670 Chapter 20 NEL
Translation
The second part of the central dogma of molecular biology (Figure 3, page 668) is the
translation of the genetic information carried by mRNA into a chain of amino acids to
form a polypeptide. Therefore, the process of translation involves protein synthesis, and
it depends on the remarkable nature of the genetic code.
Only 20 amino acids are found in proteins. The DNA in a gene codes for these 20
amino acids by combinations of the four nitrogenous bases. During translation, the
DNA code is read in groups of three nucleotides, called a codon. Each codon calls for a
specific amino acid to be placed in the growing polypeptide chain. Codons can consist
of any combination of the four nitrogenous bases, so there are 64 (4^3 = 64) possible dif-
ferent codons for the 20 different amino acids. The groups of three bases in both DNA
and mRNA are both called codons, so it is important to clarify which code is being pre-
sented when writing out a genetic sequence. The remainder of this description will use
mRNA codons.Table 1shows the mRNA codons. One of these codons (AUG) is the
start codon, where translation begins. It also codes for the insertion of the amino acid
methionine, so all polypeptide chains initially start with the methionine, but it may later
be edited out. Three other codons (UAA, UAG, and UGA) do not code for amino acids
and are called the stop codonsbecause they cause protein synthesis to stop. The other
60 codons code for one of the 20 amino acids. Some amino acids have more than one
codon; for example, both serine and leucine each have 6 different codons.Table 2, on the
next page, lists the abbreviations for the amino acids to help you look them up in Table 1.
Like transcription, translation can be divided into the same three stages: initiation, elon-
gation, and termination.Initiation
Initiation of translation occurs when a ribosomerecognizes a specific sequence on the
mRNA and binds to that site. In eukaryotes, the ribosome consists of two subunits, a
large subunit and a small subunit (Figure 6, next page). The two subunits bind to the
mRNA, clamping it between them. The ribosome then moves along the mRNA in the 5
to 3direction, adding a new amino acid to the growing polypeptide chain each time itcodonsequence of three bases in
DNA or complementary mRNA that
serves as a code for a particular
amino acid
start codonspecific codon (AUG)
that signals the start of translation
stop codonspecific codon that
signals the end of translation
Why Three Nucleotides
per Codon?
Why are there always three
nucleotides in a codon? Why not
two or four? Listen to this Audio
Clip to find out the reason behind
the triplet code found in DNA and
mRNA sequences.http://www.science.nelson.com GO+ EXTENSION
Table 1 Codons and Their Amino Acids2nd (middle) Base of a Codon
1st Base U C A G 3rd Base
U UUU Phe UCU Ser UAU Tyr UGU Cys U
UUC Phe UCC Ser UAC Tyr UGC Cys C
UUA Leu UCA Ser UAA STOP UGA STOP A
UUG Leu UCG Ser UAG STOP UGG Trp S
C CUU Leu CCU Pro CAU His CGU Arg U
CUC Leu CCC Pro CAC His CGC Arg C
CUA Leu CCA Pro CAA Gln CGA Arg A
CUG Leu CCG Pro CAG Gln CGG Arg S
A AUU Ile ACU Thr AAU Asn AGU Ser U
AUC Ile ACC Thr AAC Asn AGC Ser C
AUA Ile ACA Thr AAA Lys AGA Arg A
AUG Met ACG Thr AAG Lys AGG Arg S
G GUU Val GCU Ala GAU Asp GGU Gly U
GUC Val GCC Ala GAC Asp GGC Gly C
GUA Val GCA Ala GAA Glu GGA Gly A
GUG Val GCG Ala GAG Glu GGG Gly Sribosomean organelle composed
of RNA and protein and located in
the cytoplasm that carries out
protein synthesis