Watson and Crick DNA Model

Introduction:

The discovery of DNA’s structure is one of the most important milestones in biology. James Watson and Francis Crick proposed the double helix model of DNA in 1953, for which they received the Nobel Prize in 1962. Understanding this model helps explain how genetic information is stored, replicated, and passed on in living organisms.

Key Features of the DNA Model

1.     Double Helix Structure:
DNA consists of two strands that wrap around each other, forming a spiral staircase-like structure.

2.     Polynucleotide Chains:
Each strand is made up of unbranched polynucleotides linked by phosphodiester bonds.

3.     Antiparallel Orientation:
The two strands run in opposite directions: one from 5' to 3', the other from 3' to 5'.

4.     Base Pairing Rules:

o    Adenine (A) pairs with Thymine (T).

o    Guanine (G) pairs with Cytosine (C).

o    These pairs are held together by hydrogen bonds.

5.     Complementary Strands:
The sequence of bases in one strand determines the sequence in the other.
Example:

6.     Strand 1: A  G  A  T  G  C

7.     Strand 2: T  C  T  A  C  G

8.     Dimensions of DNA:

o    Diameter: ~20 Å

o    Length of one helix turn: 34 Å

o    Nucleotides per turn: 10

o    Distance between adjacent nucleotides: 3.4 Å

9.     Major and Minor Grooves:

o    DNA has two grooves that facilitate protein binding.

o    Major groove: Wide and deep.

o    Minor groove: Narrow and shallow.

Flowchart: DNA Structure Overview

     DNA Double Helix

                  │

 ┌─────┴─────┐

 │                               │

Strand 1                 Strand 2

 │                              │

 Bases pair via Hydrogen Bonds

 │                             │

A↔T,                   G↔C

Table: DNA Key Features at a Glance

Feature	Description
Structure	Double Helix
Chains	2 unbranched polynucleotide chains
Orientation	Antiparallel
Base Pairing	A-T, G-C
Bonds	Hydrogen bonds between bases
Diameter	20 Å
Nucleotides per turn	10
Turn length	34 Å
Grooves	Major (wide), Minor (narrow)

 Difference table of DNA types (A-DNA, B-DNA, Z-DNA)

Feature	A-DNA	B-DNA	Z-DNA
Helix Handedness	Right-handed	Right-handed	Left-handed
Diameter	23 Å	20 Å	18 Å
Base Pairs per Turn	11	10	12
Helix Shape	Shorter & wider	Normal, classic helix	Zigzag backbone
Major Groove	Deep & narrow	Wide & deep	Shallow & narrow
Minor Groove	Shallow & broad	Shallow & narrow	Flat
Typical Condition	Dehydrated DNA	High humidity (most cellular DNA)	Alternating purine-pyrimidine regions
Biological Role	Found in DNA-RNA hybrids, dehydrated DNA	Most common DNA form in cells	Gene regulation, stress response
Strand Orientation	Antiparallel	Antiparallel	Antiparallel
Occurrence	Less common	Most common	Less common, transient

Tips to Remember DNA Types

1.     A-DNA (Right-handed, dehydrated form)

o    Appears under low water conditions.

o    Mnemonic: “A for Arid” → forms in dry conditions.

o    Helix is shorter and wider than B-DNA.

o    Base pairs per turn: 11.

2.     B-DNA (Right-handed, normal form)

o    Most common DNA in cells under normal conditions.

o    Mnemonic: “B for Body” → common, normal structure in living cells.

o    Base pairs per turn: 10.

o    Major groove: wide and deep, Minor groove: shallow and narrow.

3.     Z-DNA (Left-handed, zigzag form)

o    Appears in alternating purine-pyrimidine sequences.

o    Mnemonic: “Zigzag” → left-handed, zigzag backbone.

o    Base pairs per turn: 12.

o    Plays a role in gene regulation and stress response.

Quick Memory Trick Table

DNA Type	Handedness	Base Pairs/Turn	Condition	Mnemonic
A-DNA	Right	11	Dehydrated	Arid
B-DNA	Right	10	Normal cells	Body
Z-DNA	Left	12	Special sequences	Zigzag

 

Extra Tip:

• Right-handed → standard spiral (A & B)
• Left-handed → zigzag spiral (Z)
• Think letters: A → Arid, B → Body, Z → Zigzag