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

Griffiths AJF, fearbut JH, Suzuki DT, et al. An development to hereditary Analysis. 7th edition. Brand-new York: W. H. Freeman; 2000.


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1. Define the varieties of chemical bonds and also forces of the DNA double helix.

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

2. Define what is expected by the state conservative andsemiconservative replication.

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

A primer is a quick segment the RNA that is synthesized by primase using the DNAas a template during DNA replication. ~ the primer has actually 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 can not to start DNAsynthesis and, rather, requires a 3′ end. The RNA is subsequently eliminated andreplaced v DNA so the no gaps exist in the last product.

4. What are helicases and topoisomerases?

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

Because the DNA polymerase is qualified of adding brand-new nucleotides only at the 3′end the a DNA strand and because the 2 strands room antiparallel, at least twomolecules that DNA polymerase need to take part in the replication of any specificregion the DNA. As soon as a region becomes solitary stranded, the 2 strands have actually anopposite orientation. Imagine a single-stranded region that operation from left toright. At the left end, the 3′ end of one strand points come the right, andsynthesis have the right to initiate and also continue toward the right finish of the region. Theother strand has a 5′ finish pointing towards the right, and also synthesis cannotinitiate and also continue towards the right end of the single-stranded an ar at the5′ end. Instead, synthesis must initiate what to the appropriate of the left endof the single-stranded region and move toward the left finish of the region. As first strand proceeds synthesis (continuous synthesis), the single-strandedregion extends towards the right. This now leaves the second strand unreplicatedin this brand-new region of single-strandedness, and there have to be a second initiationof DNA synthesis moving from the existing right end of the single-stranded regiontoward the an initial initiation suggest on that strand. This results in discontinuoussynthesis follow me that strand.

6. If thymine renders up 15 percent the the bases in a details DNA molecule, whatpercentage that the bases is cytosine?

7. If the GC contents of a DNA molecule is 48 percent, what are the percentages ofthe 4 bases (A, T, G, and also C) in this molecule?

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

8. E. Coli chromosomes in i beg your pardon every nitrogen atom is labeled(that is, every nitrogen atom is the hefty isotope 15N instead of thenormal isotope 14N) are allowed to replicate in an atmosphere inwhich every the nitrogen is 14N. Utilizing a solid line to stand for aheavy polynucleotide chain and a dashed line for a irradiate chain, sketch thefollowing:

a.

The hefty parental chromosome and also the products of the very first replicationafter carry to a 14N medium, assuming the the chromosomeis one DNA dual helix and that replication is semiconservative.

b.

Repeat component a, but assume that replication is conservative.

c.

Repeat part a, but assume that the chromosome is in reality two side-by-sidedouble helices, each of i m sorry replicates semiconservatively.

d.

Repeat component c, but assume that each side-by-side twin helix replicatesconservatively and that the all at once chromosomereplication is semiconservative.

e.

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

f.

If the daughter chromosomes native the first division in 14N arespun in a cesium chloride (CsCl) thickness gradient and also a solitary band isobtained, i beg your pardon of possibilities in parts a with e can be ruled out?Reconsider the Meselson-Stahl experiment: what go itprove?

9. R. Okazaki discovered that the immediate products of DNA replication in E.coli include single-stranded DNA fragments around 1000nucleotides in length after the freshly synthesized DNA is extracted and also denatured(melted). As soon as he permitted DNA replication to continue for a longer duration oftime, he found a reduced frequency the these quick fragments and also longsingle-stranded DNA chain after extraction and denaturation. Explain how thisresult might be pertained to the reality that all well-known DNA polymerases synthesizeDNA only in a 5′ → 3′ direction.

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The results imply that the DNA is replicated in short segments that aresubsequently joined by enzymatic activity (DNA ligase). Due to the fact that DNA replication isbidirectional, since there room multiple points follow me the DNA whereby replicationis initiated, and because DNA polymerases work-related only in a 5′ → 3′ direction, onestrand the the DNA is constantly in the dorn orientation because that the enzyme. Thisrequires synthetic in fragments.

10. When plant and also animal cell are offered pulses the <3H>thymidine atdifferent times during the cell cycle, heterochromatic areas on thechromosomes room invariably shown to it is in “late replicating.” have the right to you indicate what,if any, organic significance this observation could have?

11. ~ above the world of Rama, the DNA is of 6 nucleotide types: A, B, C, D, E, and F.A and also B are referred to as marzines, C and D areorsines, and E and also F room pirines. Thefollowing rules space valid in every Raman DNAs: