Question

Choose the incorrect description about DNA molecules.
a. Synthesis is semiconservative
b. Opposite strands are antiparallel
c. It contains the sugar deoxyribose
d. The number of adenine present is roughly equal to the number of thiamine
e. The number of cytosine present is roughly equal to the number of uracil

Answer 1

Deoxyribonucleic acid, also known as DNA, is a basic cell information macromolecule that stores, translates, and transmits genetic information. The three-dimensional structure of DNA, proposed by Watson and Crick, consists of two spiral strands coiled around one axis and forming a right-handed double helix.

Complete answer:
Deoxyribonucleotides are formed by stitching together three components: phosphoric acid (H3PO4), deoxyribose sugar , and a nitrogenous base. There are four different types of nitrogenous bases in DNA. They are divided into two groups: purines (nine-membered nitrogen rings at positions $1,3,7$ and $9$ ) and pyrimidines (six-membered nitrogen rings at positions $1$ and $3$ ). There are two types of purines in DNA (adenine or A and guanine or G) and two types of pyrimidines (cytosine or C and thymine or T).
Option (C) is a correct statement.

The three-dimensional structure of DNA, proposed by Watson and Crick, consists of two spiral strands coiled around one axis and forming a right-handed double helix.

Properties of three dimensional structure of DNA:
• The $5' -$ end of one polynucleotide chain and the $3' -$ end of the other polynucleotide chain are on one side and adjacent to each other. It means the two chains in DNA run antiparallel to each other. From this property it’s clear that statement( B) is correct.
• Adjacent base pairs are superimposed on each other. Together with hydrogen bonds, this arrangement ensures the stability of the helical structure.
• The helical distance is $3.4nm$ , and there are about ten base pairs per revolution. The average distance between two adjacent base pairs is about $3.4nm$.
• The backbone of each polynucleotide chain consists of alternating sugar-phosphate groups. Nitrogen base comes out. The nitrogenous bases of the two polynucleotide chains form a complementary pair. A forms bond with T and C with G.
• Chargaff $(1950)$ made observations of bases and other components of DNA. This observation or generalization is called Chargaff's rule of equality. (I) The base pairs of purine and pyrimidine have the same amount, i.e. $[A + G] = [T + C]$ , i.e. $\dfrac{{A + G}}{{T + C}} = 1$ (ii) The number of moles of adenine is always equal to the number of moles of thymine. Likewise, the molar concentration of guanine corresponds to the molar concentration of cytosine.
From the Chargaff rule it’s clear that option (D) statement is correct and (E)is incorrect.

The Watson and Crick model shows that DNA replication is semi-conserved. This shows that half of the DNA is preserved. This is the original (template) DNA strand, in which only one new strand is synthesized. Each parental strand of DNA serves as a template for a new complement strand.
Option (A) is a correct statement.

So filtering out the correct statement, the one which is an incorrect statement is, The number of cytosine present is roughly equal to the number of uracil.
Hence, the correct answer is option (e).

Note: DNA is the largest biomolecule in a cell. DNA is negatively charged and dextrorotatory. Hydrogen bonds between nitrogenous bases on additional DNA strands can be broken due to high temperatures, low pH or high. This phenomenon is called denaturation or fusion. Since the A-T base pairs only have $2$ hydrogen bonds, the regions rich in A-T base pairs can be easily denatured. In prokaryotes, DNA is mainly found in the nuclear domain. In eukaryotes, this occurs in the nucleus, mitochondria, and chloroplasts.