The genetic master plan of an organism is contained in the sequence of deoxyribonucleotides in its deoxyribonucleic acid (DNA).
RNA Structure, Synthesis, and Processing
OVERVIEW
The genetic master plan
of an organism is contained in the sequence of deoxyribonucleotides in its
deoxyribonucleic acid (DNA). However, it is through the ribonucleic acid (RNA),
the “working copies” of the DNA, that the master plan is expressed (Figure
30.1). The copying process, during which a DNA strand serves as a template for
the synthesis of RNA, is called transcription. Transcription produces messenger
RNAs (mRNAs) that are translated into sequences of amino acids (polypeptide
chains or proteins) and ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), and
additional small RNA molecules that perform specialized structural, catalytic,
and regulatory functions and are not translated. That is, they are noncoding
RNAs (ncRNAs). [Note: Only about 2% of the genome codes for proteins.] The
final product of gene expression, therefore, can be RNA or protein, depending
upon the gene. A central feature of transcription is that it is highly
selective. For example, many transcripts are made of some regions of the DNA.
In other regions, few or no transcripts are made. This selectivity is due, at
least in part, to signals embedded in the nucleotide sequence of the DNA. These
signals instruct the RNA polymerase where to start, how often to start, and
where to stop transcription. A variety of regulatory proteins is also involved
in this selection process. The biochemical differentiation of an organism’s
tissues is ultimately a result of the selectivity of the transcription process.
[Note: This selectivity of transcription is in contrast to the “all-or-none”
nature of genomic replication.] Another important feature of transcription is
that many RNA transcripts that initially are faithful copies of one of the two
DNA strands may undergo various modifications, such as terminal additions, base
modifications, trimming, and internal segment removal, which convert the
inactive primary transcript into a functional molecule.
Figure 30.1 Expression of
genetic information by transcription. [Note: RNAs shown are eukaryotic.] tRNA =
transfer RNA; rRNA = ribosomal RNA; mRNA = messenger RNA; me-7Gppp =
7-methylguanosine triphosphate “cap;” AAA = poly-A “tail,”.
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