Microscopy
What does optical density (OD) measure in a bacterial culture?
The turbidity/light scattering due to total cell mass (living + dead)
In which direction does DNA synthesis occur?
5′ → 3′
What is the main function of peptidoglycan?
Provides structural support and prevents osmotic lysis
What component makes Mycobacteria resistant to many antibiotics and stains?
Mycolic acids
What sequence does the sigma factor recognize to initiate transcription?
The −10 (Pribnow box) and −35 promoter sequences
Why is optical density not always an accurate measure of viable cell count?
Because it counts both live and dead cells and can be influenced by cell clumping
What enzyme relieves supercoiling during replication?
DNA gyrase (a topoisomerase)
What molecule transports peptidoglycan precursors across the membrane?
Bactoprenol
Why can’t Mycoplasma be Gram stained?
Lack a cell wall
What sequence does DnaA bind to in bacterial DNA replication?
The OriC (origin of replication) region.
Why is immersion oil used in light microscopy?
It reduces light refraction and increases resolution
What provides the energy for DNA chain elongation?
Hydrolysis of deoxynucleotide triphosphates (dNTPs)
Which component of Gram-negative bacteria acts as an endotoxin?
Lipopolysaccharide (LPS), specifically lipid A
Why are Mycoplasma resistant to β-lactam antibiotics like penicillin?
β-lactams target peptidoglycan, which Mycoplasma lack
What is the function of RNA polymerase?
Produces RNA from a DNA template by linking nucleotides with phosphodiester bond
What is the main limitation of electron microscopy?
It requires dead, fixed samples (no live-cell imaging)
What is the function of primers in DNA replication?
Provide a free 3′ OH group for DNA polymerase to extend
What is the role of teichoic acids in Gram-positive bacteria?
They stabilize the cell wall, give a negative charge, and help in surface attachment
Why does the lag phase occur before exponential growth?
Cells are metabolically adjusting and synthesizing necessary enzymes
Why can transcription and translation occur simultaneously in prokaryotes but not eukaryotes?
Prokaryotes lack a nucleus; ribosomes can access mRNA immediately
Why is resolution more important than magnification in light microscopy, and what physical factors limit resolution?
Resolution determines the ability to distinguish two objects as separate. It is limited by the wavelength of light and the numerical aperture of the lens. Even with high magnification, without good resolution, images appear blurred.
Explain why replication is continuous on one strand and discontinuous on the other.
DNA is antiparallel; one strand can be synthesized continuously (leading), while the other (lagging) is synthesized in fragments (Okazaki fragments).
Explain the process of endospore formation and its significance.
Bacteria form a resistant spore with a tough coat under stress; spores survive extreme heat, desiccation, and chemicals
How does dilution rate in a chemostat affect growth, and what happens if it’s too high?
Dilution rate sets nutrient flow and growth rate; if too high, cells are washed out
Explain the advantage of operons in bacterial gene expression
They allow coordinated regulation of multiple genes under one promoter, saving energy and resources