more on gametes


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  • Created by: callum
  • Created on: 31-03-12 18:12

how do gametes form?

There are 2 types of cell division

one is mitosis which produces new body cells as an organism grows and develops. mitosis retains the full amount of chromosomes called the diploid number (46 chromosomes in humans)

the other type of cell division is meiosis which produces gametes with half the number of chromosomes present (23 in humans)

Meiosis happens in the ovaries and testes of animals, and the ovaries and anthers of flowering plants.

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more on cell division

In female plants only one of the four nuclei at the end of meiosis forms an ovum- the otheres disintergrate.

In male plants the 4 haploid cells produced at the end of meiosis undergo another division, this time by mitosis, where each nucleus divides to give two haploid nuclei within the pollon grain. - one is known as the tube nucleus which controls the development of the pollon tube and the 2nd is known as the generative nucleus which will divide again to form two gamete nuclei.

Meiosis has two importantroles in biology. firstly it results in haploid cells, which are necessary to maintain the diploid number after fertilisation. secondly it helps create genetic variation among offspring.

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meiosis and genetic variation

the shuffling of existing genetic material into new combinationsuring meiosis is important in creating genetic variation.

the shuffling includes both independant assortment and crossing over.

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independent assortment

During meiosis only one chromosome from each pair ends up in each gamete.

the independent assortment of the chromosome pairs as they line up during meiosis 1 is a source of genetic variation.

this process is random, either chormosome from each pair could be in any gamete.

with organisms with many chromosomes such as humansthe gentic variation is so large that it is very unlikely that any two siblings will have the same gentic makeup  unless they are identicle twins.

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crossing over

during the first meiotic division, homologous chromosomes come togther as pairs and all four chromotids come into contact.

at these contact points the chromotids break and rejoin , exchanging sections of DNA .

the points where the chromotids break are called a chiasma, and several of these often occur along each pair of chromosomes, giving rise to a large amount of variation.

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