The Genetic Code
The Central Dogma: DNA makes RNA
makes protein
In principle: The DNA genotype does not produce the phenotype directly
A DNA gene contains the information
necessary for the production of proteins,
which is expressed biochemically through an
intermediate molecule, RNA,
which functions as a Genetic Code
The Genetic Code is ...
a messenger RNA (mRNA) code
i.e.., the code
is written in RNA
DNA is a coding molecule,
but not the 'genetic code' in the
biochemical sense
in 64 triplets (codons) : 61 for amino acids + 3 'stops' [iG1 7.19]
mRNA codons are read 5'3'
20 amino acids:
note 1- & 3-letter abbreviations
[more
on
amino
acids
&
proteins
in next section]
For example,
Degenerate: most amino acids are encoded by
more than one codon
first
two
positions
are
critical:
third position can "wobble" [see next section]
if third can be either puRine (R), or either pYrimidine
(Y)
two-fold degeneracy
if third can be any base
four-fold degeneracy
Leucine (leu) has six-fold degeneracy with
six codons in unusual arrangement
# codons / amino acid |
|
trp, met |
1 @ |
ser, arg, leu |
6 @ |
ile |
3 @ |
14 others |
2 or 4 @ |
Unambiguous: any one triplet codes for only
one amino acid
but not vice versa, because of wobble
'Always' begins with an 'start' or 'initiator'
codon:
AUG
'Always' ends with a 'stop' or 'terminator' codon: UAG, UAA, or UGA
Universal (with some important exceptions)
Five Kingdoms
(animals, plants, algae, fungi, & monera)
use
the same codes for nuclear
DNA (nucDNA)
Organelles (chloroplasts
& mitochondria) have separate genomes:
cpDNA & mitochondrial DNA codes
are evolutionarily modified
e.g., UGA codes for trp
in vertebrate mtDNA code
[iG1
7.Table 2]
Stop codons may be formed by
addition of "A"s to transcript
Lab exercises use mtDNA, so the mtDNA code is
important
Alteration & Variation in the
Genetic Code: Mutations & SNPs
Mutations
- interchanges of
one base type for another
transitions -
alternative pyrimidines [
CT ]
or purines [ AG ]
transversions -
purine
pyrimidine [C / T A / G]
Recognized in
individuals & populations as SNPs ("snips": single
nucleotide polymorphisms)
[SNPs, Mutations, & Mutants:
a note on terminology & some lessons from
history]
Alternative nucleotide sequences
of a gene
correspond to alternative alleles
or: a single gene occurs
in
variant forms (alleles)
Single-base mutations
Consequences of exon
SNPs depend on position in triplet
3rd position
typically a silent mutation -
if position "wobbles", no change to amino acid
sometimes a mis-sense
mutation -
results in different amino acids
2nd position - always a missense mutation
1st position - almost always a missense
replacement
[Leu codons are major
exception]
stop codon mutations may occur at any position: coding non-coding
triplet
non-sense (termination) mutations terminate polypeptides
prematurely
: Identify all
codons one
step away from a termination codon
mutations in non-coding DNA have variable
effects
Ex.: mutations in promoter
regions
mutations at intron / exon
splice junctions
Insertion /
Deletion ("indel")
mutations
gain or loss of one or more nucleotides alters the reading
frames
frameshift mutations
(examples)
single & double nucleotide
indel downstream amino acids change
non-sense mutation eventually (quickly) produced
triplet indel - insertion /
deletion of single amino acid
typically milder consequences
multiple triplet insertions produce major effects
Ex.: CGG repeats in "Fragile X" Syndrome
length mutations - very large
indels (102~6 bps)
Genes
are highly
polymorphic (w/ multiple alleles) wrt their SNP variation
[Concept of "wild type" allele is
erroneous]
Text material ©2016 by Steven M. Carr