Watson-Crick Model — Revision Notes

⚡ 30-Second Revision

Structure: — Right-handed double helix.
Strands: — Two polynucleotide chains, antiparallel (5'\to3' and 3'\to5').
Backbone: — Sugar-phosphate (deoxyribose + phosphate), on exterior.
Bases: — Adenine (A), Guanine (G) - Purines; Cytosine (C), Thymine (T) - Pyrimidines.
Base Pairing: — Complementary: $A=T$ (2 H-bonds), $G\equiv C$ (3 H-bonds).
Chargaff's Rules: — $A=T$, $G=C$, $A+G=T+C$.
Dimensions:

* Diameter: $2\,\text{nm}$ ($20\,\text{Å}$) * Pitch (one turn): $3.4\,\text{nm}$ ($34\,\text{Å}$) * Base pairs per turn: 10 bp * Distance between adjacent bp: $0.34\,\text{nm}$ ($3.4\,\text{Å}$)

Bonds: — Phosphodiester (covalent, backbone), Hydrogen (non-covalent, between strands), Glycosidic (sugar-base).

2-Minute Revision

The Watson-Crick model describes DNA as a right-handed double helix, a structure fundamental to life. It comprises two antiparallel polynucleotide strands, meaning one runs 5' to 3' and the other 3' to 5'.

Each strand's backbone is formed by alternating deoxyribose sugars and phosphate groups, linked by strong phosphodiester bonds. Projecting inwards from these backbones are nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).

A and G are purines (double-ring), while C and T are pyrimidines (single-ring). Crucially, complementary base pairing occurs: A always pairs with T via two hydrogen bonds, and G always pairs with C via three hydrogen bonds.

This pairing ensures a uniform helix diameter of 2 nm. One full turn of the helix is 3.4 nm long and contains 10 base pairs, with each adjacent base pair separated by 0.34 nm. This elegant structure immediately suggested the mechanism for DNA replication, where each strand serves as a template, ensuring faithful genetic inheritance.

5-Minute Revision

The Watson-Crick model, proposed in 1953, defines DNA as a right-handed double helix, a structure akin to a twisted ladder. This model was built upon crucial data from Chargaff's rules (A=T, G=C) and Franklin's X-ray diffraction images.

Each 'side rail' of this ladder is a polynucleotide strand, formed by a strong sugar-phosphate backbone. This backbone consists of repeating deoxyribose sugars and phosphate groups, linked by robust phosphodiester bonds.

The two strands are antiparallel, meaning they run in opposite directions (one 5' to 3', the other 3' to 5'), which is vital for enzyme function during replication.

The 'rungs' of the ladder are formed by specific pairings of nitrogenous bases: Adenine (A) and Guanine (G) are purines, while Cytosine (C) and Thymine (T) are pyrimidines. The pairing is highly specific: Adenine always pairs with Thymine via two hydrogen bonds, and Guanine always pairs with Cytosine via three hydrogen bonds.

This complementary base pairing ensures the helix maintains a uniform diameter of 2 nm (20 Å). The entire helix completes one full turn every 3.4 nm (34 Å), encompassing 10 base pairs. Consequently, the distance between any two adjacent base pairs is 0.

34 nm (3.4 Å).

Worked Example (Chargaff's Rule): If a DNA sample contains 25% Adenine, what are the percentages of the other bases?

1
Since A = T, then Thymine (T) = 25%.
2
Total A + T = 25% + 25% = 50%.
3
Remaining bases (G + C) = 100% - 50% = 50%.
4
Since G = C, then Guanine (G) = 50% / 2 = 25%, and Cytosine (C) = 25%.

This model's significance lies in its direct implication for DNA replication, where the complementary strands can separate and each serve as a template for synthesizing a new partner strand, ensuring accurate genetic inheritance. It also explains how genetic information is stored and provides a framework for understanding mutations and gene expression.

Prelims Revision Notes

The Watson-Crick model describes DNA as a right-handed double helix.

Components of DNA:

1
Deoxyribose Sugar: — A five-carbon sugar.
2
Phosphate Group: — Links sugars via phosphodiester bonds.
3
Nitrogenous Bases:

* Purines: Adenine (A), Guanine (G) - double-ring structures. * Pyrimidines: Cytosine (C), Thymine (T) - single-ring structures.

Key Structural Features:

Two Polynucleotide Strands: — Each strand is a polymer of deoxyribonucleotides.
Antiparallel Orientation: — One strand runs 5' to 3', the other 3' to 5'. This is crucial for replication.
Sugar-Phosphate Backbone: — Forms the outer framework, strong covalent phosphodiester bonds.
Nitrogenous Bases: — Project inwards, stacked perpendicular to the helix axis.
Complementary Base Pairing:

* Adenine (A) always pairs with Thymine (T) via two hydrogen bonds ($A=T$). * Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds ($G\equiv C$).

Chargaff's Rules: — In double-stranded DNA, $A=T$ and $G=C$. Consequently, $A+G = T+C$ (purines = pyrimidines).
Helical Dimensions (B-DNA):

* Diameter: $2\,\text{nm}$ ($20\,\text{Å}$). * Pitch (length of one full turn): $3.4\,\text{nm}$ ($34\,\text{Å}$). * Base pairs per turn: Approximately 10 bp. * Distance between adjacent base pairs: $0.34\,\text{nm}$ ($3.4\,\text{Å}$).

Grooves: — Major and Minor grooves are present, important for protein binding.

Significance: The model immediately suggested a mechanism for DNA replication (semi-conservative replication) due to complementary base pairing, explaining how genetic information is faithfully passed on.

Common Traps:

Confusing the number of hydrogen bonds for A-T vs. G-C.
Mixing up DNA dimensions (e.g., diameter vs. pitch).
Applying Chargaff's rules to single-stranded DNA or RNA (they only apply to double-stranded DNA).
Confusing phosphodiester bonds (within a strand) with hydrogen bonds (between strands).

Vyyuha Quick Recall

All Tigers Get Caught: Adenine pairs with Thymine, Guanine pairs with Cytosine. (Remember A-T has 2 H-bonds, G-C has 3 H-bonds, G-C is stronger!)