The DNA
Read the following passage carefully and answer the given questions.
DNA is a long polymer of deoxyribonucleotides. The length of DNA is usually defined as number of nucleotides present in it. A nucleotide is the combination of three different components, such as a nitrogenous base, a pentose sugar (ribose in the case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – purines and pyrimidines. When a phosphate group is linked to OH of 5′ C of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide, depending upon the type of sugar present) is formed. James Watson and Francis Crick, in 1953, proposed a very simple but famous Double Helix model for the structure of DNA based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin. One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chains. However, this proposition was also based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.
Identify the heavy isotope of nitrogen utilized by Meselson and Stahl in their experiment?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 15N
Meselson and Stahl proved in 1958 that DNA replicates semi-conservatively. They first grew E. coli for many generations in a medium with 15NH4Cl, so that its DNA took up the heavy isotope 15N. They then moved the cells to a medium with normal 14N. Heavy, hybrid and light DNA were told apart by their density in a caesium chloride gradient.
(1) 13N — Incorrect. This isotope was not used in the experiment.
(2) 14N — Incorrect. 14N is the normal (light) nitrogen, used in the second medium.
(3) 15N — Correct. 15N is the heavy isotope they used.
(4) 16N — Incorrect. This isotope was not used in the experiment.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
The Experimental Proof – …
