Biology Reference
In-Depth Information
Box 2.3 Key Points Regarding Gene Regulation
How Can a Single Genome Code for So Many Cell and Tissue Types?
Regulation
Multiple mechanisms regulate genes
. Some genes are expressed consistently (constituitively) at a specific
level in all cells (housekeeping genes), but others must be turned off and on during development or in
response to environmental challenges.
Regulatory mechanisms include
l
regulating transcription,
l
alternative splicing of mRNA transcripts,
l
DNA amplification,
l
methylation of cytosine bases, and
l
translational control.
Transcriptional regulation can be achieved by
l
a repressor binding to noncoding DNA on the strand close to or overlapping the promotor,
l
the effects of activators enhancing the interaction of RNA polymerase and a promoter, and
l
enhancers bound by activators loop DNA to bring a promoter to the initiation complex.
Epigenetic Regulation
Although DNA is the molecule that stores genetic information that is transferred to the next generation
through the germ line and soma, some inheritance is non-Mendelian (i.e., not related to the DNA
sequence in the genome).
Non-Mendelian inheritance can result from the expression of only one of the two alleles within the
genome. The DNA sequence itself is not altered, but the expression of the DNA is reversibly modified.
DNA may be reversibly altered by histone modifications, methylation of cytosines, and chromatin
remodeling and by targeting of small ncRNAs.
A fertilized egg is thought to develop into an adult organism because the single genome present in the eggs
becomes epigenetically programmed to generate distinct epigenomes in the different cell types. Transient
modification of the genome during development allows genes to be turned on and off during development.
is helping to resolve the fundamental principles of eukaryotic gene regulation
(
Harshman and James 1998
).
Some eukaryotic genes code for essential metabolic enzymes or cell structural
components and are expressed
(constituitively)
at a specific level in all cells. Such
genes are often called
housekeeping genes
, but most genes are not expressed
continuously in eukaryotic organisms. After cells differentiate, gene regulation
may be influenced by environmental cues such as hormones, nutrients, or tem-
perature. The control of gene expression is called
gene regulation
.
Different genes are regulated differently and gene regulatory mecha-
nisms are often surprisingly complex, using more than one method (
Box 2.3
).
