2.  Analyze how Watson and Crick proposed a model for the secondary structure of DNA with necessary diagrams.

answer:

Introduction :

James Watson and Francis Crick proposed that DNA must be formed like a double helix in 1953. This is called the Watson-Crick Structure of DNA.

DNA is a helical, double-stranded molecule with two strands. On the surface, it has two backbones (with alternating glycosyl and phosphate groups) that are linked together on the inside by hydrogen bonds between pairs of nitrogenous bases.

The bases are divided into four categories (A, C, G, and T), with A & T and C & G always matching.

James Watson and Francis Crick realised that these pairing principles indicated that either strand held all the information needed to build a new copy of the complete molecule and that the aperiodic sequence of bases might offer a “genetic code”. 

Watson-Crick Structure of DNA

• Watson and Crick presented a model for the DNA’s double-helix structure.
• A nucleotide polymer makes up the DNA molecule. 
• A nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate group are found in each nucleotide. 
• There are two purines (adenine and guanine) and two pyrimidines in the DNA (cytosine and thymine). 
• Two strands of DNA make up a DNA molecule. 
• Each strand is made up of nucleotides that are covalently linked by their phosphate groups and deoxyribose sugars. 
• The bases grow out of this backbone. 
• Hydrogen bonds connect the bases of strands. 
• Adenine is always linked with thymine, while cytosine is always linked with guanine.

Given below is the diagram for the Watson-Crick Structure Of DNA

Nucleic acids are long chain macromolecules which are formed by end to end polymerisation of large number of repeated units called nucleotides. Nucleic acids show a wide range of secondary structures. A secondary structure is the set of interactions between bases and sugar phosphate backbone and is responsible for the shape that nucleic acid.

James Watson and Francis Crick proposed a secondary structure of DNA molecules based on the crystallographic studies.

(i) DNA or deoxyribonucleic acid is a helically twisted double chain polydeoxyribo nucleotide macromolecule.

(ii) The two strands of DNA run anti-parallely to each other called as DNA duplex.

(iii) The spiral twisting of DNA has two types of alternate grooves, i.e., major and minor.

(iv) One turn of ${360}^o$ of the spiral has about 10 nucleotides on each strand of DNA. occupying a distance of about 3.4 nm.

(v) The nucleotides within each strand are held logether by the phosphodiester bonds between the 5' carbon of one nucleotide and the 3' carbon of the adjacent nucleotide. These strong covalent bonds holds the sugar/phosphate backbone together.

(vi) The two strands of DNA are held together by weak hydrogen bonds between the nitrogenous bases. These hydrogen bonds are base specific. That is adenine forms 2 hydrogen bonds with thymine CA=T and cytosine forms 3 hydrogen bonds with guanine (C$\equiv$G).

(vii) As specific and different nitrogen bases occur on two DNA chains, they are said to be complementary, i.e., purine lies opposite to pyrimidine. This purine-pyrimidine pairing also contributes to the thickness of strand, i.e., 2nm, and makes the two chains complementary.

• The two strands wrap around each other in a shape termed the ‘double helix’ because of the bonding. 
• The second nucleic acid found in the cells is ribonucleic acid (RNA). RNA is a nucleotide polymer with a single strand. 
• It also contains sugar ribose instead of deoxyribose and nucleotide uracil instead of thymine. 
• RNA molecules play a role in the formation of proteins using DNA’s genetic code.
• Prokaryotes have a single circular chromosome with double strands. 
• Eukaryotes have chromosomes that include double-stranded linear DNA molecules.                                                                                                               
• Nucleosomes are formed when the DNA helix wraps around proteins. 
• Protein coils get progressively more coiled, and the chromosomes become even more coiled throughout mitosis and meiosis to make movement easier. 
• Chromosomes feature two unique regions that may be differentiated by staining and are determined by whether the DNA in a region is expressed (euchromatin) or not (heterochromatin).

Features of the Watson-Crick Structure of DNA

The Watson-Crick model of DNA has the following important characteristics:

• In a DNA molecule, a right-handed double-helix is made up of a series or strands of two polynucleotides spirally wrapped around each other and twisted along a common path.
• The two strands run in opposing directions, with the fifth ending of one chain facing the third ending of the other. In other words, they are antiparallel.
• The sugar-phosphate backbones remain on the exterior, while the purines and pyrimidines bases are located at the centre of the helix.
• The two series are kept together by hydrogen bonds formed between the purines and pyrimidine bases of the opposing strands.
• Adenine (A) will always couple with thymine (T) through two hydrogen interactions, while guanine (G) will always pair with cytosine (C) via three hydrogen bonds. This complementary nature is known as the rule of the base pair. As a result, the two chains are complementary to each other.
• The base sequence varies throughout a polynucleotide chain. A certain sequence of bases carries genetic information.
• The constitution of the base of the DNA follows Chargaff’s assumptions (E.E. Chargff, 1950), according to which A = T and G = C. As a corollary, ∑ purines (A+G) = ∑ pyrimidines (C + T). Also, (A + C) = (G + T). The rules also put forth the ratio of  (A + T) and (G + C) as constants for a species (range 0.4 to 1.9).
• The DNA has a diameter of 20 nm, or 20Å. The adjoining bases are 0.34 nm or 3.4Å along the axis apart. The length of a full helix turn is 3.4 nm or 34Å, implying that there are 10b/turning.
• A small groove, called the minor groove (1.2 nm), and a deep groove, known as the major groove (2.2 nm), run across the DNA helix.

Conclusion 

Watson and Crick postulated the “double helix” molecular structure of the DNA. The precise pairing of nucleotides is a critical aspect of the Watson and Crick model of DNA. Each DNA strand, in this case, is linear and long and is made up of smaller units known as nucleotides that link together to form a chain.