Human Physiology, 14th edition (2016)

Cell Structure and Genetic Control 65

This pairing of bases, like that which occurs in DNA repli-

cation (described in a later section), follows the law of comple-

mentary base pairing: guanine bonds with cytosine (and vice

versa), and adenine bonds with uracil (because uracil in RNA

is equivalent to thymine in DNA). Unlike DNA replication,

however, only one of the two freed strands of DNA serves as

a guide for RNA synthesis ( fig. 3.16 ). Once an RNA molecule

has been produced, it detaches from the DNA strand on which it

was formed. This process can continue indefinitely, producing

many thousands of RNA copies of the DNA strand that is being

transcribed. When the gene is no longer to be transcribed, the

separated DNA strands can then go back together again.

Types of RNA

There are four types of RNA required for gene expres-

sion: (1) precursor messenger RNA (pre-mRNA), which

is altered within the nucleus to form mRNA; (2) messenger

RNA (mRNA), which contains the code for the synthesis of

DNA may be inactive or redundant, and some serves to regulate
those regions that do code for proteins.
In order for the genetic code to be translated into the syn-
thesis of specific proteins, the DNA code first must be copied
onto a strand of RNA. This is accomplished by DNA-directed
RNA synthesis —the process of genetic transcription.
There are base sequences for “start” and “stop,” and
regions of DNA that function as promoters of gene transcrip-
tion. Many regulatory molecules, such as some hormones, act
as transcription factors by binding to the promoter region of a
specific gene and stimulating genetic transcription. Transcrip-
tion (RNA synthesis) requires the enzyme RNA polymerase,
which engages with a promoter region to transcribe an indi-
vidual gene. This enzyme has a globular structure with a large
central cavity; when it breaks the hydrogen bonds between
DNA strands, the separated strands are forced apart within
this cavity. The freed bases can then pair (by hydrogen bond-
ing) with complementary RNA nucleotide bases present in the
nucleoplasm.

Figure 3.15 Chromatin structure affects gene expression. The ability of DNA to be transcribed into messenger RNA
is affected by the structure of the chromatin. The genes are silenced when the chromatin is condensed. Acetylation (addition of
two-carbon groups) produces a more open chromatin structure that can be activated by transcription factors, producing mRNA.
Deacetylation (removal of the acetyl groups) silences genetic transcription.

Condensed chromatin,

where nucleosomes

are compacted

Acetylation of chromatin

produces a more open

structure

Transcription factors

attach to chromatin,

activate genes

(producing RNA)

Deacetylation causes

compaction of chromatin,

silencing genetic transcription

Acetylation

Deacetylation

Transcription

factor

DNA region to be transcribed