22. Recombination II

Question 1

what is transduction?

Answer 1

• transfer of bacterial genes from bacteria to bacteria by viruses
• bacterial genes incorperated into phage capsid
  – due to errors in phage life cycle
• phage containing these genes injects into another bacterium
  – completing transfer

Question 2

what is the beginning process of transudction?

Answer 2

• phage attaches to host cell and injects its DNA
  – into bacterial chromosome
• phage DNA circularises
• certain factors determine next stage:
  – lytic cycle (induced)
  – lysogenic cycle (entered)

Question 3

what is the process of transduction for the lytic cycle?

Answer 3

• induced cycle
• new phage DNA and proteins synthesised
  – assembled into phages
• cell lyses
  – releasing phages
• restarts cycle when phage attaches to host cell

Question 4

what is the process of transduction for the lysogenic cycle?

Answer 4

• phage DNA integrates into bacterial chromosome
  – becomes prophage
• bacterium reproduces normally
  – copying prophage and transmitting it to daughter cells
  – many cell divisions produce large population of infected bacteria
• occasionally, prophage exits bacterial chromosome
  – initiates lytic cycle

Question 5

what are the different kinds of transduction?

Answer 5

• generalised transduction
• specialised transduction

Question 6

what is generalised transduction?

Answer 6

• occurs during lytic cycle of phages
  – can transfer any part of bacterial genome
• during assembly stage, when viral chromosome are packages into protein capsids
  – random fragments of partially digested bacterial chromosome may also be packages by mistake
• becuase capsid can only contain certain amount of DNA
  – phage DNA is left behind
• resulting virus particle often injects DNA ino another bacterial cell
  – does not induce another lytic cycle
• through homologous recombination
  – DNA incorporated into recipient chromosome
  – cell is now transformed (recombinant)

Question 7

what is the process of generalised transduction?

Answer 7

• infection
• destruction of bacteria’s DNA
• replication of viral genome
• production of viral parts
• packaging
• lysis

Question 8

what is specialised transduction?

Answer 8

• transducing particle carries only specific portions of bacterial genome
• made possibly by error in lysogenic life cycle of temperate phage
• prophage is induced to leave host chromosome
  – excision somtimes carries out improperly
• resulting genome caries portion (10-15%) of bacterial chromosome
• transducing particle will inject bacterial genes into another bacterium
  – may become stably incorporated

Question 9

what is the process of specialised transduction?

Answer 9

• temperate phage
  – reproduced in every round of binary fission
  – part of bacteria’s DNA
• replication of viral genome
• production of viral parts
• packaging
• lysis

Question 10

what is conjugation?

Answer 10

• transfer of genetic information by direct contact
  – cell-to-cell contact
• examples:
  – F+ x F- Mating
  – Hfr (high frequency) conjugation
  — requires fertility factors that are stably integrated into chromome of F+ cell

Question 11

what is F+ x F- conjugation?

Answer 11

• donor (F+)
• recipient (F-)
• gene transfer in nonreciprocal
• during F+ x F- conjugation
  – F facor replicates by roling circle replication
  – copy moves to recipient
• entering strand replicated to produce double stranded DNA
• bacterial chromosomal genes rarely transferred using this mechanism

Question 12

what is the process of F+ x F- conjugation?

Answer 12

• donor cell attaches to recipient with pilus
  – pilus draws cells together
• cells contact one another
• one strand of plasmid DNA transfers to recipient
• recipient synthesises complementary strand
  – becomes and F+ cell
  – donor synthesises complementary strand restoring to complete plasmid

Question 13

what is Hfr conjugation?

Answer 13

• homologous recombination of (Hfr) strains
  – between sequence present on both plasmid and chromosome
  — when integrated, F plasmid’s tra operon still functional
  — can direct synthesis of pili, carry out rolling circle replication, transfer genetic material to F- recipient cell
• chromosomally integrated F factor
  – integreates into bacterial chromosome at several locations
  — still carries all genes required to allow formation of pilus
• donor strains transfer bacterial genes with great efficiency
  – do not change recipients into donors
• very high efficiency of chromosomal gene transfer
  – in comparison with F+ cells

Question 14

what is the process of Hfr conjugation?

Answer 14

• F plasmid integrates into chromosome by recombination
• cells join via a conjugation pilus
• portion of F plasmid partially moves into recipient cell
  – trailing a strand of donor’s DNA
• conjugation ends with pieces of F plasmid and donor DNA in recipient cell
  – cells synthesis complementary DNA strands
• donor DNA and recipient DNA recombine
  – making a recombinant F- cell

Question 15

how does Hfr conjugation work?

Answer 15

• DNA transfer begins when integrated F factor is nicked at is site of transfer origin
• as replicated, chromosome moves through the pilus (conjugation bridge)
  – joining donor and recipient cells
• only part of F factor is transferred at start (initial break is within F factor)
  – F- recipient does not become F+
  – unless whole chromosome transferred, requiring approx. 100 min. Thus complete F factor not usually transferred.
• after replicated donor chromosome enters recipient cell
  – may be degraded or incorporated in F- chromosome
• Hfr conjugation
  – most efficient natural mehanism of gene transfer between bacteria