Biology Reference
In-Depth Information
change their function (
Baker 2011
). Correlations have been found between
gene silencing and the gene's proximity to a heterochromatic region or to the
periphery of the nucleus (
Singer and Green 1997
). Heterochromatic DNA is usu-
ally condensed (making it stain intensely) during interphase, and any genes
within heterochromatic regions usually are inactive.
The components in a typical class II eukaryotic gene are illustrated in
Figure 2.3
.
A transcriptional complex binds to the promoter to initiate RNA synthesis. The
coding regions of the gene (the exons) are interrupted by introns that are elimi-
nated in RNA processing. Transcription is regulated by enhancer elements (num-
bered 1-6) that respond to different molecules. Because enhancers may respond
to different signals or cell conditions, genes can be regulated by a combination
of different enhancers. Combinatorial control of gene transcription makes it
possible to increase the complexity of gene regulation. If transcription is deter-
mined by which pattern of binding state occurs, then a small number of regu-
latory molecules can yield a large number of different regulatory patterns for
different life stages or tissues.
Some genes have two or more promoters. Each promoter usually is active in
different cell types, thereby allowing for independent regulation of transcrip-
tion (
Figure 2.10
). The different promoters yield different primary transcripts
that code for the same polypeptide. Thus, one promoter could control transcrip-
tion in insect larvae (
Figure 2.10B
) and another promoter could control tran-
scription in adults (
Figure 2.10C
).
Different cell types can produce different quantities of a protein or dif-
ferent proteins by
alternative splicing
of mRNA. Differential splicing of exons
can result in different rates of synthesis, although the proteins may be identi-
cal. In other cases, the proteins produced are different as a result of alternative
splicing. Alternative splicing is involved in many aspects of development, includ-
ing determining the sex of an insect (see Chapter 10). Alternative splicing also
makes it possible for protein diversity to be greater than the number of genes in
a genome (
Graveley 2001
).
RNA editing
involves altering the sequence of the RNA after it has been tran-
scribed and before it has been translated. RNA editing apparently is widespread
in both prokaryotes and eukaryotes, and results in functionally distinct proteins
produced from a single gene (
Maas and Rich 2000
).
Gene regulation also can occur by controlling translation of the message into
protein.
Translational regulation
can occur in several ways: 1) inability to trans-
late an mRNA molecule unless a particular signal is present, 2) regulation of the
