Environmental Microbiology of Aquatic and Waste Systems

34 3 Aspects of the Molecular Biology of Microorganisms of Relevance to the Aquatic Environment

such as in the whites of eggs of birds and reptiles and
in seeds; they transport oxygen in the form of hemo-
globin; they are involved in contractile arrangements
which enable movement of various animal body parts,
through contractile proteins in muscles; they protect
the animal body in the form of antibodies; they are in
membranes where they act as receptors, participate in
membrane transport and antigens, and they form tox-
ins such as diphtheria and botulism. The most impor-
tant function, if it can be so termed, is that they form
the basis of enzymes which catalyze all the metabolic
activities of living things; in short, proteins and the
enzymes formed from them are the engines of life.
Notwithstanding the bewildering diversity of living
things, varying from bacteria to protozoa to algae to
maize to man, the same 20 amino acids are found in all

living things. On account of this, the principles affecting

proteins and their structure are same in all living things.

The macromolecules linked to heredity are deo-

xyribonucleic acid (DNA) and ribonucleic acid (RNA).

The genetic information which determines the poten-

tial properties of a living thing is carried in the DNA

present in the nucleus, except in some viruses where it

is carried in RNA. DNA is also present in the plant

organelles mitochondria and chloroplasts.

DNA consists of four nucleotides, adenine, cytosine,

guanine, and thymine. RNA is very similar except that

uracil replaces thymine (see Fig. 3.1). RNA occurs in

the nucleus and in the cytoplasm as well as in the

ribosomes.

The processes of protein synthesis will be

summarized briefly below. In protein synthesis, infor-

Adenine Guanine

Thymine Cytosine

Purines

Adenine

The Nucleotides of DNA

Guanosine Thymine

Pyrimidines

Cytosine

O

O

HN

N

C

C

C CH

CH 33

H

H

H

H

N

N

C C

O C O

O O O O

N

O

N N

N

N

N

N N

N

N N

H

H

H

H

HH

H

H

H

H

H

N H

N

HC HC

OH H OH H OH H OH H

O

HO CH 2 HO CH 2 HO CH 2 HO CH^2

NH 2

O

C

C

N

CH

CH

H

N

H 2 N

C

O

CC

C

CH

N N

N

H

HN

NH 2

N C

C

CH

C

N

N

N

H

HC

Fig. 3.1 The nucleic acid bases