In the cell, regulators somehow activate certain genes while leaving others
inactive. In other words, each cell must undergo a process of cellular differentiation
(structural or functional specialization of cells). This process of specialization is
directed by gene regulation.
What does it actually mean for genes to activate or deactivate? Genes determine
base sequences in specific messenger RNA (mRNA), which then determines the
sequence of amino acids in proteins. Activated genes are transcribed to RNA, and
the resulting message is translated into a specific protein. The term “gene expression” refers to the entire process of genetic information flowing from gene to
protein (Schena et al. 1995).
Unlike simple unicellular organisms such as bacteria or blue-green algae,
eukaryotes are cells with a nucleus enclosed in a membrane and specialized
organelles that perform essential functions.
Genes in eukaryotes include “housekeeping genes,” or genes that are permanently expressed structurally and code for proteins that are necessary for the cell’s
general functioning, and “luxury genes,” which are expressed on a more or less
permanent basis in cells with different specialization, but characteristically in cells
such as hemoglobin or immunoglobin that are particularly specialized due to an
induced reaction.
Most genes in eukaryotes are classified as either exons or introns. When the gene
undergoes phenotypic expression, introns typically become linked to nearby exons
to undergo RNA transcription. Alternatively, the exon of the RNA molecule
assumes the same sequence and directionality as the original gene during
post-transcription RNA processing in the nucleus, while the intron becomes
loop-shaped RNA and is removed. Typically, the 5′ end of the intron is a GU
sequence and the 3′ end an AG sequence, in what is known as Chambon’s rule.
A relatively shared base sequence exists for the mRNA precursor at the splicing
area, or the boundary section between the exon and intron parts.
RNA splicing mechanisms differ according to the type of intron included in the
RNA (precursor). In the case of splicing of the mRNA precursor encoded by the
eukaryote nucleus, a splicing complex is first created by the mRNA precursor and
small nuclear RNA (SnRNA). Simultaneously, a loop-shaped intron/exon intermediate RNA form is created by a 2′–5′ bond between the non-expressed portion’s
5′-phosphoric acid end created by truncation at the intron/exon boundary (the 5′
splicing site) and the adenosine ribose-2′–OH at the molecular position within the
intron (stage 1). This is followed by the simultaneous joining of the two exon and
formation of the loop-shaped intron portion (stage 2). Both stages are phosphoric
acid ester exchange reactions that take place due to 2′–OH (stage 1) or 3′–OH at the
truncated exon end (stage 2; see Fig. 3.1) (Black 2003).
7-methylguanosine (cap) joins the mRNA’s 5′ end, which consists solely of the
exon portion. Addition of a poly-A sequence chain at the 3′ end results in formation
of mature RNA. This mature RNA migrates into the cytoplasm (outside the
nucleus), and as the mRNA’s three-nucleotide sequences designate a single amino
acid, proteins are formed on the ribosome with the help of tRNA (Perry et al. 1987).
56
3 Fish Genetics
inactive. In other words, each cell must undergo a process of cellular differentiation
(structural or functional specialization of cells). This process of specialization is
directed by gene regulation.
What does it actually mean for genes to activate or deactivate? Genes determine
base sequences in specific messenger RNA (mRNA), which then determines the
sequence of amino acids in proteins. Activated genes are transcribed to RNA, and
the resulting message is translated into a specific protein. The term “gene expression” refers to the entire process of genetic information flowing from gene to
protein (Schena et al. 1995).
Unlike simple unicellular organisms such as bacteria or blue-green algae,
eukaryotes are cells with a nucleus enclosed in a membrane and specialized
organelles that perform essential functions.
Genes in eukaryotes include “housekeeping genes,” or genes that are permanently expressed structurally and code for proteins that are necessary for the cell’s
general functioning, and “luxury genes,” which are expressed on a more or less
permanent basis in cells with different specialization, but characteristically in cells
such as hemoglobin or immunoglobin that are particularly specialized due to an
induced reaction.
Most genes in eukaryotes are classified as either exons or introns. When the gene
undergoes phenotypic expression, introns typically become linked to nearby exons
to undergo RNA transcription. Alternatively, the exon of the RNA molecule
assumes the same sequence and directionality as the original gene during
post-transcription RNA processing in the nucleus, while the intron becomes
loop-shaped RNA and is removed. Typically, the 5′ end of the intron is a GU
sequence and the 3′ end an AG sequence, in what is known as Chambon’s rule.
A relatively shared base sequence exists for the mRNA precursor at the splicing
area, or the boundary section between the exon and intron parts.
RNA splicing mechanisms differ according to the type of intron included in the
RNA (precursor). In the case of splicing of the mRNA precursor encoded by the
eukaryote nucleus, a splicing complex is first created by the mRNA precursor and
small nuclear RNA (SnRNA). Simultaneously, a loop-shaped intron/exon intermediate RNA form is created by a 2′–5′ bond between the non-expressed portion’s
5′-phosphoric acid end created by truncation at the intron/exon boundary (the 5′
splicing site) and the adenosine ribose-2′–OH at the molecular position within the
intron (stage 1). This is followed by the simultaneous joining of the two exon and
formation of the loop-shaped intron portion (stage 2). Both stages are phosphoric
acid ester exchange reactions that take place due to 2′–OH (stage 1) or 3′–OH at the
truncated exon end (stage 2; see Fig. 3.1) (Black 2003).
7-methylguanosine (cap) joins the mRNA’s 5′ end, which consists solely of the
exon portion. Addition of a poly-A sequence chain at the 3′ end results in formation
of mature RNA. This mature RNA migrates into the cytoplasm (outside the
nucleus), and as the mRNA’s three-nucleotide sequences designate a single amino
acid, proteins are formed on the ribosome with the help of tRNA (Perry et al. 1987).
56
3 Fish Genetics