Question

3.) Describe, to the best of your ability the bond chemistry behind the following attributes of DNA:

a. the double-strandedness

b. the antiparallel nature of the two strands.

c. the twist of the ladder into a helix.

4.) Griffith discovered that S. pneumoniae caused pneumonia in mice. In his experiments, mice were injected with different strains of treated and untreated bacteria. For each of the following, indicate whether the mice lived or died and why.

a. Mice injected with heat-killed S (smooth) strain

b. Mice injected with living S (smooth) and heat-killed R (rough) strain

c. Mice injected with heat –killed S and living R

Answer 1

3. DNA are polymeric sequence of nucleotides. Each nucleotide consists of a nucleobase (that is the purines-adenine, and guanine,and the pyrimidines- thymine and cytosine), deoxyribose sugar and phosphate group. Each nucleotide are connected to each other by phosphodiester bonds. Two such polymer strands bond with each other to form the double stranded DNA.

a. The double strandedness of DNA is due to the formation of hydrogen bonds between the nucleobase of one strand with another. That is formation of complementary base pairing between purines and pyrimidines. The adenine and thymine form 2 hydrogen bonds, whereas the guanine and cytosine form 3. The DNA double strand are also maintained by intrastrand base stacking interactions that are hydrophobic and electrostatic in nature.

b. antiparallel nature of DNA are due to the hydrogen bonds that are formed between the complimentary base pairs which will not happen If the DNA were to be parallel.

c. In the DNA the sugar and the phosphate are hydrophilic in nature wheareas the bases are hydrophobic.The twisting of the DNA causes the spaces between the hydrophobic bases to minimize thereby not exposing them to fluid present surrounding the DNA. The complementary base pairing and stacking interaction also contribute to the twisting of DNA.

4. Griffith's experiment discovered that S. pneumoniae caused pneumonia and that bacteria are capable of transferring genetic information through transformation. In his experiment the mice were treated with two strains of bacteria the type III-S the smooth virulent one and type II-R the rough non-virulent one. The virulence of the bacteria is due to the presence of a polysaccharide capsule.

a. Mice injected with heat-killed S strain lived, as the bacteria had become ineffective due to heat.

b. Mice injected with living S (smooth) and heat-killed R (rough) strain died as the bacteria causing pneumonia was live and the rough strain didnot carry any virulence factor.

C. Mice injected with heat –killed S and living R died. This was due to bacterial trandormation that happened by transfer of genetic information from S train to R. The trasformed R strain now could synthesize the virulent capsule that caused pneumonia.