Essentials of Genetics Study Set 2
Quiz 9 :
Dna Structure and Analysis
Z-DNA has left handed double helix structure. In left handed double helix, double helix wind lift in zigzag pattern. Z-DNA makes a specific target during viral infection because responses to variola, human cell create small transitory stretches of Z-DNA at site that regulate genes. The specifically targeting the segment of Z-DNA and inhibiting the synthesis of the protective proteins by this reason small pox virus can bypass cellular defense.
(a)The experiments conducted by Avery, MacLeod and McCarty tested the process of transformation in bacteria. As a part of their experiment they have extracted a filtrate from the bacterial cells that retained transformation properties. The filtrate lost its transformation property after being treated with DNase. Therefore, DNA must be the transforming material. Later, to prove the transforming principle, the Hershey-Chase conducted experiment using bacteriophage T2. The Hershey-Chase labeled either DNA (deoxyribonucleic acid) or protein in a phage T2 using radioactive phosphorus, which labels DNA, or sulfur, which labels protein. The bacteria were infected with the radioactive labelled phages. The progeny viruses isolated from the bacteria were labeled with radioactive phosphorus and not with sulfur. The experimenters concluded that DNA, not protein, was passed on to the progeny cells. (b)X-ray diffraction studies revealed the structure of the DNA. The results of X-ray diffraction showed that DNA bases were stacked on top of each other that look like a helix. Chargaff's experiments concluded that DNA contains four nitrogenous bases. The nitrogenous bases are adenine (A), guanine (G), cytosine (C) and thymine (T). Chargaff also revealed that the amount of Adenine was always proportional to the amount of Thymine, and Guanine was proportional to Cytosine. Therefore, in DNA purines and pyrimidines are present in equal proportions, (A + G) and (C + T) respectively. Using the results deduced from the Chargaff's experiments, and X-ray diffraction studies, Watson and Crick built models of DNA, which resulted in the right handed helix. (c)Chargaff's experiments helped to determine the base compositions in DNA were proportional to each other. The adenine and thymine; and guanine and cytosine are equally proportional to each other. Also, the sum of A and T was usually not equal to the sum of G and C. This concluded that purine would not pair with purines, and pyrimidines with pyrimidines. It was concluded that Guanine pairs with Cytosine and Adenine pairs with Thymine, when Watson and Crick constructed the molecular structures and determined that Guanine pairs with cytosine using three hydrogen bonds, whereas adenine pairs with thymine using two hydrogen bonds. (d)Reassociation kinetics revealed the presence of repetitive DNA. Reassociation kinetics involves denaturing double helical DNA and then allowing the DNA to renature. The time taken for reassociation of the fragments was measured. Some segments of DNA reassociate more quickly than the others. That means their reassociation time is very low. This shows that the presence of more repetitive DNA, and they are more likely to collide randomly during renaturing. These segments of DNA represented repetitive DNa.
Human cell activating gene which protect themselves from viral and it is discover the Z-DNA which regulate the genes and response the variola virus. DNA sequence is more stable in the Z confirmation than B confirmation. Z-DNA does not transcribe and RNA polymerase. Z-DNA inhibiting the synthesis of the protective proteins by which small pox virus bypasses this cellular defense mechanism.