1 Structure and Function of the Human Genome
17
the connection between genetic information and cell metabolism. The flow
of information from genomic DNA to protein sequence has been itemized
by Francis Crick ( 1956) in the central dogma of molecular biology (Figure
9C; seealso Figure 10 and Figure llB).
Transcription
Figure 10 shows Transcription starting at a promoter, which is a spedfic
DNA sequence that is recognized by a multienzyme complex containing
an RNA polymerase and transcription factors. The RNA polymerase
then catalyses transcription of DNA into mRNA. As a convention, the
strand of the genomic DNA that is identical in sequence to the mRNA is
called the ( +) strand or sense strand. The complementary strand is called
the (-) strand or antisense strand.
Transcription initially Ieads to an immature heteronudear RNA
(hnRNA). hnRNA contains exons which encode amino acid sequence
and are present in mature mRNA, and introns (or intervening sequences),
which do not encode protein sequence and are absent from mature mRNA.
Introns are spliced out by spliceosomes at splice-donor and splice-acceptor
recognition sequences, and adjacent e~ons are joined to each other. Before
hnRNA leaves the nucleus and enter1j ihe cytoplasm, a poly(A) tail is added
at the 3' end by a poly(A) polymerase. The 5' end is protected by a 7 -methylguanine, constituting the so-called 5' -cap. Both mechanisms Iead to higher
stability of the mature mRNA. The mature mRNA is then translated at ribosomes into a polypeptide using the genetic code (see Figure 13). The ribosome in eukaryotes "docks" at the endoplasmic reticulum. In some instances posttranslational modification of the protein follows, such as cleavage of a signal peptide or glycosylation. The complete process from transcription to the final gene product (i.e. protein) is called gene expression of
the gene.
In prokaryotes there is co-linearity between gene, mRNA and polypeptide. This co-linearity is interrupted in eukaryotes because of the intervening introns. Furthermore, the rule of co-linearity can be broken by the presence of alternative splicing and posttranslational modification.
Exon-intron boundaries can be visualized by electron microscopy using
so called "R-loop" conditions (see above, Figure 9D).
17
the connection between genetic information and cell metabolism. The flow
of information from genomic DNA to protein sequence has been itemized
by Francis Crick ( 1956) in the central dogma of molecular biology (Figure
9C; seealso Figure 10 and Figure llB).
Transcription
Figure 10 shows Transcription starting at a promoter, which is a spedfic
DNA sequence that is recognized by a multienzyme complex containing
an RNA polymerase and transcription factors. The RNA polymerase
then catalyses transcription of DNA into mRNA. As a convention, the
strand of the genomic DNA that is identical in sequence to the mRNA is
called the ( +) strand or sense strand. The complementary strand is called
the (-) strand or antisense strand.
Transcription initially Ieads to an immature heteronudear RNA
(hnRNA). hnRNA contains exons which encode amino acid sequence
and are present in mature mRNA, and introns (or intervening sequences),
which do not encode protein sequence and are absent from mature mRNA.
Introns are spliced out by spliceosomes at splice-donor and splice-acceptor
recognition sequences, and adjacent e~ons are joined to each other. Before
hnRNA leaves the nucleus and enter1j ihe cytoplasm, a poly(A) tail is added
at the 3' end by a poly(A) polymerase. The 5' end is protected by a 7 -methylguanine, constituting the so-called 5' -cap. Both mechanisms Iead to higher
stability of the mature mRNA. The mature mRNA is then translated at ribosomes into a polypeptide using the genetic code (see Figure 13). The ribosome in eukaryotes "docks" at the endoplasmic reticulum. In some instances posttranslational modification of the protein follows, such as cleavage of a signal peptide or glycosylation. The complete process from transcription to the final gene product (i.e. protein) is called gene expression of
the gene.
In prokaryotes there is co-linearity between gene, mRNA and polypeptide. This co-linearity is interrupted in eukaryotes because of the intervening introns. Furthermore, the rule of co-linearity can be broken by the presence of alternative splicing and posttranslational modification.
Exon-intron boundaries can be visualized by electron microscopy using
so called "R-loop" conditions (see above, Figure 9D).