The Ecology Book

36 THE ROLE OF DNA

the ladder down the middle to

produce two single strands. These

act as templates for the production

of a second complementary

DNA strand on each of them by

matching up the appropriate base

pairs. The process results in two

strands of whole DNA that are

exactly the same as the original.

Since DNA remains in the

nucleus of the cell, a related molecule

called messenger ribonucleic acid

(mRNA) copies segments of DNA

coding sequence and carries the

information to the regions of the

cell where new proteins are made.

RNA is chemically related to DNA,

but the thymine base (T) is

replaced by the base uracil (U),

which is less stable but requires

less energy to make. Stable living

organisms benefit from having

DNA genomes, but RNA makes up

genomes of some viruses, where

stability can be less advantageous.

DNA is found in all living things

on Earth, from amoebae to insects,

to trees, tigers, and humans. Of

course, the sequence of base pairs

varies, and this difference allows

geneticists to trace relationships

between different species.

Good and bad errors

DNA is a highly stable molecule,

but sometimes mistakes, known

as mutations, occur. These can be

in the form of an error, duplication,

or omission in the order of the

nucleotides A, C, G, and T. Mutation

can be spontaneous—the result

of errors that occur when the DNA

is copied—or may be induced

by external influences such as

exposure to radiation or cancer-

causing chemicals. Some mutations

have no effect, but others may

change what the gene produces

or inhibit the functioning of a

gene. This can lead to problems in

the organism as a whole. Examples

Genetically

modified food

In agriculture, crops may be

engineered to enhance them

in some way. A genetically

altered crop is known as a

genetically modified organism

(GMO). Companies that

operate in this sector may

modify a plant’s DNA so that

it produces more of a certain

nutrient or a toxin specific to

a particular insect pest. The

DNA of a plant may also be

altered to become resistant to

a particular herbicide, so that

use of the chemical kills only

the weeds and not the crop.

Some ecologists argue that

there is a risk of genetically

unmodified plants becoming

contaminated by GMOs. They

also point out that the long-

term effects of eating such

foods are as yet not properly

understood. Another concern

is that in the future large

agrochemical companies could

control the world’s food supply

by patenting the GMOs that

they produce, to the detriment

of poorer nations.

New kinds of rice are being

developed through genetic

modification. This may improve

the nutritional value of the crop

or its resistance to disease.

the type of protein they go on

to make. For example, the

combination GGA is the codon for

glycine. Sixty-four possible triplets

can be made from the four base

pairs, and 61 of them code for a

particular amino acid. The other

three act as signals such as “start”

and ”stop,” which govern how

information is read by the cellular

machinery. DNA is also organized

into separate chromosomes, of

which there are 23 pairs in the

human cell.

Copying the code

When cells divide, DNA needs to

be copied. This is achieved by the

splitting of base pairs, which cuts

A DNA molecule consists of a double

helix formed by two strands, made up

of sugars and phosphates, linked by

paired base nucleotides: adenine and

thymine or cytosine and guanine.

The structure of DNA

adenine thymine

cytosine guanine

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