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Chromosome Structure

What is chromosome?

What is chromosome? i) A threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries genes and functions in the transmission of hereditary information. ii) A circular strand of DNA in bacteria (prokaryotic cells) that contains the hereditary information necessary for cell life.

History chromosomes were first observed in plant cells by a Swiss botanist Karl Wilhelm von Nageli in 1842. also discovered in Acaris worms by Belgian scientist Edouard van Beneden (1846-1910). the chromosome behaviour (e.g. mitosis) in salamander was described by Germen cytologist Walther Flemming in 1882. The word of mitosis was only invented by Germen anatomist Heinrich von Waldeyer later (in 1888).

Characteristics of eukaryotic chromosome found in the nucleus of a cell. consists of DNA and protein (histone etc.). in linear arrangement. ready to absorb dyes (it’s named after; in Greek: chroma is colour, soma is body).

Centromere Electron micrograph of one of the human chromosomes

Structure of chromosome Secondary constriction or NOR (nucleolar organizing region) Telomere Primary constriction or centromere Chromosome arm Telomere

Structure of Chromosome - Telomere telomeres are end caps of chromosomes. It is important for chromosomes that not form a ring (e.g. plasmid). telomeres are made up of both protein and DNA. the sequence motif of telomeres, (TTAGGG)n, is highly conserved across species.

Structure of Chromosome - Telomere Telomeric repeat organism TTAGGG GGGGTT GGGGTTTT AGGGTT(TC) TTTAGGG TTTTAGGG Homo sapien (man) Physarium (slime mould) Didymium (slime mould) Neurospora (filamentous fungi) Trypanosoma (protozoan) Tetrahymena (protozoan) Glaucoma (protozoan) Stylonichia (protozoan) Euplotes (protozoan) Plasmodium (protozoan) Arabidopsis (plant) Chlamydomonas (alga)

Structure of Chromosome - Telomere the ends of chromosomes must terminate in some sort of functional cap to prevent shortening of the chromosome at each round of cell division. when a chromosome breaks (losing telomeres), the ends may fuse resulting translocations or ring chromosomes (if both telomeres are lost from the same chromosome). the DNA at the extreme ends of telomeres is not in the form of double helix but a single strand to form the hairpin structure for the integrity of telomere. the enzyme responsible for replication of telomeres is telomerase. broken chromosomes somehow reform (chromosome healing) telomere using telomerase. in humans, it has been known that telomeres in germline cells are longer than those in somatic cells.

Structure of Chromosome - Centromeres the primary constriction of chromosomes. characterized by particular repeat DNA sequences (satellite DNA; repeat size ranges from 5bp to 170bp) and also associated by specific proteins (e.g. CENP-A, CENP-B, CENP-C, CENP-D). this is also the last point of separation of sister chromatids during cell division. structure kinetochore is located in centromeres. Kinetochore involves directly in cell division.

Structure of Chromosome - NORs NORs – nucleolar organizing regions. transcribed DNA. form visible structure (neucleolus) in the nucleus. the site of ribosome formation. NORs are found associated with satellited chromosomes. In human the satellited chromosomes are chromosomes 13, 14, 15, 21 and 22.

Structure of Chromosome - NORs Interphase nucleus of onion root tip through light microscope nucleolus

Structure of Chromosome - NORs electron micrograph of interphase cell from bat pancreas nucleolus