Gene Expression II:
Translation and Mutations
4 November, 2005
Text Chapter 17
mRNA molecules
are complementary
to the template
strand of the DNA.
Codons are 3-letter
genetic words that
specify amino acids.
Proteins are linear
polymers of amino
acids.
In translation, mRNA sequence
specifies protein sequence with
the help of tRNA adapter
molecules.
In the genetic code, 5’ UUU 3’
specifies Phenylalanine, so the
anticodon is 3’AAA 5’.
5’GGC 3’ specifies Glycine, so
the anticodon is ‘3 CCG 5’.
tRNA molecules are transcribed
from DNA templates (genes)
and extensively modified.
tRNA is a partially double stranded RNA polymer. It folds into a threedimensional shape with the anticodon at one end and the amino acid
attachment site at the other end.
Amino acids are loaded onto
tRNA molecules by AminoacyltRNA synthetase.
In the cell there is a different
aminoacyl-tRNA synthetase for
each of the twenty amino acids.
Ribosomes are large protein /
RNA complexes that are the site
of translation.
The structure of ribosomes
reflects ribosomal function.
Each ribosome consists of large
and small subunits, with
binding sites for mRNA and
three tRNA molecules.
During translation, the growing
polypeptide is atached to the
tRNA bound at the P-site of the
ribosome.
Translation begins when a small ribosomal subunit recognizes a start
codon. Then, an initiator tRNA-MET pairs with the start codon, and
a large ribosomal subunit completes the initiation complex.
Elongation Cycle
Termination of translation occurs when release factor binds to a
stop codon in the A-site.
Then, the completed protein, the mRNA, and the ribosomal
subunits are released.
Each mRNA may be translated thousands of times. tRNA
molecules can be re-used after recharging, and ribosomal subunits
are recycled for use translating another mRNA.
Proteins destined to be exported or transported inside the cell
contain signal sequences that direct the protein into the lumen of the
endoplasmic reticulum. Then, the protein folds into its final threedimensional shape.
Mutations are changes in DNA sequence.
These changes can lead to altered proteins. The altered proteins are
usually non-functional.
Types of Mutations
The Ames test identifies mutagenic chemicals by their
ability to cause reversion in a Salmonella histidine
auxotroph that carries a single-base missense mutation.