Clear and orderly summary of Chapter 7 of the book "Genetics: Analysis and Principles, 6th Edition by Robert Brooker". Together with all my other summaries of Genetics I got an 8,5 for this course.
Genetics Summary Chapter 7 – Genetic transfer and mapping in
Bacteria
Bacteria are asexually, researches don’t use crosses in genetic analyses.
But use genetic transfer; segment of bacterial DNA is transferred from one
bacterium to another.
7.1 Overview of Genetic Transfer in Bacteria
3 mechanisms for genetic transfer in various species of bacteria are;
1. Conjugation; physical interaction between 2 bacterial cells. One
bacterium acts a donor and transfers genetic material to a recipient
(ontvanger) cell.
2. Transduction; virus (bacteriophage) transfers bacterial DNA from one
bacterium to another.
3. Transformation; segment of chromosomal DNA introduced into a
bacterial cell OR when normal cell is converted in malignant
(kwaadaardig) cell. Bacterial cell dies, it releases fragment of DNA
into environment. This fragment taken up by recipient which
incorporate DNA into chromosome by recombination
7.2 Bacterial Conjugation
Lederberg and Tatum discovered conjugation in E.coli, by analyzing
autxotrophic strains
E.coli had different nutritional requirements for growth
1. minimal medium; contains essential nutrients for wild-type
bacterial species to grow
2. autotroph; strain that cannot synthesize particular nutrient and
needs nutrient supplemented in its growth medium (i.e strain
that cannot make amino acid methionine would not grow on
minimal medium because it doesn’t contain methionine.)
(aangeduid met -)
3. Prototroph; strain that doesn’t need a particular nutrient supplemented in its
growth medium. (aangeduid met +)
The middle plates are transferred with each other, and thus lead to colonies because
now all the things can synthesize itself.
Conjugation requires direct physical contact
Using a U-tube apparatus, Davis showed that conjugation
requires cell-to-cell contact.
Filter with pores is small enough to passage genetic material,
but too small to passage bacterial cells.
F+ strain transfers F factor to an F- strain during conjugation
F factor; factor found in strains of bacteria and has additional genetic material
besides that found in their circular chromosome.
With F factor; F+
Without F factor; F-
F factor contains genes that are required for conjugation to occur.
, Conjugation in steps;
Sex pilus (pl. Pili); acts as an attachment site to promote binding
of bacteria to each other. Gene encode the pilin protein (traA) is
located on the F factor. Once pili is made it is shortened (verkort)
drawing donor and recipient closer together. Later conjugation
bridge is formed.
Relexase; cutting of the F factor and transfer to recipient cell. It
recognizes the orgin of transfer. Only T DNA, single stranded
DNA, is transferred to recipient cell. Relexase remains bound to
the end of DNA strand, this complex is called nucleoprotein
(protein + DNA)
Coupling factor; guides DNA to the exporter
Exporter; transports it into the recipient cell
Relexase joins the ends of single stranded DNA
Replication; for double stranded DNA
Different types of plasmids
Plasmid; pieces of DNA that exist independently of the
chromosomal DNA. Most are circular few linear.
- Fertility plasmids; F factors. Allow conjugation
episomes; can replicate independently of chromosomal
DNA BUT can also integrate into a chromosome.
- Resistance plasmids; R factors. Contain genes that confer
resistance against antibiotics and toxins
- Degradative plasmids; carry genes that enable bacterium to digest an unusual
substance
- Col-plasmids; genes that encode colicins, which are proteins that kill other
bacteria
- Virulence plasmids; genes that turn bacterium into a pathogenic
(ziekteverwekker) strain
7.3 Conjugation and Mapping via Hfr Strains
Hfr have F factor integrated into bacterial chromosome
Hfr (high frequency of recombination) strain; bacterial strain in which F
factor has become integrated into bacterial chromosome. Hfr can
transfer segments of bacterial chromosome to recipient cell.
F factor in Hfr is excised (uitgesneden) from bacterial
chromosome. Looping out of the F-factor from DNA. Imprecise
excision releases an F’ factor, it carries a portion of the bacterial
chromosome.
Hfr strains can transfer via conjugation
One of the DNA strands is cut at the origin of transfer.
This cut is starting point at which one strand of Hfr
chromosome enter the F- recipient cell. It occurs via linear
transferation.
Once inside F- cell chromosomal material from Hfr can
recombine with homologous region of the recipient cell’s
chromosome.
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