CHAPTER
RNA and Protein Synthesis
Most proteins are enzymes that catalyze the myriad of
chemical reactions in cells that are necessary for life;
other proteins form structural functions as in bone and
muscle. The information for making proteins resides in
the sequence of bases in DNA in chromosomes and in
organelles such as mitochondria. Converting the informa-
tion contained in genes into proteins involves two com-
plex processes.
Transcription
is the first step in which
the sequence of bases in a gene is converted into a com-
plementary sequence of bases in a molecule of RNA.
Three chemically identical but functionally quite different
molecules of RNA are transcribed from DNA:
messenger
RNA (mRNA)
carries the genetic information contained in
a gene;
transfer RNA (tRNA)
and
ribosomal RNA (rRNA)
are also transcribed from genes but are used to convert the
information in the sequence of bases in mRNA into the
corresponding sequence of amino acids in a protein.
Translation
is the process by which an mRNA is “read”
by tRNAs,
ribosomes
(complex structures consisting of
rRNAs and ribosomal proteins), and numerous other en-
zymes. Each type of cell is programmed to synthesize only
those proteins necessary for its particular cellular func-
tions (Chapter 26). The difference between a neuron and
a liver cell is the kind of proteins that are synthesized even
though both cells contain exactly the same genetic infor-
mation. Cellular differentiation is due to differential gene
expression; a tumor cell invariably is a cell that has lost
the ability to regulate and express its genetic information
correctly and generally grows in an unregulated manner,
as opposed to normal cells whose growth is regulated.
The flow of information in all cells is from DNA to RNA
to protein, which is known as the
central dogma
of mole-
cular biology; it was formulated by Francis Crick shortly
after the discovery of the structure of DNA. Information
also can flow from DNA to DNA in both cells and among
viruses that infect cells. Information also flows from RNA
to RNA during the replication of RNA viruses such as
the polio virus. The final permitted information transfer is
from RNA to DNA, which only occurs in the case of retro-
viruses such as
human immunodeficiency virus (HIV).
The
only information transfer that is prohibited by the central
dogma is from protein to RNA or to DNA. The permitted
information transfers in cells (infected or uninfected) is
summarized below.
The synthesis of a protein in a human cell can be broadly
outlined as follows. A mRNA molecule is transcribed from
a single strand of DNA (the “sense” strand) in the nucleus.
The mRNA is processed by splicing out nontranslatable
segments of nucleotides (
introns
) and rejoining the trans-
latable segments
(exons).
Additional chemical modifica-
tions are made to both ends of the mRNA molecule and it
is transported into the cytoplasm. The mRNA is translated
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