Question

Please answer question 1b only. thanks. explanation should be deep

DNA replication

Watson and cricks idea that the double helix separates, and new babes come in to create 2 double helices, each having 1old and 1new strand, was envision at the DNA level. At the same time that Meselson and stahl were demonstrating the semi-conservative nature of DNA replication and disproving the conservative and dispersive hypothesis, other researchers were exploring DNA replication at the level of the chromosome. They use the fact that chromosome absorb dyes.

Researchers explored the DNA replication using the Easter lily in the 1950's. They labeled newly formed DNA with radioactive thymine, a precursor of thymine. When they over laid the treated DNA with photographic film, the radio activity exposed the film yielding dots that indicated the sites of replicating DNA. If a cell was allowed to divide and then the DNA was labeled, no dots formed, because the label was not incorporated- it was too late. If the cell divided once in the presence of a radioactive label and was then examined after the DNA had replicated again, both sister chromatids of each chromosome were labeled. if a cell divided once in the presence of the label, then was removed and allowed to divide again without the label, then one sister (half) of each replicated chromosome was labeled.

Questions:

1a- how would the results of the experiment have differed if the DNA replication was conservative? Dispersive

b- The researchers interpreted their results to mean that chromosomes are composed of DNA. But they were puzzled. How could a molecule that is moret han a yard long, with some 300 million twists, fold into a short, thick chromatid. Further explain how this is possible.

Answer 1

It is because of DNA packaging. We know DNA is a polynucleotide chain made up of sugar, phosphate and nitrogenous bases. The phosphate imparts a negative charge to the DNA double helix, and this has to be counterbalanced by the positive charge. Thus histone proteins are made that binds DNA and help in DNA packaging. The double-stranded DNA loops around the positively charged histone proteins about twice, forming nucleosomes (the building block of chromatin packaging) and thus takes less space within the cell. Further condensation occurs when histone proteins come in close proximity to each other. This tight winding of DNA results in the formation of tightly wrapped condensed chromosomes.