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Figure 1: Chromosome. (1) Chromatid. One of the two identical parts of the chromosome after S phase . (2) Centromere. The point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm.A chromosome is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elements and other intervening nucleotide sequences. In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, where it wraps around histones (structural proteins, Fig. 1), and where this composite material is called chromatin. During mitosis (cell division), the chromosomes are condensed and called metaphasic chromosomes. This is the only natural context in which individual chromosomes are visible with an optical microscope. Prokaryotes do not possess histones or nuclei. In its relaxed state, the DNA can be accessed for transcription, regulation, and replication. Chromosomes were first observed by Karl Wilhelm von NägeliKarl Wilhelm von Nageli ( March 27, 1817 May 10, 1891) was a Swiss biologist. He discovered what would later become known as chromosomes. Nageli was born in Kilchberg near Zurich where he studied medicine and began working with Manfred Schleiden in 1840. in 1842Events February 21 John J. Greenough patents the sewing machine. March 5 Over 500 Mexican troops led by Rafael Vasquez invade Texas briefly occupy San Antonio and then head back to the Rio Grande. This is the first such invasion since the Texas Revolution and their behavior later described in detail by Walther Flemming in 1882Events February 2 The Knights of Columbus are formed in New Haven, Connecticut February 7 In Mississippi City the last heavyweight boxing championship bareknuckle fight takes place. March 2 Robert Maclean fails to assassinate Queen Victoria at Windsor Mar. In 1910Events January events January 13 The first live musical radio program. Lee De Forest broadcasts a live performance of Enrico Caruso from the Metropolitan Opera. January 26 ? Seine floods in Paris. February events February 8 The Boy Scouts of America is in, Thomas Hunt MorganThomas Hunt Morgan ( September 25, 1866 December 4, 1945) worked in natural history, zoology, and macromutation in Drosophila. Because of his work, Drosophila became one of the major animal models in genetics. His most important contributions to science w proved that chromosomes are the carriers of genes.
1 Chromosomes in eukaryotes
Eukaryotes possess multiple linear chromosomes contained in the cell's nucleus. Each chromosome has one centromereThe centromere is a region of a eukaryotic chromosome where the kinetochore is assembled. Thus, it is the site where spindle fibers of the mitotic spindle attach to the chromosome during mitosis. It is also the site of the primary constriction visible in, with one or two arms projecting from the centromere. The ends of the chromosomes are special structures called telomereA telomere is a region of highly repetitive DNA at the end of a chromosome, which functions as an aglet. Every time linear eukaryotic chromosomes are replicated, the DNA polymerase complex stops several hundred bases before the end; if it were not for tels. DNA replication begins at many different locations on the chromosome.
2 Chromosomes in bacteria
Bacterial chromosomes are often circular but sometimes linear. Some bacteria have one chromosome, while others have a few. Bacterial DNA also exists as plasmidPlasmids are (typically) circular double stranded DNA molecules that are separate from the chromosomal DNA (Fig. They usually occur in bacteria, sometimes in eukaryotic organisms (e. the 2-micrometre-ring in Saccharomyces cerevisiae . Their size varies frs. The distinction between plasmids and chromosomes is poorly defined, though size and necessity are generally taken into account. Bacterial chromosomes initiate replication and one origin of replication.
