Chromosome Structure Notes

Chromosome Structure

Learning Objectives

  • Describe chromosome structure and organization:
    • Understanding the structural hierarchy from DNA to chromatin loops.
    • Identification of roles played by histones and condensin proteins in chromatin compaction.
  • Compare and explain chromosome staining techniques:
    • Understanding banding methods and FISH (Fluorescent In Situ Hybridization) employed in karyotyping.
    • Localizing genes within the genome using these techniques.
  • Differences between heterochromatin and euchromatin:
    • Examining histone modifications and their effects on gene expression.
    • Implications of heterochromatin spreading, position-effect variegation, and X-chromosome inactivation.

The Watson-Crick Model of DNA (1953)

  • Reference: Watson, James D., and Francis HC Crick's paper “Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid” published in Nature (1953) highlighting fundamental DNA structure.
  • Fundamental Figures: Klug Essentials of Genetics 9th Edition: Figure 9-11, Alberts Essential Cell Biology, 6th Edition, Figure 5-5.

DNA Structure

  • Deoxyribonucleic Acid (DNA):
    • Major groove and minor groove structures contribute to DNA functionality.
    • Fundamental building blocks include:
    • Nucleotides comprising
      • Adenine (A) and Thymine (T) with hydrogen bonds:
      • Base pairing: AT (2 hydrogen bonds) and CG (3 hydrogen bonds).
    • Components: Phosphate, deoxyribose sugar, and nitrogenous bases (A, T, G, C).

DNA Organization Into Chromosomes

  • Chromosome Compaction:
    • Achieving compaction necessary for fitting 50 to 250 million base pairs into a 1.4-μm chromosome requires at least four orders of packaging relative to the DNA's 2-nm double-helical chain.

Histone Proteins

  • Role of Histones:
    • Histones serve to compact DNA into a 10 nm fiber through specific structural organization.
    • Histone assembly into octamers includes two molecules each of H2A, H2B, H3, and H4.
    • Histones exhibit high percentages of positively charged amino acids facilitating binding to negatively charged DNA.

Nucleosome Structure

  • “Beads-on-a-string” Model:
    • Nucleosomes consist of DNA wound around a core of histone proteins (147 nucleotide pairs of DNA).
    • Linker DNA, released through nuclease digestion, connects nucleosome core particles.
    • High salt concentration can dissociate histone octamer from the DNA.

Non-Histone Proteins

  • Definition:
    • Non-histone proteins are chromatin components distinct from the nucleosome core.
  • Functions:
    • Regulation of gene expression.
    • Assistance in DNA replication and repair.
    • Chromosome structure organization and support.
  • Kinetochore Proteins:
    • Specialized non-histone proteins that assemble at the centromere aiding in accurate chromosome segregation during cell division.

Kinetochore Proteins

  • Centromere Specifics:
    • Centromeres embedded with specialized histone CENP-A (variant of H3) recruit kinetochore proteins for microtubule attachment during mitosis.

Higher-Order Chromosome Packaging

  • Mechanism Understanding:
    • The full mechanism remains to be elucidated; however, nucleosomes are suggested to form superhelical 30 nm fibers.
    • Process stabilization appears influenced by the linker histone H1.

Chromosome Looping

  • Formation of Loops:
    • As condensation progresses, DNA forms loops linked to a central protein scaffold, indicated by darker regions in visual illustrations of chromosomes.

Condensin Proteins

  • Role of Condensins:
    • Key components of the chromosome scaffold wrapping around DNA, consisting of structural maintenance of chromosomes (smc) proteins and other subunits.
    • Operate ATP-dependently to extrude loops of 30 nm chromatin fibers that compact DNA through anchor points on the chromosome scaffold.

Loop Extrusion Models

  • Theory of Loop-Extruding Proteins:
    • Binding stochastically to chromatin, these complexes extrude chromatin loops until they detach.
    • Chromatin fibers compact progressively during prophase and prometaphase leading to the formation of mitotic structures.

Review: The Human Karyotype

  • Karyotyping Process:
    • Staining metaphase chromosomes enables their arrangement into a karyotype.
    • Humans have 23 pairs of chromosomes; 22 autosomes and 1 pair of sex chromosomes.

Karyotyping and Chromosome Staining

  • Significant Contributions:
    • Development of chromosome banding techniques by Torbjörn Caspersson and Lore Zech in the late 1960s, notably introducing quinacrine fluorescence (Q-banding).
  • Performance Standards:
    • Measurement of approximately 5000 chromosomes from 14 healthy subjects to classify based on characteristics including length, centromere index, autoradiography, and secondary constrictions.

G-Banding

  • Method Overview:
    • Involves trypsin treatment of chromosomes followed by Giemsa dye staining, yielding characteristic dark/light banding patterns.
    • Dark bands correspond to AT-rich gene-poor areas; light bands are GC-rich and gene-rich, although the exact basis for these patterns remains not fully understood.

FISH Staining

  • Fluorescent In Situ Hybridization:
    • Localizes specific DNA sequences within the genome using fluorescent DNA probes complementary to target sequences.

Spectral Karyotyping (SKY)

  • Advancement Over FISH:
    • Each chromosome pair labeled with unique fluorescent probes to allow simultaneous visualization, facilitating detection of genome rearrangements.

Chromosome Maps

  • Mapping Genes:
    • Chromosome banding patterns assist in localizing and mapping genes to exact chromosomal regions, using a standardized system governed by ISCN.
    • Designations: p = short arm, q = long arm.

Specialized Regions of Chromosomes

  • Centromeres:
    • Constricted regions crucial for spindle fiber organization during division.
  • Telomeres:
    • Repetitive DNA sequences at chromosome ends, crucial for preventing degradation and maintaining genomic stability.

Heterochromatin vs. Euchromatin

  • Structural Functional Regions:
    • Euchromatin: Loosely packed, gene-rich, transcriptionally active.
    • Heterochromatin: Densely packed, gene-poor, transcriptionally inactive.

Constitutive Heterochromatin

  • Definition:
    • Centromere and telomere regions are composed of constitutive heterochromatin, remaining condensed throughout the cell cycle, while facultative heterochromatin can decondense under specific conditions.

Role of Telomeres

  • Maintaining Integrity:
    • Protect against DNA repair mechanisms mistakenly acting on chromosome ends, preventing them from being treated as double-strand breaks.

Histone Tail Modifications

  • Modification Mechanisms:
    • N-terminal histone tails can undergo diverse post-translational modifications (methylation, acetylation) influencing chromatin states.

The Histone Code

  • Modification Dynamics:
    • Writers, erasers, and readers (enzymes) participate in the addition, removal, and recognition of specific modifications on histones.

Impact of Histone Modifications

  • Transcriptional Influence:
    • Histone acetylation typically promotes euchromatin formation while methylation may indicate either state, dependent on the residue involved.

Heterochromatin Spreading - Drosophila Eyes

  • Phenomenon Understanding:
    • Heterochromatin can convert adjacent euchromatin into a condensed state; observed in Drosophila where heterochromatin silences specific genes.

Position-Effect Variegation (PEV)

  • Mosaic Phenotypes:
    • Genes relocated near heterochromatin can experience variegated expression patterns, leading to varied phenotypic traits.

X-Chromosome Inactivation

  • Mechanism:
    • Involvement of XIST and its role in coating future inactive X chromosomes, recruiting chromatin factors for condensation and silencing.

lncRNAs in X-Inactivation

  • Function and Recruitment:
    • Xist and Tsix transcripts regulate X-inactivation mechanism through differential expression leading to one X chromosome being inactivated.

Summary of Key Points

  • DNA Structure and Stability:

    • Double helix stability due to hydrogen bonds between complementary bases, with effective packaging essential for fitting within the nucleus.
    • Initial chromosome compaction provided by histone proteins forming nucleosomes.

  • Higher Order Packaging Mechanisms:

    • Complex looping by condensin proteins ensures tight DNA compaction for division.

  • Chromosomal Regions:

    • Centromeres and telomeres are critical for organization and stability, enriched in heterochromatin with variable histone modifications impacting their transcriptional states.

  • Heterochromatin Dynamics:

    • Ability to spread, as demonstrated in Drosophila gene expression and related to X-inactivation mechanisms.

All figures discussed are referenced from the respective texts and publications throughout this guide.