Ch. 4 Biology Homework

DNA molecules with a high percentage of guanine (G) and cytosine (C) are particularly stable. Why?

A G-C base pair has three hydrogen bonds, while an A-T base pair has two. G-C base pairs are more stable than A-T base pairs.

What feature of single nucleotides provides the energy needed for polymerization when nucleic acids are formed?

their phosphate groups

A double-stranded DNA molecule contains a total of 120 purines and 120 pyrimidines. This DNA molecule could be composed of _____.

120 thymine and 120 adenine molecules

If a DNA sample were composed of 10% thymine, what would be the percentage of guanine?

40

Which of the following pairs of base sequences could form a short stretch of a normal double helix of DNA? A. 5'-purine-pyrimidine-purine-pyrimidine-3' with 3'-purine-pyrimidine-purine-pyrimidine-5' B. 5'-AGCT-3' with 5'-TCGA-3' C. 5'-ATGC-3' with 5'-GCAT-3' D. 5'-GCGC-3' with 5'-TATA-3'

C

When double-stranded DNA is heated to 95°C, the bonds between complementary base pairs break to produce single-stranded DNA. Considering this observation, is the strand separation step required for replication of the double helix spontaneous? Why or why not?

No. A continuous input of energy is required. Chemical reactions are spontaneous if they can proceed on their own, without any continuous external influence such as added energy (in this case heat).

DNAase is an enzyme that catalyzes the hydrolysis of the covalent bonds that join nucleotides together. What would first happen to DNA molecules treated with DNAase?

The phosphodiester bonds between deoxyribose sugars would be broken.

Why is it that RNA can catalyze reactions but DNA cannot?

The sugar of RNA is much more reactive than the sugar of DNA.

Identify three possible components of a DNA nucleotide.

deoxyribose, phosphate group, thymine

The condensation reaction that forms nucleic acid polymers occurs between a _____ group on one nucleotide and a _____ group on a second nucleotide.

phosphate, hydroxyl