Biology Reference
In-Depth Information
Figure 1.8
Genetic information is contained in genes carried on one of the two strands (coding
strand). The complementary strand
in that region
is the noncoding strand. Genes can occur on differ-
ent strands at different points of the DNA molecule. Noncoding DNA between genes is called inter-
genic or spacer DNA.
possible to order four different bases (A, T, C, G) in combinations of three into
64 triplets or codons. Because there are only 20 common amino acids, the ques-
tion immediately arises as to what the other 44 codons do?
The answer is that the genetic code is
degenerate
with all amino acids, except
methionine and tryptophan, determined by more than one codon (
Table 1.1
). A,
U, C, and G represent the codons in
Table 1.1
because the genetic information in
DNA is transcribed into mRNA, which uses U instead of T.
The genetic code also contains punctuation codons. Three codons (UAA, UGA,
and UAG) function as “stop” messages or
termination codons
; they occur at the
end of a protein-coding gene to indicate where translation should stop. AUG
serves as an
initiation
or
start codon
when it occurs at the front end of a gene.
Because AUG is the sole codon for methionine, AUGs also are found in the mid-
dle of genes.
The genetic code is not universal, although it was assumed to be so initially.
In 1979, it was found that mitochondrial genes use a slightly different code
(
Knight et al. 2001
). For example, the codon AGA typically codes for arginine,
but in
Drosophila
mitochondria the codon AGA codes for serine.
Eukaryotic genes have evolutionary histories and seem to have been derived
from at least two sources. There are three domains of life: Archaea (archaebac-
teria), Bacteria (eubacteria), and Eukarya (eukaryotes).
Eukaryotes
are organ-
isms (including arthropods) that consist of cells with true nuclei bounded by
nuclear membranes. Cell division in eukaryotes occurs by mitosis, reproductive
cells undergo cell division by meiosis, and oxidative enzymes are packaged in
mitochondria with its own circular chromosome. Evidence derived from analyses
of genome sequences from the three domains strongly suggests that eukaryotic
nuclear genes are derived from both archaebacterial (informational genes) and
