Question

Choose the correct answers from the alternatives given:
In a DNA molecule
a. Bases are hydrogen-bonded to one another
b. Sugars are covalently bonded to the phosphates
c. Bases are covalently bonded to the sugars
d. All of these

Answer 1

DNA, deoxyribonucleic acid is found in chromosomes, mitochondria and chloroplasts. It carries the genetic information. It is made up of nucleotides which contains a phosphate group, a sugar group, and a nitrogen base.

Complete answer:
DNA or deoxyribonucleic acid is a molecule which is composed of polynucleotides chains that coil around each other to form a helix. It contains the hereditary material. It is located on the nucleus but can also be found in traces in mitochondria. The right-handed double helix structure of DNA was given by Watson and Crick. It has about ten nucleotide pairs per turn. Each nucleotide is composed of a sugar group, phosphate group, and a nitrogen base. The bases in DNA are- adenine, guanine (purines); thymine, cytosine (pyrimidines).

Structure of DNA: it consists of two helical polypeptide chains which are coiled around each other. The polypeptide chains are antiparallel, that is, they have opposite polarity. The backbone of each chain is made up of sugar phosphate residues which are hydrophilic in nature. The purine and pyrimidine bases are covalently bonded to the sugars and are facing inwards. The helix has two external grooves, one minor and the other major. Minor groove is the site for binding of molecules and major groove is the site for DNA binding proteins. The bases in the helical are present in pairs between purines and pyrimidines, specifically, A-T and C-G. These base pairs were founded by Chargaff and are paired according to Chargaff’s rule. There is hydrogen bonding between the bases. There are two H-bonds between A-T and three H-bonds between G-C.

Hence, the correct answer is option (D).

Note: DNA is of three types:
• A-DNA: It is the most active form. It mostly occurs in dehydrated samples of DNA.
• B-DNA: It is the Watson and Crick model of DNA.
• Z-DNA: It is said to be one of the most active double helix structures and it can be formed in-vivo.