AS AQA Biology- DNA and Meiosis

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  • Created by: Ruth Butt
  • Created on: 23-05-13 11:01

Structure of DNA

DNA is made of nucleotides, which are formed by 3 components:
1. A deoxyribose sugar
2. A phosphate group
3. An organic base,either Cytosine, Thymine, Guanine or Adenine

They are combined via a condensation reaction. The structure of DNA was founded by Watson and Crick in 1953. It is made of two strands of polynucleotides joined by hydrogen bonds.

Pairing of bases: Adenine pairs eith thymine by two hydrogen bonds, and Guanine pairs with cytosine by 3 hydrogen bonds. These pairings are therefore said to be complementary.
The double helix is a ladder like arrangement of two polynucleotides being twisted. 

The function of a DNA molecule is to pass genetic information from cell to cell.It is adapted for this by the following ways:

1. Very stable and doesn't change from generation to generation
2. Bonds connected by hydrogen bonds so can easily undo and replicate
3. Very large molecul
4, Helical shape protects genetic information 

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Triplet Code

If we only had a pair of bases, just 16 different codes would be possible which is inadequete. Three bases however will produce 64 different amino acids which is ore than enough. Therefore, it is called the triplet code. Sections of DNA which do not code for anything are known as introns.

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Chromosomes

Chromosome structure: A DNA molecule is wound around proteins, then coiled, to form loops, and finally packs together to form a chromosome.  There is always an even number of chromosomes, because they come in pairs, homologous pairs. One from each pair of chromosomes comes from the mother- maternal, and one from the father- paternal. They are therefore known as homologous pairs and are referred to as the diploid number- 46.  Alleles are differerent forms of the same gene, and will therefore code for different amino acids and polypeptides. We will inherit one from each parent 

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Meiosis

This produces 4 daughter nuclei with half the number of chromosomes as the parents cell. It is necessary for them to have half the number of chromosomes for when the gametes fuse, and produce a diploid number of chromosomes. The process involves 2 nuclear divisions: 1. Homologous chromosomes pairs up and equivalent portions of DNA may be exchanged in a process called crossing over. 2.  The second meiotic division is when the chromatids move apart, and 4 new cells are formed. Each of these cells will contain 23 chromosomes (haploid number). Three main parts to a chromosome: gene codes for a section DNA, locus is a position on a DNA molecule and an allele is a form of a particular gene.  Independent Segregation: This is when the chromosome pairs line up randomly along the equator of the cell during meiosis Crossing Over: When the chromosomes become twisted around one another. During this stage tensions are made and portions of chromosome break off. These broken portions then rejoin with the chromosome of it homologous pair. Therefore new genetic combinations are produced.

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