Transcription in eukaryotes differs from that in
prokaryotes in two main respects. In eukaryotes,
one gene codes for a single polypeptide
(monocistronic transcription unit) and the initial
transcript is processed into mature messenger
mRNA.
Sunday, April 12, 2009
Prototype of a eukaryotic structural gene
A structural gene is a gene that codes for a polypeptide
gene product. It can be divided into sections
involved in transcription (transcription
unit) and regulatory sequences. Regulatory
sequences are located both upstream (the 5!
direction) and downstream (the 3! direction) of
the gene. In addition, internal regulatory
sequences may occur in introns. Some regulatory
sequences are located far from the gene.
Together with the promoter
gene product. It can be divided into sections
involved in transcription (transcription
unit) and regulatory sequences. Regulatory
sequences are located both upstream (the 5!
direction) and downstream (the 3! direction) of
the gene. In addition, internal regulatory
sequences may occur in introns. Some regulatory
sequences are located far from the gene.
Together with the promoter
Prototype of mature eukaryotic mRNA
Mature eukaryotic mRNA is produced from its
precursor RNA by the removal of introns, addition
of a 5! cap at the 5! end, and addition of
numerous adenine nucleotides at the 3! end
(polyadenylation). A noncoding sequence (5!
leader) is located in front of the translation start
signal (AUG), and a trailer sequence, at the 3!
end in back of the translation stop signal (UAA).
Both addition of the 5! cap and polyadenylation
involve enzymatic reactions.
precursor RNA by the removal of introns, addition
of a 5! cap at the 5! end, and addition of
numerous adenine nucleotides at the 3! end
(polyadenylation). A noncoding sequence (5!
leader) is located in front of the translation start
signal (AUG), and a trailer sequence, at the 3!
end in back of the translation stop signal (UAA).
Both addition of the 5! cap and polyadenylation
involve enzymatic reactions.
7-Methyl-guanosine cap
The translation of eukaryotic mRNA is similar to
that of prokaryotic mRNA, with two distinct
differences: (1) transcription and translation
occur at different locations in the eukaryotic
cell: transcription occurs in the cell nucleus,
and translation in the cytoplasm; (2) the 5! and
3! ends of eukaryotic mRNA have special structures.
The structure at the 5! end is called a cap.
Through the action of guanosine-7-methyltransferase,
guanosine is bound by a triphosphate
bridge to the first and second ribose
groups of the precursor mRNA chain. The guanosine
is methylated in position 7, as are the
two initial ribose residues at the beginning of
the RNA chain. Except for the mRNAs transcribed
by DNA viruses, eukaryotic mRNA usually
contains a single protein-coding sequence
(monocistronic messenger).
that of prokaryotic mRNA, with two distinct
differences: (1) transcription and translation
occur at different locations in the eukaryotic
cell: transcription occurs in the cell nucleus,
and translation in the cytoplasm; (2) the 5! and
3! ends of eukaryotic mRNA have special structures.
The structure at the 5! end is called a cap.
Through the action of guanosine-7-methyltransferase,
guanosine is bound by a triphosphate
bridge to the first and second ribose
groups of the precursor mRNA chain. The guanosine
is methylated in position 7, as are the
two initial ribose residues at the beginning of
the RNA chain. Except for the mRNAs transcribed
by DNA viruses, eukaryotic mRNA usually
contains a single protein-coding sequence
(monocistronic messenger).
Polyadenylation at the 3! end
Eukaryotic termination signals have been less
well recognized than the regulators of gene activity
at the 5! end. Eukaryotic primary transcripts
are split by a specific endonuclease
shortly after the sequence AAAUAA. Subsequently,
about 100 –250 adenine nucleotides
are attached to the 3! end of the transcript by
means of a poly(A)-polymerase (polyadenylation).
The poly(A) end binds to a protein. All
mRNAs, except those that code for histone proteins,
possess a poly(A) terminus.
well recognized than the regulators of gene activity
at the 5! end. Eukaryotic primary transcripts
are split by a specific endonuclease
shortly after the sequence AAAUAA. Subsequently,
about 100 –250 adenine nucleotides
are attached to the 3! end of the transcript by
means of a poly(A)-polymerase (polyadenylation).
The poly(A) end binds to a protein. All
mRNAs, except those that code for histone proteins,
possess a poly(A) terminus.
Regulation of Gene Expression in Eukaryotes
Precisely regulated gene expression is a prerequisite
for producing and maintaining the
many different types of cells and tissues of a
multicellular organism. Cells differentiate into
their particular cell types by means of combinations
of expressed and repressed genes. During
differentiation the tightly regulated genes function
in the order, usually sequential, required
for a particular cell fate (developmental pathways).
Many regulator genes and their proteins
have been identified (cf. part III, Genetics and
Medicine). The following outlines some important
principles of the specific control of gene expression
in eukaryotic cells.
for producing and maintaining the
many different types of cells and tissues of a
multicellular organism. Cells differentiate into
their particular cell types by means of combinations
of expressed and repressed genes. During
differentiation the tightly regulated genes function
in the order, usually sequential, required
for a particular cell fate (developmental pathways).
Many regulator genes and their proteins
have been identified (cf. part III, Genetics and
Medicine). The following outlines some important
principles of the specific control of gene expression
in eukaryotic cells.
Levels of control of eukaryotic gene expression
In principle, expression can be regulated at four
distinct levels. The first and by far the most important
is primary control of transcription.
Processing to mature RNA can be regulated at
the level of the primary RNA transcript. A
frequently observed process is alternative splicing
(see D). Translation can be varied by RNA
editing (see B for an example). At the protein
level, posttranslational modifications can determine
the activity of a protein. The cleavage of
preproinsulin to form mature insulin, glycosylation
or hydroxylation, and protein folding
distinct levels. The first and by far the most important
is primary control of transcription.
Processing to mature RNA can be regulated at
the level of the primary RNA transcript. A
frequently observed process is alternative splicing
(see D). Translation can be varied by RNA
editing (see B for an example). At the protein
level, posttranslational modifications can determine
the activity of a protein. The cleavage of
preproinsulin to form mature insulin, glycosylation
or hydroxylation, and protein folding
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