Tag: structure of chromosome, cell cycle and cell division

The purine & pyrimidine pairs of complementry strands of DNA are held together by hydrogen bonds. A set of five nitrogenous bases is used in the construction of nucleotides, which in turn build up the nucleic acids like DNA and RNA.

Purines and pyrimidines, make up the two groups of nitrogenous base which plays on important role in structural backbone of DNA. Adenine and guanine are purines, while cytosine and thymine are pyrimidines. Purines pairs with pyrimidines, by formation of H-bonds. Adenine attaches to thymine, by two hydrogen bonds, while cytosine and guanine forms three hydrogen between them.

If DNA is subjected to high temperature, the hydrogen bonds between the nitrogenous bases break down and the two strands separate. This process is called as melting of DNA. More the content of guanine (G) and cytosine (C) in the DNA, higher will be the melting temperature because G forms three hydrogen bonds with C as against adenine (A) and thymine (T) which form two hydrogen bonds.
So, the correct answer is option A.

The C$ _1$ of pentose sugar in nucleotides is linked to the N1 or N9 position of pyrimidine or purine bases through a glycosidic bond. The syn and anti conformations, are the two conformational forms of nucleotides around this bond. The two forms are interconvertible by rotating the glycosidic bond without a need to break the bond.
So, the correct answer is option B.

The DNA duplex of Watson and Crick is right handed helix and is called the B-DNA. There are five more models (A-DNA, C-DNA, D-DNA, E-DNA and Z-DNA) have been proposed. The Z-DNA is left handed double helix which contains zigzag back and 12 base pairs per turn.
So, the correct answer is option A.

The base pairing confers a unique property to the polynucleotide chains. The chains are complimentary to each other, such that if the sequence of one strand is known then the sequence of other strand can be predicted. Also if each strand of DNA acts as a template, for the synthesis of a new strand, the two double stranded DNA molecule thus produced would be identical to the parent molecule. The bases in the two strands of a double stranded DNA molecule are paired through hydrogen bonds forming base pairs (bp). Adenine forms two hydrogen bonds with thymine from opposite strand and vice versa. Similarly cytosine is bonded with guanine from three H bonds. As a result, always a purine comes opposite to a pyrimidine. This generated approximately, equal distance between the two strands. If base order in one chain of DNA is "ATCGA" then the number of H bonds will be 2, 2, 3, 3 and 2 respectively for the given nucleotides to base pair. Thus, the total number of hydrogen bonds will be 12.

Wilkins X -ray diffraction showed the diameter of the DNA helix is around 20 $\mathring A$.  According to Maurice Wilkins, the structure of DNA of all species comprises two helical chains each coiled round the same axis, and each with a pitch of 34 angstroms and a radius of 10 angstroms. According to another study, when measured in a particular solution, the DNA chain measured 20 to 26 angstroms wide, and one nucleotide unit measured 3.3 long. Although each individual repeating unit is very small, DNA polymers can be very large molecules containing millions of nucleotides.
So, the correct answer is option B.

DNA is a polymer of deoxyribonucleotides. The deoxyribonucleotides are made up of phosphoric acid and deoxyribonucleosides. The deoxyribonucleosides are made up of deoxyribose sugars and nitrogenous bases. The deoxyribonucleotides are joined to each other with the help of 3', 5' phosphodiester bonds forming the backbone of single DNA chain. In a molecule of DNA, two such chains are present attached with each other with the help of hydrogen bonds between the nitrogenous bases of deoxyribonucleotides.

DNA molecule has uniform diameter due to specific base pairing between purine and pyrimidine. In DNA, the bases adenine and guanine form hydrogen bonds with their complementary pyrimidines; thymine and cytosine, respectively, this is called as complementary base pairing.

DNA is a double helical molecule made of two antiparallel strands of nucleotides which is linked by hydrogen bonds between complementary base pairs. DNA carries the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms. Adenine (A) pairs with Thymine (T) via two hydrogen bonds. Guanine (G) pairs with Cytosine (C) via three hydrogen bonds. Hence option A is correct.