Page 68 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 53

results in alternately spliced mRNAs and apply in all living organisms (from viruses
through the hierarchy of plants, inverte­
ultimately the synthesis of different proteins
VetBooks.ir from the transcription of a single gene. brates, vertebrates, and humans), these
Within the cytoplasm, the processed
techniques and procedures are widely
mRNA binds to a ribosome. This binding applied. Because of the commonality of
occurs at a specific end of the mRNA chemicals involved (DNA and RNA), it is
under the direction of a start codon at that also possible to move genetic material
site. The start codon also signals the bind­ from one species to another. For exam-
ing of an initial tRNA with a specific amino ple, mammalian DNA can be placed into
acid attached. There is at least one specific the genome of Escherichia coli, which
tRNA for each of the approximately 20 are then able to produce mammalian
amino acids in the cell. After the first proteins. Recombinant DNA is the general
tRNA is bound, a second tRNA with its term describing DNA that contains novel
attached amino acid arrives and binds to segments inserted by biotechnological
the next codon in line. Ribosomal enzymes techniques. An animal or plant that con­
then detach the amino acid from the first tains DNA from another organism is said
tRNA and link the two amino acids to to be transgenic.
begin the formation of a peptide chain. The discovery of a group of enzymes
The first tRNA can be detached from the known as restriction nucleases was a key
ribosome and transfer another amino acid. factor in the development of recombinant
The ribosome now directs the binding of a DNA techniques. These enzymes cut DNA
third tRNA with its appropriate amino into shorter segments by splitting the link­
acid and the subsequent linkage of the ages between nucleotides. The enzymes do
third amino acid to the second. Figure 2‐24 not act at random sites through the DNA
summarizes these initial steps in protein strand; instead, each individual nuclease
synthesis. acts at a specific site termed its restriction
This basic process of tRNA binding and site. If DNA from two different organisms
amino acid linking continues as the ribo­ are treated with the same nuclease, the
some moves along the mRNA strand. The nuclease will fragment the DNA at similar
result is a specific sequence of amino acids restriction sites in both. This yields seg­
that are appropriate for the codons con­ ments of DNA with similar characteristics
tained in the mRNA strand. A stop codon at their ends, but the sequences within the
at the end of the mRNA signals the ribo­ segments may be quite different. Because
some to detach the newly synthesized the ends of the segments from the two
amino acid chain from the mRNA. The organisms are similar, DNA ligase (an
mRNA remains intact and may be reused enzyme that re‐establishes the nucleotide
multiple times. The decoding of the mRNA linkages) can be used to join the different
and the synthesis of the appropriate amino DNA segments. The final result is a DNA
acid chain constitute translation. The strand that contains DNA from two different
single chain of amino acids that have been organisms.
linked will fold into the tertiary structure To produce a transgenic organism,
necessary to convey biological activity. recombinant DNA must be inserted into an
organism’s genome. In domestic animals,
this has been accomplished by microin­
Biotechnology jection of recombinant DNA into a pronu-
cleus of single‐cell embryos. The pronucleus
Genetic engineering and biotechnology are is a nucleus‐like structure in the embryo
general terms used to describe the myriad that contains genetic material from one
of techniques used to alter the genetic code parent. One‐cell embryos have two pronu­
in organisms. Because the same principles clei that ultimately fuse so that the genetic
of genetic information storage and transfer material from the two parents can be