Environmental Microbiology of Aquatic and Waste Systems

3.1 Protein Synthesis 35

mation flow is from DNA to RNA via the process of
transcription, and thence to protein via translation.
Transcription is the making of an RNA molecule from
a DNA template. Translation is the construction of a
polypeptide from an amino acid sequence from an
RNA molecule (see Fig. 3.2). The only exception to
this is in retroviruses where reverse transcription
occurs: a single-stranded DNA is transcribed from a
single-stranded RNA (the reverse of transcription).

3.1.1 Transcription

An enzyme, RNA polymerase, opens the part of the
DNA to be transcribed. Only one strand of DNA, the
template or sense strand, is transcribed into RNA. The
other strand, the anti-sense strand is not transcribed.
The anti-sense strand is used in making ripe tomatoes
to remain hard. The RNA transcribed from the DNA is
the messenger or m-RNA (see Fig. 3.3). Because some
students appear to be confused by the various types of
RNA, it is important that we mention at this stage that
there are two other types of RNA besides m-RNA.
These are ribosomal or r-RNA and transfer or t-RNA.
During this step, mRNA goes through different types
of maturation including one in which the non-coding
sequences are eliminated, in a process known as splicing.

When m-RNA is formed, it leaves the nucleus in

eukaryotes (there is no nucleus in prokaryotes!) and

moves to the ribosomes.

3.1.2 Translation

In all cells, ribosomes are the organelles where proteins

are synthesized. They consist of two-thirds of ribosomal

RNA, r-RNA, and one-third protein. Ribosomes consist

of two subunits, a smaller subunit and a larger subunit.

In prokaryotes, typified by E. coli, the smaller unit is

30S and larger 50S. S is Svedberg units, the unit of

weights recorded in an ultra centrifuge for the two parts.

The length of r-RNA differs in each. The 30S unit has

16S r-RNA and 21 different proteins. The 50S subunit

consists of 5S and 23S r-RNA and 34 different proteins.

The smaller subunit has a binding site for the m-RNA.

The larger subunit has two binding sites for t-RNA.

The messenger RNA (mRNA) is the “blueprint” for

protein synthesis and is transcribed from one strand of

the DNA of the gene; it is translated at the ribosome

into a polypeptide sequence. Translation is the synthesis

of protein from amino acids on a template of messen-

ger RNA in association with a ribosome. The bases on

m-RNA code for amino acids in triplets or codons, that

is, three bases code an amino acid. Sometimes, different

Acceptor stem

Variable loop

TψC loop

C

C

GTψ

OH

UH 2

D loop

G

G

A

A

C

C

P

5 ́

3 ́

U

U

Anticodon loop

Fig. 3.2 Cloverleaf-
shaped transfer RNA
(From The Internet
Encyclopedia of Science;
http://www.daviddarling.
info/encyclopedia/T/tRNA.
html. Reproduced with
permission)