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:

 

 

 

See a movie describing how tRNA gets acylated

 

 

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

                                                 AUG NN.

Š      Also see fig 9-7.

 

Polypeptide Synthesis

 

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                                 Initiation

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                               Termination

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.