Biology B5

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DNA is a double helix structure containing 4 different bases: A, T, C and G. The base pairs are AT and CG. The DNA unzips to form messenger RNA. The DNA is too big to pass out of the nucleus so the smaller strand of messenger RNA moves out of the nucleus into the cytoplasm. This is where it joins with a ribosome, which sticks amino acids together depending on the order of the bases on the strand. Amino acids are coded by 3 bases. Depending on the type of amino acid made, a different protein is made because the amino acids are put in a chain to form a specific protein needed by the body.

Cell Division (two types):

Mitosis is the normal cell devision where the cell splits to form 2 genetically identical new cells. New cells are needed for growth and repair. First, the DNA splits and free-floating neuceotides pair up with the matching bases of the strand of DNA. These then form conncetions (cross-links) and now there are 2 strands of DNA. Now the DNA forms chromosomes, which line up across the cell. As the cell grows, the cell fibres pull them apart. A membrane forms around each set of chromosmes, this becomes the nuclei. Lastly the cytoplasm splits and ends up producing 2 new, genetically identical cells.

Meiosis is a special cell devision used to form gametes (sex cells). Unlike normal cells, gametes have half the number of chromosomes (only 23) because when they are fertilised the zygote must contain half of each parent's genetic information. Therefore the gametes can only have half the number of chromosomes so that during fertilisation, the egg has a total of 46 chromosomes.

First, the DNA copies itself (same as in mitosis) and each chromosome is aligned with it's pair. These pairs


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