Biology 12

until a protein called a release factorbinds to the

A site. As shown in Figure 8.20, the release factor

cleaves the completed polypeptide from the tRNA.

The polypeptide then leaves the ribosome, and the

ribosome assembly comes apart. The sub-units

remain in the cytoplasm of the cell until they

bind to a new molecule of mRNA and begin the

process again.

Molecular interactions among the amino acids

along the completed polypeptide chain cause the

chain to fold into its characteristic shape. In some

cases, further modifications are required before

the protein takes on its final form and becomes

functional. The basic transfer of genetic information

from DNA to RNA to protein, however, ends with

the release of a completed polypeptide from the

ribosome-mRNA complex. The main steps in the

transfer of genetic information are reviewed in

Figure 8.21 on the next page, Table 8.2 at left

summarizes the structure and function of the nucleic

acids involved in transcription and translation.

The average protein is about 100 amino acids

long. It takes one ribosome about a minute to

synthesize a protein of this length. At this rate, if

mRNA strands were translated one at a time, it

would take a long time for the cell to synthesize

enough protein to serve its metabolic needs. The

translation process is accelerated by having all three

steps in the process take place simultaneously on

any one strand of mRNA. As soon as the first

ribosome has moved off the initiation sequence of

an mRNA molecule, a new ribosome assembly can

move in to bind to the same molecule. In this way,

dozens or even hundreds of ribosomes can attach to

a single mRNA molecule and thereby increase the

rate of protein synthesis. An mRNA molecule bound

to multiple ribosomes is called a polyribosome. A

polyribosome complex is shown in Figure 8.22 at

the bottom of the next page.

Nucleic Acid Structure Function

DNA

messenger

RNA (mRNA)

small nuclear

RNA (snRNA)

transfer RNA

(tRNA)

ribosomal

RNA (rRNA)

double helix

linear single

strand

linear single

strand

three-lobed

“cloverleaf”

linear single

strand

DNA stores genetic information.

Messenger RNA carries genetic

information from DNA to the protein

assembly line. In eukaryotes, the

precursor mRNA (pre-mRNA) must be

processed before it moves to the

cytoplasm for translation.

Small nuclear RNA combines with

proteins to create spliceosomes —

molecular structures that cleave pre-

mRNA at the ends of each intron and

join the remaining exons.

Transfer RNA carries a particular amino

acid associated with a specific mRNA

codon to the correct binding site in the

protein assembly line.

Ribosomal RNA combines with a

complex of proteins to form a ribosome,

the main structure in the protein

assembly line. Separate rRNA strands

in each ribosome sub-unit provide

binding sites for the mRNA strand

and tRNA molecules.

Table 8.2

The main nucleic acids involved in transcription

and translation

268 MHR • Unit 3 Molecular Genetics

stop codon

(UAA, UAG, or UGA)

5 ′

5 ′ 5 ′

3 ′

3 ′

3 ′

released

release polypeptide

factor

Translocation exposes a stop

codon in the A site. Instead of

tRNA, a release factor binds here.

A The binding of the release factor

causes the polypeptide to separate

from the remaining tRNA molecule.

B The ribosome assembly now

comes apart. Each element of

the assembly can be used again.

C

Figure 8.20Translation terminates when a stop codon is encountered on the mRNA.