BIO222 Molecular Genetics

Introduction to Molecular Genetics

• The course BIO222 Molecular Genetics covers topics such as regulation of transcription and translation in eukaryotic and prokaryotic cells, genome structure, stability, and organization, and molecular genetics of development.

Genetic Transfer and Mapping in Bacteria

• Genetic transfer in bacteria involves the transfer of genetic material from one bacterium to another, enhancing genetic diversity.
• There are three mechanisms of genetic transfer in bacteria: conjugation, transduction, and transformation.

Conjugation

• Conjugation involves direct physical contact between a donor and a recipient cell, with the transfer of genetic material.
• Conjugation requires direct physical contact between the donor and recipient cells, mediated by sex pili (or F pili) made only by F+ strains.
• The F factor, a small circular piece of DNA, carries genes required for conjugation, such as tra and trb genes.
• The contact between donor and recipient cells initiates conjugation, leading to the formation of a conjugation bridge between the two cells.

Mechanism of Transfer

• The mechanism of transfer involves the following steps:
  • Relaxosome recognizes the origin of transfer (DNA sequence) and makes a cut in the DNA.
  • After cutting, most accessory proteins of the relaxosome are released.
  • One protein, relaxase, remains bound to the end of the T-DNA.
  • Exporter, a complex of 10-15 proteins coded by the F factor, pumps DNA/relaxase into the recipient cell.

Plasmids

• Plasmids are extra-chromosomal DNA molecules, present in many bacteria and a few eukaryotic species.
• Plasmids can be circular or linear, ranging in size from a few thousand to 500,000 bp, and replicate independently of the bacterial chromosome.
• Plasmids are usually not required for survival but can provide growth advantages to the bacteria.
• There are five categories of plasmids: fertility plasmids, resistance plasmids, degradative plasmids, col-plasmids, and virulence plasmids.

Mechanisms of Genetic Transfer in Bacteria

• Genetic transfer in bacteria was discovered in 1946 by Joshua Lederberg and Edward Tatum
• There are three mechanisms of genetic transfer in bacteria: transduction, transformation, and conjugation

Transduction

• When a bacteriophage infects a donor cell, it incorporates a fragment of bacterial chromosomal DNA into a newly made phage particle
• The phage then transfers this fragment of DNA to a recipient cell, which incorporates the DNA into its chromosome by recombination

Transformation

• 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

Conjugation

• Conjugation requires direct physical contact between donor and recipient cells
• It is the transfer of DNA from one bacterium to another following direct cell-to-cell contact
• Many, but not all, species of bacteria can conjugate
• Only certain strains of bacteria can act as donor cells

F Factors

• F factors are small circular pieces of DNA that carry genes necessary for conjugation
• Strains containing an F factor are designated F+, while those lacking it are F-
• Genes that play a role in the transfer of DNA are named tra or trb followed by a capital letter

Conjugation Process

• The first step in conjugation is the contact between donor and recipient cells, mediated by sex pili (or F pili)
• Once contact is made, the pili shorten, and a conjugation bridge is formed between the two cells
• The successful contact stimulates the donor cell to begin the transfer process
• The mechanism of transfer involves relaxosome, exporter, and relaxase proteins
• The result of conjugation is that the recipient cell has acquired an F factor, converting it from an F- to an F+ cell

Plasmids

• Plasmids are extra-chromosomal DNA molecules that are present in many bacteria and a few eukaryotic species
• They can be circular or linear and range in size from a few thousand to 500,000 bp
• Plasmids have their own replication origin and can replicate independent of the bacterial chromosome
• They are usually not required for survival but can provide growth advantages to the bacteria
• There are five different categories of plasmids: fertility plasmids, resistance plasmids, degradative plasmids, col-plasmids, and virulence plasmids