Lecture 14 - Protein Synthesis

All Living things pass on their genetic heritage by common processes. Protein Synthesis: Lecture Outline No. 14

Transcription of DNA sequences into RNA’s

RNA polymerase makes a single stranded RNA transcript from one strand of the unwound DNA helix. Activated A, U, G and C ribonucleotide triphosphates base pair with the DNA and are linked by the RNA polymerase into RNA polynucleotides.

RNA transcripts

1. rRNA, ribosomal RNA: In large and small ribosomal subunits. rRNA-ribosome protein complexes catalyze polypeptide synthesis.

2. tRNA, transfer RNA: Contain triplet anticodons that complement the triplet codon of the messenger RNA and have an amino acid attached at the opposite end.

3. mRNA, messenger RNA: Contain amino acid sequence information as triplet codons. Eukaryotic mRNAs contain nonexpressed introns and expressed exons.

Introns must be removed before the mRNA reaches the ribosome.

The triplet genetic code

	There are 64 possible triplet codons (i.e. 4³); 61 are amino acid codons.
	AUG, the methionine codon, is the START codon
	UAG, UGA, and UAA are STOP codons.
	The genetic code is universal (with minor differences in mitochondria and ciliated protozoa) and degenerate (i.e. more than one codon per amino acid).

Translation of mRNA into polypeptides

	Each amino acid is attached to a specific t-RNA with its appropriate anticodon.
	The ribosome has two binding sites for tRNAs: an entering A site, and a leaving P site. It moves along the mRNA from 5’ to 3’ direction.
	At the START site, the charged met-tRNA enters the P site and the second charged tRNA enters the A site.
	The methionine is linked to the amino acid of the second amino acid by a peptide bond, forming a dipeptide.
	When the ribosome moves to the next codon, the uncharged met-tRNA leaves and the second tRNA with its dipeptide moves to the P site.
	The third charged tRNA enters the A site.
	The dipeptide from the second tRNA is linked to the amino acid of the third t-RNA forming a tripeptide.
	*This process continues until a STOP codon is reached. A release factor* enters the A site causing the release of completed polypeptide and the tRNA.**
	The empty ribosome moves on until it reaches the next START codon.
	Four ATP’s are required for each amino acid added to the polypeptide chain:Two to "charge" the tRNA , one to carry the charged tRNA to the ribosome and one to move the ribosome to the next codon.