DNA and RNA

Nucleic acids are essential information-carrying molecules of life. DNA (deoxyribonucleic acid) stores genetic information, while RNA (ribonucleic acid) transfers this information for protein synthesis.

Overview

DNA and RNA are crucial for genetic inheritance and protein synthesis. DNA holds the genetic blueprint, while RNA translates and helps execute these instructions to make proteins.

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Basic Structure

Both DNA and RNA are polymers of nucleotides. Each nucleotide consists of:

• A pentose sugar (5-carbon sugar)
• A phosphate group
• A nitrogenous base

Nucleotide Components

1. Pentose Sugar

   • DNA: Deoxyribose (lacks an oxygen atom at the 2' carbon)
   • RNA: Ribose (has an OH group at the 2' carbon)

2. Nitrogenous Bases

   • Purines (double-ring structure):
     • Adenine (A)
     • Guanine (G)
   • Pyrimidines (single-ring structure):
     • Cytosine (C)
     • Thymine (T) - in DNA only
     • Uracil (U) - in RNA only

3. Phosphate Group

   • Links nucleotides via phosphodiester bonds
   • Forms the sugar-phosphate backbone

DNA Structure

Double Helix

• Two antiparallel polynucleotide strands
• Held together by hydrogen bonds between complementary bases
• Right-handed helix structure

Complementary Base Pairing

• Adenine (A) pairs with Thymine (T)
  • Connected by two hydrogen bonds
• Guanine (G) pairs with Cytosine (C)
  • Connected by three hydrogen bonds
• Mnemonic: "Apples in the Tree, Cars in the Garage"

Key Features

• Sugar-phosphate backbone on the outside
• Bases face inward, forming steps of the ladder
• Major and minor grooves facilitate protein interactions
• One complete turn every 10 base pairs (~3.4 nm)
• Chargaff's rules: %A = %T and %G = %C

RNA Structure

Single-Stranded Molecule

• Generally a single polynucleotide chain
• More flexible and can fold into complex shapes
• Types of RNA:
  • mRNA (messenger RNA) - carries genetic code from DNA to ribosomes
  • tRNA (transfer RNA) - brings amino acids during protein synthesis
  • rRNA (ribosomal RNA) - structural and functional component of ribosomes

Key Differences from DNA

1. Sugar

   • RNA contains ribose sugar with an additional OH group

2. Bases

   • Uracil (U) replaces Thymine (T)
   • U pairs with A

3. Structure

   • Single-stranded but can form secondary structures via base pairing within the molecule
   • More reactive due to the extra OH group on ribose

Historical Context

Discovery of DNA Structure

• 1869: Friedrich Miescher discovered "nuclein" (DNA) from white blood cells
• 1953: James Watson and Francis Crick proposed the double-helix model
  • Based on Rosalind Franklin's X-ray diffraction images
  • Chargaff's base composition data
  • Contributions from Maurice Wilkins and others

Early Doubts About DNA

Scientists initially doubted DNA was the genetic material because:

• Simplicity: DNA seemed too simple with only four types of nucleotides
• Proteins favored: Proteins, with 20 different amino acids, were thought to carry genetic diversity
• Repetition: DNA was believed to be a repetitive polymer

Learning Objectives

Test Your Knowledge

Key Takeaways

• DNA and RNA are nucleic acids essential for genetic information storage and transfer.
• DNA is double-stranded, contains deoxyribose sugar, and uses thymine.
• RNA is single-stranded, contains ribose sugar, and uses uracil.
• Complementary base pairing is crucial for DNA replication and RNA function.
• Understanding nucleic acids is foundational in molecular biology and genetics.