Question

A 340 angstrom long segment of DNA molecules has 20 thymine nitrogen bases , which will be the number of guanine nitrogen bases on the same.
A. 160
B. 80
C. 40
D. 10

Answer 1

Hint: Double stranded DNA having four nucleotides such as adenine, thymine, guanine and cytosine. Normal length of a nucleotide is 3.4 angstrom. Adenine –thymine creates a base pair with a double hydrogen bond and guanine –cytosine creates a base pair with a threefold bond.

Complete answer :-
to answer this question you must know about DNA and chargaff's rule.
Chargaff's rule is something that relates to the DNA of a species. It is named after its founder Erwin chargaff. chargaff's rule says in DNA the number of cytosine units is equal to the guanine unit and the number of thymine units is equal to the number of adenine units .
As we know that the distance between two nucleotide is 34 angstrom and has 10 base pairs, so 340 angstrom long DNA segment would have 100 base pairs( 10 X 10), and here in the given strand 20 are thymine therefore adenine is also 20 because no. of thymine is always equal to no. of adenine.
therefore the total number of AT base pairs in a given DNA strand = 20.
Number of GC base pairs in the given DNA = total base pairs – total number of AT base pairs
                                                                             = 100 - 20 = 80.
Since the total number of GC pairs in the given DNA is 80, the total number of guanine bases in the same segment is 80,

So, the correct answer is option (b) 80.

Note:-
Chargaffs stated that DNA from any cell or organism would have a ratio 1:1 of pyrimidine and purine bases and purines is always pair with pyrimidine for example A with T and C with G. the C+G:A+T ratio varies from species to species.