The Molecular Basis of Inheritance Flashcards

The Historical Hunt for the Genetic Material

• Pre-1940s Context: The identity of the hereditary material was unknown to the scientific community until the mid-1940s.

• The Griffith Experiment: This study involved two strains of bacteria to investigate the nature of inheritance.     - S Strain (Pathogenic): This strain causes disease. If injected into mice, they die. However, if the S cells are heat-killed prior to injection, they are non-pathogenic and the mice live.     - R Strain (Non-pathogenic): This strain does not cause disease due to its lack of an outer coating. If injected, the mice live.     - The Transformation Observation: When heat-killed S cells (non-lethal) were co-injected with live R cells (non-lethal), the mice died. Subsequent analysis found live S cells in the blood of the dead mice.     - Griffith's Conclusion: The R cells were transformed into S cells by picking up "hereditary material" from the dead S cells.

• The Avery Experiment: This experiment aimed to identify the specific molecule responsible for the transformation observed by Griffith (the change in genotype and phenotype).     - Methodology: Scientists co-injected live R cells with either purified S cell protein or purified S cell DNA.     - Results:         - R cells alone: Mice live.         - Purified S cell protein & live R cells: Mice lived.         - Purified S cell DNA & live R cells: Mice died, and live S cells were recovered from the blood.     - Conclusion: The uptake of DNA, not protein, transformed the phenotype of the bacteria. Therefore, DNA is the hereditary material.

Discovery of the DNA Structure

• Functional Requirements of DNA Structure: To serve as genetic material, the structure must satisfy two primary conditions:     - It must be able to hold information.     - It must be able to be copied accurately.

• Contributors and Disciplines: The discovery involved a multidisciplinary effort including a nuclear physicist, a geneticist, a chemist, an X-ray crystallographer, and a physicist.

• Nitrogenous (N) Base Content (Chargaff’s Rules):     - The percentage of Adenine (A) relative to Guanines (G) differs across various species.     - In any given species, the percentage of Adenine ($A$) always equals the percentage of Thymine ($T$) ($\%A = \%T$).     - The percentage of Guanine ($G$) always equals the percentage of Cytosine ($C$) ($\%G = \%C$).

• X-Ray Crystallography: This technique was essential for determining the 3-D structure of crystallized nucleic acids and proteins.     - Process: X-rays are scattered by a crystal to produce a "shadow" pattern.     - Outcome: The 3D shape of the molecule is mathematically calculated from this shadow pattern. In the case of DNA, this indicated the molecule was a helix.

The Chemical Architecture of DNA

• Base Pairs (bp): A base pair consists of two nucleotides from opposite strands held together via Hydrogen bonds (H-bonds).     - Information Storage: The specific sequence of these base pairs is what holds hereditary information.     - Measurement Units: Used as a unit of length for DNA molecules, expressed as base pairs (bp) or kilo base pairs (kb).

• Watson-Crick Base-Pairing Rules:     - Adenine ($A$) H-bonds specifically with Thymine ($T$).     - Guanine ($G$) H-bonds specifically with Cytosine ($C$).     - This pairing is dictated by specific chemical and X-ray data.     - Complementarity: The nucleotides in a pair are complementary, meaning if the identity of a base on one strand is known, the identity of the base on the opposite strand is automatically known based on the rules.

The Mechanism of DNA Replication

• General Process: Replication is the process of copying DNA, which occurs during the S phase of the cell cycle.     - Strand Separation: The H-bonds between the two strands are broken.     - Template Model: Each single strand serves as a template for the synthesis of a newly complementary strand.     - Semiconservative Nature: Every resulting DNA molecule consists of one original (old) strand and one newly synthesized strand.

• Energy for Replication: The energy required to power the synthesis of a new strand comes from the hydrolysis of two phosphate ($PO_4$) groups on the incoming dNTPs (deoxynucleoside triphosphates).

• Origins of Replication: The specific location in the DNA where the strands separate to initiate replication.     - Eukaryotic DNA: Large eukaryotic DNA molecules have multiple origins of replication to facilitate faster copying.     - Replication Forks: Each origin has two replication forks where DNA unwinds and synthesis occur.     - Directionality: These forks move in opposite directions, eventually connecting to finish synthesis.

Enzymatic Machinery of DNA Replication

• Helicase: This enzyme unwinds and separates the double helix structure.

• Primase: This enzyme begins the synthesis process by creating a short stretch of complementary RNA known as the RNA primer.

• DNA Polymerase III (DNA Pol III): The primary enzyme responsible for making new DNA.     - It cannot start a strand from scratch; it requires a primer.     - It operates strictly in the $5'$ to $3'$ direction.     - It adds new nucleotides specifically to the $3'$ end of the growing strand.

• DNA Polymerase I (DNA Pol I): This enzyme is responsible for replacing the RNA primers with DNA nucleotides.

• DNA Ligase: This enzyme completes the synthesis by sealing the gaps in the sugar-phosphate backbone (joining the sugar-phosphate fragments).

• Additional Components:     - Single-strand binding proteins: Involved in stabilizing the separated DNA strands.     - Sliding clamp: Associates with DNA Pol III during synthesis.

Continuous and Discontinuous DNA Synthesis

• The Two Mechanisms: Because DNA Pol III only works in the $5'$ to $3'$ direction, two different methods of synthesis occur at each replication fork:     1. Continuous Synthesis (Leading Strand): One new strand for each template is made in a single, continuous piece beginning at the origin and moving toward the replication fork.     2. Discontinuous Synthesis (Lagging Strand): The other strand for each template is synthesized in multiple sections (fragments).

• Joining Fragments:     - DNA Pol III stops whenever it encounters the $5'$ end of an existing RNA primer.     - DNA Pol I then removes the RNA and replaces it with DNA.     - DNA Ligase joins the resulting fragments to form a continuous strand.

Questions & Discussion

• Reflection Question 1: Describe the structure of a DNA nucleotide.     - Note: Students are encouraged to write the answer down and discuss it with a peer.

• Reflection Question 2: Describe, in detail, the structure of DNA.     - Note: Students should conclude Video 2 and move to Video 3 after this.

• Reflection Question 3: Why are two mechanisms needed for DNA synthesis?     - Hint: Refer to the directional activity of DNA Polymerase III during the process.

• Study Tip: The material is best viewed in "slideshow" mode to manage overlapping figures and text. Download the file from Canvas and open with PowerPoint or a viewer.