Chromosomes and Chromatin

Abstract

AbstractChromosomes are structures within the nuclei of eukaryotic cells that containdeoxyribonucleic acid(DNA) combined with proteins. Chromatin refers to the material of the chromosomes – DNA plus proteins. Before DNA replication, each chromosome contains a single, very long DNA molecule that basically runs from one end of the chromosome to the other. Core histone proteins package segments of this DNA molecule into nucleosomes, and linker histones further compact the resulting ‘string of beads’ into a 30‐nm chromatin fibre. In mitotic chromosomes, scaffold proteins fold the chromatin fibre into loops along its length. Chromosomes are very dynamic structures and take several forms, but the basic organisation is always related to the structure of mitotic chromosomes. The location of chromosomes in the nucleus, the nature of the loops and modifications to the chromatin fibre are thought to be important in determining which DNA sequences are made available for transcription and other processes.Key Concepts:In eukaryotic cells, genes are located in chromosomes in the cell nucleus.Before the DNA has been replicated, a chromosome consists of a single very long DNA double helix that is highly coiled and folded by proteins.Five major types of histone proteins, all of them highly basic, neutralise much of the DNA's negative charge and package the DNA into nucleosomes and 30‐nm fibres.TheN‐terminal tails of core histones are intrinsically disordered, allowing them to interact with many other proteins and undergo various posttranslational modifications.Transcriptional activation of genes, or readiness for transcriptional activation, is associated with histone acetylation, and inactivation is generally associated with histone methylation.In mitotic chromosomes, the chromatin fibre is folded into loops by attachment to a scaffold of nonhistone proteins.Mitotic chromosome condensation appears to involve two components: compaction of the chromatin at the onset of mitosis, which involves phosphorylation of chromosomal proteins, and establishment of a robust architecture that can withstand spindle forces, which involves condensin.The various chromosome forms that are seen throughout the cell cycle or the life cycle of the organism have closely related structures.A chromosome during interphase retains some aspects of the structure it had during mitosis and is confined to a distinct ‘territory’ within the interphase nucleus.Chromosome banding and chromosome painting are medically important for detecting chromosome abnormalities such as breaks, fusions, translocations and aneuploidy and can be used to track chromosomal rearrangements that have occurred during evolution.