Protein synthesis

How does a gene determine the sequence of amino acids in a polypeptide?

‣ A gene consists of a specific sequence of bases.

‣ Each three bases codes for one amino acid.

‣ So the sequence of bases determines the sequence of triplets/codons,

‣ which determines the sequence of amino acids (the primary structure).

Stages of protein synthesis

1. TRANSCRIPTION (takes place in nucleus): one gene on the DNA is used to synthesise a complementary strand of mRNA.

2. TRANSLATION (takes place in cytoplasm, at ribosomes): the assembly of proteins using the sequence of codons on mRNA.

Transcription

• Hydrogen bonds are broken between the two DNA strands in the region to be copied.

‣ Strands separate, unwind and expose bases in the region to be

copied.

‣ Only one DNA strand acts as a template strand (the non-coding

strand).

‣ Free RNA nucleotides are attracted to the exposed bases

according to complementary base pairing rules (A-U, C-G).

‣ RNA polymerase joins adjacent RNA nucleotides together by catalysing condensation reactions to form phosphodiester bonds.

‣ The pre-mRNA is spliced to remove introns, forming the mature

mRNA strand.

‣ mRNA leaves the nucleus through a nuclear pore.

Translation

• A tRNA molecule with an anticodon that is complementary to the start codon on the mRNA attaches itself to the mRNA by specific base pairing.

‣ A second tRNA molecule attaches itself to the next codon on the mRNA in the same way.

‣ A ribosome can accommodate two codons at once.

‣ tRNA’s carry specific amino acids determined by their anticodon.

‣ Adjacent amino acids are joined by a condensation reaction forming a peptide bond (requires energy supplied by hydrolysis of ATP).

‣ Process continues until stop codon is reached.

‣ Polypeptide chain is released from ribosome.

Why is splicing of mRNA necessary?

• To remove introns.

‣ Introns are non-coding sequences of DNA.

‣ If introns were not spliced, the resulting polypeptide would be non-functional (or no polypeptide would be produced at all).