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Mutations in introns can be neutral or can alter gene regulation. Mutations
in the splicing signals may result in two classes of mutations. If an intron is not
spliced out, a mutant protein can be produced that functions abnormally. An
abnormal mRNA is produced if splicing occurs at a different site than normal,
and a mutant protein will be produced as a result. More than half the introns in
Drosophila and other invertebrates are < 80 nt long ( Guo and Mount 1995 ). Once
mRNA is produced, it must be transported through the nuclear envelope to the
cytoplasm where it is translated.
2.8 Translation Involves Protein Synthesis
Translation is the second stage of class II gene expression in which the informa-
tion in the mRNA is used to direct the synthesis of a polypeptide, the amino-acid
sequence of which is determined by the nucleotide sequence of the RNA.
The genetic code consists of a triplet of adjacent ribonucleotides that spec-
ify an amino acid (see Table 1.1). Translation requires ribosomes; tRNAs, a set
of enzymes (aminoacyl tRNA synthetases) to catalyze the attachment of each
amino acid to its corresponding tRNA molecule; and initiation, elongation, and
termination factors. Translation occurs in ribosomes located in the cytoplasm.
Ribosomes are cellular organelles, consisting of two subunits (a larger 60S sub-
unit and a smaller 40S subunit), each composed of ribosomal RNA and pro-
teins. (The S refers to the rate of sedimentation and is an indication of size.)
The larger 60S unit consists of three rRNA molecules (25S, 5.8S, and 5S) and 46
proteins, whereas the smaller 40S subunit includes one rRNA (18S) and 33 pro-
teins ( Ben-Shem et al. 2011 ). The smaller subunit binds mRNA and the anticodon
end of tRNAs and helps to decode the mRNA. The larger subunit interacts with
the amino-acid-carrying end of tRNAs, catalyzes the formation of the peptide
bonds, and contains the polypeptide exit tunnel ( Ben-Shem et al. 2011, Klinge
et al. 2011 ).
The mechanisms whereby ribosomes engage with the mRNA and select the
start site for translation are more complicated in eukaryotes than in prokaryotes
( Dever 1999, Kozak 1999, Preiss and Hentze 1999 ). However, the fundamental
components of translation are conserved: rRNAs are strongly conserved in both
primary and secondary structure among all organisms. The majority of ribosomal
proteins are conserved, as well as the elongation factors, the tRNAs, and the
aminoacyl-tRNA synthetases ( Kyrpides and Woese 1998 ).
Initiation sites in eukaryotic mRNAs are reached by a scanning mechanism
that predicts translation should start at the AUG codon nearest the 5 end
of the mRNA. The selection of the start codon sets the reading frame that is
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