Chapter 13: The Molecular Basis of Inheritance – Study Guide

Chapter 13: The Molecular Basis of Inheritance

Key Concepts

• DNA as the genetic material

• Proteins involved in DNA replication and repair

• Chromosome structure: DNA packed with proteins

• DNA structure and replication in genetic engineering

DNA as the Genetic Material

Discovery and Evidence

DNA replication allows genetic information to be inherited from a parent cell to daughter cells. Early experiments established DNA as the genetic material, overturning the previous belief that proteins carried genetic information.

• Chromosomes are composed of DNA and protein.

• Frederick Griffith's experiment demonstrated transformation: non-virulent bacteria became virulent when exposed to heat-killed virulent bacteria.

• Avery, McCarty, and MacLeod identified DNA as the transforming factor.

• Hershey and Chase used bacteriophages to show that DNA, not protein, is the genetic material.

Transformation and Bacteriophage Experiments

• Transformation: Uptake of genetic material from the environment by a cell.

• Bacteriophage: A virus that infects bacteria, composed of a head, tail sheath, tail fiber, and DNA.

• Hershey and Chase labeled DNA with radioactive phosphorus and protein with radioactive sulfur to trace inheritance.

DNA Structure

Chemical Composition

DNA is a polymer made of nucleotides, each consisting of a phosphate group, deoxyribose sugar, and a nitrogenous base.

• Nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

• Chargaff's rules: The amount of A equals T, and the amount of G equals C in DNA.

• Pyrimidines: Cytosine and Thymine (single ring)

• Purines: Adenine and Guanine (double ring)

• Watson and Crick's model explained base pairing and the double helix structure.

Base Pairing and Antiparallel Strands

• Adenine pairs with Thymine; Guanine pairs with Cytosine.

• DNA strands run antiparallel: one strand 5' to 3', the other 3' to 5'.

• Hydrogen bonds stabilize base pairs.

Table: Nitrogenous Bases in DNA and RNA

Nitrogenous Base	Purine	Pyrimidine	DNA, RNA, or Both?
Adenine	✔		Both
Guanine	✔		Both
Cytosine		✔	Both
Thymine		✔	DNA
Uracil		✔	RNA

DNA Replication

Models and Mechanisms

DNA replication is semiconservative: each new DNA molecule consists of one old strand and one new strand.

• Semiconservative model: Each daughter DNA has one parental and one new strand.

• Meselson and Stahl used heavy nitrogen to demonstrate semiconservative replication.

• Replication begins at origins of replication and proceeds bidirectionally.

Enzymes Involved in Replication

Enzyme	Function
Helicase	Unwinds and separates DNA strands
Single-strand binding protein	Stabilizes unwound DNA
Primase	Synthesizes RNA primer
DNA polymerase	Adds DNA nucleotides to new strand
Ligase	Joins Okazaki fragments
Nuclease	Removes damaged DNA

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized in Okazaki fragments.

• DNA polymerase synthesizes DNA in the 5' to 3' direction.

DNA Repair and Telomeres

DNA Proofreading and Repair

• DNA polymerase proofreads and corrects errors.

• Repair enzymes fix damaged DNA, such as thymine dimers caused by UV light.

Telomeres

• Telomeres are repetitive DNA sequences at chromosome ends, protecting against degradation.

• Telomerase extends telomeres in germ cells and some cancer cells.

Chromosome Structure

DNA Packing

• DNA wraps around histone proteins to form nucleosomes.

• Chromatin can be heterochromatin (condensed, inactive) or euchromatin (less condensed, active).

Genetic Engineering and Biotechnology

Plasmids and Cloning Vectors

• Plasmids are small, circular DNA molecules in bacteria, used as cloning vectors.

• Cloning vectors carry foreign DNA into host cells for replication and expression.

• Recombinant DNA technology allows insertion of genes into plasmids.

Restriction Enzymes and Gene Cloning

• Restriction enzymes cut DNA at specific sequences, creating sticky ends for ligation.

• Gene cloning involves inserting a gene of interest into a plasmid and transforming bacteria.

• Gel electrophoresis separates DNA fragments by size.

Polymerase Chain Reaction (PCR)

• PCR amplifies DNA using cycles of denaturation, annealing, and extension.

• Taq polymerase is heat-stable and enables PCR at high temperatures.

• Applications include diagnostics, forensics, and research.

DNA Sequencing and CRISPR-Cas9

DNA Sequencing

• Determines the order of nucleotides in DNA.

• Modern techniques allow rapid and large-scale sequencing.

CRISPR-Cas9 System

• CRISPR-Cas9 enables precise genome editing by targeting specific DNA sequences.

• Cas9 is guided by RNA to cut DNA at desired locations.

• Applications include gene therapy, agriculture, and research.

Summary Table: DNA Replication Models

Model	Description
Conservative	Parental DNA remains intact; new molecule is entirely new DNA
Semiconservative	Each new DNA has one old and one new strand
Dispersive	DNA strands are mixtures of old and new segments

Additional info: This study guide expands on the original questions by providing definitions, explanations, and context for each major concept in the molecular basis of inheritance, suitable for exam preparation in a General Biology course.