Ch. 16 – The Molecular Basis of Inheritance: Study Notes

Ch. 16 – The Molecular Basis of Inheritance

Concept: The Griffith Experiment

In 1928, Frederick Griffith's experiment demonstrated that some unknown genetic "factor" controls the traits of organisms. This experiment was foundational in identifying DNA as the genetic material.

• Transformation: The process by which external DNA is assimilated by a cell, resulting in a genetic and phenotypic change.

• Key Finding: Griffith showed that bacteria have the ability to acquire new traits from other bacteria.

Example: Griffith’s Experiment Showed Bacteria Can Transfer the Genetic Material.

Concept: The Hershey-Chase Experiment

In 1952, Hershey & Chase used bacteriophages to confirm that DNA is the genetic material.

• Bacteriophage: A virus that infects bacteria, consisting of an external protein shell and internal nucleic acid.

• Key Finding: Only DNA entered bacteria during infection, not protein, confirming DNA as the genetic material.

Example: The Hershey-Chase Experiment used radioactive labeling to distinguish DNA from protein.

Concept: Chargaff’s Rules

Erwin Chargaff made two important discoveries about DNA:

• DNA base composition varies between different species.

• In any species, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C):

$\%A = \%T$ $\%G = \%C$

Example:

Species	%A	%T	%G	%C
Homo sapiens (human)	31.0	31.5	19.1	18.4
Drosophila melanogaster (fruit fly)	27.3	27.2	22.5	23.0
Zea mays (corn)	26.8	26.8	23.2	23.2
Neurospora crassa (fungus)	23.1	23.6	27.1	26.2
Escherichia coli (bacteria)	24.7	23.6	26.0	25.7

Additional info: The percentages may not exactly sum to 100% because of limitations in Chargaff’s techniques.

Concept: Discovering the Structure of DNA

In the early 1950s, Rosalind Franklin used X-ray diffraction to capture images of DNA, which helped Watson & Crick deduce the double helix structure of DNA.

• Watson & Crick: Proposed the double helix model, with antiparallel strands and specific base pairing (A with T, G with C).

• Base Pairing: Hydrogen bonds form between specific pairs: A–T (2 bonds), G–C (3 bonds).

Detailed DNA Structure

• DNA consists of two strands of nucleotides wound into a double helix.

• Each nucleotide contains a phosphate group, deoxyribose sugar, and a nitrogenous base.

• Strands are antiparallel (run in opposite directions).

Concept: Meselson-Stahl Experiment

In 1958, Meselson & Stahl demonstrated that DNA replicates via the semi-conservative model:

• Semi-conservative Model: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

Example: The Meselson-Stahl Experiment confirmed semi-conservative DNA replication using isotopic labeling.

Concept: Introduction to DNA Replication

DNA replication is essential for cell division and is more complex in eukaryotes than prokaryotes.

• Replication requires a set of enzymes and proteins working together.

Enzyme / Protein	Function
Topoisomerase	Relieves DNA supercoiling ahead of replication fork.
Helicase	Unwinds DNA double helix at the replication fork.
Single-Stranded Binding Proteins	Bind and stabilize single-stranded DNA.
Primase	Creates RNA primers on which DNA polymerase can initiate synthesis.
DNA Polymerase III	Builds a new DNA strand using the old DNA as template.
DNA Polymerase I	Replaces RNA primers with DNA.
DNA Ligase	Covalently joins together Okazaki fragments in lagging DNA strand.

Concept: Origin of Replication & Replication Forks

• Replication begins at specific DNA sequences called origins of replication (ORI).

• Prokaryotes have a single ORI; eukaryotes have multiple ORIs per chromosome.

• Replication forks are Y-shaped regions where new DNA strands are synthesized.

Unwinding the DNA: Topoisomerase, Helicase & SSBs

• Topoisomerase: Cuts and rejoins DNA to relieve strain ahead of the replication fork.

• Helicase: Unwinds the DNA double helix.

• Single-Stranded Binding Proteins (SSBs): Stabilize unwound DNA strands.

Concept: DNA Polymerases

• DNA polymerase is the primary enzyme for building new DNA strands.

• It requires an RNA primer to initiate synthesis and adds nucleotides in the 5' to 3' direction.

Concept: Leading & Lagging DNA Strands

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized discontinuously, away from the fork, in short segments called Okazaki fragments.

• RNA primers are required for each Okazaki fragment.

Concept: Steps of DNA Replication

1. Topoisomerase relieves strain ahead of the fork.

2. Helicase unwinds the DNA.

3. Single-Stranded Binding Proteins stabilize the unwound DNA.

4. Primase adds RNA primers.

5. DNA Polymerase III adds nucleotides to the 3' end of the primer, synthesizing new DNA.

6. DNA Polymerase I replaces RNA primers with DNA.

7. DNA Ligase joins Okazaki fragments together.

Concept: DNA Repair

• DNA replication is not always perfect; errors can occur, but DNA polymerases have proofreading ability.

• Mismatch repair and excision repair enzymes help correct errors after replication.

Concept: Telomeres

• Telomeres are repetitive, non-coding DNA sequences at the ends of eukaryotic chromosomes.

• They protect chromosome ends from deterioration and fusion with other chromosomes.

• Telomerase is an enzyme that extends telomeres in germ cells and some stem cells.

• Telomere shortening is associated with cellular aging.

Summary Table: Key Enzymes in DNA Replication

Enzyme	Function
Helicase	Unwinds DNA double helix
Topoisomerase	Relieves supercoiling
Primase	Synthesizes RNA primers
DNA Polymerase III	Main DNA synthesis
DNA Polymerase I	Replaces RNA primers with DNA
DNA Ligase	Joins Okazaki fragments

Additional info: These notes include both conceptual explanations and practice questions to reinforce understanding of DNA structure, replication, and repair mechanisms.