mutagens mutations carcinogens and cancer

1. • Mutations, Mutagens,
Carcinogens and Cancer
• Ms. Ruth B.

2. Outline
• Mutations and their causes
• Mutagens and carcinogens
• Mutagenicity tests
• Types of Mutations
• Mutations and Cancer

3. Learning Outcomes
• Describe Mutations and their causes
• Describe types of mutations
• Distinguish mutagens from
carcinogens
• Describe mutagenicity tests
• Link mutations with cancer

4. Background history of
Mutations
• In 179, Seth Wright identified a male lamb with
abnormal short limb (Ancon sheep)
• Hugo de Vries used the term “Mutation” for the
first time in 1900
• In 1904 Morgan identified a white eye mutant of
Drosophila flies in a population of red eyed flies
• In 1927 H.J. Muller induced mutations in
Drosophila using X-rays and was awarded a
Nobel prize in 1946

5. What are Mutations?
• They are heritable changes in the genetic
materials caused by alterations of the nucleotide
sequence in the DNA.
• Mutations can happen to Somatic cells and
Germ-line cells
• They give rise to alternative forms of a gene also
known as Alleles

6. Mutations..
• Key words
• Wild type: a form of a gene which the
nucleotide sequence of that gene is
characteristic of that specie.
• E.g. AGTCGTCGAGT normal
• AGTGGTCGAGT mutant
• Also known as Standard, Normal or
Reference genotype.

7. Mutations..
• Key words
• Forward Mutations:
• Are mutations which causes a gene to change
from the wild type to the mutant form.
• Reverse Mutations:
• Are mutations which causes a gene to change
from the mutant type back to the wild type

8. What causes Mutations?
• Mutations can occur spontaneously or they can be
induced by some factors.
• Spontaneous mutation:
• They happen due to instability of DNA bases not
associated with chemicals or physical factors.
• DNA bases (A,T,C,G) can exist in alternative
chemical forms known as Tautomer

9. Spontaneous Mutations..
• Tautomer forms of DNA bases are also known
as the “Normal” and “Rare” forms of the bases.
• In its rare forms, bases can join with incorrect
partners resulting to mismatched base pairs
• e.g. rare form of Cytosine(C*) pairs with
Adenine, and rare form of thymine(T*) pairs with
Guanine

10. C---G
A---T
T---A
T---A
A---T
G---C
T---A
C---G
C---G
A---T
T---A
T---A
*A---C
G---C
T---A
C---G
C---G
A---T
T---A
T---A
A---T
G---C
T---A
C---G
Strands separate for first
round of replication
A tautomeric form of
Adenine base pair with
Cytosine

11. C---G
A---T
T---A
T---A
*A---C
G---C
T---A
C---G
C---G
A---T
T---A
T---A
A---T
G---C
T---A
C---G
G---C
T---A
A---T
A---T
C---G
C---G
A---T
G---C
Replication round two
A return to normal
form and pair with
T
A-T base pair is permanently
replaced with G-C and passed
to next daughter cells

12. Spontaneous Mutations..
• Deamination can also lead to mutations in Nucleotide
base pairs.
• Cytosine loses the Amino group to form Uracil,
which pairs with Adenine instead of a C—G bonds.
• Adenine loses the amino group to form Hypoxanthine
which base pairs with cytosine instead of thymine.
• The types of mutations arising from this processes are
known as Transitions and Tranversions

13. • Adenine loses amino
group to form
Hypoxanthine
• Cytosine loose amino
group and forms Uracil

14. Induced Mutations
• These are Mutations which are caused by
influence from environmental changes.
• Physical and Chemical Factors called
Mutagens causes the induced mutations.

15. Mutagens
• A physical or chemical agent with the ability to cause
mutations.
• Carcinogens are mutagens with the ability to cause
cancer
• Pro mutagens are not mutagenic themselves but
can form mutagenic metabolite through cellular
processes.
• Mutagens were first identified in 1927 by Muller who
used X-rays to cause mutations in fruit flies.

16. Effects of Mutagens
• They can cause changes in the DNA that affects
replication and transcription of the DNA
• They can cause a loss of function of a gene
• Some powerful mutagens can lead to
chromosomal breakage and rearrangements

17. Types of Mutagens
• Mutagens can be categorized into two types:
I. Physical mutagens
II. Chemical Mutagens
• Physical mutagens
• They include the ionizing and non-ionizing
radiations such as X-rays, Gamma rays and
UV light

18. Physical Mutagens
• X- rays and Gamma rays causes formation of
single stranded and double stranded DNA
breakages.
• Repairing of Double stranded breakages
frequently occurs improperly and causes
mutations
• UV light causes the formation of pyrimidine
dimers leading to errors of DNA replication

19. Chemical Mutagens
• They Include:
o Base analogues
o Alkylating Agents
o Intercalating agents
o DNA reactive chemicals

20. Chemical Mutagens..
• Base analogues
• They are similar to the Normal bases and can
be incorporated in DNA strands during
replication.
• 5-Bromouracil (BU) is identical to thymine
and it can exist in a “rare” form.
• In its rare form BU base pairs with Guanine
causing a change from TA to GC base
pairing.

21. • 5-Bromouracil (BU)
is identical to
thymine and it can
exist in a “rare” form.

22. Chemical Mutagens..
• Alkylating agents
• They alter one of the DNA bases and prevent it from
forming hydrogen bonds with other bases.
• Ethylmethane sulfonate adds alkyl group to the
hydrogen-bonding oxygen in Guanine preventing it
to bond with cytosine.
• Alkylated guanine bonds with thymine instead
resulting to a TA substitution from GC

23. Chemical Mutagens..
• Intercalating Agent
• They have a similar dimension to the normal
purine-pyrimidine base pair
• They include Ethidium bromide and acridine
orange
• They can insert themselves between the
nucleotide base pairs within the DNA double
helix

24. Chemical mutagens..
• Intercalating Agents
• Replication of an intercalated DNA will result to
addition of one or more bases in the new DNA
strand or a deletion of a single base
• Addition or deletion of the bases causes a
change in the ORF therefore affecting the amino
acids.
• This causes a Frame shift Mutation.

25. Chemical mutagens
• DNA reactive chemicals
• Are chemicals that interact directly with DNA to
cause mutations
• They include Deamination agents
• Nitrous Acid causes the lose of an amino group
from Cytosine resulting to Uracil
• This causes a transition mutation from a CG
base pair to AT

26. Mutagenicity test
• Ames Test (Bacterial Reverse Mutation Assay)
• Is the test to determine the mutagenic activity of
a chemical by observing whether they can cause
mutations or not.
• It was established in 1970 by Prof. Bruce Ames
• It is quick and convenient assay compared to
others which takes 2-3 years with a lot of false
positives and negatives

27. Ames Test..
• Basics of the Ames test:
• The assays uses the mutant bacteria Salmonella
typhimurium (His-) to asses the ability of a test
chemical to cause reverse mutations to the wild type
Salmonella typhimurium (His+)
• Mutant Salmonella typhimurium 40 can not grow in
the absence of histidine amino acids while the wild
type can synthesize histidine

28. Ames test..
• Requirements:
• Positive Control (chemical known to be mutagenic)
• Negative Control (chemical known to be non
Mutagenic
• Suspected mutagenic chemical
• Mutant Salmonella typhimurium (His-)
• Rat Liver enzyme ( to mimic metabolic changes a
mutagen can experience in the body
• Histidine free medium, Colony counter

29. Ames test..
• Three petri dishes containing histidine free
media are prepared
• To plate one, and two add rat liver extract, a
positive control, and Negative control
• To plate three add the suspected mutagenic
chemical and rat liver extract
• To the plates the mutant Salmonella
typhimurium bacteria is inoculated
• Incubate and observe bacterial colonies growth

30. Ames test..
• When a chemical is mutagenic:
• The mutant bacteria colonies will be observed in
histidine free media
• This is caused by the chemical capacity to cause a
reverse mutation that enables the bacteria to regain
the ability to biosynthesize the amino acid histidine
• Ethylene dibromide, 2-aminofluorene and Safrole
are examples of Ames positive chemicals

31. Ames test.

33. Gene Mutations
• A gene mutation is a change in the genetic
material which affect one or more nucleotide
bases.
• A gene mutation is also known as a Point
Mutation
• Point Mutation is further categorized into
Base Substitution Mutations and
Frame Shift Mutations

34. Base substitution
mutations
• They are changes in the Nucleotide sequences
that substitutes one Nucleotide base for the
other.
• They can be categorized in three groups based
on the effect they have on amino acids and the
formed protein
Missense
Non sense
Silent mutation

35. Missense Mutation
• They change the nucleotide sequence such that
one amino acid is replaced by the other
• If the changed amino acid is important for the
protein to be formed, the consequence of the
mutation is more pronounced
• In sickle cell anemia GAG is changed to GTG
coding for Valine instead of Glutamic acid

36. Non sense mutation
• These mutations introduces the stop codons (
UGA,UAA and UAG) to the messenger RNA
• They result to formation of protein that are
shorter than the normal protein (Truncated
proteins)
• This makes the proteins formed to be non
functional

37. Silent Mutations
• This mutations results to a different base in the
third position of a codon.
• If the new codon formed specifies the same
amino acid hence there will be no observable
changes in the amino acid composition of a
protein

38. Frame Shift Mutations
• This are mutations which affects the Open
Reading Frame (ORF) through insertion or
deletion of a single base.
• Insertion or deletion of a base pair leads to a
change in the codons.
• Frame shift mutations leads to formation of non
functional proteins, because of a substitution of
an amino acid codon to a stop codon .

39. Frame Shift Mutations
• Sometimes in-frame insertion of three bases to the
mRNA happens.
• The impacts of in-frame insertion depends on the
properties of the coded amino acid and the
position in the formed proteins.
• In-frame deletion may lead to loss of function of a
protein as it is in Cystic Fibrosis disease.
• In Cystic Fibrosis a codon TTT for phenylalanine is
deleted hence the ATP binding site of the protein
is lost.

40. Chromosomal Mutations
• They are categorized into four
groups:
Deletions
Duplications
Inversions and
Translocations.

41. Chromosomal Mutations
• Deletions:
• Are chromosomal changes in which one or more
segment of a gene is lost.
• Most chromosomal deletion are lethal unless
they affect non essential genes.
• Deletions can be caused by breaks in the
chromosomes which results from exposure to
heat, radiation or exposure to chemicals

42. Deletions

43. Chromosomal Mutations
• Duplication
• Are chromosomal changes by which one or
more copies of a gene are present on the same
chromosome
• This mutation results to doubling of a segment of
a chromosome.
• Two homologous chromosomes may have a
deletion and duplication mutation in the non-
homologous position at the same time

44. Duplication

45. Chromosomal Mutations
• Inversion :
• This mutation occurs when a segment of a
Chromosome is cut and rotated at 180 0 and re-
inserted to the chromosome
• If the cut segment carries a part of a protein
coding , the resulting protein will be drastically
changed and non-functional.

46. Inversion Mutations

47. Chromosomal Mutations
• Translocation
• This occurs when a segment of chromosome is
cut and inserted to a different non-homologous
chromosomes.
• Reciprocal translocation occurs when two non
homologous chromosomes breaks and
exchange segments.

49. Mutations and Cancer
• Cancer is caused by accumulation of genetic
mutations in genes regulating cell division
• This leads to uncontrolled cell growth.
• Three types of genes which when mutated
causes cancer
 Genes that regulate cell proliferation
 Genes that control Apoptosis
 Genes involved in repair of damaged DNA

50. Mutations and Cancer
• These genes can be put into two broad
categories
 Proto oncogenes (growth promoting)
 Tumor suppressor genes (growth inhibiting)

51. • “BEST WISHES”