Bacteria

Question 1

what is the general structure of bacteria cells?

Answer 1

1. bacterial cells vary from less than 1 um to approximately 10 um in size
2. all bacterial cells are prokaryotic, and hence lack a true nucleus and membrane bound organelles
3. all bacterial cells contain the nucleoid, ribosomes, bacterial chromosome, fimbriae and plasma membrane
4. only some bacterial cells contain the capsule and flagella

Question 2

what is the structure of the bacterial cell wall?

Answer 2

• it is composed of peptidoglycan, which is a cross-linked polymeric mesh
• the glycan component is a linear polymer of alternating monosaccharide subunits, N-acetylglucosamine (NAG), and N-acetylmuramic acid (NAM), forming the backbone of the mesh
• the peptido component is a short string of amino acids that serves to cross-link adjacent polysaccharide strands at the NAM subunits of the backbone, forming a network with high tensile strength.

Question 3

what is the structure of the bacterial cell wall in gram-positive bacteria?

Answer 3

1. presence of thick, multi-layered peptidoglycan cell walls that are exterior to the membrane
2. peptidoglycan is covalently linked to teichoic acid, which is a polymer of substituted glycerol units linked by phosphodiester bonds, and are major cell surface antigens

Question 4

what is the structure of the bacterial cell wall in gram-negative bacteria?

Answer 4

1. presence of an outer and inner cytoplasmic membrane, where peptidoglycan is located between the two membranes in the periplasmic space
2. the periplasmic space contains enzymes and various other substances
3. the outer membrane is distinguished by the presence of various embedded lipopolysaccharides. the polysaccharide portion is antigenic and can be used to identify different strains and species. the lipid A portion is an endotoxin

Question 5

what are the differences between the bacterial cell wall of gram positive VS gram negative bacteria

Answer 5

1. gram-positive bacteria have thick peptidoglycan layers, while gram-negative bacteria have thin peptidoglycan layers
2. during gram staining, gram-positive cells retain the crystal violet dye after treatment with alcohol, while gram-negative cells lose their colour and are thus counterstained with safranin (gram positive cells have thick walls of peptidoglycan which traps crystal violet)

Question 6

what is the structure of the bacterial cell membrane?

Answer 6

bacterial cell membranes are phospholipid bilayers in which specific proteins are embedded, and are generally similar in composition and function to eukaryotic cells

Question 7

flagellum structure?

Answer 7

they are found in motile bacterial cells, and are long filamentous appendages that aid in cell movement

Question 8

pilus structure?

Answer 8

they are long, thin appendages that can be used to attach one bacterial cell to another during conjugation

Question 9

do bacteria have organelles?

Answer 9

• metabolites and enzymes in bacterial cell are not interested within membrane bound organelles, but are found in cytoplasm
• ribosomes are found in cytoplasm, but are smaller at 70s compared to 80s eukaryote ribosomes

Question 10

what does the endosymbiont theory state?

Answer 10

it is an evolutionary theory that states that mitochondria and plastids of eukaryotes originated as a result of symbiosis between separate single-celled organisms

Question 11

explain serial endosymbiosis

Answer 11

1. an early ancestor of eukaryotic cell engulfed an oxygen-using non-photosynthetic prokaryotic cell, resulting in the formation of a relationships with the host cell, becoming an endosymbiont
2. the host cell and its endosymbiont merged into a single organism, which is a eukaryotic cell with a mitochondria
3. serial endosymbiosis hypothesises that mitochondria have enolved before plastids through a sequence of endosymbiotic events, forming the ancestor of eukaryotic cells that contain chloroplasts

Question 12

evidence for endosymbiont theory?

Answer 12

1. the inner membranes of both organelles have enzymes and transport systems that are similar to those found in the plasma membrane of living prokaryotes
2. mitochondria and plastids replicate by a splitting process similar to certain prokaryotes
3. mitochondria and plastids contain circular DNA which are not associated with histones, resembling chromosomes of bacteria
4. mitochondria and plastids possess cellular machinery needed to transcribe and translate their DNA into proteins
5. ribosomes of mitochondria and plastids are more similar to prokaryotic ribosomes in terms of size, RNA sequences and sensitivity to antibiotics

Question 13

bacterial chromosome structure

Answer 13

1. bacterial chromosome is typically a single, circular double stranded DNA molecule which contains essential genes required for survival
2. the bacterial chromosome is compacted approximately 1000 fold, attained by the association of DNA with positively charged histone-like proteins that aid in compacting the DNA into looped domains, as well as supercoiling of bacterial DNA
3. genes are grouped into operons where multiple genes come under the control of the same promoter and same regulatory elements
4. prokaryotic genes also lack introns, hence do not need to undergo splicing

Question 14

what is the structure of the plasmid

Answer 14

1. plasmids exist as small, circular DS extrachromosomal DNA, which may be passed onto cells of the same generation through conjugation, transduction and transformation or through its offspring via binary fission
2. plasmids are capable of replication independent of the bacterial chromosome since they possess their own origin of replication
3. plasmids contain beneficial genes which confer protective traits such as antibiotic resistance, toxin synthesis and enzyme production, and confer a selective advantage to bacteria

Question 15

what processes give rise to genetic variation in bacteria

Answer 15

transformation, conjugation, transduction

binary fission cannot give rise to genetic variation!

Question 16

describe the process of binary fission

Answer 16

1. the bacterial chromsome is attached to the plasma membrane before DNA replication
2. DNA replication begins at the ori, where the replication bubble is formed when the two strands separate. each parental strand is used as a template for the synthesis of the daughter strand in semi-conservative DNA replication. the replication bubble grows bidirectionally away from the ori until the entire bacterial chromosome is replicated
3. after DNA replication is completed, cell growth occurs, and each circular DNA molecule is attached to the cell membrane
4. cell elongates and membrane growth causes two chromosomes to be moved apart
5. cell division in bacteria is controlled by the septal ring which directs the assembly of the septum, separating the two daugher cells. the septum extends as the cell membrane invaginates as new cell membrane and cell wall materials are added to it
6. invaginating cell membrane together with the newly formed septum splits the cell into two genetically identical daughter cells by cytokinesis

Question 17

how is genetic material transferred from one cell to another

Answer 17

• DNA must be integrated into the
  chromosome of the recipient cell via homologous recombination –> needed by cells to accurately repair harmful breaks that occur on both strands of DNA.
• The transferred portion of the donor chromosome will be exchanged with a portion of the chromosome of the recipient cell that is very similar in sequence (i.e. highly homologous) –> segment of the
  recipient chromosome that is exchanged for the donor chromosome is excised and degraded

Question 18

what are the principles of transformation

Answer 18

Transformation: when recipient cell takes up small fragments of naked DNA from the surrounding environment

This DNA can originate from either:
- a donor bacterial cell which lyses and releases its DNA into the surrounding environment
- artificially constructed plasmids

how to artificially increase competency in bacteria during artificial transformation methods (plasmid DNA utilised)?
- Treatment of bacteria with calcium chloride followed by heat shock
- Electroporation

Question 19

steps involved during transformation

Answer 19

1. The donor bacterial cell lyses and releases naked DNA fragments (donor DNA fragment).
2. A competent recipient cell TAKES UP one or more of the donor DNA fragments into its cytoplasm via its competence factor. (cell surface proteins that bind to DNA fragments and aid in their uptake)
3. Homologous recombination of the donor DNA fragment takes place with a homologous section of the recipient cell’s chromosome.
4. This results in the homologous segment of the donor cell’s DNA being incorporated into the recipient cell’s chromosome and the homologous segment of the recipient cell’s
   chromosome being excised and degraded. The recipient cell is now known as a recombinant cell.

Question 20

what is transduction?

Answer 20

• bacteriophages carry bacterial genes from their first host cell (donor) to their second host cell (recipient) due to errors in the phage reproductive cycle.
• The DNA fragment of the donor cell may be incorporated into the genome /chromosome of the recipient cell via homologous recombination.

types of transduction
- generalised transduction
- specialised transduction

Question 21

what is generalised transduction

Answer 21

• when bacteriophage infects host cell, there is an accidental incorporation of a random fragment of DNA from first host cell into the phage capsid
• small fraction of the virions produced during the lytic cycle contain a random fragment of the bacterial genome instead of phage DNA.
• When such a defective phage (carrying additional host genes) infects a second host cell (recipient bacterium), the DNA of the donor bacterium is inserted into the recipient bacterium. –> donor genes integrated into the recipient cell’s genome by homologous recombination
• Transduction mediated by virulent phages is called generalised transduction, because each portion of the bacterial genome has approximately the same probability of being transferred from donor to recipient bacteria, i.e. any bacterial gene can potentially be transferred by this method.

Question 22

steps involved in generalised transduction

Answer 22

1. The virulent phage injects its DNA into its first host bacterial cell (i.e. the donor bacterium) and the first host cell’s chromosome is degraded.
2. The phage makes use of the host’s DNA replication machinery to synthesise more phage DNA + host cell’s gene expression machinery to synthesise more phage proteins
3. Occasionally, a small piece of the first host cell’s degraded DNA is accidentally packaged within a phage capsid in place of the phage genome
   during the assembly stage of the lytic cycle –> results in defective phage.
4. first host bacterium is lysed, and new phages are released into the environment.

5.The defective phage progeny which contains the first host cell’s DNA fragment may infect a second host cell (recipient bacterium) –> inject the DNA fragment acquired from the previous host cell into it.

6. The donor DNA is incorporated into the
   second host cell’s genome by homologous recombination, in which the donor DNA replaces the homologous region of the recipient cell’s chromosome. The recipient cell is now known as a recombinant cell.

Question 23

what is specialised transduction

Answer 23

During the reproduction of temperate phages (e.g.  phage), the phage genome is integrated into
the first host cell’s chromosome as a prophage during the lysogenic cycle. Upon induction, the
phage genome is excised and the cell is switched to the lytic cycle.
 The excision of the prophage is sometimes imprecise, resulting in segments of phage DNA
that lack part of the normal phage genome and contain part of the bacterial chromosome located
adjacent to the prophage attachment site.
 When such a phage infects a second host cell (i.e. the recipient bacterium), the DNA of the donor
bacterium is inserted into the recipient bacterium along with a portion of the phage
genome;
OR
 integration of the donor genes into the recipient cell’s genome by homologous recombination
can occur.
Transduction mediated by temperate phages is called specialised transduction, because only
specific portions of the bacterial genome, i.e. genes near the prophage insertion site on the host
(donor) chromosome have a high probability of being transferred from donor to recipient
bacteria.