Summary
Summary General Genetics IIB (GNE2602)
- Institution
- University Of South Africa (Unisa)
Learning units & lecture notes summaries, diagrams & pictures for better understanding the notes
[Show more]
Seller
Follow
Content preview
General Genetics IIB GNE2602 Examination NotesLearning Unit 1: Genetics of bacteria and viruses
Bacteria are living microorganisms that can reproduce on their own, but they sometimes
require alteration in their genetic material to increase their chances of survival. Bacteria can
modify their gene composition via conjugation transformation or transduction. Bacteria
can also be infected by viruses, called bacteriophages, which introduce their genetic
material into the bacterial cells to replicate. Viruses are considered non-living, and they are
not capable of reproducing on their own, thus they depend on a host cell to accomplish
replication and formation of new viral particles to complete their replication cycles.
1.1 Genetic transfer in bacteria
Genetic transfer is a process by which one bacterium transfers its genetic material to
another bacterium. This process could help enhance the genetic diversity of bacterial
species. Bacteria can exchange genetic material naturally using one of three mechanisms:
o Conjugation
o Transformation
o Transduction
Conjugation is a direct physical interaction between two bacterial cells, whereby one
bacterium transfers its genetic material (the donor) to another cell (the recipient).
Transduction occurs when a virus infects a bacterium (the donor cell) and then
incorporates the bacterial genome fragments into the newly formed virus particles.
The virus will transfer fragments of DNA to a recipient bacterial cell.
The third mode of genetic transfer in bacteria is transformation. In transformation, the
donor cell is a dead cell that released its DNA fragments into the environment. It will
come into contact with the DNA fragment, takes it up and incorporates it into its
chromosomal DNA.
1.2 Bacterial conjugation
The natural ability of some bacteria to transfer genetic material between each other was first
recognised by Joshua Lederberg and Edward Tatum in 1946. Researchers use mutant
bacteria that lack a certain gene, for example methionine or thiamine, to illustrate the
transfer of genetic material between bacterial cells. The mutated bacteria will only
grow on media supplemented with essential nutrients, which they cannot synthesise
themselves.
To conduct the experiment researchers can use two types of mutated E. coli strains; one
strain lacks the methionine gene (met-) but has the thiamine gene (thi+); the other
strain lacks the thiamine gene (thi-) but has the methionine gene (met+). The two
strains are plated separately on growth media that lack both thiamine and methionine.
Then the two strains are mixed together and plated on a single media plate still lacking
thiamine and methionine.
,The two strains did not grow when plated on separate media that lacked thiamine and
methionine. The strain with the thi- met+ genes failed to grow because there was no
thiamine in the media; and the strain with the thi+ met- genes did not grow because
there was no methionine in the media. When the two strains were mixed and plated on a
growth medium, growth of bacterial cells occurred. Lederberg and Tatum concluded that
some genetic material was transferred between the two strains that led to formation of
some bacterial cells having thi+ met+ genes, thus assisting the cells to synthesise
methionine and thiamine.
1.2.1 Conjugation requires direct physical contact
Conjugation is a process that requires the bacterial cells to be in direct contact with
each other. To demonstrate this theory Bernard Davis used a U-tube that had a filter at
the bottom of the tube. The filter separated the thi- bacterial strain from the met- strain.
The filter had pores only small enough to allow passage of genetic material but not
bacterial cells. There was alternating pressure that allows suction and pressure to
promote movement of liquid through the filter. After incubation, the bacterial strains
from either side of the tube were plated on separate growth media that had no
methionine or thiamine. In this case, no colonies grew on the plate.
,Activity 1.1: Bacterial genome transfer
Explain why no colonies grew in the Joshua Lederberg and Edward Tatum experiment
The bacteria were supposed to exchange genetic material (thi+ and met+) via conjugation,
but this would only be possible if the cells were in direct contact with each other. After the
experiment, the bacterial strains kept their initial genes that made them susceptible to dying
when plated on a media that did not have thiamine or methionine.
1.2.2 An F+ strain transfers an F factor to an F- strain during conjugation
An F factor (fertility factor) is a small circular segment of genetic material in addition
to circular chromosomes in certain donor strains of E. coli. E. coli strains that contain
the F factor are F+, while strains without the F factor are F-. The cells (F- and F+) must be in
direct contact with each other for conjugation to occur. The F factor has several genes that
are required for conjugation to occur. (For example, the traA gene encodes the pilin protein
that makes up the pilus. The pilus acts as attachment sites that promote the binding of
bacteria to each other. A direct contact between the F+ strain and the F- strain is made.)
1.2.3 Bacteria may contain different types of plasmids
F factors are one example of DNA that exists independently of the chromosomal DNA.
Generally, this type of extrachromosomal material is referred to as plasmids. Plasmids have
their own origin of replication that allows them to be replicated independently of the
bacterial chromosomes. The DNA sequence of the origin of replication also influences the
number of plasmids that can be found in the cell. Plasmids might not be necessary for the
survival of bacteria, but they are able to provide some type of growth advantage to the
cell. Most plasmids fall into five different categories:
o Fertility plasmids (F factors) – allow bacteria to conjugate with each other
o Resistance plasmids (R factors) – contain genes that confer resistance against
antibiotics and other types of toxins.
o Degradative plasmids – carry genes that enable the bacterium to digest and utilise
an unusual substance, namely toluene.
o Col-plasmids – contain genes that encode colicins, which are proteins that kill
other bacteria.
o Virulence plasmids – carry genes that turn a bacterium into a pathogenic strain
, 1.3 Conjugation and mapping via HFR strains
Bacterial Hfr (high frequency recombination) strains are formed when F factors are
integrated into bacterial chromosomes through recombination.
1.4 Bacterial transformation
Bacterial cells, which can take up foreign DNA, are known as competent cells. The cells
that accomplish transformation naturally carry genes that encode proteins called
competence factors.
Functions of the competence factor proteins are to
o Facilitate the binding of DNA fragments to the cell surface
o Assist in the uptake of DNA into the cytoplasm
o Help with the subsequent incorporation into the bacterial chromosome
A competent bacterial cell can be transformed by two kinds of genetic material, where a)
represents a bacterial cell being transformed with DNA fragments; and b) represents a
plasmid entering the bacterial cell and becoming an extra chromosomal bacterial DNA.
1.5 Bacterial transduction
Bacteriophages are viruses that infect bacteria and replicate inside the bacterial cell.
Bacteriophages bind to the surface of the bacterium and inject their genetic material
into the bacterial cytoplasm. A phage may follow:
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through EFT, credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying this summary from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller Kulanip74. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy this summary for R143,00. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
53022 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy summaries for 14 years now