Chromosome - Wikipedia

This article is about the DNA molecule. For the genetic algorithm, see Chromosome (genetic algorithm).

A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most important of these proteins are the histones. Aided by chaperone proteins, the histones bind to and condense the DNA molecule to maintain its integrity.[1][2] These eukaryotic chromosomes display a complex three-dimensional structure that has a significant role in transcriptional regulation.[3]

Chromosome (107 - 1010 bp) DNA Gene (103 - 106 bp ) Function A chromosome and its packaged long strand of DNA unraveled. The DNA's base pairs encode genes, which provide functions. A human DNA can have up to 500 million base pairs with thousands of genes.

Normally, chromosomes are visible under a light microscope only during the metaphase of cell division, where all chromosomes are aligned in the center of the cell in their condensed form.[4] Before this stage occurs, each chromosome is duplicated (S phase), and the two copies are joined by a centromere—resulting in either an X-shaped structure if the centromere is located equatorially, or a two-armed structure if the centromere is located distally; the joined copies are called 'sister chromatids'. During metaphase, the duplicated structure (called a 'metaphase chromosome') is highly condensed and thus easiest to distinguish and study.[5] In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.[6]

Chromosomal recombination during meiosis and subsequent sexual reproduction plays a crucial role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. This will usually cause the cell to initiate apoptosis, leading to its own death, but the process is occasionally hampered by cell mutations that result in the progression of cancer.

The term 'chromosome' is sometimes used in a wider sense to refer to the individualized portions of chromatin in cells, which may or may not be visible under light microscopy. In a narrower sense, 'chromosome' can be used to refer to the individualized portions of chromatin during cell division, which are visible under light microscopy due to high condensation.

Contents

• 1 Etymology
• 2 History of discovery
• 3 Prokaryotes
  • 3.1 Structure in sequences
  • 3.2 DNA packaging
• 4 Eukaryotes
  • 4.1 Interphase chromatin
  • 4.2 Metaphase chromatin and division
  • 4.3 Human chromosomes
• 5 Karyotype
  • 5.1 History and analysis techniques
• 6 Aberrations
  • 6.1 Sperm aneuploidy
• 7 Number in various organisms
  • 7.1 In eukaryotes
  • 7.2 In prokaryotes
• 8 See also
• 9 Notes and references
• 10 External links