Biochemistry : Nucleic Acids: DNA and RNA
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Example Question #84 : Macromolecule Fundamentals
If a sample of DNA contains 35% cytosine, what percentage of the DNA base pairs are thymine?
20%
30%
35%
15%
15%
Chargaff's rules state that guanine (G) = cytosine (C) and adenine (A) = thymine (T). Therefore, since the sample contains 35% C, it must also contain 35% G. 100% - 70% (G + C) leaves 30% left for A and T, or 15% T.
Example Question #84 : Macromolecule Fundamentals
What is the role of the 5’ to 3’ exonuclease activity in DNA replication?
It separates the strands to allow access of DNA polymerase.
It chews up RNA primers on the lagging strand.
It joins together the Okazaki fragments.
It synthesizes the primers for the leading strand.
It chews up RNA primers on the lagging strand.
The lagging strand is made up of Okazaki fragments due to discontinuous replication. Each of the fragments has its own primer made from RNA that needs to be removed and replaced with dNTPs. The exonuclease performs this function. A ligase comes through immediately after the exonuclease and joins the fragments together. Helicase is the enzyme that separates the DNA strands.
Example Question #85 : Macromolecule Fundamentals
Which statement best describes the role of hydrogen bonds between bases in a double-stranded DNA molecule?
Hydrogen bonding is important for the energetic stability of the DNA but contributes little to the specificity of base pairing.
Hydrogen bonding contributes little to the energetic stability of the DNA but is important for the specificity of base pairing.
Hydrogen bonding determines the melting temperature (Tm) of the DNA molecule.
Hydrogen bonding is important for both the energetic stability of the DNA and is also important for the specificity of base pairing.
Hydrogen bonding contributes little to the energetic stability of the DNA but is important for the specificity of base pairing.
Hydrogen bonds contribute solely to the base specificity of nucleic acids. Guanine and cytosine share three hydrogen bonds, while adenine and thymine share two. Incorrect base pairing will not have very favorable binding because hydrogen atoms and eletronegative atoms aren't lined up properly. The stability of DNA comes from favorable stacking interactions and hydrostatic effects of the hydrophobic bases and hydrophylic backbone.
Example Question #86 : Macromolecule Fundamentals
Which of the following is not a difference between DNA and RNA?
RNA has an extra hydroxide group on its sugar that is absent in DNA.
Adenine binds to thymine in DNA and uracil in RNA.
DNA is always double-stranded and RNA is always single-stranded.
DNA is generally more stable than RNA.
RNA frequently performs enzymatic activity in cells, while DNA does not.
DNA is always double-stranded and RNA is always single-stranded.
The correct answer is "DNA is always double-stranded and RNA is always single-stranded." While this is usually true, there are viruses that have single-stranded DNA and double-stranded RNA. All other answers are true. There is no thymine in RNA, so adenine binds to uracil instead. The ribose sugar on RNA has a hydroxide group not found on the deoxyribose sugar of DNA. This extra group makes RNA more reactive and less stable than DNA. RNA is frequently used by cells for its enzymatic activity in the form of ribosomes, while DNA is used only as a storage of information.
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