Asexual Reproduction and Genetic Variation in Prokaryotes

Question 1

How DNA is arranged in prokaroytes

Answer 1

1. DNA complexes with nucleoid associated proteins, H-NS proteins, which folds the DNA into a more compact structure organised in looped domains
2. Further compaction of bacterial chromosome is achieved through DNA supercoiling

Question 2

Binary Fission (division of a single parent bacterial cell into 2 genetically identical daughter cells)

Answer 2

1. DNA replication begins at the origin of replication (ori) resulting in the formation of a replication bubble
2. DNA double helix separates into 2 strands and each strand acts as a template for the synthesis of a daughter strand by semi-conservative replication
3. DNA synthesis progresses in both directions around the circular chromosome until the entire chromosome has been replicated
4. Newly formed ori and original ori moves to opposite poles of the cell
5. Invagination of CSM along with deposition of new peptidoglycan cell wall eventually divides the parent cell into 2 genetically identical daughter cells.

Question 3

Transformation

Answer 3

1. Transformation is the uptake of naked foreign from surrounding environment, resulting in alteration of a bacterial cell’s genotype and phenotype
2. Naked double stranded DNA from surrounding environment binds to competence factors on CSM of a competent cell. One strand is degraded by exonucleases, one strand is taken up by the bacterium
3. The single stranded DNA taken up by the bacterium can be incorporated into its chromosome via homologous recombination, by crossing over where there is sufficient homology between DNA fragments and bacterial chromosome
4. Resultant cell is a recombinant cell. If different alleles for a gene were exchanged, there will be a change in organism’s phenotype as the new allele is expressed, giving rise to a new phenotype

Question 4

Generalised Transduction (T4 phage)

Answer 4

1. During lytic cycle, phage nucleases hydrolyses host cell’s bacterial chromosome into small pieces of DNA
2. During assembly of the newly replicated DNA within the phage capsid, a small piece of host bacterial cell’s degraded DNA gets mistakenly packaged within the capsid, instead of the phage DNA. This results in a defective phage.
3. The defective phage infects another bacterial cell, and injects the DNA into the recipient cell’s cytoplasm
4. The foreign bacterial DNA can then be incorporated into recipient cell’s chromosome via homologous recombination, forming a recombinant cell.

Question 5

Conjugation

Answer 5

1. Conjugation is the direct transfer of genetic material from 1 bacterial cell to another, through a temporary link between the 2 cells, requiring cell to cell contact.
2. F+ donor cell produces a sex pilus that attaches to specific receptors on the F- recipient cell. Upon contact, sex pilus retracts, pulling the 2 cells closer together.
3. A temporary cytoplasmic mating bridge is formed between the F+ donor and F- recipient cell.
4. A single strand of F plasmid breaks at a specific point and is transferred as a single strand from the F+ donor to the F- recipient cell via the cytoplasmic mating bridge
5. Each single-stranded DNA in the F+ donor and F- recipient cell acts as a template for the synthesis of a complementary daughter strand
6. Once semi-conservative replication is complete, transferred DNA circularises by DNA ligase, resulting in a F plasmid. F- recipient cell now becomes an F+ donor cell

Question 6

Role of F plasmid

Answer 6

1. Contains F factor which carries genes to produce a sex pilus
2. Carries antibiotic resistant genes
3. Carries xenobiotic resistant genes

Question 7

Describe ways in which plasmid differs from the bacterial chromosome

Answer 7

1. Fewer genes compared to bacterial chromosome
2. Presence of antiobiotic resistant or xenobiotic resistant genes
3. Absence of genes required for cell metabolism in plasmids