Table
created by Shaun D. Black,
University of Texas Health Center at Tyler
(Color coding added by M. A. Clark)
An explanation of the
Genetic Code: DNA is a two-stranded molecule. Each strand is a polynucleotide
composed of A (adenosine), T (thymidine), C (cytidine),
and G (guanosine) residues polymerized by "dehydration" synthesis in
linear chains with specific sequences. Each strand has polarity, such that
the 5'-hydroxyl (or 5'-phospho) group of the first nucleotide begins the strand
and the 3'-hydroxyl group of the final nucleotide ends the strand; accordingly,
we say that this strand runs 5' to 3' ("Five prime to three prime")
. It is also essential to know that the two strands of DNA run antiparallel
such that one strand runs 5' -> 3' while the other one runs 3' -> 5'. At each
nucleotide residue along the double-stranded DNA molecule, the nucleotides
are complementary. That is, A forms two hydrogen-bonds with T;
C forms three hydrogen bonds with G. In most cases the two-stranded,
antiparallel, complementary DNA molecule folds to form a helical structure
which resembles a spiral staircase. This is the reason why DNA has been referred
to as the "Double Helix".
One strand of DNA
holds the information that codes for various genes; this strand is often called
the template strand or antisense strand (containing anticodons). The other,
and complementary, strand is called the coding strand or sense strand (containing
codons). Since mRNA is made from the template strand, it has the same information
as the coding strand. The table above refers to triplet nucleotide codons
along the sequence of the coding or sense strand of DNA as it runs 5' -> 3';
the code for the mRNA would be identical but for the fact that RNA contains
U (uridine) rather than T.
An example of two complementary
strands of DNA would be:
(5' -> 3') ATGGAATTCTCGCTC (Coding, sense
strand)
(3' <- 5') TACCTTAAGAGCGAG
(Template, antisense strand)
(5' -> 3') AUGGAAUUCUCGCUC (mRNA made
from Template strand)
Since amino acid
residues of proteins are specified as triplet codons, the protein sequence
made from the above example would be Met-Glu-Phe-Ser-Leu... (MEFSL...).
Practically, codons are
"decoded" by transfer RNAs (tRNA) which interact with a ribosome-bound messenger
RNA (mRNA) containing the coding sequence. There are 64 different tRNAs, each
of which has an anticodon loop (used to recognize codons in the mRNA). 61
of these have a bound amino acyl residue; the appropriate "charged" tRNA binds
to the respective next codon in the mRNA and the ribosome catalyzes the transfer
of the amino acid from the tRNA to the growing (nascent) protein/polypeptide
chain. The remaining 3 codons are used for "punctuation"; that is, they signal
the termination (the end) of the growing polypeptide chain.
Lastly, the Genetic
Code in the table above has also been called "The Universal Genetic Code".
It is known as "universal", because it is used by all known organisms as a
code for DNA, mRNA, and tRNA. The universality of the genetic code encompases
animals (including humans), plants, fungi, archaea, bacteria, and viruses.
However, all rules have their exceptions, and such is the case with the Genetic
Code; small variations in the code exist in mitochondria and certain microbes.
Nonetheless, it should be emphasized that these variances represent only a
small fraction of known cases, and that the Genetic Code applies quite broadly,
certainly to all known nuclear genes.
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