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# Solution - With the help of a neat and labelled diagram describe Watson and Crick's model of DNA. - Replication - The Experimental Proof

ConceptReplication The Experimental Proof

#### Question

With the help of a neat and labelled diagram describe Watson and Crick's model of DNA.

#### Solution

Watson Crick model of DNA:-

In 1953, James Watson and Francis Crick proposed the structure of DNA.

Their structure was based on the X-ray crystallographic studies provided by Maurice Wilkins and Rosalind Franklin.

Peculiar characteristic features of this model:-

1.DNA as a double helix:

• It consists of two long strands, coiled around a common imaginary central axis. This forms a double helix.
• The helix looks like a twisted ladder with alternating major and minor grooves.

2.Structure of each strand:

• Each DNA strand consists of several nucleotides.
• Each nucleotide is made of deoxyribose sugar, phosphate group and nitrogen base.
• The successive nucleotides of the same strands are linked by 3′–5′ phosphodiester linkages.

3.Complimentary Base Pairing:

• The nitrogen bases on one strand pair with the nitrogen bases on the opposite strand.
• The purine base ‘A’ pairs with the pyrimidine base ‘T’ by two hydrogen bonds ( A = T or T = A).
• The purine base ‘G’ pairs with the pyrimidine base ‘C’ with three hydrogen bonds (G≡ C or C ≡ G).
• This base pairing is called complimentary base pairing.
• Nitrogen bases along with hydrogen bonds constitute the rungs of the ladder.
• The distance between the two successive rungs is 0.34 nm.

4.Purine:pyrimdine ratio:

• Because of complementary base pairing, the total number of purine bases is always equal to the total number of pyrimidine bases (1:1).
• This is called Chargaff’s rule and may be represented as follows:

A+G=T+C or (A+G)/(T+C)=1

5.Polarity of strands:

• The polarity of the DNA strand is due to the presence of the 3rd and 5th carbon atoms of the deoxyribose sugar.
• There is a free phosphate group at the 5′ end, while there is a free hydroxyl group at the 3′ end.
• One of the strands runs in the 5′ to 3′ direction, while the other runs in the 3′ to 5′ direction.
• This arrangement of two strands of DNA is considered anti-parallel.

6.Major and minor grooves:

• The coiling of the double helix results in the formation of major or deep grooves.
• The twisting of two strands around one another forms minor or shallow grooves.

7.Dimensions:

• The diameter of the DNA molecule is 2 nm.
• The DNA molecule makes one complete turn of 360° after covering a distance of 3.4 nm.
• There are 10 base pairs in one complete spiral.
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#### APPEARS IN

2014-2015 (March)
Question 4.1 | 7 marks
Solution for question: With the help of a neat and labelled diagram describe Watson and Crick's model of DNA. concept: Replication - The Experimental Proof. For the courses CBSE (Science), HSC Science (General)
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