Genetics of Bacteria

Question 1

what does the bacterial genome consist of

Answer 1

• bacterial chromosome (single, double-stranded, circular DNA molecule found in the nucleoid region, associated with non-hisotne proteins and exists as a highly compact supercoiled structure), contains structural genes (code for proteins essential for bacyerial survival)
• plasmids (small, extrachromosomal, double-stranded, circular DNA) are not essential for survival but give bacteria a selective advantage

Question 2

differences between bacterial chromosome and plasmids

Answer 2

• bacterial codes for proteins essential for survival while plasmids are not essential for survival but confer a selective advantage
• bacterial chromosome is replicated and equally distributed to daughter cells during binary fission while plasmids replicate independently of the chromosome and may not be segregated equally in the daughter cell

Question 3

describe binary fission

Answer 3

• bacterial DNA begins to replicate at the origin of replication on the chromosome, producing two origins
• double-stranded DNA separates and each strand acts as template for synthesis of a daughter strand by semi-conservative DNA replication
• as the DNA continues to replicate, one origin moves rapidly toward the opposite end of the cell
• when the chromosome is replicating, the cell elongates
• when replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward to form a double layer across the long axis of the cell
• new cell wall layers are secreted between the membrane layers. the parent bacterium divides into two genetically identical daughter cells, each receiving one copy of the bacterial chromosome

Question 4

briefly describe transformation

Answer 4

• direct uptake of foreign DNA by the competent bacteria from the external environment, which requires the expenditure of ATP
• homologous regions undergo genetic recombination via homologous recombination

Question 5

outline the process of transformation

Answer 5

• transforming DNA is bound at the cell surface by a DNA-binding protein
• following this, either the entire double-stranded fragment is taken up, or a nuclease degrades one strand and only the remaining strand is taken up, depending on the organism. ATP is expended in the process
• after uptake, DNA becomes attached to a competence-specific protein, preventing the DNA from digestion by nuclease until it reaches the chromosome
• DNA replaces a homologous region of the recipient chromosome by homologous recombination

Question 6

briefly describe transduction (generalised and specialised)

Answer 6

• transfer of bacterial DNA from the donor bacterium by phage to the recipient bacterium due to packaging error during the assembly of phage components/excision error during prophage induction
• transduction occurs when bacteriophage transfers DNA fragment from donor to recipient bacteria
• via generalised transduction due to error in packaging of bacterial DNA
• or specialised transduction due to imprecise excision of prophage
• homologous regions undergo genetic recombination via homologous recombination
• transferred bacterial DNA may be integrated into the bacterial chromosome in the recipient cell during the lysogenic cycle (specialised transduction only)

Question 7

outline how transduction leads to genetic recombination

Answer 7

• generalised transduction: during assembly of phages in the lytic cycle, fragments of the bacterial chromosome are mistakenly packaged within the phage capsid
• specialised transduction: during prophage induction in the lysogenic cycle, parts of the bacterial chromosome is incorrectly excised togther with the prophage and is replicated
• upon release of phage by osmotic lysis of host cell, the phage attaches to another bacterium and injects the piece of bacterial DNA acquired from the donor cell into it
• the bacterial DNA replaces a homologous region of the bacterial chromosome in the recipient cells via homologou recombination
• bacterial DNA is integrated into the bacterial chromosome in the recipient cell during the lysogenic cycle (specialised transduction only)

Question 8

briefly describe conjugation

Answer 8

• direct contact between a F+ donor and a F- recipient bacterium via the conjugation tube, which is involved in transferring F plasmid unidirectionally
• in conjugation, F plasmid DNA is transferred from donor cell to recipient cell through a mating bridge/conjugation tube

Question 9

describe the events in conjugation

Answer 9

• donor cell forms sex pilus, which attaches to and pulls recipient cell and itself together
• a mating bridge/conjugation tube forms between the two cells
• F factor/plasmid replicates semi-conservatively by the rolling circle mechanism in the donor cell
• a single-stranded copy of the F plasmid enters the recipient cell beginning at the 5’ end via the conjugation tube and gets replicated in recipient cell to form double-stranded DNA molecule
• conjugation tube breaks and retracts, leading to recipient cell becoming a F+ cell

Question 10

describe the differences between conjugation and transformation

Answer 10

• conjugation transfers the F plasmid/R plasmid while transformation is the uptake of fragments of the bacterial chromosome
• in conjugation, donor cells carries the F plasmid/R plasmid while in transformation, donor cells may be lysed cells that release its DNA
• in conjugation, recipient cells do not carry the F plasmid/R plamid while in transformation, recipient cells must be competent
• in conjugation, direct physical contact required between two cells through sex pilus/conjugation tubewhile in transformation, no physical contact required between cells

Question 11

explain the role of the F plasmids in bacterial conjugation

Answer 11

• F plasmid is found in the donor cell and is absent in recipient cell
• F plasmid contains genes that code for synthesis of sex pilus/formation of mating bridge/conjugation tube for attachment to recipient cell during conjugation
• in the donor cell, F plasmid replicates by rolling circle mechanism, producing a single strand of F plasmid that will be transferred from the donor cell to recipient cell and is replicated, producing a double stranded F plasmid

Question 12

suggest the advantages of conjugation over transduction

Answer 12

• does not require virus for conjugation to occur
• host cell remains alive
• does not depend on chance that wrong DNA fragments are packaged in generalised transduction
• does not depend on chance that viral DNA excises imprecisely in specialised transduction
• higher chance for the genetic material to be transferred as cells contact each other while transduction relies mainly on chance
• larger portion of DNA can be transferred
• defined genes of F plasmid transmitted during conjugation as compared to random DNA fragments via general transduction/only genes/DNA fragments beside prophage via specialised transduction

Question 13

describe the differences between conjugation and transduction

Answer 13

• conjugation transfers the F plasmid/R plasmid while transduction transfers fragments of bacterial chromosome (generalised)/portion of bacterial DNA located adjacent to the prophage site (specialised)
• in conjugation, donor cells carry the F plasmid/R plasmid while in transduction, donor cells are bacterial cells which are infected by bacteriophages
• in conjugation, recipient cells do not carry the F plasmid/R plasmid while in transduction, recipient cells are bacterial cells that can be infected by the same bacteriophages
• conjugation involves direct contact between two cells through sex pilus/conjugation tube while transduction does not involve any physical contact between cells

Question 14

explain how the rolling circle replication is a form of semi-conservative DNA replication

Answer 14

• parental DNA strands serve as templates for synthesis of daughter strands and daughter DNA molecules consist of one parental DNA strand and one newly synthesised strand
• involves cutting of outer strand of DNA, followed by rolling of inner strand