Question

Describe in detail Watson - crick structure of Dna. How does this structure relate to replication , transcription, and repair

Answer 1

Watson- Crick structure of DNA

​​​​​Watson and Crick first describe the structure of DNA double helix in 1953 using x-ray diffraction data of DNA fibres obtained by Franklin and Wilkins.

One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chain. However this proposition was also based on the observation of Erwin Chargaff.

According to Chargaff, for a double stranded DNA, the ratio between adenine and thymine and guanine and cytosine are constant and equal.

The Watson Crick double helix model describe the features of the B form of DNA. The main features of the model were:

• Two long polynucleotide strands coiled around the central axis.
• Strands are wrapped plectonemically in a right handed helix.
• Strands are antiparallel that is one strand is oriented in the 5' to 3' direction and the other in 3' to 5' direction.
• Strands interact by hydrogen bonds between complementary base pairs. Guanine forms three hydrogen bonds with Cytosine. Adenine forms two hydrogen bonds with Thymine.
• Angle of interaction between base pairs result in major and minor grooves.
• The pitch of the helix 3.4 nm and there are roughly 10 base pair in each turn. Consequently, the distance between base pairs in a helix is approximately equal to 0.34nm. it is because of specific base pairing with a purine lying opposite to pyrimidine which makes two chains 2nm thick.

Figure showing double helix structure of DNA.

This double stranded DNA structure relate to replication, transcription and repair in the following ways:

The structure of DNA helped in understanding many aspects of DNA's function, such as how it was copied and how the information it carried was used by the cell to make proteins.

DNA replication is semi-conservative. This means that each of the two strands in double-stranded DNA acts as a template to produce two new strands. Replication relies on complementary base pairing, that is the principle explained by Chargaff's rules: adenine (A) always bonds with thymine (T) and cytosine (C) always bonds with guanine (G).

The double-stranded structure of DNA is critical not only for replication but also as a scaffold for the correction of errors and the removal of damage to DNA. The discovery of thymine dimerization by radiation was the key to an understanding of how DNA could be repaired.

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