Chromosomes

Chromosomes were first reported inside eukaryotic cells by Eduard Strasburger in 1875, though the actual term 'chromosome' was only coined thirteen years later, by Wilhelm Waldeyer in 1888. It took until 1916 for Calvin Bridges to actually prove that chromosomes are the physical carriers of genes, connecting the visible structure to the abstract idea of heredity. Chemically, a chromosome is made of DNA together with a set of associated proteins. Structurally, chromosomes are built from a thread-like material called chromatin, itself a combination of DNA, protein and RNA; each fully formed chromosome is made of two identical, symmetrical strands called sister chromatids, which only become visible as distinct, well-organised, definite-shaped structures once the cell enters division. A typical chromosome shows narrow, pinched-in regions called constrictions, of two kinds. The primary constriction is the site of the centromere and its associated protein complex, the kinetochore - the two sister chromatids are physically joined to each other at the centromere, and the number of centromeres a chromosome carries can vary: a monocentric chromosome has just one, while a rarer polycentric chromosome has several. The kinetochore itself is the specific region of the chromosome where the spindle fibre physically attaches during mitosis, making it essential for correctly separating the chromatids. Besides the primary constriction, a chromosome may also carry one or more secondary constrictions; these are the sites called nucleolar organizer regions, because they carry the genes for ribosomal RNA and it is around these regions that a cell's nucleoli actually form. A short chromosomal segment cut off from the rest of the chromosome by an unusually long secondary constriction is called a satellite, or SAT chromosome. Beyond the centromere and constrictions, the terminal ends of every chromosome are called telomeres; telomeres carry specific, repeated short DNA sequences (the sequence 5'TTAGGG3' repeated many times, in organisms as different as the fungus Neurospora crassa and humans), and their upkeep is directly linked to how long a cell can keep dividing and, more broadly, to processes such as ageing and cancer - telomeres also serve the simple structural purpose of preventing the ends of different chromosomes from accidentally fusing together. Based purely on where the centromere sits along a chromosome, chromosomes are classified into four types: telocentric (centromere right at the terminal end), acrocentric (centromere near, but not exactly at, the terminal end, with that end capped by a telomere), sub-metacentric (centromere off-centre, giving an L-shape with unequal arms), and metacentric (centromere exactly in the middle, giving a V-shape with equal arms). Looking more closely at the centromere itself, three distinct organisational types are recognised across eukaryotes. A point centromere is one where the kinetochore assembles at a specific, defined DNA sequence recognised by particular proteins (also called a localised centromere), and it binds only a single microtubule; this type occurs in budding yeast. A regional centromere is one where the kinetochore instead assembles across a longer, variable array of repeated DNA sequences, and it can bind several microtubules at once; this type occurs in fission yeast, humans and many other eukaryo …