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.
How many base pairs are present in the genome of Escherichia coli?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4.6 × 106
The passage says that the length of DNA is given as the number of nucleotides, or base pairs (bp), in it. This number is a feature of each organism. The bacterium Escherichia coli has 4.6 × 106 bp.
(1) 6.6 × 109 — Incorrect. This is about the DNA of a diploid human cell (twice the haploid value).
(2) 3.3 × 109 — Incorrect. This is the haploid content of human DNA.
(3) 4.6 × 106 — Correct. This is the genome size of E. coli.
(4) 6.6 × 106 — Incorrect. This is not the size of the E. coli genome.
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 DNA –
