Study Notes on Molecular Structure of DNA and RNA
Molecular Structure of DNA and RNA
Introduction to Molecular Genetics
Focus on molecular genetics, studying the structure and function of DNA at the molecular level.
Advancements in techniques have improved the understanding of molecular genetics, transmission genetics, and population genetics.
Our knowledge of genetics is rooted in the understanding of the molecular structure of DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
Identification of DNA as the Genetic Material
Criteria for Genetic Material
Information: Must contain information necessary for the development of an entire organism.
Transmission: Must be capable of being passed from parent to offspring.
Replication: Must be able to be copied for inheritance.
Variation: Must allow for changes to account for phenotypic variation within a species.
Historical Context
Many geneticists, including Mendel, supported these properties, but the definitive identification of the genetic material, DNA, came from various experimental approaches rather than genetic crosses alone.
Frederick Griffith's Experiments with Streptococcus pneumoniae
Overview of Griffith's Study
Griffith studied a bacterium known as Streptococcus pneumoniae with two strains:
Type S (Smooth):
Secretes a polysaccharide capsule that protects the bacterium from the immune system.
Produces smooth colonies on solid media.
Type R (Rough):
Unable to secrete a capsule, produces rough-looking colonies.
Transformation Principle
Griffith conducted experiments using live type S and type R strains and heat-killed type S.
Results:
Live type R + heat-killed type S → Mouse died (transformation occurred).
Live type R alone → Mouse lived.
Heat-killed type S alone → Mouse lived.
Griffith concluded that a substance from dead type S bacteria transformed type R into type S, termed the transforming principle.
Characteristics of the Transforming Principle
The transformation process demonstrated that:
The bacteria acquired information to produce the polysaccharide capsule.
Variation existed in the ability to produce the capsule.
Information for capsule production was replicable and transmitable from mother to daughter cells.
Avery, MacLeod, and McCarty Experiments
Objective and Methodology
Avery, MacLeod, and McCarty aimed to identify the transforming principle from Griffith’s findings.
In the 1940s, they isolated macromolecules from type S bacteria, discovering that only the DNA extract could transform type R to type S:
RNase/protease treatment did not eliminate transformation.
DNase treatment did disrupt transformation.
Experiment Outline
Cell extracts prepared from type S cells and purified macromolecules.
Transformation Testing:
Control: Live R-strain introduced without any treatment → transformation occurred.
RNase treatment/Protease treatment: Live R-strain → transformation occurred.
DNase treatment: Live R-strain → transformation disrupted.
Evidence from Hershey and Chase
Utilizing Isotopes
Hershey and Chase used radioisotopes to show DNA was the genetic material of T2 phage:
32P: Labels DNA specifically.
35S: Labels proteins specifically.
Results after infection:
Most 32P entered bacterial cells (links DNA to genetic material).
Most 35S remained outside (indicating proteins do not carry genetic material).
Data Representation
Extracellular Measurements:
Blending: Removed 80% of 35S from E. coli.
Significant retention of 32P demonstrated that DNA was being transferred to bacteria.
Structure of DNA and RNA
Overview
DNA and RNA are classified as nucleic acids;
First identified by Friedrich Miescher in 1869, named "nuclein" from the nucleus of cells.
Known to release H+ in water.
Nucleotides: Building Blocks of DNA and RNA
Each nucleotide consists of:
Phosphate group
Pentose sugar:
Ribose in RNA
Deoxyribose in DNA
Nitrogenous base:
Bases categorized as Purines (Adenine, Guanine) or Pyrimidines (Cytosine, Thymine for DNA; Uracil for RNA).
Terminology of Nucleic Acids
Nucleosides: Base + sugar (e.g., Adenosine = Adenine + Ribose).
Nucleotides: Nucleoside + phosphate(s):
Examples: Adenosine triphosphate (ATP), Adenosine diphosphate (ADP), Adenosine monophosphate (AMP).
Structure of DNA
Characteristics of DNA Strands
Linked through covalent bonds; ester bonds form phosphodiester linkages.
Directionality is 5’ to 3’ (sugar orientation aligned in the same direction).
Appears as a double helix; the structure folds and bends to create a three-dimensional form.
Double Helix Discovery
Watson and Crick elucidated the double-helical structure in 1953, with vital contributions from other scientists including Rosalind Franklin and Erwin Chargaff.
Key Structural Features of the Double Helix
Antiparallel strands running in opposing 5’ to 3’ directions.
Right-handed helix with approximately 10 base pairs per turn and 3.4 nm per turn.
Hydrogen bonds stabilize the structure (2 bonds between A and T; 3 between C and G).
Stabilization of the DNA Structure
Hydrogen bonds and base stacking stabilize the DNA double helix; bases oriented flat to each other in pairs.
Grooves in the DNA Double Helix
DNA features two major grooves:
Major Groove
Minor Groove
Certain proteins interact with these grooves, allowing for sequence-specific binding and recognition.
Alternative Double Helix Structures
DNA can form different secondary structures; the predominant form under physiological conditions is known as B-DNA.
B-DNA: Right-handed helix.
Z-DNA: Left-handed helix with bases tilted and a zigzag phosphate backbone.
RNA Structure
Differences from DNA
RNA uses Uracil instead of Thymine and ribose sugar with a 2' OH group.
Typically consists of several hundred to several thousand nucleotides in length.
In RNA synthesis, only one DNA strand serves as a template.
RNA can form various secondary structures through complementary base pairing (A to U; C to G).
Types of RNA Structures
Bulge loop: Abnormality in base pairing due to an unpaired base.
Stem loop: Common structure in RNA formed by base pairing.
Examination of tertiary structure considers interactions with ions and proteins.