Question 1

Which of the following best describes the central dogma of molecular biology?

Accepted Answer

DNA is transcribed into RNA, which is then translated into protein.

Question 2

A researcher isolates a DNA segment from a bacterium and finds it contains both chromosomal DNA and a small circular DNA molecule. What is the most likely identity of the small circular DNA?

Accepted Answer

Plasmid

Question 3

During transcription in prokaryotes, which of the following statements is correct regarding the coding and template strands?

Accepted Answer

The template strand is used by RNA polymerase to synthesize a complementary mRNA molecule.

Question 4

Which of the following best explains why the genetic code is described as 'degenerate'?

Accepted Answer

Multiple codons can code for the same amino acid.

Question 5

A mutation changes a codon from $$\mathrm{UAC} ($$tyrosine) to $$\mathrm{UAA} ($$stop codon). What type of mutation is this, and what is the likely effect on the protein?

Accepted Answer

Nonsense mutation; it will result in a truncated protein.

Question 6

Which of the following is a function of the lac operon structural genes in E. coli?

Accepted Answer

They code for enzymes that metabolize lactose.

Question 7

A patient is exposed to ultraviolet (UV) light, which causes the formation of thymine dimers in DNA. Which repair mechanism can directly reverse this damage?

Accepted Answer

Photolyase-mediated repair

Question 8

Which of the following best describes the process of conjugation in bacteria?

Accepted Answer

Transfer of genetic material via direct cell-to-cell contact, typically involving an F plasmid.

Question 9

Which of the following statements about the human microbiome is correct?

Accepted Answer

It is influenced by factors such as diet, antibiotics, and mode of birth delivery.

Question 10

A patient develops a Clostridioides difficile infection after taking broad-spectrum antibiotics. This is most likely due to disruption of which protective mechanism?

Accepted Answer

Microbial antagonism (competitive exclusion) by normal microbiota

Question 11

Which of the following best defines an opportunistic pathogen?

Accepted Answer

A microorganism that causes disease only when the host's defenses are compromised.

Question 12

A bacterial pathogen enters the body through a cut in the skin. Which portal of entry is being utilized?

Accepted Answer

Parenteral route

Question 13

If a toxin has an $$\mathrm{LD}_{50} of$$ $$10\ \mathrm{ng}$$ and another has an $$\mathrm{LD}_{50} of$$ $$100\ \mathrm{ng}$$, which statement is correct?

Accepted Answer

The toxin with $$\mathrm{LD}_{50} = 10\ \mathrm{ng} is $$more lethal.

Question 14

Which of the following is a characteristic of exotoxins?

Accepted Answer

They are proteins secreted by bacteria and are often highly specific in their action.

Question 15

Which of the following is NOT a physical factor of the first line of innate immune defense?

Accepted Answer

Antibody production

Question 16

Which chemical factor in the first line of defense is produced by sebaceous glands and helps inhibit microbial growth by lowering skin pH?

Accepted Answer

Sebum

Question 17

Which of the following best describes the role of toll-like receptors (TLRs) in innate immunity?

Accepted Answer

They recognize pathogen-associated molecular patterns (PAMPs) and activate immune responses.

Question 18

During inflammation, which process allows leukocytes to move from the bloodstream into infected tissues?

Accepted Answer

Diapedesis

Question 19

Which immunoglobulin class is most abundant in serum and can cross the placenta to provide passive immunity to the fetus?

Accepted Answer

IgG

Question 20

A patient is exposed to a pathogen for the second time and mounts a faster, stronger immune response. What is this phenomenon called?

Accepted Answer

Secondary (anamnestic) response

Microbiology Exam IV Study Guide – Step-by-Step Guidance

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