DNA Structure — Revision Notes

⚡ 30-Second Revision

DNA: — Deoxyribonucleic Acid, genetic material.
Monomer: — Nucleotide (Deoxyribose sugar + Phosphate + Nitrogenous Base).
Bases: — Purines (A, G), Pyrimidines (C, T).
Backbone: — Sugar-phosphate, linked by phosphodiester bonds (5'-P to 3'-OH).
Double Helix: — Two antiparallel strands.
Base Pairing: — Complementary via hydrogen bonds.

- $A=T$ (2 H-bonds) - $G equiv C$ (3 H-bonds)

Chargaff's Rules: — In dsDNA, $A=T$, $G=C$, so $A+G = C+T$.
Dimensions (B-DNA):

- Diameter: $20,\text{Å}$ ($2,\text{nm}$) - Pitch (1 turn): $34,\text{Å}$ ($3.4,\text{nm}$) - Base pairs per turn: 10 - Distance between adjacent bp: $3.4,\text{Å}$ ($0.34,\text{nm}$)

Grooves: — Major and Minor, for protein binding.

2-Minute Revision

DNA is the genetic blueprint, a double helix made of two antiparallel polynucleotide strands. Each strand is a polymer of nucleotides, where a nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), or Thymine (T).

The sugar and phosphate form the strong backbone, linked by phosphodiester bonds. The two strands are held together by specific hydrogen bonds between complementary bases: A always pairs with T (two H-bonds), and G always pairs with C (three H-bonds).

This complementary pairing is the basis of Chargaff's rules ($A=T, G=C$). The helix has a diameter of $20,\text{Å}$ and completes a turn every $34,\text{Å}$, with 10 base pairs per turn. Understanding these structural details is crucial for comprehending DNA's functions in heredity and for solving NEET problems.

5-Minute Revision

Let's quickly review the essential aspects of DNA structure for NEET. DNA is a double-stranded helical molecule, the primary genetic material. Its fundamental building block is the nucleotide, composed of a deoxyribose sugar, a phosphate group, and a nitrogenous base.

The four bases are Adenine (A), Guanine (G) (purines), Cytosine (C), and Thymine (T) (pyrimidines). These nucleotides link together via phosphodiester bonds (between the 5' phosphate of one and the 3' hydroxyl of the next) to form a strong sugar-phosphate backbone, giving each strand a 5' to 3' directionality.

The two polynucleotide strands are antiparallel, meaning they run in opposite 5'-3' directions. They are held together by hydrogen bonds between specific complementary base pairs: Adenine always pairs with Thymine (forming two H-bonds), and Guanine always pairs with Cytosine (forming three H-bonds). This is the essence of Chargaff's rules, stating that in double-stranded DNA, the amount of A equals T, and G equals C. This rule is vital for solving numerical problems.

The B-DNA double helix (the most common form) has a diameter of $20,\text{Å}$ ($2,\text{nm}$). One complete turn (pitch) is $34,\text{Å}$ ($3.4,\text{nm}$) and contains approximately 10 base pairs. Consequently, the distance between adjacent base pairs is $3.

4, ext{Å}$($0.34, ext{nm}$). The helical twisting creates major and minor grooves, which are important for protein interactions. Remember to distinguish DNA from RNA: DNA has deoxyribose and thymine, is typically double-stranded; RNA has ribose and uracil, is typically single-stranded.

Mastering these points ensures you're well-prepared for DNA structure questions.

Prelims Revision Notes

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DNA as Genetic Material: — Deoxyribonucleic acid is the primary carrier of genetic information in most organisms.
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Nucleotide Composition: — Each DNA nucleotide consists of:

* Deoxyribose sugar: A pentose sugar, lacking an oxygen at the 2' carbon. * Phosphate group: Attached to the 5' carbon of deoxyribose. * Nitrogenous base: Attached to the 1' carbon of deoxyribose via an N-glycosidic bond.

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Nitrogenous Bases:

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

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Polynucleotide Chain: — Nucleotides are linked by phosphodiester bonds (covalent) between the 5' phosphate of one nucleotide and the 3' hydroxyl of the next, forming a sugar-phosphate backbone. This creates directionality (5' end and 3' end).
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Double Helix Structure (Watson-Crick Model):

* Two polynucleotide strands coiled around a central axis. * Antiparallel: Strands run in opposite directions (one 5' to 3', the other 3' to 5'). * Complementary Base Pairing: Bases on opposite strands pair specifically via hydrogen bonds. * Adenine (A) pairs with Thymine (T) via two hydrogen bonds ($A=T$). * Guanine (G) pairs with Cytosine (C) via three hydrogen bonds ($G equiv C$).

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Chargaff's Rules: — In double-stranded DNA:

* Amount of A = Amount of T ($A/T = 1$). * Amount of G = Amount of C ($G/C = 1$). * Amount of Purines (A+G) = Amount of Pyrimidines (C+T). * $(A+T)/(G+C)$ ratio varies between species but is constant for a given species.

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Dimensions of B-DNA (most common form):

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

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Major and Minor Grooves: — Formed by the helical twisting, these grooves are sites for protein binding.
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Stability: — Hydrogen bonds (inter-strand) and base stacking interactions (hydrophobic forces) contribute to the stability of the helix.
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DNA vs. RNA: — Key differences include sugar (deoxyribose vs. ribose), base (thymine vs. uracil), and typical strandedness (double vs. single).

Vyyuha Quick Recall

To remember the base pairing rules and hydrogen bond numbers:

'AT Two, GC Three'

Adenine and Thymine pair with Two hydrogen bonds.
Guanine and Cytosine pair with Three hydrogen bonds.