Download File - NCEA Level 2 Biology

Mutation Notes:
Causes of Mutations:
Mutations can occur in all organisms spontaneously. The natural rate at which a gene
undergoes change is normally very low. This rate can be increased by environmental
factors.
A mutagen is a physical factor or a chemical substance that causes mutations. Gene
mutations are caused by mistakes during DNA replication by disrupting the base
sequence. The rate of these mistakes occurring is increased due to mutagens.
Mutagens include:
a. High energy radiation eg. Gamma rays, X-rays, ultraviolet (UV) rays, nuclear
contamination
b. Chemicals such as mustard gas, Agent Orange, benzene, formalin hydrocarbons in
cigarette smoke and many industrial chemicals.

Gametic mutations: are inherited, mutations occur in gametes cells (eg. in ovaries
and testes). If mutations occur in these cells, they can be inherited if fertilization takes
place. The offspring's cells will contain the mutation. Because of this, mutations in
gamete cells are passed from one generation to the next and can become part of
genetic variation in the gene pool ( upon natural selection can act)

Somatic mutations: occur in cells in the body other than sex organs such as ovaries
and testes. Somatic cells that undergo mutations cannot pass these on to offspring and
so have no effect on species and their evolution. An example of this a mutation in skin
cells due to exposure to UV rays to cause skin cancer. The mutation simply stays in the
individual within a population and does not affect the population as a whole.
The effects of Mutations:
Mutations introduce new alleles into a population.
Most mutations are harmful. These have a detrimental effect on the individual or
offspring.
• Cystic fibrosis - 1 base change
• sickle cell anaemia - loss of 3 nucleotides
• Albinism – caused by mutation in gene of enzyme pathway of melanin
Beneficial ones tend to occur more often in organisms with short generation times and
may have a selective advantage.
• Bacteria – antibiotic resistance through mutation, transfer between bacterial
species
• Superbugs such as MRSA have arisen this way
• RNA viruses – such as HIV – mutates it’s protein coat so that the host human is
unable to make antibodies quick enough against it
•
Many may be silent – Virtually impossible to detect because no observable effect
and may only be selected for or against at a later date.
•
Neutral mutations make no change at all. Neither harmful or beneficial to the organism
but may be important in an evolutionary sense
Types of mutations:
During replication, mistakes can be made in the copying process. This can result in an
altered sequence of DNA bases and the formation of an alternative allele of a gene.
These gene mutations can be of 3 types: a substitution mutation, a deletion mutation and
an insertion mutation. If the mutation only involves one nucleotide they are called point
mutations. Insertion and deletions usually cause major changes in the amino acid sequence
of the protein.
Chromosome mutations: during meiosis chromosomes can break apart and recombine in
ways that cause genes to be arranged in new sequences often causes severely harmful
effects:
Classified into four types:
Deletion - where part of a chromosome breaks off and a gene is effectively lost from
the chromosome eg. Crit-du-Chat syndrome (a deletion occurs in a chromosome from
pair 5);
Duplication - where the same section of a chromosome is copied or repeated and
occurs twice. In some cases a piece of a chromosome may break off and join onto other
chromosome of the homologous pair;
Inversion - where the order of the genes is reversed. This can occur due to part of a
chromosome breaking off, rotates 1800 and then rejoining in the opposite direction; The
gene will be in reverse order for this segment of the chromosome.
Translocation - where a part of one chromosome breaks off and migrates to another
chromosome where it becomes attached eg. breakages in chromosomes 15 and 21 can
result in Down syndrome.
CHROMOSOME MUTATIONS
These can only occur during meiosis.
a.k.a. Block mutations.
Deletion:
Inversion:
Examples:
Examples:
Cru-Du-Chat, Prader-Willi
On chromosome 2 – unviable offspring
Translocation:
Duplication:
A section of one chromosome is
lost to another one
A section of one chromosome
is lost to its Homologue
Chromosomes 9 and 22 – chronic
leukaemia
Page 149/50
Whole Chromosomal mutations occur during meiosis as a result of homologous pairs of
chromosomes not separating during the first division. This is called non-disjunction and
results in some gametes with extra chromosomes and others with less than the normal
number. A whole chromosome, or whole set of chromosomes are added or lost.Will have dire
consequences.

Trisomy An example of this in humans is Down syndrome. During meiosis, the two
number 21 chromosomes (usually in the mother) do not separate and the resulting
ovum contains two number 21 chromosomes instead of just one, with a total of 24
instead of the normal haploid number of 23. When this ovum is fertilized by a normal
haploid sperm with one number 21 chromosome (total 23), the resulting zygote is a
trisomy and will have three number 21 chromosomes (total of 47 instead of the
normal diploid number of 46). Down syndrome is also called trisomy-21 as there are
three number 21 homologous chromosomes.
Another example of trisomy in humans is Klinefelter’s syndrome (XXY) where the ovum
has two x chromosomes and is fertilized by a normal sperm with one Y.

Polyploidy is the result of a diploid (2N) gamete being fertilized by a haploid (N)
gamete to produce a triploid (3N) zygote, or even two diploid gametes producing a
tetraploid (4N) zygote. These types of chromosomal non-disjunctions are the result of
all homologous chromosomes not separating during meiosis with one gamete having all
(2N) chromosomes and the others having none.
Polyploidy is common in plants such as modern hybrid bread wheat (Triticum aestivum)
which has 6N number of chromosomes. In the weed Datura stramonium, the trisomic
effect can be detected by the phenotypes of the seed capsule such as rolled, elongate,
globe. Many garden flowering plants such as tulips and daffodils are polyploids and they
mainly reproduce asexually by bulbs.