102
UNIT 1
Organization of the Body
3
Messenger RNA (mRNA)
, relatively long nucleotide strands
resembling “half-DNA” molecules (one of the two strands of
a DNA molecule coding for protein structure). mRNA car-
ries a transcript of the code to the cytoplasm, where protein
synthesis occurs.
Ribosomal RNA (rRNA)
, along with proteins, forms the ri-
bosomes, which consist of two subunits—one large and one
small. Te two subunit types combine to form functional ri-
bosomes, which are the sites of protein synthesis.
Transfer RNA (tRNA)
, small, roughly L-shaped molecules
that ferry amino acids to the ribosomes. Tere they decode
mRNA’s message for amino acid sequence in the polypeptide
to be built.
All types of RNA are formed on the DNA in the nucleus in
much the same way as DNA replicates itself: Te DNA helix
separates and one of its strands serves as a template for synthe-
sizing a complementary RNA strand. Once formed, the RNA
molecule is released from the DNA template and migrates into
the cytoplasm. Its job done, the DNA recoils into its helical,
inactive form.
Approximately 2% of the nuclear DNA codes for the synthe-
sis of short-lived mRNA. DNA in the nucleolar organizer re-
gions (mentioned previously) codes for the synthesis of rRNA,
which is long-lived and stable, as is tRNA coded by other DNA
sequences. Because rRNA and tRNA do not transport codes
for synthesizing other molecules, they are the final products of
the genes that code for them. Ribosomal RNA and tRNA act
together to “translate” the message carried by mRNA.
Essentially, polypeptide synthesis involves two major steps:
1.
Transcription
, in which DNA’s information is encoded in
mRNA
2.
Translation
, in which the information carried by mRNA is
decoded and used to assemble polypeptides
Figure 3.34
summarizes the information flow in these two
major steps. Te figure also indicates the “RNA processing” that
removes introns from mRNA before this molecule leaves the
nucleus and moves into the cytoplasm.
Transcription
A transcriptionist converts a message from a recording or short-
hand notes into a written copy. In other words, information is
transferred from one form or format to another.
In cells,
transcription
transfers information from a DNA
base sequence to the complementary base sequence of an
mRNA molecule. Te form is different, but the same informa-
tion is being conveyed. Once the mRNA molecule is made, it
detaches and leaves the nucleus via a nuclear pore, and heads for
the protein synthesis machinery, the ribosome.
±ranscription cannot begin until gene-activating chemicals
called
transcription factors
stimulate histones at the site-to-be
of gene transcription to loosen. Te transcription factors then
bind to the promoter. Te
promoter
is a special DNA sequence
that contains the
start point
(beginning of the gene to be tran-
scribed). It specifies where mRNA synthesis starts and which
DNA strand is going to serve as the
template strand
(Figure 3.35,
Most genes of higher organisms contain
exons
, which are
amino acid–specifying informational sequences. Exons are of-
ten separated by
introns
, which are noncoding, o²en repeti-
tive, segments that range from 60 to 100,000 nucleotides. Once
considered a type of “junk DNA,” intron DNA is believed to
serve as a reservoir or scrapyard of ready-to-use DNA segments
for genome evolution, as well as a rich source of small RNA
molecules. Te rest of the DNA (indeed, most of it) is essen-
tially “dark matter” whose function is still a mystery. It is in
these regions that
pseudogenes
(false genes) are found. Pseudo-
genes are genetic fossils that are the remains of damaged genes.
Tey “look like” real genes but have deficits that render them
functionless.
The Role of RNA
By itself, DNA is like a CD recording: Te information it con-
tains cannot be used without a decoding mechanism (a CD
player). Furthermore, most polypeptides are manufactured at
ribosomes in the cytoplasm, but in interphase cells, DNA never
leaves the nucleus. So, DNA requires not only a decoder, but a
messenger as well. Te decoding and messenger functions are
carried out by RNA, the second type of nucleic acid.
As you learned in Chapter 2, RNA differs from DNA: RNA
is single stranded, and it has the sugar ribose instead of deoxy-
ribose, and the base uracil (U) instead of thymine (±). Tree
forms of RNA typically act together to carry out DNA’s instruc-
tions for polypeptide synthesis:
Nuclear
pores
mRNA
Pre-mRNA
RNA Processing
Transcription
Translation
DNA
Nuclear
envelope
Ribosome
Polypeptide
Figure 3.34
Simplified scheme of information flow from the
DNA gene to mRNA to protein structure during transcription
and translation.
(Note that mRNA is first synthesized as pre-mRNA,
which is processed by enzymes before leaving the nucleus.)
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