Lecture 18: Bacterial Structure And Functions

Question 1

What does a typical bacterial cell contain?

Answer 1

• cytoplasmic membrane + cytoplasm
• chromosomal DNA
• cell wall
• outer membrane
• flagellum
• pili (fimbriae)
• Sex pili
• Plasmid
• Capsule

Question 2

What is the cytoplasm?

Answer 2

• contains everything
• enzymes for providing energy & supporting cellular growth
• DNA that helps in protein synthesis (occurs in ribosomes)

Question 3

What is the composition and function of cytoplasmic membrane?

Answer 3

• composed of lipid bilayer (polar lipids)
• controls movement of nutrients in/out of cell
• contains transport proteins for movement of nutrients against conc. Gradient

Question 4

Describe the cell wall

Answer 4

• 90% of bacteria contain cell wall
• peptidoglycan (strength and porous for small molecules)
• if any change happens to cell wall, lysis occurs

Question 5

Describe Bacterial Ribosomes:

Answer 5

• supramolecular structures for protein synthesis
• contain rRNA and r-proteins
• 70S ribosomes that split into 50 S and 30 S
• large (50S) subunit has two rRnas ( 23S and 5S ) and 34 proteins
• small (30S) subunit has one rRNA (SSU RNA, 16A) and 21 proteins

Question 6

Describe Eukaryotic Ribosomes

Answer 6

• large in size (80S) and split into 60S and 40S subunits
• large 60S subunit has 3 rRNAs (28S,5.8S and 5S) and 50 proteins
• small 40S subunit has one rRNA ( SSU RNA,18S ) and 33 proteins

Question 7

Describe Archael Ribosomes

Answer 7

• 70S and split into 50 S and 30S subunits
• three rRNA (16s, 23s, 5s) and 50-70 r-proteins
• primary structure closer to eukaryotes than bacteria
• all archael r-proteins represented in eukaryotes
• no r-proteins are shared by archae and bacteria only

Question 8

What are the roles of ribosomes in protein synthesis?

Answer 8

• protein synthesis requires mRNA to attach to ribosomes and interact with tRNA
• the ribosomes contain rRNA and provide binding sites for mRNA and tRNA

Question 9

What are the 4 stages of protein synthesis

Answer 9

• initiation
• elongation of polypeptide chain
• termination
• recycling of subunits
• each stage assisted by protein factors

Question 10

What is the role of the SSU?

Answer 10

• decoding process

Major functional sites:

• mRNA tunnel to conduct mRNA
• Decoding center ( selection of aminoacyl tRNA)
• tRNA binding sites (A,P and E)

Question 11

What is the role of LSU?

Answer 11

• peptide bond formation

Major functional sites:

• tRNA binding sites (A,P and E)
• Peptidyl transferase center
• peptide exit tunnel

Question 12

What are the 3 types of cell wall, and based on what?

Answer 12

• gram positive
• acid fast
• gram-negative

Question 13

Describe the gram-positive bacterial cell wall:

Answer 13

• 1 cytoplasmic lipid bilayer membrane
• lower lipid content
• thick peptidoglycan layer
• peptidoglycan polymer has repeating units (monomers)
• repeating units are cross linked during cel, walls ynthesis to provide strength

Question 14

What do peptidoglycan (PG) monomers contain in gram positive bacteria ?

Answer 14

• Two amine sugars: N-acetyl glucosamine (NAG) and N-acetyl Nuramic (acid NAM)
• pentapeptide attached to NAM

Question 15

What is the process of cell wall synthesis?

Answer 15

• PG monomers synthesized in cytosine and bind to bactoprenol
• BP transports PG monomers across membrane and inserts monomers into existing PG
• glycosidic bonds link PH monomers into growing PG chain
• transpeptidases re-form peptide cross links between rows and layers of PG

Question 16

Describe cross lining in gram positive cell wall:

Answer 16

• transpeptidases first remove terminal D-Alanine from pengapeptides to form tetrapeptides
• this provides energy to link penta glycine bridge
• transpeptidases connect pentaglycine to D alanine of one monomers to L-lysine of the other to provide cell wall strength

Question 17

Describe the structure of acid-fast bacteria cell wall:

Answer 17

• gram positive during staining but stain poorly
• resist decolonization with acid-alcohol mixture , retaininh initial dye and appear red
• thin inner layer of peptidoglycan linked to a layer of arabinogalactan, linked to outer atypical membrane containing mycolic acids overlaid with polypeptides and glycolipids (impede entry of chemicals)

Question 18

What are the unique functions of Acid-Fast Bacteria?

Answer 18

• arabinogalactan linked to peptidoglycan linked to mycolic acid outer membrane provides additional cell wall strength
• fewer poring and much longer pores than gram negative cell wall lowering permeability of acid fast bacteria
• mycolic Acids and other glycolipids slow down entry of chemicals (nutrients) hence slow growth
• slow growth makes it more resistant to anti-bacterial agents and lysosomal phagocytes

Question 19

Describe the gram negative bacterial cell wall:

Answer 19

• 2 (cytoplasmic and outer) lipid bilayer membranes
• higher lipid content (LPS)
• thinner peptidoglycan (repeating cross linked monomers to provide cell wall strength)

Question 20

What do peptidoglycan monomers contain in gram negative bacteria?

Answer 20

• two amino sugars : N-acetyl glucosamine (NAG) and N-acetylene muramic acid (NAM)
• pentapeptide attached to NAM. D- glutamic acid in place of D- glutamine
• meso-Diaminopimelic acid in place of L-lysine
• Terminal D alanine removed for energy for cross linking
• D-alanine of one monomer is directly linked to meal diaminopimelic acid of the other

Question 21

What is the gram stain?

Answer 21

Differentiates gram positive and negative .