Download Mutations

Microbial Genetics

In bacteria genetic transfer (recombination) can
happen three ways:
 Transformation
 Transduction
 Conjugation

The result is a recombinant cell that has a
genome different from either the donar or the
recipient.
A segment of DNA is
transferred from one
bacterial cell to another
by a bacterial virus
called “bacteriophage”
or phage.
Once the
bacteriophage”
attaches to the
bacterial cell wall, it
injects its nucleic acid
into host cell.
A phage enzyme is
produced that breaks
down the host DNA
into small fragments
Phage DNA is
replicated and phage
coat proteins are
produced
During the formation of
mature phage
particles, a few phage
heads surround
fragments of bacterial
DNA instead of phage
DNA.
The phage particles
carrying the bacterial
DNA infects another
bacterial cell,
transferring the
bacterial DNA to a new
cell.
The bacterial DNA
being injected to a new
host cell
When the bacterial
DNA introduced into a
new host cell, it can
become integrated into
bacterial chromosome
, thereby transferring
genes to the recipient.
Microbial Genetics
Genetic Variations and Mutations

Roughly 99.9 % of human genomes (3.2
billion bases) are the same between any two
people. Amazing!!!!!!!!!!!

The remaining tiny fraction of the genome, 0.1
% (several million bases)-makes a person
unique. This small amount of variation
determines how a person looks, or the
diseases he or she develops.

Heritable variation
within and between
populations of
organisms

Where does genetic
variation come from?

Mutations (ultimate
sources of all genetic
variations)

Recombination
of chromosomes that
occurs during sexual
reproduction

Permanent change in the DNA sequence
Outcome depends on:

What gene(s) is (are) affected

where in the gene the change occurs, (i.e., in
the coding or non-coding region)

the exact nature of the change.

Most mutations have no known effect at all
because they occur in non-coding regions of the
DNA
In addition, there are some mutations that do
occur in coding regions of DNA, yet they have
no known effect
All these are silent mutations

Some of the mutations that occur in the
coding regions of genes have "harmless"
effects.

They can, for example, change the way a
person "looks." Some people have blue eyes,
others brown; some are tall, others short; and
some faces are oval, others round.



There are a group of mutations in coding
regions that result in harmful effects.
They cause disease because changes in the
genome's instructions alter the functions of
important proteins that are needed for
health.
For example, diabetes, cancer, heart disease,
and hemophilia all result from mutations that
cause harmful effects.

There are genetic mutations that have
"latent" effects. These variations, found in
coding regions, are not harmful on their own,
However, such mutations cause some people
to be at higher risk for some diseases such as
cancer, but only after exposure to certain
environmental agents. They may also explain
why one person responds to a drug treatment
while another does not.

Finally, there are genetic mutations that have
“Helpful" effects. These variations, usually
induced by scientist either to study a
particular gene or correct abnormal gene.
This is called “gene therapy”.

Spontaneous Mutations:
 occur in the natural environment without the
addition of mutagens (agents
mutations)
 Occur randomly and spontaneously

that
cause
Induced Mutations:
 Mutations that are created by the addition of
mutagens

Two types:

DNA Mutations: affect one base pair in the
DNA

Chromosomal Mutations: affect
section of DNA on the chromosome
entire

Point
mutation
(single
nucleotide
polymorphism): a mutation that alters ONE
base of DNA sequence

Types of point mutations:
 Substitution
 Deletion
 Insertion

Most common type of point mutation

Mistake during DNA replication, incorrect
base incorporated into DNA
Point mutation:
Substitution
Silent mutation
The substitution results in a
codon that codes for the
SAME amino acid as the
original.
Therefore
the
protein
structure and function is not
altered.
Point mutation:
Substitution
Missense mutation
A base substitution results
in a different codon.
Therefore
a
different
amino acid is coded for.
This can alter the structure
and function of the overall
protein.
like in Sickle Cell Anemia.
Point mutation:
Substitution
Nonsense mutation
The substitution results in
the original codon being
converted into a STOP
codon
This does not “make
sense” to the translation
machinery so translation
STOPS
This results in a truncated
(shortened) protein

Insert or delete a nucleotide- very disastrous

Shifts codons of DNA when transcribed into
RNA (also called frame shift mutation)

All nucleotides downstream of mutation will
be grouped into improper codons, and wrong
amino acids will be added

Protein will be non-functional

Chromosomal mutations: permanent changes in
the DNA that alter the chromosome itself
•
Types
•
Deletion
•
Duplication
•
Inversion
•
Translocation

The loss of a portion of a chromosome
Deletion in chromosome 5



Cri-du-chat syndrome
Infant cries like a cat
Respiratory problems,
early death

A portion of a
chromosome
is
duplicated (copied)
more than once

Can be detrimental
if it occurs within a
gene region

A segment of a chromosome is broken in two
places, reversed, and ligated back together

Detrimental if it occurs in the middle of a
gene

A piece of a chromosome is
broken off and joined to a
DIFFERENT chromosome

They can cause problems
during metaphase of meiosis I
with homologous pairing

Can change the expression of
genes
 Some leukemias can result from
translocations

Mutations are induced by either certain
chemical mutagens or physical mutagens

Sometimes scientists intentionally mutate
DNA to study it

Mutagens are agents or substances can cause
mutations.

Example: nitrous acid (HNO2)

Converts adenine so it no longer pairs with
thymine

Instead pairs with cytosine

Compounds that resemble bases closely
Repair of
thymine
dimers