Biology · MU Sofia

4. RNA types – structure and functions.

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 · 20 flashcards · 5 min

Topic 4 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), with 20 flashcards.

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.

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