DNA, Chromosomes, and the Nucleus: Structure, Function, and Organization

Chapter 16: DNA, Chromosomes and the Nucleus

General Flow of Genetic Information

The flow of genetic information is fundamental to cell biology, involving the transmission and expression of genetic material within and between cells.

• Cell Division: Genetic information is passed between generations through cell division.

• Central Dogma: Within a cell, genetic information flows from DNA to RNA (transcription) and from RNA to protein (translation).

• Gene Expression: The process by which genetic information is used to synthesize gene products (proteins or RNA). A gene is a segment of DNA that encodes a functional product.

DNA

Nucleotide Structure

DNA is a polymer composed of nucleotide monomers, each with distinct structural features.

• Nucleotide Components: Each nucleotide consists of a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, or thymine).

• Pyrimidines and Purines: Pyrimidines (cytosine, thymine) have a single ring; purines (adenine, guanine) have a double ring.

• Nucleoside vs. Nucleotide: A nucleoside is a base plus sugar; a nucleotide is a base, sugar, and phosphate.

• Numbering: The carbons on deoxyribose are numbered to distinguish the base, the OH, and the phosphate group.

DNA as a Polymer

DNA forms a long chain through the polymerization of nucleotides.

• Phosphodiester Bond: Nucleotides are joined by condensation reactions between the 3' hydroxyl group of one nucleotide and the 5' phosphate of the next, forming a sugar-phosphate backbone.

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

• Abbreviations: kb (kilobase, 1,000 bases), Mb (megabase, 1,000,000 bases), Gb (gigabase, 1,000,000,000 bases).

• Genome Size: The human genome is approximately 3.2 billion base pairs (3.2 Gb).

The Double Helix

The double helix is the fundamental structure of DNA, elucidated by Watson and Crick in 1953, based on Rosalind Franklin's X-ray diffraction data.

• Antiparallel Strands: Two complementary DNA strands run in opposite directions, forming the double helix.

• Base Pairing: Adenine pairs with thymine (A-T) via two hydrogen bonds; guanine pairs with cytosine (G-C) via three hydrogen bonds (Chargaff's rules).

• Stabilization: Hydrogen bonds and hydrophobic interactions (base stacking) stabilize the helix.

• Major and Minor Grooves: The double helix has major and minor grooves, which are important for protein-DNA interactions.

• Forms of DNA: B-DNA (right-handed, most common), A-DNA (right-handed, more compact), Z-DNA (left-handed, less common).

Example: The B-form of DNA is the predominant form in cells, with about 10 base pairs per turn and a helical pitch of 3.4 nm.

Melting Temperature (Tm) and Base Composition

The melting temperature (Tm) is the temperature at which half of the DNA helix denatures into single strands.

• Base Pair Effect: DNA with higher G-C content has a higher Tm due to the three hydrogen bonds in G-C pairs compared to two in A-T pairs.

Hybridization and FISH Technique

Hybridization refers to the process by which complementary nucleic acid strands pair to form a double helix.

• FISH (Fluorescence In Situ Hybridization): A technique that uses fluorescent probes to detect specific DNA sequences on chromosomes, useful for gene mapping and diagnostics.

Eukaryotic Chromatin

Chromatin Structure

Chromatin is the complex of DNA and proteins (mainly histones) that packages eukaryotic DNA into the nucleus.

• Charge: Histones are positively charged; DNA is negatively charged, facilitating tight binding.

• Nucleosome: The basic unit of chromatin, consisting of DNA wrapped around a histone octamer (H2A, H2B, H3, H4).

• Beads-on-a-String: Nucleosomes appear as "beads" on a DNA "string" under electron microscopy, with about 147 base pairs per nucleosome and 10 nm fiber diameter.

• 30 nm Fiber: Nucleosomes further coil to form a 30 nm chromatin fiber, stabilized by histone H1.

• Chromosome Scaffold: Higher-order folding involves loops attached to a protein scaffold, forming metaphase chromosomes.

Heterochromatin vs. Euchromatin

Chromatin can exist in two main forms, affecting gene expression.

• Heterochromatin: Highly condensed, transcriptionally inactive.

• Euchromatin: Less condensed, transcriptionally active.

Alterations in Chromatin Packing

Chromatin structure can be dynamically modified to regulate gene expression.

• Histone Modifications: Histones can be chemically modified (e.g., methylation, acetylation), affecting chromatin accessibility and gene activity.

• Chromatin Remodeling: ATP-dependent complexes can reposition nucleosomes, altering DNA accessibility.

Eukaryotic Chromosomes

During cell division, chromatin condenses to form visible chromosomes.

• Chromosome Structure: Each chromosome consists of a single, continuous DNA molecule and associated proteins.

• Centromere and Telomere: Specialized regions important for chromosome stability and segregation.

• Karyotype: The complete set of chromosomes in a cell, visualized during metaphase.

Supercoiled DNA

Supercoiling refers to the overwinding or underwinding of DNA, affecting its compactness and function.

• Positive Supercoiling: DNA is overwound.

• Negative Supercoiling: DNA is underwound (more common in cells).

• Topoisomerases: Enzymes that relieve supercoiling by cutting and rejoining DNA strands.

Bacterial Chromosomes

Bacterial DNA is organized differently from eukaryotic DNA.

• Circular DNA: Bacterial chromosomes are typically circular and supercoiled.

• Plasmids: Small, circular DNA molecules that replicate independently of the bacterial chromosome.

Key Experiments: Avery and Griffith

Classic experiments established DNA as the genetic material.

• Griffith's Experiment: Demonstrated transformation in bacteria, suggesting a "transforming principle."

• Avery, MacLeod, and McCarty: Identified DNA as the transforming principle, confirming its role as genetic material.

DNA Form	Handedness	Major Features
B-DNA	Right-handed	Most common, 10 bp/turn, major and minor grooves
A-DNA	Right-handed	More compact, 11 bp/turn, found in dehydrated samples
Z-DNA	Left-handed	Less common, may play a role in gene regulation

Additional info: The notes have been expanded with definitions, context, and examples to provide a comprehensive yet concise study guide for college-level cell biology students.