AS biology unit 2, AQA

DNA- Triplet code

Gene: section of DNA that codes for a polypeptide chain
Polypeptide chain: a sequence of amino acids.
Sequence of DNA can code for an amino acid by: Triplet code
Triplet code: 20 naturally occurring amino acids, and 4 bases
4 to the power of 3: 64, 64 different codes can satisfy 20 naturally occuring amino acids. There are 64 possible codes and 20 amino acids- some triplet codes can code for the same amino acid.
INTRONS: DNA that doesn't code for an amino acid
Degenerate code: when one amino acid has more than one triplet code

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Prokaryotic cells:- not associated with proteins, the DNA is smaller, they do not have chromosomes, DNA is arranged in a circular format
Eukaryotic cells:-chromosomes present when cell is dividing, larger DNA, arranged in a linear manner, are associated with proteins.
Chromosomes: visible when cell is dividing, two threads (chromatin) joined by a centromere
Chromosome number: normally an even number (homologous pairs)
Homologous chromosomes: A pair a chromosomes which code for the same characteristics(same genes) yet with different alleles. Homologous pair: one paternal and one maternal. Need to be even (diploid number)- for mitosis
Allele: One of the different forms of a particular gene

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Four daughter nuclei are produced with a haploid number of chromosomes
Meiosis is necessary to maintain the diploid number in the offspring: (46)
Meiosis 1: homologous chromosomes pair up (randomly) and the chromatids wrap round each other: an equivalent amount is swapped to form a recombination.(CROSSING OVER) The homologous chromosomes split off with 1 chromosome from each pair going into a new daughter cell
Meiosis 2: Chromatids split apart and go into 4 new daughter cells
Genetic variation in meiosis: Crossing over and independent assortment

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INDEPENDENT ASSORTMENT
In meiosis 1: homologous pairs align to form 23 pairs of chromosomes
when homologous chromosomes arrange themselves with theirs this is completely random
Which homologous chromosome ends up in which cell depends on how the pairs have lined up: this was random and so the combination of chromosomes that go into the daughter cells at the end of meiosis 1 is independent: independent assortment,
CROSSING OVER
During meiosis 1when chromosomes align in homologous pairs the chromatids wrap around each other, this causes a tension which allows equivalent parts of the chromatids to be exchanged with the homologous pair.
A new genetic recombination is produced which increases genetic variety
FUSION AT FERTILISATION
gametes produced at fertilisation will be genetically different (one from mother and one from father)
the haploid gametes will fuse randomly at fertilisation
So: two different gametes with different genetic make-ups combine -more variety

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