What is the smallest unit of heredity?
Answer: B
Correct answer: gene
A gene is the smallest unit of heredity - the stretch of DNA that specifies one product and is inherited as a unit. A chromosome carries many genes; a codon is three bases within one; a nucleotide is a single chemical building block and carries no heritable information alone.
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A nucleotide contains which of the following?
Answer: D
Correct answer: all of these
A nucleotide has three parts: a five-carbon sugar (deoxyribose in DNA, ribose in RNA), a nitrogenous base, and one or more phosphate groups. Sugar plus base alone is a nucleoside; adding the phosphate makes it a nucleotide.
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The nitrogen bases in DNA are bonded to the
Answer: B
Correct answer: deoxyribose
The base is joined to the 1’ carbon of the deoxyribose by an N-glycosidic bond. The phosphate attaches to the sugar too, at the 5’ and 3’ carbons, so the sugar is the hub of the nucleotide: base on one side, backbone on the other.
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DNA replication is semiconservative because the ______strand will become half of the ________molecule.
Answer: B
Correct answer: template, finished
Each parental strand serves as the template for a new partner and is then retained as half of the finished duplex. That is what “semi-conservative” means - half of each daughter molecule is conserved from the parent - and it is what Meselson and Stahl demonstrated.
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In DNA, adenine is the complementary base for _____, and cytosine is the complement for ______.
Answer: C
Correct answer: thymine, guanine
The pairing is fixed by hydrogen bonding: A with T through two bonds, and C with G through three. Uracil replaces thymine in RNA, so it plays no part in DNA.
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The base pairs are held together primarily by
Answer: B
Correct answer: hydrogen bonds
The two strands are held to each other by hydrogen bonds between the paired bases, reinforced by the stacking of the flat rings. Individually these bonds are weak, which is what lets helicase separate the strands; collectively, over thousands of pairs, they hold the duplex firmly. The covalent bonds run along each backbone, not across between them.
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Why must the lagging strand of DNA be replicated in short pieces?
Answer: C
Correct answer: the DNA polymerase can synthesize in only one direction
DNA polymerase can add nucleotides only to a free 3’-OH, so it works 5’→3’ and nothing else. On the lagging strand that direction runs away from the advancing fork, so synthesis has to be done in short bursts - the Okazaki fragments - each begun near the fork and extended backwards, and afterwards joined by ligase.
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Ribosomal RNA _______
Answer: A
Correct answer: contributes to the structure of ribosomes
Ribosomal RNA is a structural and catalytic component of the ribosome: with the ribosomal proteins it forms the two subunits, and it is the rRNA itself, not any protein, that catalyses peptide bond formation. Carrying the code from DNA is the job of mRNA, and matching codons to amino acids that of tRNA.
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