Biology Unit 2 - DNA

DNA - Properties and Structure

Meiosis -  Cell Division - Genetically Different, Variation

Mitosis - Cell Division - Genetically Identical

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DNA - Properties and Structure

  • DNA carries information (genetic code) from which essential proteins are made (protein synthesis)
  • DNA can replicate, without this ability there would be no cell division hence no growth, repair or reproduction.
  • DNA is a nucleic acid and a stable polynucleotide. Stable as does not denature until heated to around 50-90 deg. (Compared to a basic protein denatures around 40-50 deg).
  • In eukaryotic cells the DNA is linear and attached to organising proteins called histones. In prokaryotic cells, DNA is circular and not attached to histones.
  • DNA is a polymer: monomers are nucleotides, nucleotide has 3 components: a deoxyribose sugar (5 carbon - pentagon shaped), a phosphate molecule and an organic base - 1 of 4 nitrogen containing compounds - adenine, thymine, guanine and cytosine. Nucleotides are joined by condensation reactions to form a single DNA chain.
  • Nucleotides are arranged in a double helix (imagine a twisted ladder) The 2 sides of the ladder are chains of alternating sugar-phosphate groups, while the steps are made up of pairs of bases bonded together by hydrogen bonds.
  • For protein synthesis and replication it's important that the strands can seperate and rejoin without damaging the molecule, one one part of one strand in a double stranded DNA molecule - the sense strand - is used to make proteins, the other side stablilises the molecule.
  • Covalent bonds between sugar-phosphate groups are stronger than hydrogen bonds in bases.
  • The four bases in a DNA molecule always bond the same way, A-T and C-G. The bases are held together by hydrogen bonds, a double bond A-T and a triple bond C-G
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  • Definition - A gene is a section of DNA (deoxyribose nucleic acid) that codes for the manufacture of a particular polypeptide or protein.
  • Different versions of genes are called alleles and these arise by mutation. For example, a flower colour gene could have an allele for red flowers and an allele for white flowers. A fault in the copying of the base sequence in a particular gene will probably change the amino acid sequence, in turn producing a different polypeptide/protein.
  • Polypeptide - a chain of amino acids that will fold and bend into a particular shape, proteins are made from one or more polypeptides.
  • These slightly altered proteins may function as normal but it is also possible they may not work correctly and this will cause implications in the molecule, for example a non-functional enzyme may cause a metabolic block which can result in the death of an organism.
  • A sequence of three bases, called a triplet, codes for one particular amino acid, for example the DNA triplet TAC codes for the amino acid methionine.
  • Not all the DNA in eukaryotic cells code for proteins, these sections of DNA are called introns (everything that does code for proteins are called exons) Introns are removed during protein synthesis, their purpose isn't known for sure. Eukaryotic DNA also contains regions of multiple repeats outside of genes, these are DNA sequences that repeat over and over, these areas dont code for proteins either.
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Meiosis - Unit 2

  • Two key features to meiosis: it creates genetic variation and it halves the number of chromosomes in the cell. Therefore it is completely different to mitosis, which does neither. 
  • Every diploid human cell has 23 pairs of chromosomes, these pairs are homologous, which is another way of saying they have the same genes in the same positions but the alleles of those genes may differ. 
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Tone down the colour of the font, but apart from that, v. useful

Marta Kohls


This is AS not A2- please change the level of study

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