Polypeptide Synthesis

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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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.