Major types of mutations
Major types of mutations
Basis of Mutation type Major features
Origin Spontaneous Absence of known mutagen
Induced Presence of known mutagen
cell type Somatic Non-reproductive cells
Germ-line Reproductive cells
expression Conditional Under restrictive conditions
Unconditional Under permissive conditions
Effect on function Loss-of-function Eliminating normal function
Hypomorphic Reducing normal function
Hypermorphic Increasing normal function
Gain-of-function Expressed at incorrect time
or in inappropriate cell type
Major types of mutations
Basis of Major types Major features
Molecular change Transition Pyrimidine (T,C) to Pyrimidine
Transversion Pyrimidine to purine (or opposite)
Insertion One or more nucleotides present
Deletion One or more missing nucleotides
Effect on Silent No change in amino acid
translation Missence Change in amino acid
Nonesense Creating stop codon
Frameshift Shift reading of codon incorrectly
• Example for conditional mutation is temperature-sensitive
Usually the restrictive temperature is high (29°indrosophila)
and the phenotype is mutant above it. The permissive
temperature is low (18° in drosophila) and the phenotype is
wildtype under it.
• loss-of-function is also called knock out or null mutation.
• A common type of frameshift mutation is a single-base
addition or deletion.
CUG CUG CUG
Leu Leu Leu
CUG CAU GCU G
Leu His Ala
CGG = codon on mRNA for Argenine (AA)
UGG = codon on mRNA for Tryptophan (AA)
wild type mutant
DNA: 5’ ……CGG…… .…..TGG……3’
DNA(T): 3’……GCC…… .…..ACC……5’
mRNA: 5’……CGG…… ……UGG......3’
So the Aa (Argenine) will be replaced by the Aa (
Tryptophan), that means mutant protein.
• X-linked form of mental retardation shows
dynamic mutations. The chromosome X
tends to fracture near the end of the long
arm and it is called fragile-X and the
disease is called fragile-X syndrome.
• This syndrome affects 1 in 2500 children
and is second only to Down syndrome as
a cause of inherited mental impairment.
• Males are more affected than females.
• The molecular basis is trinucleotide
repeat of the form CGG (or CCG on the
other strand) in the part where the
breakage takes place.
• Normal X-chromosome has 6 to 54
repeating unit. Affected persons have
230 to 2300.
• The premutation has an intermediate
number of copies ranging from 54 to
• The premutation increases in copy number
when transmitted through only females and
is called trinucleotide expansion. This occurs
not in germ line but in somatic cells of the
• Amplification to 230 copies or more causes
silence to the gene FMR1 (fragile-mental
retardation-1) which is expressed usually in
brain and testes.
• Different extent of amplification in different
somatic cells accounts for the variation in
severity of this syndrome among affected
• The molecular mechanism of trinucleotide
expansion is a process called replication
slippage (also called slipped-strand
• The mechanism of inactivation of FMR1 is
Methylation of Cytosine in the full mutation.
• FMR1 protein regulates translation of some
mRNA related to development of facial
bones and the nervous system. FMRP also
functions in learning and memory.
DNA damage can be repaired
• Mismach-repair system fixes incorrectly matched
• AP endonuclease system repairs nucleotide sites at
which the base has been lost (Apyrimidine site and
• Special enzymes repair damage caused to DNA by
• Postreplication repair skips over damaged bases.
The cell cycle is divided into three-part
interphase composed of G1 (gap1), S
(DNA synthesis), and G2 (gap2),
followed by M (mitosis).
The essential functions of mitosis are:
1) Replication of DNA once per cycle.
2) Distribution of the replicas (the sister
chromatids) equally to the two daughter
Methods to study cell cycle control
• Genetic control has been approached via
the methods of biochemistry, cell biology,
• Budding yeast, Saccharomyces cerevisiae
(S. cerevisiae) is good example for
experiments on cell cycle control.
• It has 5538 genes and we can analyze the
transcription pattern by gene microarray.
Cell division cycle (cdc) mutants
• Cdc mutant is a mutant whose phenotype
is to arrest the cell cycle at a specific point.
• Cdc mutants are wildtype at 23° ( the
permissive temperature) and unable to
complete cell cycle at 36° ( the restrictive
• Cdc13 causes arrest at the G2/M
boundary because of a defect of telomere
• So after the temperature shift, single cells
or cells with small buds will give a pair of
large cells configuration.
• Cells which were nearing the end of
division and had large buds will give
quartets after the temperature shift.
Cyclins and cyclin-CDK
• In the early stages of cell cycle,
progression from one phase to the next is
controlled by characteristic protein
complexes that are called cyclin-CDK
• They are composed of cyclin subunits
combined with cyclin-dependent protein
kinase (CDK) subunits.
Routes of regulation
• When cyclin subunit binds to the protein
substrate, the CDK component
phosphorylates the substrate.
• Phosphorylation activates some proteins
and inactivate others.
• Phosphorylation of different site might
inactivate the same activated protein.
Phosphatase enzymes dephosphorylate
proteins that CDKs have phosphorylated,
reversing the effects of CDKs.
Some important Cyclin-Cdks
Cyclin D-Cdk4 and cyclin D-Cdk6complexes
appear in the early or middle part of G1.
They promote entry into S phase.
Cyclin A-Cdk2 and cyclin E-Cdk2 appear
later in G1.
Cyclin B-Cdk2 complex carries the cell
through the G2/M transition.
Retinoblastoma (RB) protein
The normal role of RB is to maintain cycling
cells at a point in G1 called the G1
restriction point or start until the cell has
attained proper size.
RB binds to the transcription factor E2F
which is needed for further progression in
the cell cycle.
RB is phosphorylated by D-Cdk4,6, E-Cdk2
kinase as cells approach G1/S transition.
RB phosphorylation eliminates its ability to
bind the E2F transcription factor.
Release of E2F results in transcription of
enzymes responsible for DNA synthesis
and E2F itself (as positive autoregulation).
Through S-phase, E2F is phosphorylated by
cyclin A-Cdk2 and inhibits it binding to
DNA, so inactivating its function as a
Cyclin B-Cdk2 complex (also called
maturation promoting factor) controls the
progression from G2 to M transition.
Protein degradation (proteolysis)
Cell cycle go forward by protein degradation
that complements the periodic activation
of cyclin-CDK complexes.
Progression of cell cycle needs destruction
of previous proteins.
Protein degradation entails:
-Sister chromatids to separate.
-Cell to exit from mitosis.-
Checkpoints allow repair or death
A DNA damage checkpoint. (G1/S
A centrosome duplication checkpoint. (G2/M
A spindle checkpoint. (Anaphase
P53 for DNA damage checkpoint
DNA damage checkpoint acts via three
stages in the cell cycle:
P53 transcription factor
It is responsible to stress in general, and to
DNA damage in particular.
In normal cells, activated p53 protein level is
very low. However, p53 mRNA and p53
protein are present.
P53 must go phosphorylation and
acetylation to be active .
Mdm2 and p53
The protein Mdm2 inactivates p53 by
preventing it from phosphorylation and
subsequent steps of its activation.
Activated P53 triggers the transcription of a
number of genes and the repression of
Genes activated by p53
14-3-3 σ which plays a role to arrest cell at
P21 which plays a role at G1/S transition
and S checkpoint.
Apaf1 and Bax which promote apoptosis.
Maspin which inhibits angiogenesis
(formation of blood vessels) and
Means programmed cell death. (suicide)
DNA damage also triggers activation of the
pathway of apoptosis.
A cascade of proteins involved in the lysis of
the cell initiate suicide.
They are called caspases.
Oncogene is a gain-of-function mutation in a
cellular gene, called proto-oncogene,
whose normal function is to promote
proliferation or inhibit apoptosis;
oncogenes are often associated with
Oncogenes and Bcl2
Oncogenes can increase the level of
activated (phosphorylated) Bcl2, which
prevents apoptosis and allows the affected
cells to grow and divide indefinitely.
Tumor suppressor genes
Tumor suppressor gene is a loss-of-function
mutation in a cellular gene, whose normal
function is to inhibit cell division or to
Tumor suppressor genes
Tumor suppressor alteration consequence
p53 mutation Loss of G1/S, S, and
p21 mutation Loss of G1/S and S
RB mutation Promote proliferation;
Bax mutation Failure of apoptosis
Centrosome duplication checkpoint
and the spindle checkpoint
Centrosome is the organelle around which
the bipolar spindle is organized.
Failure in centrosome duplication checkpoint
might result in polyploidy.
Failure in spindle checkpoint might cause
APC is the anaphase-promoting complex
needed for entry into anaphase.
Correct attachment to the spindle causes
inactive spindle checkpoint (inactive Bub,
Mad, and Mps) and so, active APC.
Incorrect or unbalanced attachment to the
spindle activates spindle checkpoint and
so, inactive APC.
Loss of heterozygosity
Loss of the presence of the wildtype allele,
or loss of its function, in a heterozygous
cell, enabling the phenotype of a recessive
mutant allele to be expressed.