Enzymatic proteins are essential to metabolic processes within the cell in that they speed up cellular reactions that would otherwise take too long. Defensive proteins help fight disease, receptor proteins respond to stimuli, motor proteins help with cell movement, and transport proteins move substances across the cell.  

In a fasting state, the body will attempt to break down macromolecules into smaller molecules to help generate ATP. Nitrogen is abundant in proteins, and thus excessive amounts of nitrogen in the urine likely came from the breakdown of proteins secondary to starvation. The other answer choices would not explain the presence of nitrogen in the urine.

Essential amino acids are termed "essential" because humans cannot biologically manufacture them. Therefore, they must be ingested exogenously (from sources outside of the human body). There are ten essential amino acids, and these ten are obtained from the human diet (e.g., foods we ingest)

The ringed structure indicates that the molecule in question is in fact a steroid molecule. For example, cholesterol has is an important steroid molecule in the human body. It contains four rings in its structure. Steroids are easy to identify by their ringed structure.

Chargaff found that in double-stranded DNA, the number of guanine bases should be equal to the number of cytosine bases, and the number of adenine bases should equal the number of thymine bases. These rules proved to be important pieces of evidence for the idea of complementarity, the theory that each DNA base pairs only with a specific other base on its opposite strand.

According to Chargaff's rules, the statement regarding guanine and cytosine bases is correct. The two other statements that are similarly worded are not correct because they do not compare the frequencies of two bases that are complementary to each other (adenine will not bind cytosine and guanine will not bind thymine). Finally, guanine-cytosine bonds are more stable than adenine-thymine bonds.

An amino acid may be represented by multiple different codon sequences of base pairs of DNA. This codon degeneracy is what allows for the greater variability of DNA as compared to amino acid sequences.

For example, a DNA sequence may show variability without affecting the variability of the coded amino acids.

Organism 1 DNA: 5'-AAAGCAGGC-3' // Organism 2 DNA: 5'-AAGGCTGGT-3' // Differences: 3

Organism 1 RNA: 3'-UUU-CGU-CCG-5' // Organism 2 RNA: 3'-UUC-CGA-CCA-5' // Differences: 3

Organisms 1 Amino Acids: Phe-Arg-Pro // Organism 2 Amino Acids: Phe-Arg-Pro // Differences: 0

The backbone of DNA is made up of deoxyribose sugars linked to phosphate groups. These units are joined by phosphodiester bonds into chains. Nitrogenous bases are bound to the sugars of these groups and join DNA strands together by hydrogen bonds with their complementary base pairs.

Amino acids are the building blocks of proteins, and are not found in DNA. Alpha-linked glucose residues describe a type of polysaccharide, namely glycogen. Glycerol and fatty acids describe a type of lipid known as a triglyceride. Triglycerides and glycogen are primarily used in energy storage.

By looking at the name "phosphodiester bond" you should realize that a phosphate group is involved. A phosphodiester bond occurs between the phosphate group and a pentose sugar in the DNA backbone. The phosphate group from one nucleotide binds to the 3’ carbon on pentose sugar from the other nucleotide.

Nitrogenous bases are attached to the 1' carbon of the ring by a glycosidic linkage.

Remember that all nucleic acids contain nucleotides, composed of a pentose (a five-carbon sugar), a nitrogenous base, and a phosphate group. DNA and RNA must both contain a five-carbon sugar. RNA contains the pentose ribose, while DNA contains the pentose deoxyribose.

Glycogen does not contain a five-carbon sugar because it is made up of glucose subunits. Glucose contains six carbons.

ATP stands for adenosine triphosphate and GTP stands for guanosine triphosphate. Both of them are nucleic acids, meaning that they must contain a pentose sugar, a nitrogenous base, and phosphate groups. Both ATP and GTP contain three phosphate groups. The only difference between ATP and GTP is their nitrogenous base. ATP contains adenine whereas GTP contains guanine. Recall that adenine and guanine are both purines.

ATP and GTP do not contain any pyrimidines (cytosine, thymine, and uracil).