lecture 3 - Growth and reproduction of Prokaryotes Flashcards
How do most prokaryotes divide?
Most prokaryotes divide by binary fission
The cell splits into two equal-sized daughter cells, each containing a copy of the mother’s genetic material.
Start of process to end of process is called “Cell Cycle”
What are the stages of the cell cycle of a prokaryotic cell?
B or G1 period - cell growth
C period - chromosome replication
D period - septum formation
cell division
Explain the role of FtsZ in cell division
Cell division is initiated by FtsZ
Acts as an organizer protein for cell division in most prokaryotes
In the presence of GTP FtsZ forms polymers able to associate into multi-stranded flexible structures
Forms a ring at midcell (Z ring) and recruits all the other cell division proteins at that site.
Full mechanism is unclear / different in different species
How does FtsZ know where midcell is?
MinCDE system properly positions the septum in the middle of the cell
MinCDE system repeatedly builds up and is broken down in polar parts of cell.
Because Min C prevents Z ring formation and concentration of MinC is higher in poles that at midcell, Z ring can only form in middle.
Process is probably slightly different in different bacteria.
Interesting target for new antibiotics
How is peptidoglycan made?
During cell growth and division, we need both enzymes that make and enzymes that break the cell wall. One carrier molecule called bactoprenol plays an important role in the synthesis of the peptidoglycan. Bactoprenol is an hydrophobic alcohol (C55) that binds and transports the building blocks of murein (1 NAG plus NAM with attached pentapeptide) across the cytoplasmic membrane. Once in the periplasm, enzymes called transglycosylases catalyze the insertion of the precursor into the growing point of the cell wall and catalyze glycosidic bond formation.
How do chromosome replication & cell division take place?
Chromosome replication and cell division are two highly coordinated events of the prokaryotic cell cycle.
The cell division septum is formed only after the two chromosomes have been replicated and segregated in the cell.
The septum is NEVER positioned on top of a chromosome (nucleoid occlusion), but it is located in the space between the two newly replicated chromosomes.
How do bacterial chromosomes replicate?
-bidirectionally from a single origin-
Origin is a distinctive region on the chromosome where special DNA sequences are repeated.
Initiation involves the binding of multiple copies of an initiation protein (called DnaA) to repeated DNA sequences, bending the DNA into a loop.
Replication proteins then add to this complex to form two replisomes (replication forks).
Termination of chromosome replication occurs when both replisomes reach a special region, the terminus.
What is interdivision time?
Interdivision time is period between the birth of a cell and the moment in which it divides into two daughter cells.
What does interdivision time depend on?
Depends on the conditions of the environment in which the bacteria are growing:
Rich environment, full of nutrients=Fast growth (short interdivision t.)
Poor environment, scarcity of nutrients=Slow growth (longer interdivision t.)
Which stage of the cell cycle has a variable length?
B or G1
C and D always the same
How do cell cycles overlap in fast growing bacteria?
Cells start a new round of chromosome replication before the stage of cell division in order to speed up the doubling time.
This way the newborn cell acquires a chromosome that is already partly replicated.
What is a rich medium?
a medium containing a mixture of different organic compounds like amino acids, vitamins, purines, pyrimidines etc.
What is a minimal medium?
a medium containing mineral salts of important bioelements like sulfur, nitrogen and phoshorous. Generally supplemented with a single organic compound that acts as source of carbon and energy (e.g. glucose).
How do you use spectrophotometry to measure bacterial growth?
Spectrophotometer shines light of a chosen wavelength through a specimen and determines the amount of light that gets through.
Turbidity or optical density of a culture is defined as:
OD = log (I0/I)
I0 = the amount of light entering the specimen
I = the amount of light emerging from it.
