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You are watching: What holds the two helixes in dna together

Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

1. Describe the types of chemical bonds and forces of the DNA double helix.

The DNA double helix has two types of bonds, covalent and hydrogen. Covalentbonds exist within each linear strand and strongly bond bases, sugars, andphosphate groups (both within each component and between components). Hydrogenbonds exist between the two strands and form between a base, from one strand anda base from the second strand in complementary pairing. These hydrogen bonds areindividually weak but collectively quite strong.

2. Explain what is meant by the terms conservative andsemiconservative replication.

3. What is meant by a primer, and why are primers necessary for DNAreplication?

A primer is a short segment of RNA that is synthesized by primase using the DNAas a template during DNA replication. After the primer has been synthesized, DNA polymerase then adds DNA to the 3′ end of the RNA. Primers are required becausethe major DNA polymerase catalyzing DNA replication is unable to initiate DNAsynthesis and, rather, requires a 3′ end. The RNA is subsequently removed andreplaced with DNA so that no gaps exist in the final product.

4. What are helicases and topoisomerases?

5. Why is DNA synthesis continuous on one strand and discontinuous on the oppositestrand?

Because the DNA polymerase is capable of adding new nucleotides only at the 3′end of a DNA strand and because the two strands are antiparallel, at least twomolecules of DNA polymerase must take part in the replication of any specificregion of DNA. When a region becomes single stranded, the two strands have anopposite orientation. Imagine a single-stranded region that runs from left toright. At the left end, the 3′ end of one strand points to the right, andsynthesis can initiate and continue toward the right end of that region. Theother strand has a 5′ end pointing toward the right, and synthesis cannotinitiate and continue toward the right end of the single-stranded region at the5′ end. Instead, synthesis must initiate somewhere to the right of the left endof the single-stranded region and move toward the left end of the region. As thefirst strand continues synthesis (continuous synthesis), the single-strandedregion extends toward the right. This now leaves the second strand unreplicatedin this new region of single-strandedness, and there must be a second initiationof DNA synthesis moving from the current right end of the single-stranded regiontoward the first initiation point on that strand. This results in discontinuoussynthesis along that strand.

6. If thymine makes up 15 percent of the bases in a specific DNA molecule, whatpercentage of the bases is cytosine?

7. If the GC content of a DNA molecule is 48 percent, what are the percentages ofthe four bases (A, T, G, and C) in this molecule?

The frequency of both A and T is (52%) = 26%.

8. E. coli chromosomes in which every nitrogen atom is labeled(that is, every nitrogen atom is the heavy isotope 15N instead of thenormal isotope 14N) are allowed to replicate in an environment inwhich all the nitrogen is 14N. Using a solid line to represent aheavy polynucleotide chain and a dashed line for a light chain, sketch thefollowing:


The heavy parental chromosome and the products of the first replicationafter transfer to a 14N medium, assuming that the chromosomeis one DNA double helix and that replication is semiconservative.


Repeat part a, but assume that replication is conservative.


Repeat part a, but assume that the chromosome is in fact two side-by-sidedouble helices, each of which replicates semiconservatively.


Repeat part c, but assume that each side-by-side double helix replicatesconservatively and that the overall chromosomereplication is semiconservative.


Repeat part d, but assume that the overall chromosome replication isconservative.


If the daughter chromosomes from the first division in 14N arespun in a cesium chloride (CsCl) density gradient and a single band isobtained, which of possibilities in parts a through e can be ruled out?Reconsider the Meselson-Stahl experiment: what does itprove?

9. R. Okazaki found that the immediate products of DNA replication in E.coli include single-stranded DNA fragments approximately 1000nucleotides in length after the newly synthesized DNA is extracted and denatured(melted). When he allowed DNA replication to proceed for a longer period oftime, he found a lower frequency of these short fragments and longsingle-stranded DNA chains after extraction and denaturation. Explain how thisresult might be related to the fact that all known DNA polymerases synthesizeDNA only in a 5′ → 3′ direction.

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The results suggest that the DNA is replicated in short segments that aresubsequently joined by enzymatic action (DNA ligase). Because DNA replication isbidirectional, because there are multiple points along the DNA where replicationis initiated, and because DNA polymerases work only in a 5′ → 3′ direction, onestrand of the DNA is always in the wrong orientation for the enzyme. Thisrequires synthesis in fragments.

10. When plant and animal cells are given pulses of <3H>thymidine atdifferent times during the cell cycle, heterochromatic regions on thechromosomes are invariably shown to be “late replicating.” Can you suggest what,if any, biological significance this observation might have?

11. On the planet of Rama, the DNA is of six nucleotide types: A, B, C, D, E, and F.A and B are called marzines, C and D areorsines, and E and F are pirines. Thefollowing rules are valid in all Raman DNAs: