book 7 (bacteria)

Question 1

outline the mechanism of binary fission in a prokaryote

Answer 1

1. a single double-stranded chromosomal DNA attaches to the plasma membrane, at the origin of replication
2. DNA replicates via bidirectional semi-conservative DNA replication whereby from one origin of replication, DNA replication progresses in both directions around a circular chromosome until the 2 replication fork meet on the other side at the termination region of replication
3. at each replication fork, one strand is made continuously as a leading strand
4. the lagging strand is made discontinuously in the form of Okazaki fragments
5. enzymes cut, twirl and reseal the double helix to prevent the DNA from tangling
6. newly synthesised DNA’s origin of replication moves and attaches to the plasma membrane at the other pole of the cell
7. the 2 bacterial chromosomes’ origin of replication will end up in the opposite poles of the cell
8. bacterial call then elongates
9. when DNA replication is complete and the cell is twice of its original size, the plasma membrane invaginates, pinching off the 2 halves of the cell
10. new cell wall material is synthesised
11. this process gives rise to 2 genetically identical daughter cells

Question 2

describe the process of transformation which give rise to variation in prokaryotic genomes

Answer 2

1. double-stranded foreign DNA binds to cell-surface proteins of competent bacteria
2. one of the two strands is degraded as it passes into the cell by a nuclease
3. uptake of naked DNA through the bacterial cell wall and plasma membrane
4. the single strand aligns with a homologous segment of DNA in the bacterial chromosome and is integrated in via homologous recombination or site-specific integration

Question 3

definition of transformation

Answer 3

transformation is a mechanism of gene transfer in which naked DNA is taken up by competent recipient bacteria through its cell wall and plasma membrane from external environment

Question 4

definition of transduction

Answer 4

refers to a mechanism of gene transfer between bacterial in which the fragments of the donor DNA is introduced into the recipient cell via a bacteriophage

Question 5

describe the process of generalised transduction

Answer 5

1. the lytic phage recognises and binds to specific receptors on bacterial cell wall and injects viral DNA into bacterial cell
2. lytic phage breaks down donor DNA into smaller pieces
3. accidental random packaging of donor bacterial genes, together with viral DNA into the capsid of newly assembled phages occur
4. host cell lysed, releasing transducing phages along with other phages
5. transducing released phages containing the bacterial donor DNA fragments then recognises and binds to another bacterium, and it will inject this foreign DNA into its new hsot
6. donor DNA incorporated into bacterium’s DNA via homologous recombination which occurs between the donor DNA and the recipient DNA of the new host

Question 6

describe the process of specialised transduction

Answer 6

1. lysogenic phage recognises and binds to specific receptors on bacterial cell wall and injects viral DNA into bacterial cell wall and injects viral DNA into bacterial cell
2. viral DNA integrates into host bacterial chromosome at insertion sites to form a prophage
3. under stress, prophage excised from host chromosome
4. donor bacterial genes adjacent to either side of the prophage insertion sites may be excised together with the prophage due to an error
5. during viral assembly, phage DNA along with the bacterial DNA replicated and is packaged into the phage capsid
6. host cell lysed, releasing transducing phage progenies
7. newly released phages containing the bacterial donor genes along with the phage’s genome thn recognises and binds to specific receptors on another bacterium and it will inject this foreign DNA into it’s new host
8. donor DNA incorporated into bacterium’s DNA via homologous recombination of site-specific integration

Question 7

definition of conjugation

Answer 7

refers to the mechanism of gene transfer between bacteria that involves physical contact between 2 bacterial cells, physical contact can be achieved via the formation of a cytoplasmic channel known as a sex pilus through which the transfer of F plasmic from donor to recipient cell occurs

Question 8

describe the process of conjugation

Answer 8

1. F+ cell contains the F plasmid that carries a F factor
2. the F factor contains genes encoding for proteins required for the formation of a sex pilus
3. F+ cell initiates conjugation through the formation of the sex pilus which extends towards the F- cell
4. sex pilus recognises and binds to specific receptor site on the cell wall of the F- cell and provides physical contact between cells. it then retracts, pulling the donor and recipient cells close together. a temporary conjugation tube is formed between the 2 cells
5. one strand of F plasmid is cut by an endonuclease at the origins of transfer and begins to move into the recipient cell via the mating bridge
6. as the transfer occurs, each original strand in the donor and recipient cell is used as a template to synthesise a complementary strand, forming a double stranded F plasmid in both cells
7. in the donor cell, the complementary strand is synthesised as a leading strand by adding deoxyribonucleotides to the 3’OH ends, via a rolling circle mechanism
8. concurrently, in the F- cell, RNA primers binds to nicked strand which allows DNA polymerase to form a complementary DNA strand, via the synthesis of a lagging strand
9. sex pilus is then broken, the plasmid in F- cell circularises to produces a new F plasmid in the F- cell. both cells now contains F plasmid and are F+ cells