Fund of Microbiology (BIOL 107 606)

Correct Answer E: Telophase

Detailed Explanation of the Correct Answer:

E. Telophase is the correct answer because the image shows two distinct nuclear regions forming within a single cell, which is a defining feature of telophase. During this phase, the separated chromosomes reach the opposite poles of the cell and begin to de-condense back into chromatin. At the same time, nuclear envelopes start to re-form around each set of chromosomes, effectively restoring the nuclei in each emerging daughter cell. The cell also begins to show signs of cytokinesis, as indicated by the clear indentation or cleavage between the two nuclear regions in the image.

Explanation of Why the Other Options Are Incorrect:

A. Interphase – This is the phase where the cell is not actively dividing. The DNA is in the form of uncondensed chromatin, and the nucleus is intact. No visible chromosomes or nuclear division occurs in this phase, which contrasts with what is shown in the image.

B. Metaphase – In metaphase, chromosomes align in the middle of the cell along the metaphase plate. The image does not show any such alignment; instead, the chromosomes are already separated.

C. Prophase – Prophase is characterized by the condensation of chromatin into chromosomes and the beginning of spindle fiber formation. However, no nuclear envelope breakdown or chromosome condensation is occurring in the image; the chromosomes are already separated into two regions.

D. Anaphase – In anaphase, sister chromatids are pulled apart toward opposite poles of the cell. The image shows that the chromatids have already been moved to opposite ends and are now enclosed in forming nuclei, indicating that anaphase is complete and telophase has begun.

Summary:

The image depicts two forming nuclei within one cell, with chromosomes enclosed and the cell beginning to divide. These characteristics are consistent with telophase, the final stage of mitosis before cytokinesis. Therefore, the correct answer is E. Telophase.

Correct Answer: Homozygous dominant

Explanation:

At locus A, both homologous chromosomes (one red and one blue) carry the dominant allele "A"

Since both alleles are the same (A and A), the individual is homozygous.

Because the allele is uppercase "A", it represents the dominant form of the gene.

Therefore, the individual is homozygous dominant at locus A.

Why the other options are incorrect:

Homozygous recessive: This would require two recessive alleles ("a a"), which is not the case here.

Heterozygous: That would involve one dominant and one recessive allele (A a), but both alleles are A in this image.

Correct Answer B: Acetyl-CoA production

Detailed Explanation of the Correct Answer:

B. Acetyl-CoA production (also called pyruvate oxidation) is the correct answer because this step does not directly produce any ATP. During this stage, each pyruvate molecule (from glycolysis) is converted into acetyl-CoA, releasing one molecule of CO₂ and generating one molecule of NADH per pyruvate. However, no ATP is synthesized during this step.

Explanation of Why the Other Options Are Incorrect:

A. Krebs cycle – This cycle produces 2 ATP per glucose (1 per turn), along with NADH and FADH₂, which are used later in the electron transport chain.

C. Glycolysis – This anaerobic step breaks down glucose into two molecules of pyruvate and yields a net gain of 2 ATP per glucose molecule.

D. Electron transport chain – This is the main ATP-producing step, generating about 32–34 ATP molecules through oxidative phosphorylation.

Summary:

The acetyl-CoA production step is essential for preparing molecules for the Krebs cycle but does not generate any ATP directly. Therefore, the correct answer is B. Acetyl-CoA production.

Correct Answer D: Cell wall

Detailed Explanation of the Correct Answer:

D. Cell wall is correct because the cell wall is a rigid outer layer found in plant cells (and some fungi and bacteria) that provides structural support and acts as a protective barrier. It is primarily made of cellulose, a complex carbohydrate that gives the wall its strength. The cell wall helps maintain the shape of the plant cell, prevents excessive water intake, and supports the overall structure of the plant.

Explanation of Why the Other Options Are Incorrect:

A. Xylem – Xylem is a type of vascular tissue in plants that transports water and minerals, but it is not a part of an individual plant cell’s structure.

B. Cell membrane – While the cell membrane is a selective barrier that controls what enters and exits the cell, it does not provide the rigid support that the cell wall does.

C. Chloroplast – Chloroplasts are the organelles where photosynthesis takes place. They help the plant make food but do not act as a barrier or support structure.

Summary:

The cell wall is the part of a plant cell that provides support and acts as a protective barrier, making the correct answer D. Cell wall.

Correct Answer B: Mitochondrion

Detailed Explanation of the Correct Answer:

B. Mitochondrion is correct because the Krebs cycle (also known as the citric acid cycle) takes place in the matrix of the mitochondria in eukaryotic cells. The mitochondrion is often called the "powerhouse" of the cell because it is where most of the cell’s ATP (energy) is produced. During the Krebs cycle, acetyl-CoA is broken down, and high-energy electron carriers NADH and FADH₂ are produced, which then feed into the electron transport chain to generate ATP.

Explanation of Why the Other Options Are Incorrect:

A. Cytoplasm – The cytoplasm is where glycolysis occurs, not the Krebs cycle. Glycolysis happens before the pyruvate enters the mitochondrion.

C. Nucleus – The nucleus houses the cell's DNA and controls gene expression but is not involved in energy production or the Krebs cycle.

D. Chloroplast – The chloroplast is found in plant cells and is responsible for photosynthesis, not cellular respiration or the Krebs cycle.

Summary:

The Krebs cycle occurs in the mitochondrion, specifically in its matrix, where it plays a central role in energy production. Therefore, the correct answer is B. Mitochondrion.

Correct Answer A: Flagellum

Detailed Explanation of the Correct Answer:

A. Flagellum is correct because the flagellum is a long, whip-like tail that enables the sperm cell to move through fluid. In human sperm, the flagellum generates movement by whipping back and forth, propelling the sperm toward the egg for fertilization. This structure is critical for sperm motility and successful reproduction.

Explanation of Why the Other Options Are Incorrect:

B. Cilium – Cilia are short, hair-like structures found on the surface of some cells (like in the respiratory tract) that move substances across the cell surface. They are not involved in the movement of sperm cells.

C. Centrosome – The centrosome is an organelle that organizes microtubules and helps in cell division, but it does not play a direct role in cell movement or motility.

D. Centriole – Centrioles are components of the centrosome involved in spindle formation during cell division. While the basal body of a flagellum is derived from a centriole, the centriole itself is not the structure that causes movement.

Summary:

The flagellum is the specialized structure that enables sperm cells to swim, making A. Flagellum the correct answer.

Correct Answer A: Oxidized

Detailed Explanation of the Correct Answer:

A. Oxidized is the correct answer because oxidation is defined as the loss of electrons. In this reaction, NADH donates two electrons and one hydrogen ion (H⁺) to enzyme complex I in the electron transport chain, converting into NAD⁺. Since NADH loses electrons, it is said to be oxidized.

Explanation of Why the Other Option Is Incorrect:

B. Reduced – This would mean gaining electrons, which is not the case here. NAD⁺ is the oxidized form, and when it gains electrons to become NADH, it is reduced. But in this situation, the reverse is happening—NADH is losing electrons.

Summary:

Because NADH donates electrons and a hydrogen ion to the electron transport chain, it is oxidized to become NAD⁺. Therefore, the correct answer is A. Oxidized.

Correct Answer E: Atoms 2 and 3

Explanation:

Isotopes are atoms that have the same number of protons (same atomic number) but a different number of neutrons. This means they are the same element but differ in atomic mass.

Atom #2 and Atom #3 both have 10 protons, meaning they are the same element. However:

Atom #2 has 11 neutrons

Atom #3 has 10 neutrons

Because they differ in neutrons but have the same number of protons, they are isotopes.

Why the other options are incorrect:

A) Atoms 1 and 4:

Both have 11 protons, but also have different numbers of electrons (10 vs. 11). This makes them ions, not isotopes.

B) Atoms 1 and 2:

Different number of protons (11 vs. 10), so they are not the same element and therefore cannot be isotopes.

C) Atoms 1 and 3:

Different number of protons (11 vs. 10), so they are not isotopes.

D) Atoms 3 and 4:

Different number of protons (10 vs. 11), so they are different elements, not isotopes.

Correct Answer B: Genotype

Detailed Explanation of the Correct Answer:

B. Genotype is the correct answer because it refers to the genetic makeup of an organism, specifically the sequence of nucleotides (DNA bases: A, T, C, G) in a gene that determines a particular trait. The genotype provides the instructions for building proteins that contribute to the organism’s characteristics.

Explanation of Why the Other Option Is Incorrect:

A. Phenotype – This refers to the observable traits or characteristics of an organism, such as eye color or height, which result from the interaction of the genotype with the environment. It is the expression, not the genetic code itself.

Summary:

The genetic code or nucleotide sequence that determines a trait is called the genotype. Therefore, the correct answer is B. Genotype.