Explanations
Explanations

1.      D     

Adenosine triphosphate, ATP, is the molecule that provides most of the energy used by cells. When the bond between the second and third phosphate molecules in ATP is hydrolyzed, or broken, energy is released. The by-product of this process is a molecule of ADP, adenosine diphosphate, which can be recycled into another ATP molecule through the process of cellular respiration. Respiration uses oxygen to break down glucose and releases water as a by-product.

2.      A     

Glycolysis, the first step in aerobic cellular respiration, takes place in the cytoplasm. The citric acid cycle, electron transport chain, and oxidative phosphorylation constitute the remaining steps in aerobic respiration and take place in the mitochondria. Photosynthesis occurs in the chloroplasts of plant cells.

3.      B     

A codon, the nucleotide sequence on mRNA used to specify an amino acid, is composed of three nucleotides. Although there are some amino acids that can be indicated by more than one codon, every three-nucleotide codon specifies only one amino acid.

4.      B     

In RNA, cytosine pairs with guanine, and adenine pairs with uracil. Therefore, the tRNA anticodons corresponding to the mRNA AUGCUG are UAC and GAC.

5.      B     

To solve this question, trace the sequence back through mRNA and from there to DNA. GUA is the mRNA sequence that corresponds to CAU. (Remember: C pairs with G and A pairs with T in DNA; in RNA A pairs with U.) The DNA sequence that would code for this mRNA codon is CAT. If you memorize the rule that tRNA sequence is the same as DNA sequence only with U substituted for T, you can answer this type of question without doing any scratch work.

6.      C     

DNA replication occurs during interphase. During this phase, chromosomes are replicated to provide a full set of genetic material for both daughter cells. Mitosis is the phase during which the cell splits into two daughter cells, but the DNA is replicated before the cell enters this process. Prophase is the first stage of mitosis, while metaphase is the second stage of mitosis.

7.      D     

Chromosomes are composed of deoxyribonucleic acid (DNA), a polymer made up of nucleotides strung together through covalent bonding. Adenosine triphosphate (ATP) is the primary energy source of cellular processes, and adenosine diphosphate (ADP) is the by-product formed when the third phosphate bond is broken to release ATP’s energy. The phospholipid bilayer is the major component of all biological membranes.

8.      B     

The haploid number is the number of unique chromosomes, or homologous pairs, in a cell. The human haploid number is 23. A normal human somatic (not sperm or ovum) cell has 46 total chromosomes in its nucleus. However, these chromosomes are found in pairs, so there are actually 23 pairs of what are called homologous chromosomes.

9.      A     

The part of the cell cycle in which cell division occurs is called cytokinesis. During prophase, homologous chromosomes begin to separate with the help of the spindle. Metaphase is marked by the formation of the metaphase plate and the lining up of the chromosomes along the midline of the cell. During anaphase, each chromosome splits into its two chromatids, which are separated to opposite poles of the cell. Telophase is the last phase of mitosis before the cell splits in two.

10.      E     

In the Krebs cycle, also called the citric acid cycle, acetyl-CoA produced from pyruvate in the pyruvate deoxygenase complex combines with oxaloacetic acid to form citric acid. At the end of the cycle, oxaloacetic acid is formed again, making it both a reactant and a product. NADH is only a product of the Krebs cycle and is broken down after glycolysis.

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