Genetics
Unit 4P
Translation
– Elongation
With
the met-tRNA in the P-site, a new tRNA
enters the ribosome complex carrying the amino acid corresponding to the codon exposed in the A-site. The anticodon of
the tRNA pairs with the codon
of the mRNA with the aid of an elongation factor. The energy source for this
step is provided by GTP. An enzyme catalyzes the formation of a peptide bond
between the methionine carried on the tRNA at the P-site and the incoming amino acid carried on
the tRNA at the A-site. The newly formed dipeptide
sits on the tRNA at the A-site.
The
met-tRNA is released from the P-site and the tRNA with its dipeptide is then translocated to the P-site. The tRNA
and mRNA are bound together – anticodon to codon – so they move together as a unit. GTP is the energy
source for the translocation. The mRNA
is moved along the ribosome in a 5’ to 3’ direction, and the polypeptide is
built in an amino (NH2) to carboxyl (COOH) direction. The next codon to be translated is now exposed at the A-site.
Practice
see also http://www.emc.maricopa.edu/faculty/farabee/BIOBK/ELONGATION3.gif
To
synthesize a polypeptide with three amino acids, a message containing four codons (12 NTPs) is required. The
codons read: AUG-UUU-UGG -UAA.
1.
met-tRNA reads AUG and sits on the P-site.
2.
phe-tRNA reads UUU and sits on the A-site. The
phenylalanine is attached to the methionine.
3.
The empty met-tRNA is released from the P-site.
4.
The phe-tRNA carries a met-phe
dipeptide. It is translocated
to the P-site.
5.
A trp-tRNA reads UGG and sits at the A-site. The met-phe is attached to the tryptophan.
6.
The empty phe-tRNA is released from the P-site.
7.
The trp-tRNA carries met-phe-trp
and is translocated to the P-site.
8.
A stop codon UAA is exposed at the A-site. Elongation
has ended.
Termination
see also http://www.emc.maricopa.edu/faculty/farabee/BIOBK/termination_5.gif
When
a termination (stop) codon is reached on the mRNA at
the A-site, the termination cycle begins. There are no corresponding tRNAs for any stop codon (UAG, UAA, or UGA). A release factor recognizes the
stop codon and sits at the A-site. The release factor
hydrolyzes the bond between the polypeptide and the tRNA
at the P-site. The polypeptide, the empty tRNA, and
the release factor leave the ribosome. Finally, the two ribosomal subunits and
the mRNA disengage.
RECAP
The
central dogma of genetics http://www.emc.maricopa.edu/faculty/farabee/BIOBK/cendogma.gif
