1 Structure and Function of the Human Genome
19
Regulation of transcription
Transcription is highly regulated by transcription factors (Figure llA),
which are proteins that recognize specific regulatory DNA sequences. Those
regulatory sequences, called enhancers or silencers in most instances lie
"upstream" from the transcription start site in the so called 5' flanking region. The presence or absence of such protein factors in a cellleads to tissuespecific gene expression and allows the cell to function in a tissue specificmanner. In addition, these factors endow the cell with the ability to alter
gene expression in response to external signals. The glucocorticoid receptor
may serve as an example: After binding to the steroid hormone, the cytoplasmic receptor is translocated to the nucleus and binds to a glucocorticoid
responsive element ( GRE). The GRE functions as an enhancer for neighboring genes.
genom ic
p
DNA
- ------------------------j transcri pT t ion
hnRNA
spli cing -
mRN A
_.,._.., _ _._...,.~ AAAA
5' '
translat io n
protei n
+
posllraoslati onal modjfjcat jon ·
- c teavage
- g lycosylation
Fig. 10. Aspects of colinearity between gene and gene products. For details see text. P =promoter; arrows denote exons; hooks denote introns.