Mutations and Mutagens
A mutation is a change in the DNA sequence. A block mutation
involves a large piece of DNA, while a point mutation involves a
single DNA base.
Occasionally, errors can occur during DNA replication as DNA is
being copied. This means that the instructions carried by the code are
not followed exactly.
This may be the result of an incorrect pairing of bases; the
substitution of a different nucleotide; or the deletion, inversion or
insertion of a nucleotide.
The consequence of such a point mutation is that it can change the
genetic message. This may result in a different amino acid being
coded, leading to the production of a different or non-functional
protein.
Sickle-cell anaemia is a disease caused by a point mutation. In the
DNA sequence a T gets replaced by an A, thereby changing the
amino acid formed. This results in sickle shaped red blood cells which
clump together and block blood vessels.
Only mutations that have occurred in the germline cells such as the
sex cells or gametes (sperm and ova) are inherited.
In sexually reproducing organisms, mutations that occur in somatic
cells are not passed on to the next generation.
Mutations can happen by chance or have a particular cause.
When the cause of the mutation cannot be identified it is called a
spontaneous mutation, and when it can be identified it is referred
to as an induced mutation.
A factor that triggers mutations in cells is called a mutagen or
mutagenic agent.Examples of mutagenic agents include radiation
(e.g. ultraviolet radiation, nuclear radiation and X-rays) and
some chemical substances such as formalin and benzene (which
used to be common in pesticides).
As a result of the thinning of the ozone layer in the atmosphere, we
are exposed to increasing amounts of UVB radiation that can
damage (or mutate) our DNA. This can lead to the development of
skin cancers. Protective clothing and sunscreens can help reduce our
exposure to this dangerous, potentially mutagenic environmental
radiation.
GENETIC TESTING
Why is genetic testing done?
 for gender determination
 carrier screening for genetic mutations
 diagnosis, prediction or pre-disposition to particular diseases
These tests can be done prenatally, on newborn babies, children or
adults.
Pre-natal testing
Chorionic villus sampling- collection of foetal cells ( 10- 12 weeks
old )
Amniocentesis- collection of fluid from the uterus that contains
cells shed by the foetus (14-16 weeks old)
These tests are conducted to detect diseases like Down Syndrome.
Risks- miscarriage or damage to the foetus
Testing newborns
Blood test done for cystic fibrosis, phenylketonuria and
hypothyroidism.
Risks associated with genetic testing
 People might be discriminated against on the basis of their
genetic make-up.
 Pregnancies might be terminated.
 It may cause undue stress if the person has a predisposition to
a disease — they may spend a lot of time worrying.
DNA Fingerprinting
A large percentage of our DNA is non-functional or non-coding and is
known as junk DNA.
It contains patterns of repetitive base sequences called
microsatellites.
DNA fingerprinting is based on variations in the patterns of these
repeating base sequences in DNA of different individuals.
DNA fingerprinting can be used in forensic investigations,
paternity tests and evolutionary studies.
Cloning of Animals
Cloning of Dolly the sheep
Steps:
 A donor cell is taken from a sheep's udder — this is the animal
being cloned.
 An egg cell is taken from an adult sheep.
 The nucleus of the egg cell is removed.
 The two cells are fused using an electric shock.
 The fused cells begin dividing normally to become an embryo.
 The embryo is placed in the uterus of a foster mother.
 The embryo develops normally into a lamb — Dolly.
TRANSGENIC ORGANISMS
Transgenic organisms are organisms with foreign genes( genes
from another species ) inserted into their genome.
Risks
 Not enough known about genes jumping species barrier
 could affect food chains
 could interact with host genes
 causing unanticipated results- transferred genes could mutate
and affect other species
 legal problems, e.g. patenting living things