Nov 20 Chapter 10 (continued)

 

2. Mechanism of gene expression (continued)

d. Translation

i)        transfer RNA (tRNA) (See the figure below.)

-          Small RNAs that deliver amino acids to the translation machinery.

-          Anticodon at one end and an amino acid attachment site at the other end.

-          Anticodons are complementary to codons on mRNA.

Example: The anticodon of the codon UAG is AUC.

-          An enzyme attaches specific amino acids to specific tRNA.  (This reaction requires ATP.)  (Fig 10.15, p199)

 

Example: The tRNA with the anticodon AUC carries the amino acid Tyr (tyrosine).

 

Which amino acid is attached to the tRNA molecule with the anticodon UCC?

 

ii)      Ribosome

-          Site of protein synthesis.

-          Found in the cytoplasm (may or may not be attached to ER).  Assembled in the nucleous.

-          Protein – RNA complex.

-          Protein components: two subunits (small and large), each consisting of multiple proteins.

-          RNA components: ribosomal RNA (rRNAs)

-          Binds to mRNA and tRNA.

-          Two tRNA binding sites

o       P site – holds tRNA carrying growing polypeptide.

o       A site – holds tRNA carrying the next amino acid to be added.

 

iii)    Coding sequence

–        The sequence that codes for a polypeptide.

–        From the start codon to a stop codon. (Fig 10.13A).

 

 

 

 

 

 

 

 

 

 

 

 

iv)    Stages of translation

Three stages of translation: initiation, elongation and termination

(1)   Initiation (Fig 10.13B, 199)

Step1: An mRNA molecule binds to a small ribosomal subunit.  The initiator tRNA, carrying methionine (Met), binds to the start codon.

Step 2: A large ribosomal unit binds to the small one.  The initiator tRNA fits into the P site on the ribosome.

 

(2)   Elongation (Fig 10.14, p199)

Step 1 Codon recognition: The anticodon of tRNA pairs with the corresponding mRNA codon in the A site of the ribosome.

 

Step 2 Peptide bond formation: The polypeptide at the P site separates from tRNA and attaches to the amino acid at the A site.  The ribosome catalyses the peptide bond formation.

 

Step 3 Translocation: The P site tRNA leaves the ribosome and the ribosome moves the A site tRNA to the P site.  The ribosome is now ready to accept another tRNA at the A site.

(3)   Termination

- The ribosome reaches a stop codon (UAA, UAG or UGA).  Completed polypeptide is released from the last tRNA.  The ribosome splits into its subunits.

 

e. Gene expression review (Fig 10.15, p 200)

Stage 1: transcription – initiation, elongation, termination

Additional stage for eukaryotes: RNA processing – CAP, TAIL and splicing

Stage 2: Attachment of amino acids to the corresponding tRNAs.

Stages 3 – 5: translation – initiation, elongation (codon recognition, peptide bond formation and translocation), termination

 

f. Mutation - Change in the nucleotide sequence.

i)        Substitution of base

-          Replacement of a nucleotide with another.

-          Does not always result in change in amino acids because of the redundancy in the code. 

Example:

The codon CAG codes for Gln (glutamine).  The base substitution of G to A would not change the meaning because the CAA also codes for Gln.  However, the base substitution of C to A would result in the amino acid Lys (lysine) instead.

 

-          Base substitution of a single nucleotide could cause harmful effects.

Example: The sickle cell disease results from a single mutation.  (Fig 10.16A)

 

- Not all mutations are harmful. 

 

ii)      Deletion or insertion of base

-          Often have harmful effects.

 

Translate the three mRNA sequences shown below and think of the reason why deletion or insertion causes more serious effects.

 

Reading frame – grouping of triplets

Deletion or insertion of a nucleotide within the coding sequence will result in the shift in the reading frame and will change the amino acid sequence after the mutation.  This is also known as frame-shift mutation.

 

Mutagenesis – creation of mutation

Mutagens – agents that induce mutagenesis

Examples: UV light, X-rays, cigarette smoke, etc.

Most mutagens are also carcinogens (cancer causing agents).