Chapter 9 – Translation
“The synthesis of every protein molecule in a cell is directed by that cell’s DNA”
There are two aspects to how this is accomplished:
Introduce the Players and their roles:
See fig. 9-1 for an overview of how translation occurs. Note that there is a direction: the mRNA is read 5’ ą 3’.
Also note that the new protein (nascent protein) is made amino-terminus first.
Please see figures 9-2 and 9-3 for a few more specific details:
The Genetic Code – Table 9-1
What is tRNA? – See fig 9-4 and 9-5.
The tRNA molecule has several important characteristics:
Let’s review the general structure of mRNA
Let’s review polycistronic RNA again:
What are “overlapping” genes?
The book refers to the phage fX174 – a single-stranded DNA virus.
Š Analysis revealed that the genome of this virus was too small to account for all the proteins that it could make.
Š The problem was solved by finding that some of fX174’s genes are overlapping:
o The same nucleotide sequence could be read in multiple reading frames:
o The sequence below could be read differently depending on which AUG is used as a start codon:
AUGNNNNAUGNNNNNNNAUGNN… … …
AUG NNN NAU GNN NNN NNA UGN N.. or
AUG NNN NNN NAU GNN … or
Š Also see fig 9-7.
Š Prokaryotic ribosome structure – 70s ribosomes
Š 30s subunit – 16s rRNA plus 21 polypeptides
Š 50s subunit – 23s rRNA, 5s rRNA and 32 polypeptides
Š Eukaryotic ribosome structure – 80s ribosomes
Š 40s subunit – 18s rRNA plus 30 proteins
Š 60s subunit – 5s rRNA, 5.8s rRNA, 28s rRNA and 50 proteins
Š The 16s rRNA of the 30s subunit binds to the “Shine-Delgarno” sequence.
§ The Shine-Delagarno sequence (AGGAGGU) is also known as the “translation initiation region.
§ The eukaryotic counterpart is the 5’-cap of the eucaryotic message.
§ (See fig 9-11) The “pre-initiation complex” forms: the 30s subunit + mRNA + f-met-tRNA + initiation factors (proteins) + GTP.
§ Then the 50s subunit binds and “elongation” begins.
Š Note the A-site and the P-site in the figure
Š Elongation (fig. 9-13)
Š With f-met-tRNA in the P-site and the next acylated-tRNA in the A-site ---- a peptidyl transferase forms a peptide bond between the two amino acids.
Š F-met is cleaved from its tRNA and the tRNA leaves, leaving the P-site open.
Š The second tRNA “translocates” into the P-site from the A-site.
Š A new acylated tRNA moves into the empty A-site to base-pair with the codon in that site.
Š This process repeats over and over until a nonsense codon is presented in the A-site.
Š Release factors cleave everything is the A-site is unoccupied for too long.
Š In polycistronic mRNA, the next AUG is not too far away and the ribosome reinitiates to synthesize the next protein.
What are polysomes?
Read about Antibiotics and ribosomes on page 188.