Mutations ariane

Which picture do you think is a case of
Mutation?

Key Idea:
Mutation:
• Changes in DNA that affect genetic
information

Objectives:
• Determine the different types of Mutations
and how it affects the gene expression of an
organism.
• Determine the different mutagens and how it
affects the proteins during translation phase.
• Explain the concept and mechanisms of
transposons as a one of the factors of
Mutation

Protein Synthesis and Mutation
•
•

Gene Regulation in Prokaryotes
Mutation(Permanent, heritable DNA changes)
– Point mutation (base substitutions)
• Missense mutation
• Nonsense mutation (premature stop)
• Silent mutation
– Insertions/deletions
• Frameshift mutation
– Dramatic change in amino acids
– Run-ons, premature stops (nonsense mut.)
•
The Creation of Mutation (mutagenesis)
– Spontaneous mutation
• Occurs in DNA replication (1 in 109 bp)
– Chemical mutagens
• Base pair changers (nitrous acid)
• Base analogues (e.g.. 5 bromouracil)
• Frameshift mutagens (aflatoxin, benzpyrene)
– Radiation
• X rays, gamma rays break DNA, bases
• UV light causes knots in DNA strand
Proteins are made by translation of genetic instructions by a ribosome. Mutations in the genetic instructions usually
changes the resultant protein with deleterious effects.

Mutation: Some Definitions
•
•
•
•

A heritable change in the genetic material
Mutations may be neutral, beneficial, or harmful
Mutagen: Agent that causes mutations
Spontaneous mutations: Occur in the absence of a
mutagen

Mutation: Base Substitution (Point
Mutations)

G
G
C
C

Glu
(a) Silent mutation
(d) Run-on mutation

Regulation of Bacterial Gene Expression
•

Steps in Translation of mRNA
– Initiation, Elongation, Termination
•
• Silent mutation
•
– Radiation

Mutation: Insertions and Deletions

THEBIGCATATETHERAT
THEBIGCBATATETHERAT

Figure 8.17a, d

Summary of Mutation Types

Run-on mutation
Stop codon lost so
protein is extra long

(can also produce
nonsense and run-ons)

Protein Synthesis and Mutation
•

Steps in Translation of mRNA
– Initiation, Elongation, Termination
•
• Silent mutation
•
– Radiation

What causes MUTATION?
• Reasons
1. Spontaneous errors in DNA replication or
meiotic recombination

2. A consequence of the damaging effects of
physical or chemical mutagens on DNA

Errors in Meiosis (Dr.RS)
• Errors during DNA Replication
– Errors during meiosis introduce variation in the DNA sequence. The
effect of this variation depends on a number of factors:
• The size of the variant. It may be as small as a change to a single
base or as large as the rearrangement of whole chromosomes.
• The pathogenicity of the variant. It may have no effect on gene
function or may severely disrupt the function of the gene.
– Some variants, such as single nucleotide polymorphisms (SNPs) are
relatively common in the population. In other
cases,mutations responsible for rare genetic conditions may be
specific to an individual family.

• Errors during Recombination
– Recombination involves the 'swapping' of genetic material between both
chromosomes of the same pair (homologous chromosomes). They align sideby-side and pair exactly, break, swap lengths of DNA and rejoin.
– If chromosomes of the same pair misalign, the exchange of material may
result in duplications (extra genetic material) or deletions (missing genetic
material).
– If a chromosome mispairs with a chromosome from a different pair (nonhomologous chromosomes) the exchange of material will lead to a
chromosome translocation. This may involve the swapping of material
(a reciprocal translocation) or result in chromosomes becoming stuck
together end-to-end (a Robertsonian translocation).
– Translocations may lead to gene dosage problems. Extra copies of genes can
cause disease through overexpression, resulting in disrupted cell
function. Loss of genetic material may lead to the cell missing copies of
genes essential to its activity.

Errors during segregation
• Errors can occur when the chromosomes segregate into the
gametes during meiosis resulting in egg or sperm with too
many or too few chromosomes.
• As a result, fertilised eggs and the ensuing embryos may
have trisomy (an extra chromosome of a particular pair) in
each cell or monosomy (one chromosome fewer in each
cell).
• For example in trisomy 21 Down syndrome, cells have an
extra copy of chromosome 21 (trisomy 21). In Turner
syndome cells have only one X chromosome (monosomy X)
and no Y chromosome as shown in the following karyotype.

Spontaneous and Induced Mutation

• Spontaneous mutation rate = 1 in 109 (a billion)
replicated base pairs or 1 in 106 ( a million) replicated
genes. Mistakes occur during DNA Replication just
before cell division. This is natural error rate of DNA
polymerase.
• Mutagens increase mistakes to to 10–5 (100 thousand)
or 10–3 ( a thousand) per replicated gene

Effects of Spontaneous Mutation
• Congenital Heart Disease
• Mental Retardation

Mutagen
• Mutation relevant

• Cause DNA damage that can be converted to
mutations.

1 Mutaagenesis

Physical mutagens
High-energy ionizing radiation: X-rays and g-rays

strand breaks and base/sugar destruction
Nonionizing radiation : UV light

pyrimidine dimers

Chemical mutagens

Base analogs: direct mutagenesis
Nitrous acid: deaminates C to produce U
Alkylating agents
Intercalating agents

Lesions-indirect mutagenesis

Chemical Mutagens
Base pair altering chemicals (base
modifiers) deaminators like
nitrous acid, nitrosoguanidine,
or alkylating agents like cytoxan
cytoxan

Nitrous acid

Base analogues “mimic”
certain bases but pair with
others - E.g. 5fluorouracil, cytarabine

Acts like a “C”
cytarabine

Deaminating Agent
• *Deaminating agent - Nitrous acid - removes the anime
group from Adenine and Cytosine

• Nitrous acid is a deaminating agent that converts cytosine
to uracil, adenine to hypoxanthine, and guanine to
xanthine. The hydrogen-bonding potential of the modified
base is altered, resulting in mispairing.

Alkylating agents
• Alkylating agents like EMS/MMS(ethyl/methly
methyl sulphonate) add methyl groups to
Guanosine . Bulky attachment to the side
groups or bases.

Hydroxilating Agents
• Addition of OH (Hydroxyl Group)
hydroxylamine (HA)

Intercalating Agents
• Intercalation agents are compounds that can
slide between the nitrogenous bases in a DNA
molecule.
• This tends to cause a greater likelihood for
slippage during replication, resulting in an
increase in frameshift mutations.
• Example (Sodium Azide)

Chemical Frameshift Mutagens Intercalate into DNA

Carboplatin
(anti-cancer drug)
Benzpyrene in
cigarette smoke

Aflatoxin from
Aspergillus fungus
growing on corn

AT
GC
TA
GC
CG

AT
GC
TA
GC
CG

AT
GC
CG
TA
GC
CG

Daunarubicin
(anti-cancer drug)

Bleomycin (anti-cancer
drug produced by
Streptomyces)

Mutation: Ionizing Radiation

• Ionizing radiation (X rays, gamma rays, UV light) causes
the formation of ions that can react with nucleotides and
the deoxyribose-phosphate backbone.
• Nucleotide excision repairs mutations

X-rays and Gamma Rays Cause Breaks
in DNA

Ionizing Radiation: UV
• UV radiation
causes thymine
dimers, which
block replication.
• Light-repair
separates thymine
dimers
• Sometimes the
“repair job”
introduces the
wrong nucleotide,
leading to a point
mutation.
Figure 8.20

Transposable elements in eukaryotes:
Barbara McClintock (1902-1992)
Cold Spring Harbor Laboratory, NY
Nobel Prize in Physiology and Medicine 1983
“for her discovery of mobil genetic elements”
•

Studied transposable elements in corn (Zea mays) 1940s-1950s
(formerly identified as mutator genes by Marcus Rhoades 1930s)

•

Also known for work demonstrating crossing over as part of the chromosomal basis
of inheritance.

McClintock’s discovery of transposons in corn:
•

c/c = white kernels and C/- = purple kernels

•

Kernal color alleles/traits are “unstable”.

•

If reversion of c to C occurs in a cell, cell will produce purple pigment and a spot.

•

Earlier in development reversion occurs, the larger the spot.

•

McClintock concluded “c” allele results from a non-autonomous transposon called
“Ds” inserted into the “C” gene (Ds = dissassociation).

•

Autonomous transposon “Ac” controls “Ds” transposon (Ac = activator).

21.1 Introduction
 transposon (transposable element) : a DNA sequence able to
insert itself at a new location in the genome, without having any
sequence relationship with the target locus.

 Transposons fall into two general classes: transposons and
retrotransposons.
 Transposons that mobilize via DNA are found in both prokaryotes
and eukaryotes. A genome may contain both functional and
nonfunctional (defective) elements. Often the majority of elements in
a eukaryotic genome are defective. A eukaryotic genome contains a
large number and variety of transposons. The fly genome has >50
types of transposon, with a total of several hundred individual
elements.
 Transposable elements confer neither advantage nor disadvantage
on the phenotype, but could constitute “selfish DNA,” concerned only
with their own propagation.

Figure 21.1 A major cause
of sequence change within
a genome is the movement
of a transposon to a new
site. This may have direct
consequences on gene
expression. Unequal
crossing-over between
related sequences causes
rearrangements. Copies of
transposons can provide
targets for such events.

 insertion sequences (IS) : the simplest small bacterial transposon,
each of which codes only for the proteins needed to sponsor its own
transposition.
 inverted terminal repeats : the short related or identical sequences
present in reverse orientation at the ends of some transposons.
 direct repeats : identical (or closely related) sequences present in two
or more copies in the same orientation; they are not necessarily
adjacent.
 transposase : the enzyme involved in insertion of transposon at a new
site.

Figure 21.2 Transposons
have inverted terminal
repeats and generate direct
repeats of flanking DNA at
the target site. In this
example, the target is a 5 bp
sequence. The ends of the
transposon consist of
inverted repeats of 9 bp,
where the numbers 1
through 9 indicate a
sequence of base pairs.
the most common
length is 9 bp

 replicative transposition : the element is duplicated during the
reaction, so that the transposing entity is a copy of the original
element (Figure 21. 6).
 resolvase : the enzyme involved in site-specific recombination
between two transposons present as direct repeats in a cointegrate
structure.
 nonreplicative transposition : the transposing element moves as a
physical entity directly from one site to another and is conserved
(Figure 21. 7).
 conservative transposition : another sort of nonreplicative event, in
which the element is excised from the donor site and inserted into a
target site by a series of events in which every nucleotide bond is
conserved (Figure 21. 8).

Figure 21.5 The direct
repeats of target DNA
flanking a transposon are
generated by the
introduction of staggered
cuts whose protruding
ends are linked to the
transposon.

Figure 21.6 Replicative transposition creates a copy of the transposon,
which inserts at a recipient site. The donor site remains unchanged, so
both donor and recipient have a copy of the transposon.
Requires resolvase as well as transposase

Fig. 7.20, Integration of IS element in chromosomal DNA.

Host repair
system required

Figure 21.7 Nonreplicative transposition allows a transposon to
move as a physical entity from a donor to a recipient site. This leaves
a break at the donor site, which is lethal unless it can be repaired.
Requires only a transposase

Figure 21.8 Conservative transposition involves
direct movement with no loss of nucleotide bonds;
compare with lambda integration and excision.
Resembles the mechanism of lambda integration

Models of transposition:
•

Similar to that of IS elements; duplication at target sites occurs.

•

Cointegration = movement of a transposon from one genome (e.g.,
plasmid) to another (e.g., chromosome) integrates transposon to
both genomes (duplication).

•

Transposition may be replicative (duplication) or non-replicative
(transposon lost from original site).

•

Result in same types of mutations as IS elements: insertions,
deletions, changes in gene expression, or duplication.

•

Crossing-over occurs when donor DNA with transposable element
fuses with recipient DNA.

Control of Transposons
• Autoregulation: Some transposases are
transcriptional repressors of their
own promoter(s)
• e.g., TpnA of the Spm element

• Transcriptional silencing: mechanism not
well understood but correlates
with
methylation of the promoter (also
methylation of the IRs)

Biological Significance of Transposons
• They provide a means for genomic change and
variation, particularly in response to stress
(McClintock’s "stress" hypothesis)
(1983 Nobel lecture, Science 226:792)

• or just "selfish DNA"?
• No known examples of an element playing a
normal role in development.

• Transposable elements cause genetics changes
and make important contributions to the
evolution of genomes:
• Insert into genes.
• Insert into regulatory sequences; modify gene
expression.
• Produce chromosomal mutations.

Mutations ariane

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