bacteria: structure and genetic variation

Question 1

describe the bacterial chromsome

Answer 1

one double stranded, circular DNA molecule
circular DNA forms loop domains by associating with DNA binding proteins and further supercoils to form highly condensed DNA to fit nucleoid

Question 2

describe nucleoid

Answer 2

where chromosomal DNA is confined to
not bound by a membrane

Question 3

describe storage granules

Answer 3

nutrients and chemical reserves may be stored in the cytoplasm in the form of granules

Question 4

describe plasmid

Answer 4

a small circular autonomously replicating DNA molecule
is extrachromosomal
contains genes that confer advantages for bacterial living in stressful environments

Question 5

describe bacterial plasma membrane

Answer 5

phosholipid bilayer
where electron transport system and ATP synthase are embedded to produce ATP

Question 6

difference in plasma membrane between a prokaryotic and eukaryotic cell

Answer 6

prokaryotic cell does not have membrane bound organelles hence the cell membrane is involved in making ATP
eukaryotic cell has membrane bound organelles which are the mitochondrion and chloroplasts that specifically produce ATP during r/p and p/s respectively so cell membrane is not involved in making ATP

Question 7

describe and state function of cell wall

Answer 7

made up of peptidoglycan

protects cell from osmotic lysis confers rigidity and shape to cell

Question 8

what implications are there as a result of the absence of a nuclear envelope in a bacterial cell

Answer 8

1. does not have a nuclear envelope that separates DNA from ribosomes
2. mRNA is translated into protein as soon as it is transcribed as there is no nuclear envelope to act as a barrier to prevent it from being immediately transcribed
3. results in mature mRNA being formed immediately and no post transcriptional modification in bacteria
4. simultaneous transcription and translation in bacteria

Question 9

describe capsule and slime layer

Answer 9

layer of polysaccharides (glycolyx) to the exterior of the cell wall
capsule: is a distinct layer as glycolyx is firmly attached to cell wall and is a organised mass
slime layer is a diffused mass and glycolyx is loosely attached to cell wall

Question 10

function of capsule/slime layer

Answer 10

protects bacteria from being taken in via phagocytosis by the phagocytes which are unable to recognise the bacteria due to the capsule

enables bacteria to adhere to one another or to particular surfaces

contains water to prevent the bactera from drying out and prevent its dessication

Question 11

descibe fimbriae

Answer 11

short and bristle like fibres
evenly distributred over entire cell surface
for attachment to surface or to other bacteria

Question 12

describe pili

Answer 12

longer and fewer than fimbriae
involved in conjugation and motility: pilus makes contact with surface and retracts to pull the bacteria forward

Question 13

describe flagella

Answer 13

hollow cylindrical protein thread that propels the bacterium by rotation

Question 14

describe binary fission

Answer 14

1. DNA replication begins at the origin of replication (ori) where DNA is unzipped by breaking hydrogen bonds between bases of the 2 strands to form a
   replication bubble
2. DNA replicates by semi-conservative replication where each original strand serves as template for synthesis of daughter strands by complementary
   base pairing
3. 2 newly formed ori move to opposite poles of the cell and attach to the plasma membrane
4. Cell elongates to prepare for division.
5. DNA is circular with no free ends, and the 2 daughter DNA molecules will be interlocked with the completion of replication.
6. Enzyme topoisomerase cut, separate and reseal the two DNA molecules
7. Invagination of the plasma membrane and the deposition of new cell wall (division septum) eventually divide the parent cell into two daughter cells
8. each daughter cell inherits a complete genome (genetically identical)

Question 15

describe process of transformation

Answer 15

1. Fragments of foreign naked DNA from dead lysed bacterial cells present in surrounding medium
2. Naturally competent bacteria with cell-surface proteins bind to the DNA and are able to take up the DNA into the cell.
3. Artificially-competent bacteria can be made competent through immersion in a medium with CaCl2 followed by a heat shock treatment
4. Foreign DNA incorporated into host cell chromosome through crossing over at 2 homologous regions found on the bacterial chromosome
5. Result: recombinant cell
6. If different alleles for a gene were exchanged, the new allele will be expressed, there will be a permanent change in genotype & phenotype

Question 16

describe generalised transduction

Answer 16

1. A phage infects a bacterium, injecting its viral genome(DNA)
   into the host cell
2. The bacterial DNA is degraded into small fragments, one of
   which may be randomly packaged into a capsid head during
   the spontaneous assembly of new viruses, forming a defective phage
3. Upon cell lysis of donor bacterium, the defective phage will infect another
   bacterium and inject bacterial DNA from the previous host cell
   into the new bacterium
4. Foreign bacterial DNA can replace the homologous region in the recipient cell’s chromosome through homologous
   recombination, allowing the expression of a different allele
   from the previous host

Question 17

describe specialised transduction

Answer 17

1. Temperate phage infects a bacterium, injecting its viral genome into the host cell
2. The viral DNA is integrated into bacterial chromosome forming a prophage
3. prophage may be improperly excised to include adjacent segment of bacterial DNA and not the entire phage DNA during an induction event, forming hybrid DNA
4. Hence phage-bacterium hybrid DNA may be packaged into a capsid head during the spontaneous assembly of new viruses, forming recombinant phages
5. upon cell lysis, the defective recombinant phages will infect a recipient bacterium by injecting the hybrid DNA into it
6. New alleles from the previous bacterial cell can be incorporated into the genome
   of the new host by homologous recombination
7. or integration of phage-bacterium hybrid DNA as defective phage enters the lysogenic cycle
8. expression of new alleles will result in change of phenotype in recipient bacterial cell

Question 18

describe conjugation

Answer 18

1. Sex pilus (coded for by F factor) of F+ bacterial cell makes contact with a F- cell and retracts to bring the 2 cells closer
2. The hollow pilus then acts as a cytoplasmic mating bridge between the 2 cells
3. One of the 2 strands of the plasmid DNA is nicked and transferred from the F+ cell to the F- cell through the bridge
4. Replication of the plasmid occurs via rolling circle DNA replication
5. The single stranded F plasmid DNA recircularizes in the F-cell and is used as a template to synthesize a complementary strand for a double-stranded
   plasmid DNA, by semiconservative DNA replication
6. The F-recipient cell is now a F+ cell

Question 19

describe rolling circle DNA replication mechanism

Answer 19

1. One strand of ds F plasmid is nicked by a nuclease 
2. free 3’OH end is then used as a primer for strand elongation by DNA polymerase using the
   unnicked/intact strand as a template 
3. elongation process is facilitated by the displacement of the 5’ end of the nicked strand and is transferred across the mating bridge to the recipient bacterium 4. Upon completion of a unit length of the plasmid DNA (after 1 round), another nick occurs to release the original strand and to end the replication of the newly synthesied strand