Chapter 16: The Molecular Basis of Inheritance – DNA Structure, Replication, and Chromosome Packing

Chapter 16: The Molecular Basis of Inheritance

Introduction to DNA and Genetic Information

DNA replication is essential for the inheritance of genetic information from parent to daughter cells, both during mitosis and across generations. Each gene is a unit of hereditary information, consisting of a specific DNA sequence. Replication begins at multiple sites along the chromosome, resulting in duplicated and condensed chromosomes that are distributed to daughter cells.

• Unduplicated chromosome: Contains one DNA molecule and associated proteins.

• Duplicated chromosome: Contains two DNA molecules and proteins, ready for cell division.

• Gene: A segment of DNA encoding hereditary information.

Discovery of DNA as Genetic Material

Experiments demonstrated that traits can be transferred between bacteria, providing evidence that DNA is the genetic material.

• Griffith's Experiment: Showed that nonpathogenic bacteria could acquire pathogenic traits when mixed with heat-killed pathogenic bacteria, resulting in the death of mice.

• Transformation: The process by which genetic material is transferred between cells.

• Application: This experiment laid the foundation for understanding DNA as the molecule of inheritance.

DNA Structure Review

DNA Nucleotide Structure

DNA is a polymer composed of nucleotides, each consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base. The backbone is formed by alternating sugar and phosphate groups, with nitrogenous bases projecting inward.

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

• Phosphodiester bonds: Link nucleotides together in a strand.

• Directionality: DNA strands have a 5' end (phosphate) and a 3' end (hydroxyl group).

Double Helix and Base Pairing

DNA consists of two antiparallel strands forming a double helix. The strands are held together by hydrogen bonds between complementary bases.

• A-T base pair: Two hydrogen bonds

• C-G base pair: Three hydrogen bonds (stronger)

• Antiparallel orientation: One strand runs 5' to 3', the other 3' to 5'

Chargaff's Rules

Chargaff discovered that the amount of adenine equals thymine, and the amount of guanine equals cytosine in DNA.

• %A = %T

• %G = %C

• Example: If DNA is 40% A, then it must be 40% T.

Concept 16.2 – DNA Replication

Overview of DNA Replication

DNA replication is the process by which DNA is copied before cell division. Each strand of the double helix serves as a template for the synthesis of a new complementary strand.

• Unwinding: The double helix unwinds and separates.

• Template: Each parental strand acts as a template for a new strand.

Semiconservative Model

DNA replication follows the semiconservative model, where each new DNA molecule consists of one parental and one newly synthesized strand.

• Semiconservative replication: Each daughter DNA contains one old and one new strand.

Origin of Replication in Eukaryotes

Replication begins at specific sites called origins of replication, forming replication bubbles and forks.

• Multiple origins: Eukaryotic chromosomes have many origins to speed up replication.

• Replication fork: The Y-shaped region where DNA is actively unwound and replicated.

Enzymes Involved in DNA Replication

Several enzymes are required for DNA replication to proceed efficiently and accurately.

• Helicase: Unzips the double helix and separates the two DNA strands.

• Single-strand binding proteins: Stabilize unwound DNA strands and prevent them from reannealing.

• Topoisomerase: Relieves strain from twisting of the double helix as it is unwound.

• Primase: Synthesizes short RNA primers needed to start DNA synthesis.

• DNA polymerase: Adds nucleotides to the new DNA strand in the 5' to 3' direction.

• Ligase: Joins Okazaki fragments together on the lagging strand.

Leading and Lagging Strand Synthesis

DNA replication is continuous on the leading strand and discontinuous on the lagging strand, resulting in Okazaki fragments.

• Leading strand: Synthesized continuously toward the replication fork.

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

• Okazaki fragments: Short DNA segments synthesized on the lagging strand.

Steps in Lagging Strand Synthesis

1. Primase synthesizes RNA primer for each fragment.

2. DNA polymerase III makes Okazaki fragments.

3. DNA polymerase I replaces RNA primers with DNA.

4. DNA ligase forms bonds between DNA fragments.

Proofreading and DNA Repair

DNA polymerase proofreads newly synthesized DNA and corrects errors. Additional repair mechanisms fix mismatches that escape proofreading.

• Proofreading: DNA polymerase detects and corrects mismatches during replication.

• Mismatch repair: Nuclease enzymes remove incorrect bases, DNA polymerase fills the gap, and DNA ligase seals the strand.

Telomeres and Telomerase

Linear chromosomes have repetitive sequences at their ends called telomeres. Telomerase is an enzyme that extends telomeres, preventing chromosome shortening during replication.

• Telomeres: Repetitive DNA sequences at chromosome ends that protect genetic information.

• Telomerase: Adds telomeric repeats to the ends of chromosomes, especially in germ cells and some stem cells.

Concept 16.3 – Chromosome Packing

Chromatin Structure

Eukaryotic chromosomes consist of linear DNA molecules associated with large amounts of protein, forming chromatin. Chromatin is organized into nucleosomes, the smallest unit of DNA packaging in the nucleus.

• Histone proteins: Positively charged proteins that DNA wraps around to form nucleosomes.

• Nucleosome: DNA wrapped around a core of histone proteins; "beads on a string" structure.

• Chromatin: The complex of DNA and proteins that forms chromosomes.

Enzyme/Protein	Function
Helicase	Unwinds and separates DNA strands
Single-strand binding protein	Stabilizes unwound DNA
Topoisomerase	Relieves strain ahead of replication fork
Primase	Synthesizes RNA primer
DNA polymerase III	Synthesizes new DNA strand
DNA polymerase I	Replaces RNA primer with DNA
DNA ligase	Joins Okazaki fragments
Telomerase	Extends telomeres

Key Equations and Concepts

• Base Pairing: $\%A = \%T$, $\%G = \%C$

• Semiconservative Replication: Each new DNA molecule: $1$parental strand$+$$1$ new strand

Additional info: Chromatin structure is dynamic and plays a role in gene regulation and accessibility for transcription, replication, and repair.