AS biology unit 2, AQA

As unit 2 biology, revision cards

  • Created by: Bela
  • Created on: 25-04-11 11:46

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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Chromosomes and DNA structure

  • 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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Genetic Variety is caused by (independent assortme

  • 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,
  • 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
  • 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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Alina Sheikh


Thx! That really helped!! :D



Really helpful thanks!




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