7. The Size of DNA molecules. The DNA double helix contains exactly ten nucleoti

Question

7. The Size of DNA molecules. The DNA double helix contains exactly ten nucleotide pairs per turn, and each turn adds 3.4 nm to the overall length of the duplex. The circular DNA molecule of E. coli has about 4 million nucleotide pairs and a molecular weight of about 2.8A109.
a) What is the circumference of the E. coli DNA molecule? What problems might this pose for a cell that is only about 2 :m long?
b) Human mitochondrial DNA is also a circular duplex, with a diameter of about 1.8 :m. About how many nucleotide pairs does it contain? If it takes about 1000 nucleotides (pairs) of DNA to encode (specify the amino acid sequence of) a typical polypeptide, how many polypeptides can mitochondrial DNA encode?
c) What is the weight in grams of one nucleotide pair? How many nucleotide pairs are present in the 6 picograms (6A10-12 gm) of double stranded DNA present in the nucleus of most cells in your body?
d) What is the total length of the 6 picograms of DNA present in a single nucleus?

Explanation / Answer

a) Given that, DNA double helix each turn have 10 nucleotide pairs. Each turn = 3.4 nm circular DNA molecule of E. coli has about 4 million nucleotide pairs Circumference = d number of turns = 4 million nucleotide pairs / 10 nucleotide pairs = 4 x 10^5 Total distance = 4 x 10^5 x 3.4 x10^-9 m = 13.6 x10^5 nm Circumference = 22/7 x 13.6 x 10^5 nm                            = 42.7 x 10^5 nm If the circumference of the DNA is more than cell diameter, then the acomadation of DNA will be difficult. To overcome this condensation of the DNA occurs generally. b)diameter =1.8 m Circumference = d = 22/7 x 1.8 = 5.65 m = 5.65 x 10^9 nm Each turn = 3.4 nm Therefore number of turns = 5.65 x 10^9 nm / 3.4 nm                                               = 1.66 x  10^9 Each turn contains 10 nucleotide pairs = 20 nucleotides, so total number of nucleotides are =1.66 x  10^9 / 20 = 0.83  x  10^8 = 83 x  10^6 nucleotide pairs --it takes about 1000 nucleotides (pairs) of DNA to encode a single polypeptide. Number of polypeptides the mitochondrial DNA can encode = 83 x  10^6 nucleotide pairs / 1000                                                                                                        = 83,000 c) weight in grams of one nucleotide pair = 660daltons = 1056 x 10^-24 Number of nucleotide pairs are present in the 6 picograms = 6 x 10^-12 /1056 x 10^-24 = 5 x10^9 d) Length = 3.4 nm x 5 x10^9                  = 17 x 10^ 9 nm

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