Question

Describe Watson and Crick's model of DNA.

Answer 1

DNA is a double helical structure consisting of two long strands, coiled around a ventral axis. This structure was discovered by Watson and Crick. Each strand is a polynucleotide chain, composed of many nucleotides. Each is formed by a pentose, deoxyribose sugar, a phosphate group and one nitrogen base.,

Complementary pairing : Nitrogen bases are double ring purines or single pyrimidines. Adenine, guanine are purines and cytosine, thymine are pyrimidines.

(1) Sugar and nitrogeneous bases are connected to each other by glycosidic bond while sugar and phosphate are joined by phosphodiester bond, known as $3^{\prime }\to 5^{\prime }$ phosphodiester linkage.

(2) There is complementary pairing of purines and pyrimidines. Adenine, thymine pair with double $H$ bond, while cytosine and guanine pair with the triple $H$ bond. $H$ bond form the rings in DNA molecule.

(3) Total number of purines always equals to the total number of pyrimidines, so $A+G=T+C$. This is known as Chargaff's law.

Polarity of strands : One strand of DNA has $3^{\prime }\to 5^{\prime }$ polarity, while opposite strand has polarity $5^{\prime }\to 3^{\prime }$. $3^{\prime }$ and $5^{\prime }$ are carbon atoms. Due to the fact that $3^{\prime }\to 5^{\prime }$ and $5^{\prime }\to 3^{\prime }$ direction, DNA strands are called antiparallel.

Diameter of DNA is $2nm$. For one complete rotation of ${360}^o$, DNA molecules cover a distance of $3.4nm$, whereas the distance between two successive rings are $0.34nm$, that means there are $10$ base pairs in one complete spiral turn.