Fund of Microbiology (BIOL 107 606)

Correct answer D: smaller, circular

Detailed explanation

Bacterial genomes are typically smaller than eukaryotic genomes and are usually organized into a single circular DNA molecule. This circular chromosome contains most of the genetic material needed for the bacterium to survive and reproduce. In contrast, eukaryotic genomes are generally larger, more complex, and composed of multiple linear chromosomes found within a membrane-bound nucleus.

Key differences:

Size: Bacterial genomes range from thousands to a few million base pairs, while eukaryotic genomes can contain billions.

Shape: Most bacteria have a circular chromosome, while eukaryotic DNA is arranged in linear chromosomes.

Why the other options are incorrect

A. smaller, linear

While bacterial genomes are smaller, they are usually not linear. Linear chromosomes are more characteristic of eukaryotes.

B. larger, linear

Bacterial genomes are not larger than eukaryotic genomes, and although linear genomes exist in some rare bacterial species, this is not the norm.

C. larger, circular

Again, bacterial genomes are not larger than eukaryotic ones. While the shape is correctly identified as circular, the size is incorrect.

Summary

Bacterial genomes are typically smaller and circular, which makes D smaller, circular the correct answer.

Correct Answer E: 31

Explanation:

Atomic Mass=Number of Protons+Number of Neutrons

The atomic mass (also called mass number) is the sum of protons and neutrons in an atom.

15 protons + 16 neutrons = 31

Atomic Mass=15 p⁺+16 n⁰=31

Atomic Mass=15p⁺+16n⁰=31

Electrons are not included in the atomic mass calculation because their mass is negligible compared to protons and neutrons.

Why the other options are incorrect:

A) 15:

This is the atomic number, representing the number of protons, not the mass.

B) 32:

This value is one more than the correct mass number and does not reflect the actual count of subatomic particles.

C) 17:

This is the number of electrons, which are not counted in the atomic mass.

D) 33:

This is higher than the actual mass and not supported by the subatomic particle count.

F) 0.48:

This is not a valid atomic mass; atomic mass numbers are whole numbers based on proton and neutron counts.

G) 16:

This is the number of neutrons, not the atomic mass.

Correct Answer A: Phagocytes like neutrophils and macrophages

Explanation:

Phagocytes, including neutrophils and macrophages, are key players in the innate immune response. They recognize, engulf, and destroy invading pathogens (like bacteria and viruses) through the process of phagocytosis. This involves surrounding the pathogen, enclosing it in a vesicle, and breaking it down with digestive enzymes.

Why the other options are incorrect:

B) Red blood cells:

Red blood cells transport oxygen and have no role in immune defense or phagocytosis.

C) B-cells:

B-cells are part of the adaptive immune system, producing antibodies to target specific pathogens, but they do not perform phagocytosis.

D) T-cells:

T-cells are also part of the adaptive immune system, and they help regulate immune responses or directly kill infected cells—but they do not engulf pathogens

Correct answer C: binary fission

Detailed explanation

Binary fission is a method of asexual reproduction in bacteria where one cell divides into two genetically identical daughter cells. It does not involve the exchange or recombination of genetic material between organisms. Therefore, it does not contribute to genetic variation or recombination.

The other three options do involve genetic recombination:

A conjugation

This is a form of horizontal gene transfer where two bacteria connect using a pilus, and one transfers a plasmid or part of its chromosome to the other. This leads to genetic variation.

B transformation

In this process, a bacterium takes up free DNA fragments from its environment (often from dead cells) and incorporates them into its own genome. This results in new genetic traits.

D transduction

This occurs when a bacteriophage (virus) transfers DNA from one bacterium to another. The bacterial DNA carried by the virus gets incorporated into the genome of the new host, creating genetic variation.

Summary

Binary fission simply copies and divides the bacterial genome without introducing new genetic combinations. Therefore, the correct answer is

C binary fission.

Correct answer B: to determine if a chemical or agent is a mutagen and able to cause mutations in DNA

Detailed explanation

The Ames Test is a biological assay used to assess the mutagenic potential of chemical compounds. It is widely used in genetic toxicology and pharmaceutical safety testing. The test uses specially engineered strains of bacteria, typically Salmonella typhimurium, that carry mutations making them unable to synthesize a particular amino acid (often histidine).

The principle behind the Ames Test is simple:

If a chemical causes mutations that reverse the original mutation (a reversion mutation), the bacteria regain the ability to grow on media lacking that amino acid.

An increased number of colonies on a test plate compared to the control indicates that the chemical likely induced mutations, suggesting it is a mutagen.

Why the other options are incorrect

A to determine how well bacteria can use the lac operon

This refers to a different genetic system used to study gene regulation and operon function, not mutation detection.

C to determine if bacteria have antibiotic resistance

This is tested using antibiotic susceptibility assays, not the Ames Test. The Ames Test is focused on mutation, not resistance genes.

D to amplify a specific section of DNA in a test tube

This describes PCR (Polymerase Chain Reaction), which is used for DNA amplification, not mutation detection.

Summary

The Ames Test is specifically designed to evaluate whether a substance causes DNA mutations. Therefore, the correct answer is

B to determine if a chemical or agent is a mutagen and able to cause mutations in DNA.

Correct Answer B: cholesterol

Why this is correct:

Cholesterol is embedded within the phospholipid bilayer of animal cell membranes, where it plays a critical role in modulating membrane fluidity and stability. It provides extra support by preventing the fatty acid chains of the phospholipids from packing too closely together, especially at lower temperatures. At higher temperatures, it helps stabilize the membrane and reduces excessive fluidity, making the membrane more flexible but not too permeable.

In summary, cholesterol acts as a buffer, maintaining optimal membrane consistency across temperature changes and ensuring the membrane remains functional and structurally sound.

Why the other statements are false:

A) glucose

False. While glucose is a source of energy and may be part of glycoproteins or glycolipids on the membrane surface, it does not provide structural support or flexibility to the membrane.

C) transport proteins

False. Transport proteins are involved in moving substances across the membrane, not in providing support or flexibility. Their function is primarily about selective permeability.

D) glycoproteins

False. Glycoproteins are involved in cell recognition and signaling, not structural support. They help with cell-cell communication and immune responses but do not influence membrane flexibility.

Summary:

The only component listed that directly contributes to support and flexibility of the animal cell plasma membrane is cholesterol. Therefore, the correct answer is:

cholesterol.

Correct answer A: Transduction

Detailed explanation

Transduction is a method of horizontal gene transfer in which DNA is transferred from one bacterium to another via a bacteriophage, which is a type of virus that infects bacteria.

How it works:

A bacteriophage infects a bacterial cell and mistakenly packages fragments of the host bacterium’s DNA into new virus particles.

When these phages infect another bacterium, they inject the previous host’s DNA into the new cell.

This foreign DNA may become integrated into the recipient’s genome, resulting in genetic recombination.

Why the other options are incorrect

B. Conjugation

This involves direct contact between two bacterial cells, typically through a pilus, and the transfer of DNA, often plasmids. It is a different form of horizontal gene transfer, but not mediated by a virus.

C Transformation

This occurs when bacteria absorb free DNA from their environment (usually from lysed cells). It’s also a form of horizontal gene transfer but does not involve bacteriophages.

D Binary fission

Binary fission is a form of asexual reproduction, not horizontal gene transfer. It produces identical offspring and does not create genetic variation by recombination.

Summary

The method of horizontal gene transfer in which a bacteriophage virus transfers DNA between bacteria is called

A Transduction.

Correct Answer E: Multiple alleles and codominance

Detailed Explanation of the Correct Answer:

E. Multiple alleles and codominance is correct because the ABO blood group system is governed by a single gene (I) that has three different alleles: Iᴬ, Iᴮ, and i. This makes it an example of multiple alleles. Additionally, Iᴬ and Iᴮ are codominant, meaning that if a person inherits both (genotype IᴬIᴮ), both A and B antigens are expressed on the surface of red blood cells. This results in blood type AB, where both alleles are fully and simultaneously expressed, rather than blending.

Explanation of Why the Other Options Are Incorrect:

A. Balanced polymorphism and codominance – While codominance is correct, "balanced polymorphism" refers to a situation where multiple alleles are maintained in a population due to evolutionary advantages. That concept does not fully define the ABO system.

B. Balanced polymorphism but not codominance – This is incorrect because codominance is a key feature of the ABO system.

C. Multiple alleles but not codominance – This is partially true (multiple alleles), but codominance is an essential part of the ABO blood group, so this answer is incomplete.

D. Multiple alleles and incomplete dominance – This is incorrect because the ABO system does not involve incomplete dominance. In incomplete dominance, heterozygous individuals show a blended phenotype, which is not the case here.

Summary:

The ABO blood group is determined by three alleles (Iᴬ, Iᴮ, i) and features codominance between Iᴬ and Iᴮ, making the correct answer E. Multiple alleles and codominance.

Correct answer B: flagellum

Detailed explanation:

The flagellum is a long, whip-like appendage that extends from the surface of many bacteria and is primarily responsible for motility — allowing bacteria to move toward or away from stimuli, a phenomenon known as taxis. The movement of the flagellum is powered by a rotary motor embedded in the bacterial cell membrane, and it enables bacteria to swim through liquids, such as in water or bodily fluids. Flagella are essential for the motility of many bacteria, and they allow bacteria to move in response to various environmental cues like nutrients (chemotaxis) or light (phototaxis).

Flagella can be located in various positions on the bacterial cell: they can be monotrichous (one flagellum), lophotrichous (multiple flagella at one end), amphitrichous (flagella at both ends), or peritrichous (flagella distributed all over the surface of the cell).

Why the other options are incorrect:

A. pilus

A pilus (or pili in plural) is a hair-like appendage found on the surface of many bacteria, but it is not involved in motility. Pili are mainly used for attachment to surfaces or for genetic exchange (such as in conjugation), not for movement.

C. glycocalyx

The glycocalyx is a gel-like outer layer composed of polysaccharides (or occasionally proteins) that surrounds the cell. It is involved in adhesion, protection, and biofilm formation, but it does not contribute to bacterial motility. The glycocalyx can help bacteria stick to surfaces, but it does not enable movement.

D. fimbriae

Fimbriae are short, bristle-like appendages on the bacterial surface that allow the bacterium to adhere to surfaces or host tissues. Like pili, they are primarily involved in attachment rather than motility.

Summary:

The flagellum is the cellular appendage that allows bacteria to be motile. It enables bacteria to swim and move in response to environmental stimuli. The other options (pilus, glycocalyx, and fimbriae) are not involved in motility. Thus, the correct answer is B flagellum.