Test: The DNA (NCERT) - NEET MCQ

DNA (deoxyribonucleic acid) is an acidic biomolecule.

The DNA helix makes one complete spiral turn every 3.4 nm and has a diameter of 2nm. The distance between adjacent stacks is 0.34 nm.

In eukaryotes, DNA organisation is complex. There is a set of positively changed, basic porteins called histones. Histones are rich in the basic amino acid residues lysine and arginine. There are five types of histone proteins-H₁,H₂A,H₂B,H₃ and H₄. Four of them (H₂A,H₂B,H₃ and H₄) produce histone octamer called nu body or core of nucleosome . The negatively charged DNA is wrapped around the positively charged histone octamer to form nucleosome. DNA connecting two adjacent nucleosomes is called linker DNA which bears H₁ histone proteins.

In a DNA molecule, the number of cytosine molecules is equal to guanine molecules and the number of adenine molecules are equal to Thymine molecules. Thus, If a double stranded DNA has 20% of cytosine, it has 20% of guanine. Thus, cytosine plus guanine make 40% of the total bases. The remaining 60% includes both adenine and thymine which are in equal amounts. Therefore, the percent of adenine is 30%.

In prokaryotes, the DNA is not scattered throughout the cell but is held with proteins in a region termed as 'nucleoid'.

Adenine always pairs with thymine and cytosine always pairs with guanine. Thus, sequence of bases on complementary strand would be TACGTACGT.

A typical nudeosome contains 200bp of DNA helix. Nudeosomes constitute the repeating unit of a structure in nucleus called chromatin. Chromatin is held over a scaffold of non-histone chromosomal (or NHC) proteins. At some places chromatin is densely packed to form darkly stainined heterochromatin. At other places chromatin is loosely packed. It is called euchromatin. It is transcriptionally active chromatin whereas heterochromatin is transcriptionally inactive and late replicating or heteropycnotic.

2OH group present in ribose sugar of every nucleaotide of RNA is a reactive group. Uracil present in RNA is less stable as compared to thymine (= methyl uracil) of DNA. Being unstable, RNA mutates at a much faster rate and there is no repairing system.Consequently, viruses having RNA genome and having shorter life span mutate and evolve faster.

In a DNA strand, the complementary based are linked together by hydrogen bond

In the DNA double helix, adenine (A) pairs with thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three hydrogen bonds. These hydrogen bonds are crucial for maintaining the specific pairing between bases, ensuring that the two strands of DNA are held together in a stable and specific manner, thus contributing to the stability of the helical structure and the accurate preservation and replication of genetic information

A nucleotide is composed of three components: a nitrogenous base, a pentose sugar (ribose in RNA or deoxyribose in DNA), and a phosphate group. Amino acids are the building blocks of proteins and are not components of nucleotides.

A nitrogenous base is linked to the OH group of the 1' carbon of the pentose sugar through an N-glycosidic linkage, forming a nucleoside. When a phosphate group is added to the 5' carbon of this nucleoside, a nucleotide is formed.

Uracil is found in RNA in place of thymine, which is present in DNA. Cytosine is common to both DNA and RNA, while adenine and guanine are also present in both DNA and RNA.

Purines are a category of nitrogenous bases that include adenine and guanine. The other bases listed (cytosine, thymine, and uracil) are pyrimidines.

DNA is a polymer of nucleotides each of which has a pentose sugar, a nitrogenous base and phosphate group. The base of a nucleotide is joined covalently (at N-1 of pyrimidines and N-9 of purines) in an N –β- glycosyl bond to the 1’ carbon of sugar. The phosphate is linked by a phosphodiester linkage to the 5’ carbon. Two complementary bases of two strands of DNA are paired by hydrogen bonding; adenine makes two hydrogen bonds with thymine and guanine makes three hydrogen bonds with cytosine. Two successive nucleotides of DNA are joined by phosphodiester linkage in which the 5’-phosphate group of one nucleotide unit is joined to the 3’-hydroxyl group of the next nucleotide. Thus, phosphate and pentose sugars of DNA are joined by phosphodiester linkage; the correct answer is B.