Question

Describe the 3 structures the DNA double helix could exist in.

Answer 1

DNA or Deoxyribonucleic acid is a biomolecule that exist in three-dimensional structure, by undergoing intricate folding. The basic unit undergoes folding and interactions to form other structures.

The three structures of double stranded DNA molecule are:

A. Primary structure: Consist of linear form of DNA, units joined by phosphodiester linkages. Composed of 3 components.

1. Nitrogenous bases:

Purines: Forms glyosidic bonds between 9 Nitrogen with 1’ OH.

• Adenine (A)
• Guanine (G)

Pyrimidines: Forms glyosidic bonds between 1 Nitrogen with 1’ OH.

• Cytosine (C)
• Thymine (T)

2. 5-Carbon sugar: Deoxyribose

Sugar +base is called Nucleoside.

3. Phosphate groups: 1 to 3

Nucleoside + phosphate is called Nucleotide.

B. Secondary structure: Two strands of DNA joined by hydrogen bond.

• Nucleotides of one strand is paired with nucleotides of the other strand.
• The double helix structure is formed due to secondary conformation. The double helix is formed by wrapping of the two strands around each other.
• The strands also hold with the force of stacking interaction of bases, stabilized by Vander Waals forces.
• Adenine pairs with Thymine with double bonds (A=T)
• Guanine pairs with Cytosine with triple bonds (G=C)
• Forms major groove and minor groove.

C. Tertiary structure: The different conformation that the tertiary structure takes, result in 3 forms of DNA:

1. A-DNA:

• Discovered by Rosalind Franklin.

• Rare structural conformation may be formed during dehydration.
• Shorter and compact.
• Right handed helix.
• Helical diameter- 26 Angstrom.
• Height of helical turns (helical pitch)- 28.6 Angstrom.
• Number of base pairs per helical turn- 11.6
• Helical twist per base pair- 31°.
• Distance between each base pair (helical rise)- 2.9 Angstrom.
• Major groove- narrow and deep.
• Minor groove- wide and shallow.
• Ribose sugar conformation- C₃ endo.
• Glyosidic bond conformation- anti.

2. B-DNA:

• Discovered by Watson and Crick.
• Most common and predominate form of DNA.

• Right handed helix.
• Helical diameter- 20 Angstrom.
• Number of base pairs per helical turn-10
• Height of helical turns (helical pitch)- 34 Angstrom.
• Helical twist per base pair- 36°.
• Distance between each base pair (helical rise)- 3.4 Angstrom.
• Major groove- wide and deep.
• Minor groove- narrow and deep.
• Ribose sugar conformation- C2₃ endo.
• Glyosidic bond conformation- anti.

3. Z-DNA:

• Discovered by Wang and Rich.

• Forms a zig-zag pattern.
• Left handed helix.
• Helical diameter- 18 Angstrom.
• Height of helical turns (helical pitch)- 44 Angstrom.
• Number of base pairs per helical turn- 12
• Helical twist per base pair- 9-51°.
• Distance between each base pair (helical rise)- 7.4 Angstrom.
• Major groove- flat.
• Minor groove- narrow and deep.
• Ribose sugar conformation- C₃ endo for purine, C2 for pyrimidine.
• Glyosidic bond conformation- anti for pyridine and syn for purine.

D: Quaternary structure: Formed by association with proteins, like histones.