Cellular Control

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Genes code for polypeptides, including enzymes.

Genetic code: The sequence of the bases on a gene is a code with instructions for the construction of proteins. It has a number of characteristics:

  • It is a triplet code - three bases code for an amino acid
  • It is a degenerate code - all amino acids bar one have more than one code
  • Some codes don't code for amino acids but are 'stop' codons - they indicate the end of the polypeptide chain
  • It is widespread but not universal - codons generally always code for the same amino acid in every organism, but this is not always the case

DNA is copied into mRNA by a process called transcription.
Only one strand of DNA is copied by complementary base-pairing (hydrogen bonds).
The code is a triplet code which is read in threes (a codon is 3 bases).
The base sequence determines the amino acid sequence.
The mRNA is released from the DNA and passes out of the nucleus through a pore in the nuclear envelope to a ribosome.

1. A molecule of mRNA binds to a ribosome. Two codons are attached to the small subunit of the ribosome and exposed to the large subunit. The first exposed mRNA codon is always AUG. Using ATP energy and an enzyme, a tRNA molecule with the amino acid methionine and the anticodon UAC forms hydrogen bonds with this codon.
2. A second tRNA molecule, bearing a different amino acid, binds to the second exposed codon with its complementary anticodon.
3. A peptide bond forms between the two adjacent amino acids This is catalysed by an enzyme.
4. The ribosome now moves along the mRNA reading the next codon. A third tRNA brings another amino acid and a peptide bond forms between it and the dipeptide. The first tRNA leaves and is able to collect and bring another of its amino acids.
5. The polypeptide chain grows until a stop codon is reached, for which there are no corresponding tRNAs and the polypeptide chain is complete.

Mutations cause changes to the sequence of nucleotides in DNA molecules.
A 13 base pair deletion has serious consequences as it could cause a frameshift. As the genetic code is triplet (read in groups of 3 bases), it alters all amino acids that are coded for after the mutation.
A 21 base pair deletion causes 7 amino acids to be lost. 
A substitution changes one or possibly no amino acids.

Beneficial effects: The mutation changes the sequence of amino acids and therefore the phenotype, but…

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