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Genetics summary chapter 7 VU Amsterdam
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This summary covers complete chaper 7 of genetics at the Vrije Universiteit Amsterdam.
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Chapter 7 Genetic transfer and mapping in bacteriaBecause bacteria reproduce asexually, researchers do not use crosses in genetic analysis of bacterial species.
Instead, they rely on a similar mechanism, called genetic transfer – a segment of bacterial DNA is transferred
from one bacterium to another/ process in which one bacterium transfers genetic material to another
bacterium. The advantage of genetic transfer is that it enhances the genetic diversity in bacterial species.
Bacteria can transfer genetic material naturally via three mechanisms:
1. Conjugation: it involves a direct physical interaction between two bacterial cells. One bacterium acts
as a donor and transfers genetic material to a recipient bacterium cell. Requires a direct contact
between donor and recipient cell. The donor cell transfers a strand of DNA to a recipient. The DNA is
known as a plasmid and it is being transferred to the recipient cell.
2. Transduction: during transduction, a virus infects a bacterium and then transfers bacterial genetic
material from one bacterium to another bacterium. When a virus infects a donor cell, it incorporates a
fragment of bacterial chromosomal DNA into a newly made virus particle. The virus then transfers this
fragment of DNA to a recipient cell, which incorporates the DNA into its chromosome by
recombination.
3. Transformation: transformation is a process in which genetic material is released into the
environment when a bacterial cell dies. This material then binds to a living bacterial cell, which can
take it up. When a bacterial cell dies, it releases a fragment of its DNA into the environment. This DNA
fragment is taken up by a recipient cell, which incorporates the DNA into its chromosome by
recombination.
Bacterial conjugation (physical contact)
The natural ability of one bacterial cell to transfer genetic material to another bacterial cell was first recognized
by Joshua Lederberg and Edward Tatum. They were studying strains of Escherichia coli that had different
nutritional requirements for growth.
- Minimal medium: growth medium that contains essential nutrients for a wild-type (nonmutant)
bacterial species to grow.
Researchers often study bacterial strains that protect mutations and cannot grow on a minimal medium. A
strain that cannot synthesize a particular nutrient and needs that nutrient to be added to its growth medium is
called an auxotroph. For example, a strain that cannot make methionine would not grow on a minimal
medium because the minimal medium does not contain methionine. Such a strain would need methionine in
order to grow and would be called a methionine auxotroph.
A strain that could make methionine would be termed a methionine prototroph. A prototroph does not need a
particular nutrient included in its minimal medium. A plus superscript ( +) indicates a functional gene, and a
minus superscript (-) indicates that a mutation has causes the gene or gene product to be inactive. Because no
colonies were observed on either plate in which the two strains were not mixed, Lederberg and Tatum
concluded that some genetic material was transferred between the two strains.
Bernard Davis conducted experiments showing two strains of bacteria must make physical contact with each
other to transfer genetic material. He used the U-tube. At the bottom of the U-tube is a filter with pores small
enough to allow the passage of genetic material (DNA molecules) but too small to permit the passage of
bacterial cells.
1. One side of the filter, Davis added a bacterial strain with a certain combination if nutritional
requirements. On the other side, he added a different bacterial strain.
2. The application of alternating pressure and suction promoted the movement of liquid though filter.
Because the bacteria were too large to pass through the filter, the movement of the liquid did not
allow the two types of bacterial strains to mix with each other. However, any genetic material that
was released from the bacterium could pass through the filter.
3. After incubation in a U-tube, bacteria from either side of the tube were placed on a medium that could
select for the growth of the cells that were met + bio+ thr+ leu+ thi+. This minimal medium lacked
methionine, biotin, threonine, leucine, and thiamine, but contained all other nutrients that are
essential for growth.
4. In this case, no bacterial colonies grew on the plates. The experiment showed that, without physical
contain, the two bacterial strains did not transfer genetic material to another bacterium.
, Chapter 7 Genetic transfer and mapping in bacteria
Certain donor strains of the E.coli contain a small circular segment of genetic material known as the F factor in
addition to their circular chromosome. Strains of E.coli that contain an F factor are designated as F +, and strains
without an F factor are designated as F-. F factors carry several genes that are required for the conjugation to
occur. The proteins that are encoded by the genes on the F factor are needed to transfer a strand of DNA from
a donor cell to a recipient cell. Sex pili are produced by the F+ strains. The gene that encodes for the Sex pilus
(traA) is on the F factor located. The sex pili act as attachment sites that promote the binding of bacteria to
each other. In this way, a F+ strain makes physical contact with a F- strain. Once contact is made, the sex pili
shorten, thereby drawing the donor and recipient cells closer together.
A conjugation bridge is later formed between the two cells, which provide a passageway for DNA transfer. The
successful contact between donor and recipient cells stimulates the donor cell to begin the transfer process.
Genes within the F factor encode a protein complex called the relaxosome. This protein complex first
recognizes a DNA sequence in the F factor known as the origin of transfer. Upon recognition, the relaxosome
also catalyzes the separation of the DNA strands, and only the cut DNA strand, called the T DNA, is transferred
to the recipient cell. As the DNA strands separate, most of the proteins within the relaxosome are released, but
one protein (relaxase) remains bound to the end of the cut DNA strand. The complex between the single—
stranded DNA and the relaxase is called a nucleoprotein because it contains both nucleic acid (DNA) and a
protein (relaxase).
The next phase of conjugation involves the export of the nucleoprotein complex from the donor cell to the
recipient cell. To begin this process, the T DNA/Relaxase complex is recognized by a coupling factor that
promotes the entry of the nucleoprotein into the exporter, a complex of proteins that spans both inner and
outer membranes of the donor cell. In various bacterial species, this complex is composed of 10-15 different
proteins that are encoded by the genes within the F factor. The exporter pumps the T DNA/relaxase complex
through a conjugation bridge into the recipient cell.
Once the DNA/relaxase complex (nucleoprotein) is pumped out of the donor cell, it travels through the
conjugation bridge and then in the recipient cell. The other strand of the F factor DNA remains in the donor
cell, where DNA replication restores this DNA to its original double-stranded condition. After the recipient cell
receives a single strand of the f factor DNA, relaxase catalyzes the joining of the ends of this linear DNA
molecule to form a circular molecule. This single-stranded DNA is replicated in the recipient cell to become
double-stranded. The result of the conjugation is that the recipient cell has acquired an F factor, converting it
from F- to an F+ cell. The genetic composition of the donor cell has not changed.
F factors are also called plasmids. Most known plasmids are circular, although some are linear. Some plasmids,
such as the F factors can integrate into a chromosome. These plasmids are called episomes. A plasmid has its
own origin of replication that allows it to be replicated independently of the bacterial chromosome. The DNA
sequence of the origin of replication influences how many copies of the plasmid are found within a cell. Some
origins are said to be very strong because they result in many copies of the plasmid. Plasmids are necessary for
bacterial survival. However, in many cases, certain genes within a plasmid provide some type of growth
advantage to the cell.
By studying plasmids in many different species, researchers have discovered that most plasmids fall into 5
categories:
1. Fertility plasmids (F factors), that allow bacteria to conjugate with each other.
2. Resistance plasmids (R factors), that contain genes that confer resistance against antibiotics and other
types of toxins.
3. Degradative plasmids carry genes that enable the bacterium to digest and utilize an unusual
substance.
4. Col-plasmids contain genes that encode colicins, which are proteins that kill other bacteria.
5. Virulence plasmids carry genes that turn a bacterium into a pathogenic strain.
Conjugation and mapping via HFR strains
Certain donor strains of E.coli, called Hfr strains, are capable of conjugation. Luca Cavalli-Sforza discovered a
strain of E.coli that was very efficient at transferring many chromosomal genes to recipient F - strains. He called
this bacterial strain as Hfr strain (high frequency of recombination). An F factor may align with a similar region
found in the bacterial chromosome. Due to recombination, the F factor may integrate into the bacterial
chromosome. F factors can integrate into several different sites that are scattered around the E.coli
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