Page 66 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 51

A gene is a segment of DNA that con­
To pass on the genetic information to tains the triplet codes for all amino acids
the next generation of cells or animals, the
VetBooks.ir DNA double helix must be replicated. in one or more proteins and the signal
Replication of DNA begins with unwind­
sequences necessary to regulate the pro­
ing of the helix and splitting of the chain at cessing of the DNA segment. However,
the point of junction of complementary interspersed within a gene are also
bases. Each separated strand serves as a sequences of nucleotides that are not reg­
template, or model, for the formation of ulators of the process and do not contain
its complementary strand, which produces necessary triplet codes. These noncoding,
two double DNA helices, replicas of the nonregulatory segments are introns. The
original. Each new double helix consists of DNA coding segments in a gene are exons.
one strand of the original double helix and A single gene may have multiple exons and
one newly synthesized strand (Fig. 2‐22). introns throughout its length.
Errors in the duplication of DNA strands
during replication give rise to genetic
mutations. Errors may occur spontane­ RNA: Transcription and Translation
ously, or their frequency may be increased
through the effects of numerous external The processes by which the genetic code is
factors, or mutagens (e.g., ionizing radia­ interpreted and proteins are synthesized
tion, exposure to certain chemicals). require the participation of three forms of
The genetic information in DNA is coded RNA. The three forms of RNA are messenger
by the specific sequence of purine and RNA (mRNA), transfer RNA (tRNA), and
pyrimidine bases in a DNA molecule. This ribosomal RNA (rRNA). Like DNA, all
sequential arrangement of bases and its three forms of RNA consist of nucleotide
control of heredity, both on the cellular and units that contain a sugar (ribose), a phos­
the species level, have been called the genetic phate, and a purine or pyrimidine base.
code, or the language of life. The interpreta­ The two purines that are found in DNA,
tion of this code results in the synthesis of adenine and guanine, are also found in
specific proteins and this process is consid­ RNA, as is the pyrimidine cytosine. How­
ered the central dogma of molecular biology. ever, RNA does not contain the pyrimidine
The only cellular constituent whose synthe­ thymine that is found in DNA. Instead,
sis is specifically directed by the genetic RNA contains the pyrimidine uracil.
code is protein. Such proteins include those Whereas the structure of DNA was two
that are secreted as cellular products, those strands or chains of nucleotides joined
that are found in the cell membrane, and together in a double helix, RNA exists only
those that function within the cytosol or as a single strand.
within cellular organelles. The first step in the interpretation of the
The DNA code is said to be a triplet genetic code, transcription, results in the
code, for each group of three nucleotides formation of a mRNA. This process is
in the DNA chain ultimately calls for a similar to DNA replication except that
specific amino acid in the process of DNA now serves as a template for the
protein synthesis. There are approximately synthesis of mRNA instead of a new
20 amino acids in the cell. With four bases complementary strand of DNA. A specific
that may be included in a triplet, there are nuclear enzyme, RNA polymerase II, and
more than enough potential triplet codes other nuclear proteins collectively known
to represent the 20 amino acids. Other tri­ as general transcription factors bind
plet codes in the DNA serve as signals to together at a specific site on the DNA to
demarcate the segment of the DNA chain initiate the transcription of a specific gene.
that represents a particular protein and to The site on the DNA at which binding
regulate the initial and terminal steps in occurs and transcription begins is known
protein synthesis. as a promoter. A special segment of DNA