Question

Describe Watson and Crick’s model of DNA.

Answer 1

Hint: DNA is the genetic material and forms the molecular basis of heredity in all organisms, except in certain viruses like tobacco mosaic virus RNA is the genetic material. Nucleic acids exhibit a wide variety of secondary structures. For example, one of the secondary structures exhibited by DNA is the famous Watson - Crick Model.

Complete answer:
Watson - Crick Model of DNA
- This model says that DNA exists as a double helix. A DNA molecule has two branch polynucleotide strands. Each polynucleotide strand or chain consists of a sequence of nucleotides linked together by phosphodiester bonds. The polynucleotide strands are antiparallel, i.e, run in the opposite direction
- The two strands are not coiled upon each other but the double strand is coiled upon itself around a common axis like a spiral staircase with base pairs forming steps while backbones of the two strands rooms railing.
- The nitrogen bases are projected more or less perpendicular to the sugar-phosphate backbone but face inside.
- The base-pairing is specific. Adenine is always paired with thymine and guanine is always paired with cytosine. Thus, all base-pairs consist of purine and one pyrimidine.
- The two polynucleotide strands are held together in their helical structure by hydrogen bonding between bases in opposing strands.
- One end of the strand is called 5` end where the fifth carbon of the pentose sugar is free and the other end is called 3` end where the third carbon of pentose sugar is free.

Additional information:
At each base pair, the strand turns 36 degrees. One full turn of the helical strand 360 degrees, would involve ten base pairs i.e, one turn of 360 degrees of the helical strand has about 10 nucleotides on each strand of DNA. The base-pairs in DNA are stacked 3.4 nm. This form of DNA with the above mentioned salient feature is known as B-DNA.

Note: Once the sequence of bases in one strand of a DNA double helix is known, the sequence of the bases in the other strand is also known because of the specific base pairing. The two strands of a DNA double helix are thus said to be complementary (not identical). This is known as a complementary base pairing. Adenine and thymine form two hydrogen bonds. Guanine and cytosine form three hydrogen bonds.