements about meiosis are true?
What is the role of meiosis?
Stages Involved in Meiosis
Which of the following statements about meiosis are true?
What is separated during meiosis I?