Gene expression

A gene is a segment of double-stranded nuclear DNA.
The transfer of genetic information from DNA into
protein constitutes gene expression. Apart from
housekeeping genes, which are expressed in all tissues,
genes are expressed in a highly controlled manner
so that only those proteins needed by a specifi c
cell for a specifi c purpose are synthesized. Each step
in the pathway of protein synthesis may be regulated.
Controlled processes include transcription, RNA
processing, RNA transport to the cytoplasm, mRNA
stability, selection of mRNAs for translation, and
post-translational modifi cation of the protein product.
Most genes are regulated in part at the transcriptional
level.
In adult mammalian cells, the majority of the DNA
is methylated, specifi cally at the cytosines of CpG
dinucleotides. DNA methylation is associated with
repression of gene expression (see Section 6.6.5 for
the mechanism) and is retained after cell division.
Most tissue-specifi c genes are methylated (and thus
repressed) in every cell type, except those that actually
express the gene. Housekeeping genes have a nonmethylated
CpG island tightly associated with their
promoter and are thought not to be regulated by DNA
methylation (Laird & Jaenisch, 1994).
The pattern of gene expression is determined to
a large extent by the conditions of the extracellular
environment. Growth factors, cytokines and other
extracellular agents stimulate cell-surface receptors,
which initiate a complicated biochemical pathway of
signal transduction to the nucleus. Steroid hormone
receptors can enter the nucleus directly. The transduced
signals can cause gene expression to be either
enhanced or suppressed. In this manner, a transient
external stimulus can be converted into a long-term
biological response.