Black Chapter 8-Gene transfer and genetic Engineerings

hollandmercifulΒιοτεχνολογία

11 Δεκ 2012 (πριν από 4 χρόνια και 10 μήνες)

176 εμφανίσεις

Black Chapter 8
-
Gene transfer and genetic Engineerings


THE TYPES AND SIGNIFICANCE OF GENE TRANSFER

Gene transfers refers the movement of genetic information between organisms.

Vertical gene transfer
passes genes from parent to offspring
.

Lateral gene tran
sfer

passes genes to other cells in the same generation. It occurs in
bacteria by transformation, transduction, and conjugation.

Gene transfer is significant because iy increases genetic diversity within a population,
thereby increasing the likelihood that

some members of the population will survive
environmental changes.


TRANSFORMATION

The discovery of T
ransformation

Bacterial transformation was discovered in 1928 by Griffith, who showed that a mixed
culture of live rough and heat
-
killed smooth pneumococc
i could produce live smooth
pneumococci capable of killing mice.

Avery later showed that a capsular polysaccharide was responsible for virulence and that
DNA was the substance responsible for transformation Watson and Crick determined the
structure of DNA,

which led to studies showing that a cell’s genetic information is
encoded in its nucleic acids.


The Mechanism of T
ransformation

Transformation involves the release of naked DNA fragments and their uptake by other
cells at a certain stage in their growth
cycle;

(1)

Uptake of DNA requires a protein called
competence factor
to make recipient
cells ready to bind DNA.

(2)

Endonucleases cut double
-
stranded DNA uinto units; the strands separate, and
only one strand is transferred.

(3)

Ultimately, donor DNA is spliced into
recipient DNA. Leftover recipient DNA is
broken down, so a cell’s total DNA remains constant.


The Significance of Transformation

Transformation is significant because

(1) it c
ontributes to genetic diversity


(2) it can be used to introduce DNA into an or
ganism, observe its effects, and study gene
locations;

(3) it can be used to create recombinant DNA.


TRANSDUCTION

The Discovery
of
Transduction

In transduction, genetic material is carried by
bacteriophage

(phage)
.


The Mechanism of Transduction

Phages c
an be virulent of temperate.

(1).
Virulent
phages

destroy a host cell’s DNA, direct synthesis of phage particles, and
cause lysis of the host cells in the
lytic cycle.

(2).
Temperate phages

can replicate themselves as a
prophage
-
part of a bacterial
chromo
somes
-
or eventually produce new phage particles and lyse the host cell.

Persistence of the phage in the cell without the destruction of the host cell is called
lysogeny.

Prophage
can be incorporated into the bacterial chromosome, or it can exist as a plasm
id,
a piece of extrachromosomal DNA. Cells that containsa prophage are called
lysogenic
cells because they have the potential to enter the
lytic cycle.


Transduction
can be specialized or generalized.


(1) In
specialized transduction
, the phage is incorpor
ated into the chromosome and can
transfer only genes adjacent to the phage.


(2) In
generalized transduction
, the phage exists as a plasmid and can transfer any
DNA fragments attached to it.


The Significance of T
ransduction

Transduction is significant bec
ause it transfers genetic material and demonstrates a close
evolutionary relationship between prophage and host cell DNA. Also, its persistence in a
cell suggests a mechanism for the viral origins of cancer, and it provides a possible
mechanism for studyin
g gene linkage.


CONJUGATION

The Discovery of Conjugation

In
conjugation

large quantities of DNA are transferred from one organism to another
during contact between donor and recipient cells.

Conjugation was discovered by L
ederberg in 1946 when he observed

that mixing strains
of E. coli with different metabolic deficiencies allowed the cells to overcome
deficiencies.

Plasmids
are extrachromosomal DNA molecules.


The Mechanisms of Conjugation

Three mechanisms of conjugation have been observed:

(1) In the tra
nsfer of
F plasmids
, a piece of extrachromosomal DNA (a
plasmid
) is
transferred.

(2). In
high
-
frequencccy recombinations

, parts of F plasmids that have been
incorporated into the chromosome (
the initiating segment
) are transferred along with
adjacent bact
erial genes.

(3) An
F


plasmid

incorporated into the chromosome and subsequently separated
becomes an
F


plasmid

and transfers chromosomal genes attached to it.


The Significance of Conjugation

The significance of conjugation is that it increases genetic

diversity, it may represent an
evolutionary stage between asexual and sexual reproduction, and it provides a means of
mapping genes in bacterial chromosomes.

Promiscuous

-

those can transfer to other species.



Gene Transfer
Mechanisms

Compared

Genetic t
ransfer mechanisms differ in the quantity of DNA transferred.


PLASMIDS

Characteristics of Plasmids

Plasmids are circular, self
-
replicating, double
-
stranded extrachromossomal DNA that
carry information that is usually not essential for cell growth.


Resist
ance Plasmids

Resistance (R) plasmids
carry genetic information that confers resistance to various
antibiotics to certain heavy metals. They generally consist of a
resistance transfer factor
(RTF)
and one or more
Resistance (R) genes.


Tra
n
sposons

R genes
that move from one plasmid to another in a cell or become inserted in the
chromosome are part of a
transpons

because they transpose, or change, their locations.


Bacteriocinogens

Bacteriocinogens

are plasmids that produce
bacteriocins
, which are proteins t
hat inhibit
growth of other strains of the same species or closely related species.


GENETIC ENGINEERING

Genetic Engineering
is the

manipulation of genetic materia
l to alter the characteristics of
an organism.


Genetic
Fusion

Genetic fusion allows transpos
ition of genes from one location on a chromosome to
another, sometimes deleting a portion, thereby causing the jointing of genes from two
different operons.


Protoplast Fusion

Protoplast Fusion

combines
protoplast
s
(organisms without cell walls)

and allows

mixing of genetic information.


Gene Amplication

Gene amplication
involves the addition of plasmids to microorganisms to increase yield
of useful substance.

Recombinant DNA Technology

Recombinant DNA
is DNA produced when genes from one kind of organism ar
e
introduced into the genome of a different kind of organism. The resulting organism is a
transgenic
, or recombinant organism.

Recombinant DNA has proven especially useful in medicine, industry, and agriculture.

1).The donor segments are incorporated into
a
vector (plasmid
)

2) It can be introduced into cells by heating in a solution of calcium chloride or by
electroporation
-

use electrical pulses to produce pores in the cell

membrane
.


3).

Restriction endonuclease (restriction enzymes)

cut specific DNA sequ
ences

create
restriction fragment.

4). Restriction Fragment length polymorphisms (RFLPs)
DNA sequences can be
slightly different between members of the same species.


Medical Applications of R
ecombinant DNA

1). To make bacterial cells produce human protein
s, a human DNA (synthesize the
protein) is inserted into vector. Interferon (treat viral diseases and cancer) and insulin,
human growth hormone,
vaccines can be produced.

2). Diagnosis of genetic defects in a fetus by studying enzymes in fetal cells from
a
mniotic fluid. By using recombinant DNA with a known nucleotide sequence to find
errors in the nucleotide sequence in fetal DNA segments.

3). Gene therapy : inserting the gene in a defective gamete (egg or sperm) might prevent
offspring from inheriting a
genetic disease, like in severe combined immunodeficiency
disease.

4). Make a blood substitute

Inject copies of human hemoglobin genes into 1
-
day
-
old pig embryos that had been
removed from their mother’s uteri.

The embr
y
os were then implanted into a second

pig’s
uterus to grow until delivery.

Two days after birth, the piglets were tested to see whether
they produced human hemoglobin with pig hemoglobin (transgenic, 0.5% succeed).The
transgenic pigs produced blood containing 50% human hemoglobin.

The pigs ar
e bled and red
blood cells are ruptured and pur
ified.

It has storage life of
months instead weeks.
Because naked hemoglobin does not have any antigens like red
blood cells, it can be transfused without blood typing. It can ensure safety from human
pathogen
s, and use as immediate sources of oxygen during disaster.


Industrial Application of
Recombinant

DNA

Fermentation processes used in making wine, antibiotics
might be greatly improved by
the use of recombinant DNA.


Agricultural Application of recombinant
DNA

Some bacteria are being engineered to control insects that destroy crops.

Pseudomonas fluorescens

(colonizes the roots of corn)

has been induced to carry genetic
information inserted into it from Bacillus thuringiensis, allowing P.fluorescens to
synthe
size a protein that kills insects. Toxins made by B.thuringiensis has been extracted
and used as insecticide.

Pseudomonads could replace the use of chemicals insecticides to
control black cutworm and other insect larvae that damage crops.



Hybridomas

Fusi
on
of a myeloma (bone marrow cancer) cell with an antibody
-
producing white blood
cells. It can produce monoclonal antibodies.


Weighing the Risks and Benefits of Recombinant DNA

When recombinant DNA techniques were first developed, scientist

were concerne
d that
v
irulent pathogens might be created, and they developed containment procedures. As
research proceeded and no illness caused by recombinants were observed, most scientists
came to believe that the benefits of recombinant DNA techniques outweigh the risks.