Transcription, Translation, and Regulation of Eukaryotic DNA - Insect Molecular Genetics - page 59

Figure 2.9 Amino-acid sequence of a portion of a protein containing zinc fingers drawn to illustrate
the finger motif. Zinc-finger proteins are able to recognize specific DNA sequences. The fingers can
intercalate into the DNA molecule, and they are important in gene regulation.
Hormone-receptor complexes and transcriptional activator proteins bind with
specific DNA sequences called enhancers. Enhancer sequences can be found in a
variety of sites in relation to the target gene ( Figure 2.3 ). Enhancers can be long
or short distances upstream (5 ′ ) from the target gene; they may be included in
introns within the coding region of the target gene, or even at the 3 ′ end of
the gene. Some enhancers respond to molecules produced inside the cell during
development, and many genes are under the control of several enhancers, so
they can respond to a variety of internal and external molecular signals.
In the living cell, the DNA of the chromosome is tightly bound up with pro-
teins called histones and other proteins that can make transcription of the genes
difficult by preventing the association of proteins with the DNA that are needed
for gene transcription ( Grunstein 1997, Pennisi 1997, Workman and Kingston
1998, Henikoff and Shilatifard 2011 ). At least four different enzymes, called
nuclear histone acetylating enzymes, are associated with the transcription com-
plex and add simple chemical groups known as acetyls to the histones to open
the DNA up to transcription. In addition, there are five more enzymes that can
undo the reaction and remove acetyls from the histones, thus making transcrip-
tion more difficult.
The physical location of a gene within the nucleus during interphase is impor-
tant in gene regulation ( Cockell and Gasser 1999, Baker 2011, Henikoff and
Shilatifard 2011 ). Furthermore, the locations of genes may shift as the cells