Covers the two main nucleic acids, DNA and RNA, starting with the historical experiments that established DNA as genetic material: Griffith, Hershey-Chase and the Chargaff rules leading to the double helix. It then describes nucleotide and polynucleotide structure, the A, B and Z forms of DNA, and the main RNA classes with their roles. The note closes with short RNAs such as microRNA, siRNA, antisense RNA and CRISPR, and includes a mnemonic for the major RNA types.
Topic 4 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 958 words (7 min of reading), with 20 flashcards.
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Nucleic acids. DNA. RNA
🧬 Nucleic Acids – Summary Cheat Sheet
💊 Medical Applications
Nucleic acids. DNA. RNA
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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