Another important feature of the genetic code:

1
Another important feature of the genetic
code It is non-overlapping.
2
There are 64 - 3 61 codons. This might imply
that there are 61 anticodons, and therefore 61
different tRNA molecules. Is this really the
case? There are actually fewer than 61 tRNA
molecules. A single anticodon (tRNA
molecule) can frequently pair with more than
one codon---partially explaining the degeneracy
of the code. The rules of WC base pairing are
relaxed for the 5 base of the anticodon
(corresponding to the 3 base of the codon).
This base is less constrained than the others, it
can form a greater variety of hydrogen bonds.
In Cricks terms, it can wobble.
3
Wobble rules for codon-anticodon pairing in E.
coli Anticodon 5 prime base
Codon 3 prime base A
U
C
G U
A or G G
C or U
I
A or C or U
4
DNA Replication errors insults
Mispairing, Damage Repair
Systems MUTATIONS
Repaired DNA
5
Xeroderma pimentosum is an inherited defect in
DNA repair, partiularly of UV-induced damage.
(This is a very good reason for worrying about
depletion of the ozone layer)
6
(No Transcript)
7
Mutations at the molecular level
8
Other Ways of Classifying or Thinking About
Mutations germline vs somatic
spontaneous vs induced conditional
(permissive vs nonpermissive conditions) point
mutations vs deletions, insertions, inversions,
etc. forward vs back mutations (back
mutations include reversions and true reverse
mutations)
9
The Ames Test is the most widely used method of
detecting chemical mutagens. It is based on
His revertants in the bacterium Salmonella.
His- His (his- sup- his-
sup) This reversion rate is easily measured
(how..?). An increase in the rate of
reversion is evidence that a particular
substance has mutagenic properties. Note
There is a good general correlation among
mutagens, carcinogens and teratogenic agents.
The Ames Test is therefore the first line of
detection/defense for all of these .
10
Please read about the mutagenic effects of
ionizing radiation, p 250 - 253 in the
text. Then read about the Cernobyl incident and
consider what price we might pay for nuclear
power. Remember this in the years ahead when
the power industry and certain politicians
promote nuclear power.