Biology · MU Sofia

3. Nucleic acids. DNA structure.

Covers the two classes of nucleic acids and the historical experiments behind DNA as genetic material, including Griffith's transformation study and the Hershey-Chase experiment. It then sets out Chargaff's rules and the Watson and Crick double helix, explains nucleotide and polynucleotide structure, and describes the A, B and Z forms of DNA. Later sections turn to the main RNA types, their functions, and short RNAs such as siRNA, miRNA and CRISPR.

Inside: 1 section · 3 labelled pictures · 5 min

Topic 3 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 1 section, about 770 words (5 min of reading).

In this note

  1. Introduction

Introduction

Nucleic Acids: Decoding the Blueprint of Life

  • Nucleic acids, the molecules responsible for storing, transmitting, and retrieving genetic information, come in two main forms:
  • Deoxyribonucleic acid (DNA): Encodes the genetic instructions for an organism's development and function.
  • Ribonucleic acid (RNA): Plays diverse roles in protein synthesis, gene regulation, and other cellular processes.
  • Discovery of DNA as the Genetic Material:
  • Late 19th century: Nucleic acids were identified in the nucleus, leading to speculation about their link to heredity.
  • Griffith's experiment (1928): Demonstrated the "transforming factor" in heat-killed bacteria, suggesting the transfer of genetic material.
  • Griffith injected mice with two types of pneumococci bacteria:
  • Smooth strain: Deadly due to a capsule.
  • Rough strain: Harmless without a capsule.
  • Smooth strain killed mice, rough strain didn't.
  • Heat-killed smooth strain mixed with rough strain killed mice, showing the transformation.
  • Later discovered by McCarty: DNA was the transforming factor.
  • This showed DNA's role in inheritance and biology.
  • Hershey-Chase experiment (1952): Used radioactive tracers to confirm DNA as the genetic material, not protein.
  • DNA labelled with 35S, Protein with 32P.
  • Bacteriophage-infected bacteria.
  • Blend to separate phage.
  • Centrifuge and check radioactivity.
  • Bacterial pellet is highly radioactive.
  • Unraveling the DNA Code:
  • Chargaff's rules (1950): Revealed specific base-pairing patterns in DNA, hinting at a structured organization.
  • Watson and Crick's model (1953): Proposed the iconic double helix structure based on X-ray crystallography and Chargaff's rules.
  • Complementary base pairing: A-T and G-C pairs hold the two DNA strands together, allowing for information storage and replication.
  • Structure of Nucleic Acids:
  • Nucleotides: Subunits of nucleic acids, composed of a sugar, a nitrogenous base, and a phosphate group.
  • DNA: Deoxyribose sugar, A-G-C-T bases.
  • RNA: Ribose sugar, A-G-C-U bases.
  • Polynucleotide chains: Chains of linked nucleotides forming the backbone of nucleic acids.
  • DNA: Two antiparallel strands forming the double helix.
  • RNA: Typically single-stranded.
  • Nitrogenous base binds to 1' atom of pentose→Nucleoside formed.
  • Example: Adenine + Ribose = Adenosine.
  • The phosphate reacts with 5’ hydroxyl group of pentose→Nucleotide formed.
  • Types of DNA:
  • A-DNA: Dehydrated form with a wider diameter.
  • B-DNA: The most common form, right-handed double helix.
  • Z-DNA: Left-handed double helix found in specific regions.
  • Diversity of RNA and its Functions:
  • Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis.
  • Transfer RNA (tRNA): Decodes the mRNA code and delivers specific amino acids for protein assembly.
  • Ribosomal RNA (rRNA): Forms the structural core of ribosomes, essential for protein translation.

Continue reading free

The whole note is free with an AceMed account. Premium adds flashcard recall, highlights and Ask AI.

Start freeAlready a member? Open the note