Study Notes on Chromosome Structure and Function

Chromosome Structure \n## Course: BIOL210 - Genetics and Genomics \n### Instructor: Professor Cori Anderson \n \n--- \n\n# Learning Objectives \n- Describe chromosome structure and organization, from DNA and nucleosomes to higher-order loops. \n - Highlight the roles of histones and condensin proteins in achieving different levels of chromatin compaction. \n- Compare and explain major chromosome staining techniques, such as banding methods and FISH (fluorescent in situ hybridization). \n - Techniques are used to construct karyotypes and localize genes within the genome. \n- Understand the differences between \n - Heterochromatin and euchromatin. \n - The role of histone modifications. \n - How heterochromatin spreading influences gene expression, including: \n - Position-effect variegation. \n - X-chromosome inactivation. \n \n--- \n\n# Watson-Crick Model of DNA (1953) \n- Key reference: Watson, James D., and Francis H.C. Crick's paper titled "Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid," published in Nature 171.4356 (1953): 737-738. \n- See figures from Klug, Essentials of Genetics (9th Edition) and Alberts, Essential Cell Biology (6th Edition). \n \n--- \n\n# DNA Structure \n## Deoxyribonucleic Acid (DNA) \n- Detailed description of DNA structure: \n - Major and minor grooves, base pairs include adenine, thymine, cytosine, and guanine. \n - DNA is a double helix formed by base pairs held together by hydrogen bonds. \n - Fundamental components: \n - Phosphate group. \n - Sugar-phosphate backbone. \n - Nitrogenous bases (A, T, C, G). \n - Visualization of structure can be found in genome resources. \n \n--- \n\n# DNA Organization Into Chromosomes \n- DNA compaction allows for the fitting of \n - 50 to 250 million base pairs into an individual chromosome of 1.4 μm seen at metaphase. \n- Requires at least four orders of packaging above the 2-nm double-helical chain. \n - Source: Sciencedirect.com. \n \n--- \n\n# Histone Proteins \n- Histones serve to compact DNA into a 10 nm fiber. \n - Assemble into an octamer comprised of \n - 2 each of H2A, H2B, H3, and H4 (total of eight proteins). \n - Histones bind DNA due to a high percentage of positively charged (basic) amino acids. \n \n--- \n\n# Non-Histone Proteins \n- Definition: Proteins found in chromatin that are not part of the nucleosome. \n- Functions: \n - Regulation of gene expression (activation/repression). \n - Assisting in DNA replication and DNA repair. \n - Organizing and supporting chromosome structure. \n \n## Kinetochore Proteins \n- Specialized type of non-histone protein. \n - Assemble at the centromere during mitosis and meiosis. \n - Ensure accurate chromosome segregation. \n - Incorporate specialized histone CENP-A, an H3 variant for microtubule attachment. \n \n--- \n\n# Higher-Order Chromosome Packaging \n- Mechanism for chromatin condensation is not fully understood. \n - Proposed model: nucleosomes fold into a superhelical 30 nm fiber. \n - Stabilization thought to be aided by the linker histone H1. \n \n--- \n\n# Chromosome Looping \n- DNA condenses further into loops that extend from a central protein scaffold. \n - Loops are attached to scaffolding proteins at the base of the chromosome. \n \n--- \n\n# Condensin Proteins \n- Major component of the chromosome scaffold wrapping around the DNA. \n- Complexes consist of structural maintenance of chromosomes (smc) proteins and other associated proteins. \n - Condensins bind the 30 nm chromatin fiber and utilize ATP to extrude loops, compacting DNA during mitosis. \n \n--- \n\n# Loop Extrusion Models \n- Loop-extruding protein complexes bind to chromatin/DNA and extrude loops until they unbind randomly. \n- Process leads to the compaction of chromatin into dense mitotic chromosomes. \n--- \n\n# Review: The Human Karyotype \n- Analysis involves staining chromosomes from a metaphase cell and arranging them in a karyotype. \n- Humans possess 23 pairs of chromosomes – 22 autosomes and 1 pair of sex chromosomes. \n \n--- \n\n# Karyotyping and Chromosome Staining \n- Contributions by Torbjörn Caspersson and Lore Zech in developing chromosome banding techniques during late 1960s. \n- Introduction of Q-banding technique for distinctive banding patterns in chromosomes. \n - 5000 chromosomes from 14 healthy subjects were measured, characterized by length, centromere index, autoradiography, etc. \n \n--- \n\n# G-Banding \n- Method used to identify chromosomes through Trypsin treatment followed by Giemsa staining. \n - Produces a pattern of dark (AT-rich, gene-poor) and light (GC-rich, gene-rich) bands. \n - Patterns allow chromosome identification and structural analysis. \n--- \n\n# FISH Staining \n- Fluorescent in situ hybridization (FISH) allows localization of specific DNA sequences in the genome. \n - Involves hybridizing a fluorescently labeled DNA probe to target sequences. \n--- \n\n# Spectral Karyotyping (SKY) \n- Advanced FISH where each chromosome is labeled with unique fluorescent probes. \n - Enables simultaneous visualization of all chromosomes in distinct colors, aiding in detection of complex genome rearrangements. \n--- \n\n# Chromosome Maps \n- Banding patterns assist in localizing genes to specific regions. \n - Each chromosome arm is divided into numbered regions and bands, defined by the International System for Human Cytogenetic Nomenclature (ISCN). \n - “p” represents short arm (petit) and “q” represents long arm. \n--- \n\n# Specialized Regions of Chromosomes \n- Centromeres: Constricted regions crucial for organizing and attaching spindle fibers during cell division. \n- Telomeres: Repetitive DNA at the ends of linear chromosomes; protect against degradation and maintain genomic stability. \n--- \n\n# Centromeres and Telomeres as Heterochromatin \n- Chromosomal organization into euchromatin (loosely packed, gene-rich, active) and heterochromatin (densely packed, gene-poor, inactive). \n- Constitutive heterochromatin remains condensed, while facultative heterochromatin can decondense. \n--- \n\n# Role of Telomeres \n- Maintain chromosome integrity and prevent erroneous repairs. \n- Form protective caps at chromosome ends against degradation by nucleases. \n--- \n\n# Histone Tail Modifications \n- The N-terminal tails of histone proteins are subject to various post-translational modifications. \n - Modifications include methylation, acetylation, etc. \n--- \n\n# The Histone Code \n- Enzymatic activities categorized into: \n - Writers (add modifications). \n - Erasers (remove modifications). \n - Readers (recognize modifications). \n--- \n\n# Histone Tail Modifications Alter Chromosome Structure \n- Acetylation generally promotes transcriptionally active euchromatin. \n- Methylation can mark either chromatin state depending on residue and methyl groups. \n--- \n\n# Heterochromatin Spreading – Drosophila Eyes \n- Heterochromatin spreading transforms adjacent euchromatin into a transcriptionally silent state. \n - Example: silencing of the white and RST genes in Drosophila via chromosomal inversion. \n--- \n\n# Position-effect Variegation (PEV) \n- Phenomenon where genes silenced in some cells but active in others due to proximity to heterochromatin. \n--- \n\n# Histone Modifications in Heterochromatin Spreading \n- H3K9 methylation serves as a mark for transcriptional silence. \n- HP1 binds these methylated histones and recruits further HMTs to propagate the repressive mark. \n--- \n\n# Review: X-Inactivation \n- Early development: random choice of which X-chromosome to inactivate, maintained in descendants. \n--- \n\n# X-Chromosome Inactivation \n- Inactivation process involves conversion of one X chromosome into heterochromatin. \n- Expression of XIST recruits chromatin-modifying factors to establish transcriptional silencing. \n--- \n\n# Long noncoding RNAs (lncRNAs) in X-Inactivation \n- XIST initiates inactivation while antisense transcript Tsix keeps the other X active. \n- Stochastic process initially selects which X to inactivate. \n--- \n\n# RepA Role in X-Inactivation \n- RepA from XIST recruits PRC2 to initiate silencing and deposit repressive histone marks, catalyzing XIST coating and heterochromatin formation. \n--- \n\n# Summary of Key Points \n- DNA double helix stabilized by hydrogen bonds. \n- Efficient packaging essential: first step involves histones forming nucleosomes (8 core histones). \n- Non-histone proteins assist in gene regulation, chromosome structure, and mitosis. \n- Higher-order chromosome packaging involves complex looping, with condensins extruding DNA loops. \n- Human karyotype includes 46 chromosomes, with staining methods for identification. \n- Centromeres and telomeres are heterochromatin-rich, affecting chromosome stability and activity. \n- Chromatin structure is dynamic, with examples of heterochromatin spreading impacting gene expression, including PEV and X-inactivation.