Page 65 - Anatomy and Physiology of Farm Animals, 8th Edition

50 / Anatomy and Physiology of Farm Animals

blood cells. This lack of nucleus is associated together. Adenine is always paired with
thymine, and guanine is always paired with
with their short life span of only 120 days.
VetBooks.ir are (1) to regulate protein synthesis in the cytosine. The two strands are joined by
The primary functions of the nucleus
hydrogen bonds between the bases. The
cell, thereby regulating the biochemical two strands of the DNA double helix are not
activities of the cell, and (2) to ensure the identical but are complementary (Fig. 2‐22).
passage of genetic material (the chromo­ In other words, whenever adenine appears
somes and their component genes) to on one strand, thymine is in the same posi­
subsequent generations of cells and/or tion on the opposite strand, and whenever
organisms. guanine is on one strand, cytosine is in the
same position on the other strand.
DNA and DNA Replication
Typical deoxyribonucleotide:
The genetic material necessary to direct
cellular functions is primarily composed of NH 2
chains of DNA. The chains of DNA are Base (cytosine)
formed by joining small units (nucleotides), O N
each containing a phosphate, a sugar (deox­
yribose), and either a purine or pyrimidine –O P O CH 2 O
base. The purine bases in DNA are adenine N
and guanine, and the pyrimidine bases are O– O
thymine and cytosine (Fig. 2‐21). Phosphate C H H C
Watson and Crick determined the struc­
ture of DNA to be a double helix, some­ H C C C H
thing like a spiral staircase or twisted metal
H
OH
ladder. The outside rails consist of two long Sugar (deoxyribose)
chains of sugar–phosphate molecules, and
the rungs are made up of paired bases that Figure 2-21. General structure of a nucleotide
hold the two parts of the double helix (subunits of DNA and RNA).

C A T G C C G A G C T A A
T G G C
C T C G A G
G T
C T C
G
C
Parent A DNA polymerase
G C
T A
A T
G
C C
G
G
Nucleotides G
T
A Adenine C Replicas
T Thymine G A A C
T C
G Guanine T G G C G A G C T
C Cytosine T C G A C G T A G C A T C G
Figure 2-22. Replication of DNA. The nitrogen bases form complementary strands. As the parent
strands separate during DNA replication, each strand serves as a template for the formation of two
complementary strands.