Transcription and the Role of Messenger RNA
Transcription and the Role
of Messenger RNA
Information is coded in DNA, but DNA does not participate directly in protein synthesis. It is obvious that an intermediary is required. This intermediary is another nucleic acid called messenger RNA (mRNA). The triplet codes in DNA are transcribed into mRNA, with uracil substituting for thymine (Table 5-3).
Ribosomal, transfer, and messenger RNAs are transcribed directly from DNA, each encoded by different sets of genes. In this process of making a complementary copy of one strand or gene of DNA in the formation of mRNA, an enzyme, RNA polymerase, is needed. (In eukaryotes each type of RNA [ribosomal, transfer, and messenger] is transcribed by a specific type of RNA polymerase.) The mRNA contains a sequence of bases that complements the bases in one of the two DNA strands, just as the DNA strands complement each other. Thus A in the coding DNA strand is replaced by U in RNA; C is replaced by G; G is replaced by C; and T is replaced by A. Only one of the two chains is used as the template for RNA synthesis because only one bears the AUG codon that initiates a message (Table 5-3). The reason why only one strand of the double-stranded DNA is a “coding strand” is that mRNA otherwise would always be formed in complementary pairs, and enzymes also would be synthesized in complementary pairs. In other words, two different enzymes would be produced for every DNA coding sequence instead of one. This certainly would lead to metabolic chaos.
Only one strand of DNA serves as the coding strand in all DNA except that found in plasmids. Messenger RNA can be transcribed from both DNA strands in one region of plasmid DNA, and this is the only known example of proteins being encoded in both DNA strands.
Genes on the DNA of prokaryotes
are coded on a continuous stretch of
DNA, which is transcribed into mRNA
and then translated (see the following
section). It was assumed that this was
also the case for eukaryotic genes until
the surprising discovery that some
stretches of DNA are transcribed in the
nucleus but are not found in the corresponding
mRNA in the cytoplasm. In
other words, pieces of the nuclear
mRNA were removed in the nucleus
before the finished mRNA was transported
to the cytoplasm (Figure 5-18).
It was thus discovered that many genes
are split, interrupted by sequences of
bases that do not code for the final
product, and the mRNA transcribed
from them must be edited or “matured”
before translation in the cytoplasm.
The intervening segments of DNA are
now known as introns, while those
that code for part of the mature RNA
and are translated into protein are
called exons. Before the mRNA leaves
the nucleus, a methylated guanine
“cap” is added at the 5´ end, and a tail
of adenine nucleotides (poly-A) is
often added at the 3´ end (Figure 5-
18). The cap and the poly-A tail are
characteristic of mRNA molecules.
In mammals the genes coding for the histones and for interferons are on continuous stretches of DNA. However, we now know that genes coding for many proteins are split. In lymphocyte differentiation the parts of the split genes coding for immunoglobulins are actually rearranged during development, so that different proteins result from subsequent transcription and translation. This partly accounts for the enormous diversity of antibodies manufactured by the descendants of the lymphocytes.
Base sequences in some introns are complementary to other base sequences in the intron, suggesting that the intron could fold so that complementary sequences would pair. This may be necessary to control proper alignment of intron boundaries before splicing. Most surprising of all has been the discovery that, at least in some cases, RNA can “self-catalyze” the excision of introns. The ends of the intron join; the intron thus becomes a small circle of RNA, and the exons are spliced together. This process does not fit the classical definition of an enzyme or other catalyst since the molecule itself is changed in the reaction.
Information is coded in DNA, but DNA does not participate directly in protein synthesis. It is obvious that an intermediary is required. This intermediary is another nucleic acid called messenger RNA (mRNA). The triplet codes in DNA are transcribed into mRNA, with uracil substituting for thymine (Table 5-3).
 |
Ribosomal, transfer, and messenger RNAs are transcribed directly from DNA, each encoded by different sets of genes. In this process of making a complementary copy of one strand or gene of DNA in the formation of mRNA, an enzyme, RNA polymerase, is needed. (In eukaryotes each type of RNA [ribosomal, transfer, and messenger] is transcribed by a specific type of RNA polymerase.) The mRNA contains a sequence of bases that complements the bases in one of the two DNA strands, just as the DNA strands complement each other. Thus A in the coding DNA strand is replaced by U in RNA; C is replaced by G; G is replaced by C; and T is replaced by A. Only one of the two chains is used as the template for RNA synthesis because only one bears the AUG codon that initiates a message (Table 5-3). The reason why only one strand of the double-stranded DNA is a “coding strand” is that mRNA otherwise would always be formed in complementary pairs, and enzymes also would be synthesized in complementary pairs. In other words, two different enzymes would be produced for every DNA coding sequence instead of one. This certainly would lead to metabolic chaos.
Only one strand of DNA serves as the coding strand in all DNA except that found in plasmids. Messenger RNA can be transcribed from both DNA strands in one region of plasmid DNA, and this is the only known example of proteins being encoded in both DNA strands.
 |
| Figure 5-18 Transcription and maturation of ovalbumin gene of chicken. The entire gene of 7700 base pairs is transcribed to form the primary mRNA, then the 5 cap of methyl guanine and the 3 polyadenylate tail are added. After the introns are spliced out, the mature mRNA is transferred to the cytoplasm. |
In mammals the genes coding for the histones and for interferons are on continuous stretches of DNA. However, we now know that genes coding for many proteins are split. In lymphocyte differentiation the parts of the split genes coding for immunoglobulins are actually rearranged during development, so that different proteins result from subsequent transcription and translation. This partly accounts for the enormous diversity of antibodies manufactured by the descendants of the lymphocytes.
Base sequences in some introns are complementary to other base sequences in the intron, suggesting that the intron could fold so that complementary sequences would pair. This may be necessary to control proper alignment of intron boundaries before splicing. Most surprising of all has been the discovery that, at least in some cases, RNA can “self-catalyze” the excision of introns. The ends of the intron join; the intron thus becomes a small circle of RNA, and the exons are spliced together. This process does not fit the classical definition of an enzyme or other catalyst since the molecule itself is changed in the reaction.
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