DNA Helix

 

Definition & Overview

• DNA (Deoxyribonucleic Acid) is the genetic material that carries hereditary information in living organisms.
• The double helix is the three-dimensional structure of DNA, first proposed by James Watson and Francis Crick in 1953, based on
• Rosalind Franklin’s X-ray diffraction 
• Chargaff’s base-pairing rules.
• The DNA helix is the twisted, ladder-like structure of deoxyribonucleic acid (DNA), the molecule that carries genetic instructions.
• It consists of two antiparallel strands coiled into a right-handed double helix (B-DNA form).

Discovery & Nobel Prize

• 1953: Watson and Crick proposed the model using Franklin’s X-ray data.
• 1962: Watson, Crick, and Wilkins received the Nobel Prize in Physiology or Medicine (Franklin had passed away by then).

Key Components

a) Sugar-Phosphate Backbone

• Made of alternating deoxyribose sugar and phosphate groups.
• Provides structural stability.

b) Nitrogenous Bases (4 Types)

• Purines (Double-ring): Adenine (A), Guanine (G)
• Pyrimidines (Single-ring): Thymine (T), Cytosine (C)
• Base Pairing Rule (Chargaff’s Rule):
• A pairs with T (2 hydrogen bonds)
• G pairs with C (3 hydrogen bonds)

Types of DNA Helices

• B-DNA: Most common form, right-handed helix, 10 base pairs per turn.
• A-DNA: Shorter and wider, found in dehydrated DNA (e.g., in crystals).
• Z-DNA: Left-handed helix, forms under high salt conditions or in alternating GC sequences.

Functions of the Helix Structure

• Stable Storage of Genetic Info (H-bonds & hydrophobic core).
  
• Efficient Replication (strands separate easily for copying).
• Transcription & Translation (template for RNA synthesis).

Alternative Helical Forms Beyond A, B, and Z-DNA

• C-DNA: Forms under low humidity, right-handed with 9.3 base pairs per turn.
• D-DNA: Rare, seen in synthetic DNA with no guanine (all A-T pairs).
• E-DNA: An extended form observed under high tension or chemical treatments.
• P-DNA: A "stretched" form with phosphate backbones pulled inward, seen under extreme mechanical force.

DNA Can Form Triple Helices (H-DNA)

• Triplex DNA occurs in Hoogsteen base pairing, where a third strand binds to the major groove of B-DNA.
• Found in telomeres and gene regulation regions, possibly used in antigene therapy

Four-Stranded DNA (G-Quadruplexes)

• Forms in G-rich sequences (e.g., telomeres, oncogene promoters).
• Stabilized by metal ions (K⁺, Na⁺).
• Potential cancer therapy target (e.g., inhibiting telomerase).

6. DNA with No Helix: Single-Stranded (ssDNA) and Cruciform DNA

• ssDNA: Found during replication & repair, used by viruses (e.g., parvoviruses).
• Cruciform DNA: Self-complementary inverted repeats form hairpin loops, common in regulatory regions.

MCQ's :

1. Who first proposed the double-helix model of DNA?

1. a) Rosalind Franklin
   b) James Watson & Francis Crick 
   c) Linus Pauling
   d) Erwin Chargaff

2. 2. Which scientist’s X-ray diffraction data was critical for determining DNA’s structure?
   a) Maurice Wilkins
   b) Gregor Mendel
   c) Rosalind Franklin
   d) Frederick Sanger

3. 3. What type of bond connects the two strands of the DNA double helix?
   a) Ionic bonds
   b) Peptide bonds
   c) Hydrogen bonds 
   d) Covalent bonds

4. 4. In B-DNA, how many base pairs are present per complete turn?
   a) 8
   b) 10 
   c) 12
   d) 15

5. 5. Which of the following is NOT a nitrogenous base in DNA?
   a) Adenine
   b) Uracil (RNA base)
   c) Thymine
   d) Cytosine

6. 6. Which DNA form is left-handed?
   a) A-DNA
   b) B-DNA
   c) Z-DNA 
   d) D-DNA

7. 7. What stabilizes G-quadruplex DNA structures?
   a) Calcium ions (Ca²⁺)
   b) Potassium ions (K⁺) (Correct)
   c) Magnesium ions (Mg²⁺)
   d) Zinc ions (Zn²⁺)

8. 8. Which enzyme resolves DNA supercoils and knots?
   a) Helicase
   b) DNA polymerase
   c) Topoisomerase 
   d) Ligase

9. 9. Triple-helix DNA (H-DNA) is formed by:
   a) Complementary base pairing
   b) Hoogsteen base pairing 
   c) Van der Waals forces
   d) Phosphodiester bonds

10. 10. Which synthetic DNA alternative uses L-sugars?
    a) PNA
    b) XNA
    c) L-DNA 
    d) TNA

11. 11. Z-DNA is associated with which biological process?
    a) Photosynthesis
    b) Antiviral defense *( via ZBP1 protein)*
    c) Protein synthesis
    d) Cellular respiration

12. 12. Which organism can survive extreme radiation by repairing shattered DNA?
    a) Escherichia coli
    b) Deinococcus radiodurans 
    c) Streptococcus pneumoniae
    d) Mycobacterium tuberculosis

13. 13. Cruciform DNA structures arise from:
    a) Tandem repeats
    b) Inverted repeats 
    c) Random mutations
    d) Methylation

14. 14. Mirror-image DNA (L-DNA) is resistant to:
    a) Heat
    b) Enzymatic degradation 
    c) UV radiation
    d) Acidic conditions

15. 15. Pandoravirus is notable for having:
    a) Single-stranded DNA
    b) Exceptionally large DNA (2.5 million bp)
    c) No nucleic acids
    d) Triple-helix DNA