Account
It's free!

User



Login
Register


      Forgot password?
Share
Notifications

View all notifications
Books Shortlist
Your shortlist is empty

Solution - With the help of a neat and labelled diagram describe Watson and Crick's model of DNA. - Replication - 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.
Is there an error in this question or solution?

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)
S