section 25.2
Messenger RNA
(a) The currently accepted “standard” tRNA cloverleaf with its bases numbered. A few bases present in almost all tRNA
molecules are indicated, (b) Schematic diagram of the three-dimensional structure of yeast tRNAphe. (Courtesy of Dr.
Sung-Hou Kim.)
initiate translation of a polypeptide and the codons UAG,
UGG, and UAA to terminate translation. Prokaryotic
mRNAs are polycistronic (polygenic) and usually carry
information for the synthesis of several polypeptides from
a single mRNA. The triplet codons in prokaryotic mRNA
are transcribed from the sense strand of DNA and subse-
quently are translated continuously from the
0 4
of the mRNA to the 3'-OH end. Since prokaryotic DNA
is not separated from the cytoplasm by a nuclear mem-
brane, translation begins on mRNA molecules before tran-
scription is completed. Thus, transcription and translation
are coupled in prokaryotes. Synthesis of each polypeptide
chain in a polycistronic mRNA is determined by an AUG
initiation codon and one or more nonsense codons that
release the finished polypeptide from the ribosome.
Eukaryotic mRNAs differ from prokaryotic mRNAs in
several respects (Figure 25-2). Eukaryotic genes invari-
ably contain information for only a single polypeptide
but each gene may consist of millions of nucleotides be-
cause eukaryotic genes contain introns and exons. The
mRNA that is transcribed (primary transcript) is processed
in several ways:
1. The
(intervening sequences) are spliced out of
the primary transcript and the
sequences) are joined together. The splicing reactions
and removal of introns from the primary transcript are
carried out by
small nuclear ribonucleoproteins
2. While transcription is in process, the 5' end of the
mRNA is capped with a methyl guanine nucleotide
3. After the primary transcript is complete, a poly A tail
(-AAA,, Aqh) is added to the 3' terminus.
Prokaryotic mRNA
Gene 1
Gene 2
Gene 3
Eukaryotic mRNA (Primary transcript)
I = intron
E = exon
F I G U R E 2 5 - 2
Structures of prokaryotic and eukaryotic primary transcripts (mRNAs).
Prokaryotic mRNAs are polygenic, do not contain introns or exons, and are
short lived in the cell. Eukaryotic mRNAs are monogenic, contain introns
and exons, and usually are long lived in the cell.
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