section 25.2

Messenger RNA

565

FIGURE 25-1

(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

5

'-P

0 4

end

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

introns

(intervening sequences) are spliced out of

the primary transcript and the

exons

(expressed

sequences) are joined together. The splicing reactions

and removal of introns from the primary transcript are

carried out by

small nuclear ribonucleoproteins

(snRNPs).

2. While transcription is in process, the 5' end of the

mRNA is capped with a methyl guanine nucleotide

(m

7

Gppp).

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)

A-3'

.tail

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.