The Molecular Basis of Inheritance
The Molecular Basis of Inheritance
Introduction
Overview of the history and discovery of DNA.
Key figures include James Watson, Francis Crick, Frederick Griffith, and others.
Historical Timeline
1857: Early studies in genetics begin.
1859: Charles Darwin publishes "On the Origin of Species."
1910: Thomas Hunt Morgan provides evidence that chromosomes are the basis of heredity.
1953: Discovery of the double helical structure of DNA by Watson and Crick.
DNA Overview
Deoxyribonucleic Acid (DNA):
Introduced as a carrier of hereditary information in 1953.
Encoded hereditary information is reproduced in all cells.
Directs biochemical, anatomical, physiological, and behavioral traits of organisms.
The Search for the Genetic Material
Frederick Griffith's Experiment (1928):
Studied two strains of Streptococcus pneumoniae:
S strain (pathogenic, smooth with sugar coat, virulent).
R strain (non-pathogenic, rough without sugar coat).
Key Results:
Living S cells killed the mouse.
Living R cells left the mouse alive.
Heated S cells left the mouse alive.
Combination of heated S cells with living R cells killed the mouse.
Found living S cells in blood of the mouse, indicating transformation.
Transformation Process:
Defined as a change in genotype and phenotype because of the assimilation of foreign DNA.
DNA as the Transforming Principle
Avery, McCarty, and MacLeod (1944):
Investigated what component of the heated S strain transformed R strain bacteria:
Tested sugar coat, proteins, RNA, and finally DNA, which was confirmed to initiate transformation.
Experiment Breakdown:
Heat-kill S strain, treat with enzymes (RNase, protease, DNase).
Treat samples with enzymes that destroy respective components, and add to R strain cultures.
Only samples treated to eliminate DNA did not transform R strain.
Bacteriophages and DNA
Hershey and Chase Experiment (1952):
Labeled DNA and protein components of T2 bacteriophage to see which entered E. coli during infection.
Confirmed that DNA, not protein, is the genetic material of T2 bacteriophage.
Bacteriophage Structure:
Composed only of DNA and protein, confirming the need to identify the genetic material being injected into the bacterial cell.
DNA Composition
Erwin Chargaff's Findings (1947):
Identified that DNA is a polymer of nucleotides, each consisting of:
Nitrogenous base (adenine (A), thymine (T), guanine (G), cytosine (C)).
Sugar (deoxyribose).
Phosphate group.
Chargaff’s Rules:
Base composition varies between species.
The number of A equals T; G equals C.
Structure of DNA
X-Ray Crystallography (1950s):
Maurice Wilkins and Rosalind Franklin utilized X-ray diffraction, discovering key structural measurements:
DNA is double stranded, helical (turns every 3.4 nm), and has a consistent width (2 nm).
Nitrogenous bases spaced at intervals of 0.34 nm.
Enabled Watson and Crick to model the double helix structure.
Watson and Crick Model
Base Pairing:
Watson and Crick deduced that:
Adenine (A) pairs with Thymine (T) through 2 hydrogen bonds.
Guanine (G) pairs with Cytosine (C) through 3 hydrogen bonds.
This explains Chargaff's rules.
Structural Features:
Sugar-phosphate backbone faces outward with nitrogenous bases facing inward.
Strands run in antiparallel directions (5' to 3' and vice versa).
DNA Replication
Concept of DNA Replication:
Each daughter molecule has one old strand (semiconservative model) and one new strand.
Alternative models:
Conservative Model: parent strands rejoin.
Dispersive Model: each strand is a mix of old and new.
Meselson and Stahl Experiment (1958):
Used heavy and light nitrogen isotopes to label strands and prove that DNA replication is semi-conservative.
DNA Replication Process
Initiation:
Enzyme Functions:
Helicases unwound the helix.
Single-strand binding proteins keep strands separate.
Topoisomerase alleviates tension ahead of the replication fork.
Primase synthesizes RNA primers required for DNA polymerase action.
Elongation:
DNA Polymerases:
Synthesizes new strands by adding nucleotides only at the 3' end of the growing strand.
Rates: E. coli at 500 nt/sec; Humans at 50 nt/sec.
Leading vs. Lagging Strands:
Leading strand synthesized continuously, while lagging strand is synthesized in fragments (Okazaki fragments).
Finishing Up the Lagging Strand:
DNA Polymerase I replaces RNA primers with DNA.
DNA Ligase joins Okazaki fragments together.
DNA Repair Mechanisms
Proofreading:
DNA polymerases can correct mismatched nucleotides during synthesis with a final error rate of 1 in 10 billion base pairs.
Mismatch Repair:
Enzymes correct errors after DNA replication has occurred using specialized repair enzymes.
Nucleotide Excision Repair:
Involves a nuclease that cuts out sections of erroneous DNA, which are then replaced and resealed by polymerases and ligases.
Telomeres and Aging
Telomeres:
Protect the ends of chromosomes by preventing erosion of genes during replication (short repeats, e.g., TTAGGG).
Telomerase:
Enzyme that extends telomeres in germ cells, not present in somatic cells.
Chromatin Structure
Types of Chromatin:
Euchromatin (loosely packed) allows for gene expression.
Heterochromatin (densely packed) is more compacted and less accessible.
Changes in Chromatin:
Occur during the cell cycle: euchromatin exists during interphase while chromatin condenses into metaphase chromosomes during mitosis.
Summary
The structure and function of DNA are tightly interconnected, with the double helix model laying the groundwork for understanding DNA replication and repair mechanisms. The information encoded within DNA serves as the basis of inheritance, guiding not only individual traits but also the evolutionary potential through genetic variation.
Griffith’s Transformation Experiment
Design: Frederick Griffith (1928) studied two strains of Streptococcus pneumoniae: S strain (pathogenic, smooth) and R strain (non-pathogenic, rough).
Findings: Living S cells killed mice, living R cells left them alive, heated S cells left them alive, but a combination of heated S cells and living R cells killed the mice, indicating transformation where R strain took on the properties of S strain.
Chargaff’s Rules
Rules:
The number of adenine (A) equals thymine (T).
The number of guanine (G) equals cytosine (C).
Application: If an organism’s genome has 20% adenine, then it has 20% thymine, and to find the percentages of guanine and cytosine, 60% is left to be divided equally, giving 30% each for guanine and cytosine.
Meselson and Stahl’s Experiment
Design: Used heavy and light nitrogen isotopes to label DNA and observe replication in E. coli.
Findings: Their results showed a banding pattern indicative of semi-conservative replication, where each daughter molecule contains one old strand and one new strand.
Nucleotide Excision Repair
Process: Involves recognizing and removing damaged DNA segments.
Enzymes involved:
Nucleases cut out erroneous sections.
DNA polymerases fill in the gaps.
DNA ligases seal the new segments into the existing DNA strand.
Telomeres
Definition: Repetitive nucleotide sequences at the ends of chromosomes that protect them from erosion.
Impact on Cellular Aging: Shortening of telomeres over time can lead to cellular aging and loss of function.
Telomerase: An enzyme that extends telomeres in germ cells, which is not present in somatic cells, allowing those cells to divide more without losing important DNA.
Chromatin Structure
Euchromatin: Loosely packed, accessible for gene expression.
Heterochromatin: Densely packed, less accessible, generally inactive.
DNA Replication Enzymes
Enzymes involved in DNA replication include:
Helicases: Unwind the DNA helix.
Single-strand binding proteins: Keep unwound strands separate.
Topoisomerase: Alleviates tension ahead of the replication fork.
Primase: Synthesizes RNA primers for DNA polymerases.
DNA Polymerases: Add nucleotides to the growing strand, synthesizing new DNA.
DNA Ligase: Joins Okazaki fragments on the lagging strand.