2.6 Structure of DNA and RNA Notes

Introduction

In the realm of molecular biology, understanding the structure and function of nucleic acids such as DNA and RNA is fundamental. DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) are the molecules that carry genetic information essential for the growth, development, functioning, and reproduction of all living organisms and many viruses. This study note will delve into the intricate structures of DNA and RNA, their components, and their roles in genetic information storage and transmission.

Basic Structure of Nucleotides

Both DNA and RNA are polymers of nucleotides. A nucleotide is the basic building block of nucleic acids and consists of three components:

1. Pentose Sugar: A five-carbon sugar.
2. Nitrogenous Base: An organic molecule with nitrogen, which can be a single or double ring of atoms.
3. Phosphate Group: A negatively charged, acidic group.

Pentose Sugar

• DNA contains deoxyribose sugar, which lacks one oxygen atom compared to ribose.
• RNA contains ribose sugar.

Nitrogenous Bases

There are four nitrogenous bases in DNA and RNA:

• DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
• RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

Phosphate Group

The phosphate group links the nucleotides together by forming a covalent bond with the pentose sugar of the next nucleotide, creating a sugar-phosphate backbone.

Formation of Nucleic Acid Strands

Nucleotides join together to form long chains, or strands:

• DNA: Double-stranded, forming a double helix.
• RNA: Usually single-stranded.

Sugar-Phosphate Backbone

The backbone is formed by alternating sugar and phosphate groups. The bases project out from the sugar-phosphate backbone.

$$ \text{Sugar} - \text{Phosphate} - \text{Sugar} - \text{Phosphate} - \text{Sugar} - \text{Phosphate} $$

Base Pairing in DNA

In DNA, the two strands are held together by hydrogen bonds between complementary bases:

• Adenine pairs with Thymine (A-T)
• Cytosine pairs with Guanine (C-G)

Base Pairing in RNA

In RNA, base pairing occurs within the same strand or between different RNA molecules:

• Adenine pairs with Uracil (A-U)
• Cytosine pairs with Guanine (C-G)

DNA Structure

Double Helix

DNA's structure is often described as a double helix, which looks like a twisted ladder. The sugar-phosphate backbones form the sides of the ladder, and the base pairs form the rungs.

Antiparallel Strands