Evolution and Genetic Engineering - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 699

TABLE 14.2 mRNA Codons and Resulting Protein Expressed

First
letter

Second letter Third

letter U C A G

UUU UCU UAU UGU U

Phe Tyr Cys

UUC UCC UAC UGC C

U Ser

UUA UCA UAA Stop UGA Stop A

Leu

UUG UCG UAG Stop UGG Trp G

CUU CCU CAU CGU U

His

CUC CCC CAC CGC C

C Leu Pro Arg

CUA CCA CAA CGA A

Gin

CUG CCG CAG CGG G

AUU ACU AAU AGU U

Asn Ser

AUC Ile ACC AAC AGC C

A Thr

AUA ACA AAA AGA A

Lys Arg

AUG Met ACG AAG AGG G

GUU GCU GAU GGU U

Asp

GUC GCC GAC GGC C

G Val Ala Gly

GUA GCA GAA GGA A

Glu

GUA GCA GAA GGA G

does not alter the amino acid that it codes. To code for arginine, if first two bases are “C and
G” third base can be any one out of all the four RNA bases. In addition, both AGA and AGG
also code for arginine.
Missense mutationsd Exchange of one nucleotide base with another one results in
a codon that codes for a different amino acid. For example, in codon “CGU” (arginine),
exchanging cytosine with uracil (U) will result in codon UGU (cysteine).
Nonsense mutationsd One type of point mutation that usually has a profound effect
results in a nonsense or stop codon. For example, the codon “UGU,” exchanging the third
base “U” with an “A,” will result in a codon “UGA” that codes for a stop and can truncate
the protein molecule before completion. This results in the premature termination of trans-
lation and an incompletely formed protein.

14.1.2.3.2. INSERTIONS
Addition of one or more extra nucleotides or nucleotide pairs into the DNA of an organism
is called insertion. The insertion elements are usually about 700 e 1400 bp (base pairs) in
length; in Escherichia coli about five different insertion sequences are known and are present
on the chromosome. These elements can move on the chromosome from essentially any site
to another. Often they will insert in the middle of a gene, totally destroying its function.
Single base pair insertions usually occur due to errors in DNA replication or replication of
repeating sections of DNA (e.g. AT base pair repeats). Larger insertions involving long
sections of DNA are caused by transposable elements. Insertions in the coding region of
a gene (even a single base pair insertion) cause a shift in the reading frame (frameshift). Inser-
tions can also result in the altered splicing of the mRNA, because posttranscriptional splicing
of mRNA requires specific sequences of nucleotides. Both of these effects can alter the gene
product very significantly and are mostly deleterious.