Question

Hello! Can someone help me answer this please?

1. Answer the following questions about chromosomal and extra-chromosomal DNA: A. A previously undiscovered single-cell organism was found living at a great depth on the ocean floor. Its nucleus contained only a single linear chromosome with 7* 106 nucleotide pairs of DNA coalesced with three types of histone-like proteins. A short micrococcal nuclease digestion yielded DNA fractions of 700, 1400, and 2100 bp. Predict what these fractions represent. What conclusions can be drawn? B. After salivary gland cells from Drosophila are isolated and cultured in the presence of radioactive thymidylic acid, autoradiography is performed, revealing the presence of thymidine within polytene chromosomes. Predict the distribution of the grains along the chromosomes.

Answer 1

A. Micrococcal Nuclease is an endonuclease thst digests sDNA or RNA, preferntially at AT- or AU-rich regions. These enzymes are responsible for fragmentation of chromatins. In the given expt, the total DNA of 7*10^6 bp are digested to yield 700, 1400 and 2100. Chromosomes are made of nucleosomes which are histone octamers bound to the DNA. Micrococcal nucleases cleave between the nucleosomes initially. So the 2100 fragments are the mononucleosomes. This is followed by end-trimming on further incubation that reduces the length of DNA. So 1400 fragment is the trimmed nucleosome particle. After prolonged incubation, nucleases cleave this trimmed nucleosome to final core particles that are represented by the 700 fragments.

B. Radioactive thymidine is used to elucidate the structure od Polytene chromosome. The details of polytene grains (dark areas of enriched light and dark bands) are explained in the picture below:

a Polyline chromosome x chronos are Right arm of chi (BR) Chr IV Band I paterband romocenters (Helero Reft arm A ch] (IL) chronation) Nucleotus -arm Right 1 Chr. 5 arm of air m. (LTL) Balbiani (OR) -found (chiring asonal puffs) insalivary glands. specialised interphate elsomosomes. repeated frounds of DNA replicath without well division. Puffs ) Transcriptionally active RNA synthesizer Interband Each chromosome made up of light & dank bands alternate Dark bands rich in DNA, RNA, Š.

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