Chapter 14.1 aqa biology

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Features of Genetic code

  • each amino acid in a protein is coded for by a sequence of three nucleotide bases on mRNA called a codon
  • A few amio acids have only a single codon.
  • the code is Degenerate ; most amino acids have more than one codon.
  • three codons do not code for any amino acid and are called stop codons.
  • each base in the sequence is read only once, so the code is non-overlapping.
  • it is a universal code; it is the same in all organisms.
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Structure of Ribonucleic acid (RNA)

RNA is a polymer made up of mononucleotide sub-units which forms a single strand in which each nucleotide is made up of:

  • the pentose sugar ribose.
  • one of the organic bases ( A, G, C, U)
  • a phosphate group

The two types of RNa that are important in protein synthesis are ;

  • messenger RNA (mRNA)
  • transfer RNA (tRNA)
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Messenger RNA (mRNA)

mRNA is a long strand comprised of thousands of mononucleotides that is arranged into a single helix. It is manufactured when DNA forms a mirror image of part of one of its two strands. once formed, mRNA leaves the nucleus hrough nuclear pores and associates with ribosomes in the cytoplasm. There it acts as a template upon which proteins are built. It is easily broken down and so only exists as long as it is needed to produce a protein.

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Transfer RNA (tRNA)

tRNA is a small molecule made up of around 80 nucleotides. It is a single stranded chain folded into a clover leaf shape with one end of the chain extending beyond the other. This is the part of the tRNA molecule to which amino acids can easily attach. There are several types of tRNA each able to carry a single amino acid. At the opposite end of the tRNA molecule is a sequence of three other organic bases known as the anticodon. For each amino acid there is a different sequence of organic bases on the anticodon.

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