Eukaryotic chromatin and chromosome

Eukaryotic chromatin and chromosome

Polytene chromosomes are common in:

Polytene chromosomes arise from repeated rounds of endoreplication without chromatid segregation or cytokinesis, producing thousands of aligned chromatids remaining synapsed, visible as giant banded structures with interbands and transcriptionally active puffs. Best described in dipteran salivary glands, especially Drosophila melanogaster third instar larvae where tissue-specific gene amplification supports massive glue secretion and high metabolic demand. Banding pattern provides physical cytological map for gene localization and transcription mapping. Unlike mammalian liver polyploidy forming binucleate cells, true polytene with visible Balbiani rings occurs primarily in flies and some plants.

Ref: Bridges CB Polytene Maps 1935; Lodish et al., Chapter 8: Polytene Chromosomes Common in Drosophila Salivary Gland

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Eukaryotic chromatin and chromosome

Lampbrush chromosomes are seen in:

Lampbrush chromosomes represent exceptionally extended diplotene bivalents observed during prolonged prophase I arrest in vertebrate oocytes, particularly Xenopus, axolotl and birds. They consist of large chromomeres from which pairs of lateral loops containing actively transcribed DNA expand, coated with nascent ribonucleoprotein matrix visible by light microscopy as brush-like structure. High transcription from loops produces maternal RNAs stored for early embryogenesis while homologous chromosomes remain paired and recombined. Giant size due to active decondensation makes them excellent models for transcription visualization, not found in mitotic somatic cells.

Ref: Morgan TH and Callan Lampbrush; Alberts et al., Molecular Biology of the Cell, Chapter 21: Lampbrush Seen in Meiosis I Oocytes

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Eukaryotic chromatin and chromosome

Histone acetylation causes:

Acetylation of epsilon amino groups on histone tail lysines by HAT complexes like p300, CBP and GCN5 neutralizes positive charge, weakening interaction with negatively charged phosphate backbone and loosening nucleosome wrapping around DNA gyres. Bromodomain-containing proteins specifically recognize acetyllysine and recruit ATP remodelers and polymerase associated factors. Acetylation at H4 K16 directly disrupts interaction with acidic patch on neighboring H2A-H2B dimer required for 30 nanometer fiber formation. Consequently acetylation produces open relaxed chromatin permissive for transcription, replication and repair, opposing condensation and methylation-driven silencing pathways effectively.

Ref: Allfrey et al., 1964; Alberts et al., Molecular Biology of the Cell, Chapter 4: Histone Acetylation Causes Relaxation

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Eukaryotic chromatin and chromosome

DNA methylation usually causes:

DNA methylation at 5-methylcytosine within CpG context catalyzed by DNMT1 maintenance during replication and DNMT3A/3B de novo enzymes generally correlates with stable gene silencing. Methyl-CpG recruits methyl-binding domain proteins MeCP2, MBD1, MBD2 which recruit histone deacetylase complexes and H3K9 methyltransferases, generating compact repressive chromatin impeding transcription factor access. Promoter methylation blocks initiation and maintains X inactivation and genomic imprinting. Removal via TET-mediated hydroxymethylation restores activity, confirming methylation predominantly represses transcription rather than activating replication or recombination alone in differentiated tissues.

Ref: Bird and Wolffe 1999 Methylation Mechanisms; Lodish et al., Chapter 8: DNA Methylation Usually Causes Repression

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Eukaryotic chromatin and chromosome

CpG islands are:

Mammalian gene promoters frequently overlap CpG islands, stretches of 300 to 3000 base pairs with elevated GC content, high CpG dinucleotide frequency and low DNA methylation when active. Most housekeeping and many regulated promoters reside within islands, providing platforms for Sp1 and other transcription factors and H3K4 trimethylation deposition protecting from silencing. Bulk genome shows CpG depletion due to spontaneous deamination of 5-methylcytosine to thymine over evolution, but islands remain unmethylated via TET dioxygenase activity and CXXC domain proteins. Satellite DNA and centromeres distinct AT-rich repeats, not GC islands.

Ref: Bird AP 1986 Nature CpG Islands; Alberts et al., Molecular Biology of the Cell, Chapter 7: GC-rich Promoter Regions

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Eukaryotic chromatin and chromosome

SWI/SNF complex is involved in:

Chromatin structure generally impedes transcription factor binding, requiring active remodeling. ATP-dependent remodeling complexes utilize energy from ATP hydrolysis to slide, evict or restructure nucleosomes exposing DNA. SWI/SNF family, containing BRG1 or BRM ATPase, disrupts histone-DNA contacts creating nucleosome-depleted regions at enhancers and promoters, essential for inducible transcription activation and DNA repair. Mutations in SWI/SNF subunits drive rhabdoid tumors and other cancers. It cooperates with histone acetylation opening chromatin, contrasting with ISWI and CHD families spacing nucleosomes regularly. SWI/SNF governs accessibility rather than histone synthesis.

Ref: Clapier et al., Nat Rev Mol Cell Biol 2017; Alberts et al., Chapter 4: SWI/SNF Complex Chromatin Remodeling

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Eukaryotic chromatin and chromosome

Polycomb proteins cause:

Polycomb group proteins maintain developmental regulators silenced through epigenetic memory. PRC2 complex containing EZH2, EED, SUZ12 catalyzes H3K27 trimethylation, PRC1 containing BMI1, RING1B ubiquitinates histone H2A at lysine 119 and compacts chromatin, blocking SWI/SNF remodeling and preventing RNA polymerase II elongation. Targets include HOX clusters and lineage transcription factors. Compaction renders chromatin inaccessible. Polycomb loss results in ectopic gene activation and homeotic transformations. Unlike constitutive activators, Polycomb establishes facultative heterochromatin reversible during differentiation and reprogramming processes, ensuring cell identity maintenance.

Ref: Lewis Polycomb 1978; Lodish et al., Molecular Cell Biology, Chapter 8: Polycomb Proteins Cause Gene Repression

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Eukaryotic chromatin and chromosome

Which histone modification activates transcription?

Active promoter epigenetic signature includes specific histone methylation. H3 lysine 4 trimethylation deposited by SET1 and MLL complexes at transcription start sites recruits chromatin remodelers, histone acetyltransferases and general transcription factor TFIID via TAF3 PHD finger recognition, reducing nucleosome stability and stimulating preinitiation complex assembly and pause release. Conversely H3K9 trimethylation and H3K27 trimethylation deposited by SUV39H and Polycomb EZH2 cause repression. H4K20 trimethylation marks pericentric heterochromatin. Thus H3K4 trimethylation functions as universal epigenetic hallmark of actively transcribed euchromatic genes across eukaryotes.

Ref: Santos-Rosa et al., Nature 2002; Alberts et al., Molecular Biology of the Cell, Chapter 4: H3K4me3 Active Transcription

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Eukaryotic chromatin and chromosome

H3K9 methylation marks:

Histone code marks specific chromatin states. H3 lysine 9 di and trimethylation catalyzed by SUV39H1 and SUV39H2 methyltransferases creates high affinity binding site for heterochromatin protein 1 alpha via its chromodomain, inducing spreading, compaction and transcriptional repression. H3K9 trimethylation enriched at pericentric satellite repeats, subtelomeric repeats and retrotransposons characterizes constitutive heterochromatin, contrasting with H3K4 trimethylation at active promoters. HP1 oligomerization and interaction with DNMTs maintain domain stability. Disruption causes position effect variegation, ectopic transcription and chromosome missegregation defects compromising genome stability severely.

Ref: Bannister et al., Nature 2001; Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: H3K9 Methylation Heterochromatin Mark

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Eukaryotic chromatin and chromosome

Histone tails are site of:

Histone octamer contains flexible unstructured N-terminal tails and protruding C-termini extending beyond DNA gyres. These lysine, arginine and serine rich segments undergo extensive covalent post-translational modifications including acetylation, methylation, phosphorylation, ubiquitination and sumoylation catalyzed by writer enzymes and erased by erasers. Modifications alter electrostatic charge and create binding sites for effector proteins containing bromodomains, chromodomains and PHD fingers, forming combinatorial histone code that governs accessibility, transcription activation, repression, replication timing and DNA repair without altering fundamental histone DNA wrapping structure.

Ref: Strahl and Allis 2000 Histone Code Hypothesis; Alberts et al., Chapter 4: Histone Tails Site of Modifications

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Eukaryotic chromatin and chromosome

Kinetochore assembles at:

Kinetochore is large multiprotein structure assembling exclusively at centromeric chromatin containing CENP-A nucleosomes. During mitosis inner kinetochore components CENP-C, CENP-T, CENP-I constitutively associate with centromere, outer KMN network including Ndc80, Mis12, KNL1 complexes binds plus ends of spindle microtubules. This linkage transmits pulling forces to separate sister chromatids and activates spindle assembly checkpoint via Mad2, BubR1 until biorientation achieved. Telomeres cap ends, replication origins fire throughout genome, NORs build nucleolus, whereas kinetochore location solely defines segregation platform at centromere region.

Ref: Cheeseman and Desai 2008 Nat Rev Mol Cell Biol; Lodish et al., Chapter 19: Kinetochore Assembles at Centromere

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Eukaryotic chromatin and chromosome

Telomere shortening leads to:

Telomeres shorten each cell division because lagging strand synthesis cannot fully replicate chromosome terminus, known as end replication problem. Somatic cells lacking telomerase lose 50 to 200 base pairs per division. When telomere length reaches critical threshold, uncapped ends recognized as double strand breaks trigger DNA damage response activating p53 and p16 pathways, inducing replicative senescence or apoptosis. This divisional counting acts as mitotic clock explaining Hayflick limit. Severe shortening associates with aging syndromes, bone marrow failure, pulmonary fibrosis, whereas cancer cells bypass by reactivating telomerase or alternative lengthening.

Ref: Hayflick Limit and Telomere Attrition Hypothesis; Alberts et al., Molecular Biology of the Cell, Chapter 20: Telomere Shortening Causes Aging

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