GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Nucleic Acids
Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology
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Example Questions
Example Question #31 : Biochemistry
Which of the following nucleotides is present in RNA, but not DNA?
Uracil
Guanine
Cytosine
Adenine
Uracil
Uracil is one of the nucleotide bases that composes RNA. It is replaced by thymine in DNA.
Uracil, thymine, and cytosine are pyrimidine residues, capable of bonding and pairing with the purines adenine and guanine via hydrogen bonding. During DNA replication, thymine matches with adenine. During transcription, uracil matches with adenine.
Example Question #32 : Biochemistry
In DNA, which of the following nucleotides forms hydrogen bonds with guanine?
Thymine
Adenine
Uracil
Cytosine
Cytosine
Nucleotides (DNA monomers) and ribonucleotides (RNA monomers) are formed from a pentose sugar, phosphate group, and nitrogenous base. Each nitrogenous base has a complement that allows it to form hydrogen bonds to the template strand. This allows for the proper sequence of genetic code in DNA replication and RNA transcription.
Purine residues will always pair with pyrimidine residues. The purines are adenine and guanine. The pyrimidines are cytosine and thymine in DNA, and cytosine and uracil in RNA. Adenine will match with thymine or uracil, forming two hydrogen bonds, while cytosine will match with guanine to form three hydrogen bonds.
Example Question #33 : Biochemistry
Which of the following processes allows DNA mismatch repair enzymes to distinguish between old and new DNA strands?
Methylation
Heterochromatin
Histone acetylation
Euchromatin
Methylation
Template strand cytosine and adenine are methylated in DNA replication, which allows DNA mismatch repair enzymes to distinguish between old and new DNA strands.
In contrast, histone acetylation relaxes DNA coiling and allows for the DNA to be transcribed.
You can remember that methylation makes DNA mute, and acetylation makes DNA active.
Example Question #34 : Biochemistry
Which of the following amino acids is NOT necessary for purine synthesis?
Aspartate
Glutamine
Glycine
Tyrosine
Tyrosine
Purines are defined by their two-ring structure. A six-member ring with two amine groups and a five-member ring with two amino groups join to form each purine molecule. Addition substituents on the rings (often ketones or other amines) determine purine identity.
Glycine, aspartate, and glutamine are necessary for purine synthesis, along with phosphoribosyl pyrophosphate (PRPP). Glycine is incorporated into the final purine product structure, while glutamine is converted to glutamate and aspartate is converted to fumarate. The final purine product is used to make useful molecules, such as adenine and guanine for nucleotide synthesis.
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