8122 ‎(2)‎

• UCREFRP Catalog Number: 8122
• Author: Kapuscinski, A. R. and L. D. Jacobson.
• Title: Genetic Guidelines for Fisheries Management.
• USFW Year: 1987.
• USFW - Doc Type: St. Paul, Minnesota.
• Copyright Material: NO

Expression of genes at the molecular level
A gene is a sequence of nucleotides occupying a specific position (locus) on a DNA molecule.
There are three classes of genes including: 1) structural genes that code for proteins, 2) genes that
code for molecules that are involved in protein synthesis, and 3) regulatory genes that do not code for
a specific molecule but regulate the functioning of other genes. The immediate product of a structural
gene is a specific chain of amino acids called a protein. The one gene - one protein concept
provides another useful definition for a structural gene: a structural gene is the DNA that codes for a
single protein.
The genetic code
Sequences of nucleotides in structural genes are the templates for amino acids in proteins. The
genetic code is a triplet code because nucleotide triplets code for individual amino acids. A triplet of
nucleotides is called a codon. The same genetic code is shared by almost all organisms.
Figure 2 illustrates the correspondence between nucleotides, codons, and amino acids in
proteins. Note that the order of amino acids in the protein is the same as the order of the respective
codons in the DNA molecule.
Protein synthesis
Genetic control of biological processes begins with the synthesis of proteins. An intermediate
molecule (messenger RNA) and two processes (transcription and translation) are involved (Figure
2). Messenger RNA (mRNA) is a chain of nucleotides in a single strand, much like a single strand of the
DNA helix. During transcription, a strand of RNA is produced which is complementary to one strand in the
DNA helix; all of the information in the DNA is transferred to RNA. This property makes RNA an
appropriate template for protein synthesis. During translation, enzymes link individual amino acids
together using the mRNA as a template. The resulting chain of amino acids is shaped by other enzymes
to form the final protein product.
Organization of genetic information in cells
Two types of DNA are found in cells: nuclear and cytoplasmic DNA. Nuclear DNA is found in
chromosomes that are located in the nucleus of a cell. Cytoplasmic DNA is found outside of the nucleus
in various organelles (e.g., mitochondria) within the cytoplasm of the cell. All of the cytoplasmic
DNA in an individual fish is thought to originate from the mother through the cytoplasm of the egg. The
sperm of the male is thought to contribute no cytoplasm to the fertilized egg. Most genetic tools and
issues of interest to hatchery and fisheries managers involve nuclear DNA. Information about cytoplasmic
DNA should not be neglected, however. New techniques, particularly for stock identification, are being
developed that make use of cytoplasmic DNA. Furthermore, the suitability of an organism for a particular
environment depends on both its nuclear and cytoplasmic DNA.
Chromosomes - structure
Most of the DNA in fish is packaged in chromosomes that reside in the cell nucleus. Each
chromosome contains a single strand of DNA. Prior to and during cell division, the chromosomes are
condensed. The DNA in condensed chromosomes is coiled so that the chromosomes assume a
characteristic form and occupy a minimal amount of space. Condensed chromosomes are relatively easy
to see using a light microscope.
Heterochromatin are regions of a condensed chromosome that stain more darkly than other
regions when chromosomes are prepared for microscopic examination. Heterochromatin gives stained
chromosomes a banded appearance with alternating dark (heterochromatin) and light bands (Figure 3).
Heterochromatin bands can be useful as markers for stock identification.
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