Genetic Code

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  • Watson and Crick, along with the structure, proposed a theory for the replication of this DNA.
  • This is the semi-conservative model.
  • This was later proven by Meselsohn and Stahl:
    -Growing bacteria on heavy N15, which when centrifuged caused a band at the bottom.
    -Then transfering this bacteria to material containing only N14.
    -With every generation taking the bacteria out and centrifuging a sample.
    -The first generation provided one band between that of N15 and N14, wiping out the possibility for the theory of conservative replication.
    -Then with the second generation there were two equal bands at N14 and the midpoint.
    -And the third provides a thicker N14 band a thin midpoint, removing possibility of other theories.
  • The accepted method is as such:
    -H bonds break and the two strands seperate.
    -DNA unwinds and DNA polymerase catalyses the addition of free nucleotides.
    -Each chain is a template for a new strand to be attached.
    -The result is two DNA molecules, both consisting of one original and one new strand.
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The Genetic Code

  • DNA acts as a store for genetic information, divided up into genes.
  • The code is triplet because there are 20 amino acids and 3 corresponding bases as a code provides enough possibilites for all amino acids, whereas 2 does not.
  • Characteristics:
    -Contains 3 bases: triplet code.
    -The code is degenerate, as most amino acids have more than one code.
    -Three codes are STOP CODES, and are the end point of a code, like a full stop almost; the terminating signal.
    -The portion of DNA that corresponds to a whole polypeptide is called a gene.
    -Each 3 bases is a codon.
    -The codons are universal, in all living organisms.
    -The code is non-overlapping. 
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Protein Synthesis

  • Basic outline:
    -DNA acts as a template for the instructions to form long sequences of nucleotides.
    -A complementory section is made into mRNA by transcription.
    -The mRNA acts as a template for complementory tRNA to attach amino acids, translation.
  • Transcription:
  • DNA does not leave the nucleus, acting as a template for the formation of mRNA; which carries the genetic information into the cytoplasm.
    -DNA helicase acts on a specific region called the cistron on the DNA and breaks the H bonds, causing the two strands to seperate.
    -RNA polymerase links the template strand to the begining of the sequence, unwinding and then unzipping in the relavent region.
    -Free RNA nucleotides attach themselves opposite correpsonding bases.
    -So G-C, C-G, T-A, A-U.
    -The DNA moves along attaching these bases, synthesising an mRNA strand.
    -The DNA rezips behind the forming bonds, and then this all ends at a stop code.
    -The mRNA then moves this information to a ribosome outside the nucleus. 
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Protein Synthesis (Cont.)

  • Translation:
  • Involves translating codons on the mRNA into a sequence of amino acids, on a ribosome.
  • Two tRNA are associated with a ribosome at one time, the ribosome acts as framework.
    -Ribosome becomes attached to the starting codon on the mRNA.
    -The first tRNA with the complementory anticodon attaches to the ribosome, the second tRNA with the complentory anticodon for the next code also attaches; these are close enough for a peptide bond to form between them.
    -The tRNA leaves and the ribosome moves one along.
    -One site binds tRNA to the growing polypeptie, the other site carries tRNA carrying the next amino acid in sequence.
    -Translation by ribosomes allows the assembly of amino acids in accordance with the original DNA, the ribosome moving along the mRNA one code at a time.
    -Each time the ribosome moves, a new molecule of polypeptide is produced.
    -Usually more than one ribosome is associated with the mRNA at one time, this is called the polysome system and so many molecules can be formed at the same time. 
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  • The anticodon determines which amino acid attaches to the binding site.
    -ie. CCC = glycine will attach to the tRNA
  • This anticodon will combine with codon GGG on the mRNA molecule.
  • This codon therefore translates into glycine.
  • Once the tRNA is released from the mRNA it is free to collect another amino acid from the amino acid pool in the cell, this requires ATP.
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