From DNA to Protein


The Central Dogma


The Players


Basic Anatomy of a Gene


Transcription


The Genetic Code


A More Detailed Look at tRNA

 

A tRNA

tRNA is the interpreter of the codons contained in a mRNA
  • The function of tRNA is to transfer amino acids from teh cytoplasm's amino acid pool to a ribosome
  • The Cell keeps its cytoplasm well stocked with all 20 amino acids, either by synthesizing them or by taking them up from the surrounding solution
  • A charged tRNA is one which has a specific amino acid bound to the acceptor arm 
  • Each tRNA has a three-base sequence called the anticodon which binds to a complementary triplet on the mRNA according to the base-pairign rules
Wobble - Some Relaxation of Strict Base Pairing
  • There is some "flexibility" in the third position of codon-anticodon base pairing - the third base of a codon is known as the "wobble position"
    • example - the base U of tRNA anticodon can pair with either A or G in the third position of an mRNA codon
  • The most versatile tRNA's are those with Inosine (I), a modified base, in the wobble position
    • I can hydrogen bond with U, C, or A.
  • Wobble explains why the synonymous condons for a given amino acid usually only differ by the the third position

 


Aminoacyl-tRNA Synthetases


When a ribosome pairs a "CGC" tRNA with "GCG" codon, it expects to find an alanine carried by the tRNA. It has no way of checking; each tRNA is matched with its amino acid long before it reaches the ribosome. The match is made by a collection of remarkable enzymes, the aminoacyl-tRNA synthetases. These enzymes charge each tRNA with the proper amino acid, thus allowing each tRNA to make the proper translation from the genetic code of DNA into the amino acid code of proteins.

Twenty Flavors

Most cells make twenty different aminoacyl-tRNA synthetases, one for each type of amino acid. These twenty enzymes are widely different, each optimized for function with its own particular amino acid and the set of tRNA molecules appropriate to that amino acid.

Surprises from Genome Analyses

Recent analyses of entire genomes revealed a big surprise: some organisms don't have genes for all twenty aminoacyl-tRNA synthetases. They do, however, use all twenty amino acids to construct their proteins. The solution to this paradox revealed, as is often the case in living cells, that more complex mechanisms are used. For instance, some bacteria do not have an enzyme for charging glutamine onto its tRNA. Instead, a single enzyme adds glutamic acid to all of the glutamic acid tRNA molecules and to all of the glutamine tRNA molecules. A second enzyme then converts the glutamic acid into glutamine on the latter tRNA molecules, forming the proper pair.

 


Ribosomes


Translation


Mutation

If a base in DNA is changed, there may be a corresponding change in the RNA However, a change in DNA does not always lead to a change in the coded protein Note that even though the RNA was changed, both codons still coded for Leu, so there was no net change in the protein

Neutral (silent) mutation - a base pair changes, but there is no change in the protein
Missense mutation - A mutation that alters a codon for a particular amino acid to one specifying a different amino acid
Nonsense mutation - A point mutation that causes the change of an amino acid codon into a stop codon
Frameshift mutation - A mutation that alters the normal triplet reading frame so that codons downstream from the mutation are out of register and not read properly.



What you should know how to do:

1.  Identifiy a molecule as either DNA or RNA
2.  Create a complementary strand of DNA given the template strand
3.  Given a piece of DNA (coding or template), generate the protein that is coded in the DNA
4.  Given a piece of mRNA, translate into protein
5.  Determine how a given mutation will affect the protein