Double Helix Structure of DNA

Deoxyribonucleic acid (DNA) is the molecule that carries genetic information in all living organisms. Its structure, famously described by James Watson and Francis Crick in 1953, is a double helix—a twisted ladder-like shape that provides stability and efficiency in storing genetic instructions.

1.0DNA - A Polymer of Deoxyribonucleotides.

• DNA is typically double-stranded and has a double-helix structure. It is found in chromosomes, mitochondria, and chloroplasts and serves as the genetic material in most organisms, carrying essential genetic information.
• In 1868, Friedrich Miescher first identified DNA as an acidic substance in the nucleus and named it "nuclein."
• In 1953, James Watson and Francis Crick proposed the now-famous Double Helix model of DNA structure.
• Their discovery was built upon the scientific contributions of other researchers who laid the foundation for their breakthrough.
• Linus Pauling
• Rosalind Franklin and Maurice Wilkins
• Erwin Chargaff

2.0DNA Structure

DNA structure is often divided into four levels: primary, secondary, tertiary and quaternary.

DNA has three main components

1. Deoxyribose (a pentose sugar)
2. Base (there are four different ones)
3. Phosphate

3.0The Nitrogenous Bases

• Nitrogenous bases in DNA and RNA are classified into two groups: pyrimidines and purines.  
• Pyrimidines (consist of a single six-membered ring):  1.Thymine   2. Cytosine  
• Purines (consist of a six-membered ring fused to a five-membered ring):  1. Adenine  2. Guanine  
• These rings are composed of more than just carbon; they also contain nitrogen.

Nitrogenous Bases of DNA & RNA

Nucleotide Structure

• Nucleotides are composed of a sugar, a phosphate group, and one of the four nitrogenous bases, joined through a condensation reaction. 
• The illustration below depicts the structure of a single nucleotide.

• Base + sugar  → nucleoside
• Example : Adenine + ribose = Adenosine
• Adenine + deoxyribose = Deoxyadenosine
• Base + sugar + phosphate(s) → nucleotide
• Example: Deoxyadenosine monophosphate (dAMP)
• Deoxyadenosine diphosphate (dADP)
• Deoxyadenosine triphosphate (dATP)
• Nucleotides are joined by covalent bonds known as phosphodiester linkages.

4.0DNA Double Helix & Hydrogen bonding

• DNA consists of two polynucleotide chains, with a sugar-phosphate backbone and nitrogenous bases extending inward.  
• The two DNA strands run in opposite directions, exhibiting antiparallel polarity, where one strand is oriented 5' to 3', and the other 3' to 5'.  
• Base pairs are held together by hydrogen bonds (H-bonds):  
• Adenine (A) bonds with Thymine (T) through two hydrogen bonds.  
• Guanine (G) bonds with Cytosine (C) through three hydrogen bonds.  
• Chargaff’s Rule states that in double-stranded DNA, the amount of Adenine equals Thymine and Guanine equals Cytosine.  
• A hydrogen bond is a type of chemical interaction where a hydrogen atom is shared between two electronegative atoms (such as oxygen or nitrogen), involving both covalent and ionic characteristics.

• The two strands of DNA are twisted into a right-handed (clockwise) helix.  
• The helix has a pitch of 3.4 nm with approximately 10 base pairs (bp) per turn.  
• The spacing between consecutive base pairs is about 0.34 nm.  
• Base pairs are stacked on top of each other, and this stacking, along with hydrogen bonding, enhances the stability of the helical structure.  

The DNA helix also features two asymmetrical grooves on its outer surface:  

• Major groove – wider and deeper  
• Minor groove – narrower and shallower
• A groove refers to a shallow indentation or channel along a structure. Certain proteins can bind within these grooves, allowing them to interact with specific base sequences.

5.0Chargaff's rule

• Erwin Chargaff, an Austrian biochemist, expanded Levene's work and found additional information on the structure of DNA. 
• Chargaff found that the nucleotide composition of DNA varies among species. 
• He concluded that, despite variations in DNA composition across different organisms and tissue types, certain fundamental properties remain consistent.  
• Using paper chromatography, he discovered that the amount of Adenine (A) is typically equal to Thymine (T), while the amount of Guanine (G) is approximately equal to Cytosine (C).  
• This means that the total number of purines (A + G) is always equal to the total number of pyrimidines (C + T), a principle known as Chargaff's Rule.  
• In summary, A + T = G + C, a rule that applies to all DNA molecules.

6.0Structure of Double-helix

Three major forms:

• B-DNA
• A-DNA
• Z-DNA

B-DNA

• It is a -helix, meaning that it has a right-handed, or clockwise, spiral.
• Complementary base pairing
• A-T
• G-C
• In ideal B-DNA, each complete turn of the helix (360° rotation) contains 10 base pairs, meaning each base is twisted 36° relative to its adjacent base. 
• Base pairs are spaced 0.34 nm apart, resulting in a full helical turn of 3.4 nm.  
• The axis runs through the centre of each base pair.  
• The minor groove is narrow and shallow, while the major groove is wide and deep.  
• This structure forms when DNA is surrounded by ample water and lacks unusual base sequences, conditions commonly found in cells.  
• B-DNA is the most stable conformation for a random sequence of nucleotides under physiological conditions.

A-DNA

• Right-handed helix, but wider and flatter than B-DNA, with 11 base pairs per turn.  
• Bases are tilted away from the central axis of the molecule.  
• Features a narrow, deep major groove and a broad, shallow minor groove.  
• Typically observed under low-water conditions, such as dehydration.  
• A-DNA has been found in two contexts:  
• Within the active site of DNA polymerase (~3 base pairs).  
•  During sporulation in Gram-positive bacteria.

Z-DNA

• Left-handed helix, observed under high salt concentrations.  
• The sugar-phosphate backbone forms a zigzag pattern, named Z-DNA.  
• Contains 12 base pairs per turn.  
• Features a deep minor groove but lacks a distinct major groove.  
• Found in certain active genes, suggesting a potential role in gene transcription regulation.