Bacteria (Structure and division)

What is the structure of bacterial chromosome?

• 1 double-stranded circular DNA molecule

• Forms loop domains by associating with proteins → Supercoiling → Highly condensed DNA

What is the nucleoid?

Region of the bacterial cell where the chromosomal DNA is confined to

What are ribosomes in bacteria?

• 70s

• For protein synthesis

What are storage granules in bacteria?

• Nutrients / chemical reserves that are stored in the cytoplasm

• May have glycogen, lipids and ions

What are plasmids in bacteria?

• Small, circular, autonomously replicating DNA molecule (not part of bacterial chromosome, but part of the genome)

• May contain genes that confer advantages for the bacteria living in stressful environments

• There may be more than 1 copy of the same plasmid in a cell

• Used extensively in genetic engineering as vectors

What is the cell membrane of bacteria?

• Phospholipid bilayer

• Where the electron transport chains and ATP synthase are embedded to produce ATP during photosynthesis / respiration

What is the cell wall of bacteria?

• Consists of peptidoglycan (carbs + proteins)

• Functions

  • Protects cell from osmotic lysis

  • Confers rigidity and shape

What is the structure of glycocalyx in bacteria?

• A layer of polysaccharides on the exterior of the bacterial cell wall

• Can be a distinct layer → Capsule

• Can be a diffused mass → Slime layer

What are the functions of capsules (some bacteria)?

• Protects the bacteria from being taken in via phagocytosis by phagocytes: Unable to recognise bacteria due to capsule

• Enables bacteria to adhere to one another or to particular surfaces → biofilm

• Prevent desiccation of bacteria with water to prevent bacteria from drying out

• Protection from antibiotics

What are the bacterial appendages?

• Fimbriae (s: fimbria)

• Pili (s: pilus)

• Flagella (s: flagellum)

What are fimbriae?

• Short, bristle-like fibres extending from the cell surface

• Usually evenly distributed over the entire cell surface or at poles

• For attachment to surfaces or other bacteria / organisms

What are pili?

• Involved in conjugation

• Involved in motility: A pious makes contact with a surface and retracts to pull the bacteria forward → Jerky, intermittent movement

• Longer and fewer in numbers than fimbriae

What are flagella?

• For motility

• Hollow cylindrical protein thread that propels the bacterium by rotations

• Some bacteria possess more than one flagellum

What is binary fission?

Asexual means by which bacterial cells produce genetically identical offspring

How does binary fission begin?

1. Origin of replication

2. Double helix separates to form a replication bubble made of 2 single DNA strands → Replication outward from ORI in both directions → 2 replication forks with leading and lagging strand

3. ORI move to opposite poles of the cell and attach to plasma membrane

4. Cell elongates to prepare for division

What is formed from binary fission?

1. DNA is circular with no free ends → Interlocking structure made of 2 daughter DNA molecules

2. Topoisomerase cuts, separates and reseals the two DNA molecules

What happens at the end of binary fission?

• Invagination of plasma membrane + deposition of new cell wall

• Parent cell is divided to 2 daughter cells each with a complete genome