Lecture No 6

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Dec 14, 2012 (4 years and 6 months ago)

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NAJRAN UNIVERSITY

College of Medicine

Microbiology &Immunology Course

Lecture No.
6

By

Dr. Ahmed
Morad

Asaad

Associate Professor of Microbiology

II
-

Genotypic

Variations :


These
are permanent
(irreversible)

variations, which are heritable,
i.e. they will be transmitted among generations. They may be due
to
:


1
-

mutation

2
-

gene transfer

Genetic transfer

Bacterial DNA may be transferred within or between bacterial
cells.



A.
Transfer of DNA within bacterial cells:


Transposition:

Transposons

or jumping genes can move from one site in a DNA
molecule to other target chromosomal or plasmid sites in, the
same or a different DNA molecule. This process is termed
transposition and it results in insertion or deletion mutations.



B. Transfer of DNA between bacterial cells:

Bacterial DNA may be transferred between bacterial cells
by three mechanisms:


transformation
,
transduction
,
and
conjugation
.

Transformation:


The

transfer

of

DNA

from

one

cell

to

another

by

either

of

2

mechanisms
:


1
-

In

nature,

dying

bacteria

may

release

their

DNA

which

may

be

taken

by

another

cells


2
-

In

the

lab,

DNA

may

be

extracted

from

one

bacterial

cell

into

another

one

Transduction:


Transfer

of

bacterial

DNA

by

means

of

a

bacterial

virus

(
bacteriophage
)
.


2

types

of

transduction
:

generalized

and

specialized


Generalized

transduction
:


This

occurs

when

the

bacterial

virus

carries

a

segment

from

any

part

of

the

bacterial

chromosome
.

This

occurs

because

cell

DNA

is

fragmented

after

phage

infection

and

a

piece

of

DNA

is

incorporated

into

the

virus
.

Specialized

transduction:


This

occurs

when

the

bacterial

virus

that

has

integrated

into

the

cell

DNA

is

excised

and

carries

with

it

an

adjacent

part

of

the

cell

DNA

Conjugation:


1
-

It

is

the

mating

of

2

bacterial

cells

during

which

DNA

is

transferred

from

the

donor

to

recipient

cell
.



2
-

The

mating

process

is

controlled

by

F

(fertility)

plasmid

(F

factor)

which

carries

genes

for

synthesis

of

pilin

and

form

sex

pilus
.


3
-

Mating

begins

when

the

sex

pilus

of

donor

male

bacteria

carrying

F

factor

(F+)

attach

to

to

recipient

female

bacteria

(F
-
)
.


4
-

Cleavage

of

F

factor

DNA
:

one

strand

to

recipient

cell
.

The

process

is

completed

by

synthesis

of

a

complementary

strand

in

each

cell


Genetic engineering


*
-

A

method

to

isolate

genes

coding

for

certain

properties

and

join

them

together

to

form

new

combinations
.

*
-

Also

called

genetic

recombination,

recombinant

DNA

technology,

DNA

cloning
.

*
-

Major

3

steps

(It

requires)
:

1
-

Separation

of

required

gene

(by

restriction

endonuclease
)
.

2
-

Carrying

this

gene

by

a

vector

3
-

Introducing

the

gene

into

a

host

cell

(by

transformation)

Restriction

endonucleases
:

*
-

Enzymes

from

bacteria

and

fungi

that

can

recognize

and

cut

DNA

fragments

(genes)

at

specific

sites

Vectors
:

1
-

Plasmids

2
-

Bacteriophage

3
-

Cosmids
:

circular

double
-
stranded

DNA

molecule

constructed

from

plasmid

DNA+phage

DNA
.

They

carry

large

genes

4
-

Retroviruses

and

adenoviruses

Recombinant

DNA

technique
:

1
-

Chromosomal

DNA

is

extracted

and

cleaved

by

Restriction

endonuclease

which

cut

at

specific

sites

to

separate

the

required

fragment

containing

the

required

gene

(insert)

2
-

The

vector

(e
.
g
.
,

plasmid)

is

cleaved

by

the

same

step

3
-

The

insert

+

vector

are

mixed

under

certain

conditions
.

This

results

in

recombinant

plasmid

=

plasmid+insert

4
-

By

transformation,

the

recombinant

plasmid

is

introduced

into

a

suitable

host

(bacteria

or

yeast

cell)

which

can

replicate

autonomousely

Applications

of

recombinant

DNA

technology
:

1
-

Extensive

chromosomal

and

genes

studies

2
-

Preparation

of

probes

for

diagnostic

purposes

3
-

Production

of

proteins

of

medical

importance

(large

amount

+

low

coast)

4
-

production

of

recombinant

vaccines

5
-

Gene

therapy

(virus

vectors)