DNA and meiosis

- structure of DNA

- Triplet code

- chromosomes


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Structure of DNA

DNA is formed by 2 chains of nucleotides, joined together by hydrogen bonds.

Nucleotides are composed of a deoxyribose (pentose) sugar, a phosphate group and an organic base.

Nucleotides join to each other to form a chain by forming phosphodiester bonds.

Nucleotide chains join to each other by forming hydrogen bonds within the organic bases

There are 4 organic bases: Adenine, Cytosine, Guanine, and Thymine.

Adenine joins to Thymine by making 2 hydrogen bonds

Cytosine joins to Guanine by making 3 hydrogen bonds.

DNA has a double helix structure.

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Function of DNA

Functions of DNA include:

1. Being very stable it can be passed on generation to generation without being unchanged.

2. It is an extremely large molecule and therefore can contain lots of genetic material.

3. The base pairs are in the centre of the helix and so are protected from damage by the deoxyribose-phosphate backbone.

4. The nucleotide chains are only joined by hydrogen bonds and so can be broken apart easily by DNA helicase as part of protein synthesis.

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The triplet code

Scientists concluded that there must be 3 bases for each amino acid:

There are 20 known amino acids,

4^1 results in 4 amino acids

4^2 would result in 16 amino acids, which is still inadequate,

4^3 would result in 64 amino acids, which is more than adequate, meaning that 3 bases produces 1 amino acid.

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DNA and chromosomes

The structure of a chromosome is as follows: DNA is coiled around protie, and then the protein-DNA complex is further coiled and loop to form a chromosome, which is 2 chromatids joined at the centromere.

The number of chromosomes in most species is usually an even number as chromosomes occur in homologous pairs. A homologous pair is always 2 chromosomes that determine the same genetic characteristic, for example, eye colour, but not the same eye colour, one chromosome could code for brown eyes, wile the other could code for blue eyes.

During meiosis the chromosome divides in a manner which ensure it receives on chromosome from each pair.

Each gene exists in two or more forms (1 from mother 1 from father) 1 of these genes is called an allele, as in the mother carries the allele for blue eyes whilst the father carries the allele for brown eyes.

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Meiosis is necessary to ensure offspring have the correct amount of chromosomes. If Meiosis did not occur, every generation would have double the number of chromosomes than the one before it.

Meiosis occurs in 2 stages, meiosis 1 and meiosis 2

Meiosis1: The homologous chromosomes pair up and crossing over occurs, the chromosomes then split up, with one chromosome going into each daughter cell.

Meiosis 2: the chromatids move apart, forming 4 daughter cells, each with half a chromosome in them.

Meiosis provides genetic variation within offspring, as due to crossing over, an infinite number of combinations can be made.

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Independent segregation of chromosomes is when, during meiosis 1, the homologous pairs line up next to each other and one of each pair goes into a daughter cell. The chromosomes can line up in any order, and so there are a number of possibilities. the pairs are lined up at random, and so the combination of chromosomes in the daughter cell is random.

There is much variety simply due to the amount of combinations that can be made by the chromosomes. and alleles.

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