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Polypeptide Synthesis

Biology: Cambridge International A Level, topic 6.2.2

Transcription

Transcription is the first step in protein synthesis. As a result of this process, mRNA that is a copy of the target gene is produced in the nucleus. The stages involved are:

1) Binding of RNA polymerase

1) Binding of RNA polymerase

  • RNA polymerase is the enzyme that allows transcription to take place.
  • RNA polymerase binds to the locus of the gene to be transcribed (the target gene).
2) Separation of DNA strands

2) Separation of DNA strands

  • When RNA polymerase binds to DNA, DNA helicase unwinds the double strands and the hydrogen bonds that bind the two strands together break.
  • The DNA strands separate.
  • The bases of the target gene are exposed.
3) Binding to template strand

3) Binding to template strand

  • RNA polymerase binds free-floating RNA nucleotides to the template strand.
  • The template strand is the DNA strand that is complementary to the base sequence of the target gene.
  • The RNA nucleotides form a strand of mRNA that is complementary to the template strand.
  • The template strand is complementary to the gene so this means mRNA is a copy of the gene.
4) Joining the nucleotides

4) Joining the nucleotides

  • The free-floating nucleotides are joined together by RNA polymerase.
  • Phosphodiester bonds form between the nucleotides to form the completed strand of mRNA.
5) STOP codon

5) STOP codon

  • RNA polymerase eventually reaches the triplet of bases that signal 'stop'.
    • E.g. UAG encodes a STOP codon.
  • The RNA polymerase stops separating the DNA and producing mRNA.
6) Removal of the mRNA

6) Removal of the mRNA

  • The mRNA strand is separated from the template strand by RNA polymerase.
  • The hydrogen bonds between the two strands of DNA form again and the strands join together.
7) mRNA leaves the nucleus

7) mRNA leaves the nucleus

  • The completed mRNA strand leaves the nucleus and enters the cytoplasm.
  • mRNA is used in translation, the next step in protein synthesis.

Translation

Translation is the second step in protein synthesis. Translation takes place in the cytoplasm. The steps involved are:

1) Attachment to the ribosome

1) Attachment to the ribosome

  • mRNA that has been produced during transcription binds to a ribosome in the cytoplasm.
  • The ribosome is the site of protein synthesis.
2) Binding of tRNA

2) Binding of tRNA

  • Six bases (two codons) can fit inside the ribosome at one time.
  • One molecule of tRNA binds to the first codon in the ribosome.
  • The tRNA molecule has an anticodon that is complementary to a specific codon.
  • The anticodon allows the correct tRNA molecule to bind to the correct codon.
3) Bringing in amino acids

3) Bringing in amino acids

  • Each tRNA molecule carries a specific amino acid into the ribosome.
  • The amino acid is bound to tRNA using ATP.
4) Binding of the second tRNA

4) Binding of the second tRNA

  • A tRNA molecule binds to the second codon in the ribosome.
  • When a tRNA molecule binds to mRNA, the corresponding amino acid is brought into the ribosome.
  • The two amino acids in the ribosome form a peptide bond.
5) Movement of the ribosome

5) Movement of the ribosome

  • When the two amino acids bind together, the ribosome moves along the mRNA strand so that a new codon enters the ribosome.
  • A complementary tRNA molecule binds to the new codon.
  • A new amino acid is brought into the ribosome.
  • A peptide bond forms between the new amino acid and the existing chain of amino acids (a polypeptide chain).
6) STOP codon

6) STOP codon

  • When the ribosome reaches a STOP codon (e.g. UAG) there is no corresponding tRNA molecule.
  • The polypeptide chain is released from the ribosome.
7) Completion of the polypeptide

7) Completion of the polypeptide

  • The polypeptide chain has been formed and is ready to complete its function (e.g. as a protein channel).
  • Some polypeptide chains are joined to other chains or a prosthetic group is added.
  • E.g. Each haemoglobin molecule is made up of four polypeptide chains and each polypeptide has an iron prosthetic group.

Test yourself on Polypeptide Synthesis

  • Steps Involved in Transcription

  • Which of the following statements about transcription are true?

  • What enzyme is used to separate the DNA strands?

  • How many codons can fit inside the ribosome at one time?

  • Which of the following statements about translation are true?

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