Cellular Control

5.1.1 OCR A2 Biology

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How DNA codes for proteins

A gene is a length of DNA that codes for polypeptides.

The sequence of nucleotide bases on a gene provides the code, which gives instructions for the construction of the polypeptide or protein.

This genetic code is:

  • a triplet code
  • a degenerate code
  • widespread but not universal.

Messenger RNA is a copy of the genetic code, which is made in order for the proteins to be assembled in the cytoplasm at ribosomes because it can pass through a pore in the nuclear envelope.

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Protein Synthesis

Stage 1 - Transcription

Occurs in the nucleus of a cell.

  • A length of DNA unwinds and unzips.
  • Free activated RNA nucleotides pair up and, using hydrogen bonds, bind their complementary, exposed bases on the template strand of unwound DNA.
  • The sugar-phosphate groups are bonded together to form the sugar-phosphate backbone.
  • This single-stand of mRNA produced, which is a coding strand of the length of DNA, leaves the nucleus through a pore in the nuclear envelope.
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Protein Synthesis

Stage 2 - Translation

Occurs in the cytoplasm.

A sequence of nucleotides becomes a sequence of amino acids.

  • mRNA binds to a ribosome and attracts tRNA with the complementary anticodon.
  • Ribosome moves along mRNA onto the next codon and attracts another tRNA with complementary anticodon.
  • A peptide forms, by a condensation reaction, between the amino acids which are bonded onto the tRNA molecules. The tRNA molecules then detatch from the amino acids that have bonded.
  • The polypeptide chain grows until a stop codon is reached.
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A mutation is a random change to the genetic material.

There are 2 main classes of DNA mutations:

  • Point mutations - when one base pair replaces another, also called substitution.

When the gene containing the error is next replicated, at least one copy will retain the error, the incorrect base will be paired with its complementary nucleotide to complete the mutation. A substitution is only a minor mutation, as the primary structure of the protein is only changed slightly and only affects a maximum of one amino acid.

  • Insertion/Deletion mutations - these cause a frameshift.

The DNA polymerase 'slips' causing it to either add or miss one or more nucleotide in the new strand. When the gene containing the error is next replicated the extra or missing base will have a knock on effect on the triplet code on the rest of the strand. The primary structure of the protein will be changed, which could be fatal.

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good notes here

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