Chapter 9: Gene Transfer, Genetic Engineering, and Genomics

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11 Δεκ 2012 (πριν από 4 χρόνια και 8 μήνες)

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Chapter 9

Outline

Gene Transfer, Genetic Engineering, and Genomics


Introduction

9.1 Genetic Recombination in Prokaryotes



Genetic Information in Prokaryotes Can Be Transferred Vertically and
Horizontally



Vertical gene transfer (VGT) is the transfer of gen
etic material from
parent cell to daughter cell



Horizontal gene transfer (HGT) is the transfer of DNA from a donor cell
to a recipient cell



Transformation Is the Uptake and Expression of DNA in a Recipient Cell



By integration of a new DNA fragment, the rec
ipient has gained some
ability it previously lacked



Transformation was first described by Frederick Griffith in 1928



Competence is the ability of a recipient cell to take up DNA from the
environment



Conjugation Involved Cell
-
to
-
Cell Contact for Horizontal
Gene Transfer



In conjugation, a donor cell transfers DNA directly to the recipient



The donor cell forms a conjugation pilus to make contact with the
recipient



The F factor DNA in the donor replicates by the rolling
-
circle mechanism



Conjugation also Can Tra
nsfer Chromosomal DNA



High frequency of replication (Hfr) strains can donate chromosomal genes
rather than just the F plasmid



The F factor attaches to the chromosome using an insertion sequence



Conjugation is usually interrupted before the entire chromosom
e is
transferred,



The recipient remains F
-

(called a recombinant F
-
)



If an integrated F plasmid breaks from the chromosome, taking a fragment
of chromosomal DNA, it is called an F' plasmid



Transduction Involves Viruses as Agents for Horizontal Transfer of

DNA



In transduction, a virus (bacteriophage) carries a chromosomal DNA
fragment from donor to recipient



In the lytic cycle, virulent phages



destroy the host chromosome



replicate themselves



destroy the cell



In the lysogenic cycle, temperate phages integrat
e their DNA into the host
chromosome (as a prophage)



Virulent phages perform generalized transduction



A fragment of host cell DNA ends up in the phage during
packaging, which they transfer to a new host cell




In the lysogenic cycle, the prophage eventuall
y excises itself from the host
chromosome




Sometimes it takes a few flanking host genes and leaves a few
phage genes behind

9.2 Genetic Engineering and Biotechnology



Genetic Engineering Was Born from Genetic Recombination



Genetic engineering involves chang
ing the genetic material in an organism
to alter its traits or products



A recombinant DNA molecule contains DNA fragments spliced together
from 2 or more organisms



Specific fragments can be obtained by cutting short stretches of
nucleotides with a restric
tion endonuclease



The fragments are joined by DNA ligase



Genetic Engineering Has Many Commercial and Practical Applications



The genes responsible for producing human insulin can be cloned into
bacteria



Bacteria could be genetically engineered to



break down

toxic wastes



produce antibiotics



Plants have been engineered using microbial genes for



herbicidal activity



viral resistance



Cows produce more milk when injected with bovine growth hormone produced
by engineered bacteria



Specific nucleotide sequences in
pathogens allow us to identify them using DNA
probes



Recombinant vaccines can be safer than traditional vaccines using killed
or attenuated microbes



DNA vaccines are being developed in which a gene serves as the vaccine

8.3 Microbial Genomics



Many Microbia
l Genomes Have Been Sequenced



Hundreds of microbial genomes have been sequenced since the first in
1995



Many of which are pathogens



Segments of the Human Genome May Have “Microbial Ancestors”



As many as 200 of the 35,000 human genes are essentially identic
al to
those of
Bacteria



They were passed down from early ancestors of humans



Microbial Genomics Will Advance Our Understanding of the Microbial World



Knowing genomes of bacteria that cause food
-
borne diseases can help us



develop detection methods



make food

safer



It can help us identify microbes that cannot be cultured in the lab



Environmental genomics helps us understand how microbial communities
function



Improved biosensing



Comparative Genomics Brings a New Perspective to Defining Infectious Diseases
and S
tudying Evolution



Functional genomics attempts to discover



the function of proteins coded for in a genome



how the genes interact, allowing the microbe to grow and
reproduce



Comparative genomics compares the DNA sequence of one microbe to
another similar or

dissimilar organism



Metagenomics is identifying the previously unseen microbial world



Techniques are now being developed to analyze and understand all the
genomes within a microbial community