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Encyclopedia of Physical Science and Technology EN017F-788 August 3, 2001 16:27

32 Translation of RNA to Protein

consecutive nucleotides termed a codon. Decoding of The polypeptide chains of proteins are also asymmetri-

mRNA takes place in the 5 → 3 direction, and the cal polymers in which the amino acid residues are linked

polypeptide is synthesized from the amino to the car- by peptide bonds between their alpha amino and carboxyl
boxyl terminus. groups (Fig. 2), leaving a free alpha amino group at one
Translocation Thestepwiseadvanceofaribosomealong end (the amino terminus) of the polymer and a free alpha
mRNA, one codon at a time, with simultaneous transfer carboxyl group at the opposite end (the carboxyl termi-
of peptidyl-tRNA from the A site to the P site of the nus). The significance of the polarity of RNA and proteins
ribosome. will become evident when the process of protein biosyn-
thesis is explained below.
The genetic information stored in DNA is not usable
PROTEINS are essential to the structure and function directly for making proteins. Rather, it must be copied
of living cells. The assembly of polypeptide chains from into a primary RNA transcript containing the mRNA by an
amino acids and their subsequent modifications, leading enzymatic transcription of segments of DNA containing
to the final three-dimensional protein structure, are ex- the genes. In prokaryotes, the primary transcript is also the
ceptionally complex processes; many components are in- messenger; that is, it can be used directly in polypeptide
volved and much of the cell’s energy is utilized. Each synthesis. In contrast, in eukaryotes the primary transcript
peptide bond requires the expenditure of four high-energy is often much larger in size than the mature mRNA and
phosphate bonds. This value excludes the energy used requires extensive processing involving the excision of
for initiation and release of the polypeptide chains and intervening and other noncoding sequences.
the cost of synthesizing and processing mRNA. The lin- Messenger RNA serves as the template for protein syn-
ear amino acid sequence of a protein is encoded within thesis; that is, the linear nucleotide sequence of the mRNA
the gene as a linear deoxyribonucleotide sequence. Early dictates the amino acid sequence of the polypeptide
steps in the biosynthesis of a protein include transcrip- encoded originally by the gene. Conventionally, gene
tion of the gene and appropriate processing of the tran- and mRNA nucleotide sequences are written in the 5
script leading to the production of mature messenger to 3 direction, which corresponds to the direction in

RNA (mRNA). We describe the mechanisms involved which mRNA is decoded during polypeptide synthesis:

in translating mRNA to produce a polypeptide chain The mRNA is read in the 5 to 3 direction, and the
which has the amino acid sequence specified by the polypeptide is synthesized from the amino- toward the
gene. carboxyl-terminus.
The mechanism whereby RNA is translated into protein
is complex, and the cell devotes considerable resources to
I. INTRODUCTION the translational machinery. The components include 20
different amino acids, transfer RNAs, aminoacyl-tRNA
A gene or cistron is defined as the region of DNA that synthetases, ribosomes, and a number of protein factors
is transcribed into a functional RNA. The transcript func- which cycle on and off the ribosomes and facilitate various
tions either as such (e.g., tRNA, rRNA, snRNA) or as a steps in initiation of translation, elongation of the nascent
messenger (mRNA), which, after processing or editing polypeptide chain, and termination of synthesis with re-
as required, normally codes for one or more polypep- lease of the completed polypeptide from the ribosome.
tide chains in the translation process. A polynucleotide The process depends on a supply of energy provided by
such as RNA is an asymmetrical polymer assembled ATP and GTP. The rate of protein synthesis is typically
from nucleoside triphosphates by a stepwise mechanism in the range of 6 (immature red blood cells of the rabbit)

linking the 5 position of one nucleotide by a phos- to 20 (Escherichia coli growing optimally) peptide bonds

◦
phate bridge to the 3 position of the adjacent nucleotide. per sec. at 37 C.
In the finished polynucleotide chain the first nucleotide

residue has a 5 position which is not linked to an- II. mRNA STRUCTURE AND
other nucleotide, whereas the last nucleotide has an un- THE GENETIC CODE

linked 3 position. Thus, polynucleotide synthesis pro-

ceeds from the 5 to the 3 terminus and the polymer A. Structure

is said to have a 5 to 3 polarity. Usually, linear RNA

sequences are written with the 5 terminus on the left The sequence information of a gene is copied (transcribed)

and the 3 terminus on the right (Fig. 1A). Within the into the nucleotide sequence of RNA from the com-
RNA chain some bases may form antiparallel base pairs plementary strand of DNA, called the template strand.
(Fig. 1B). The primary transcript is a single strand of RNA, which