http:www'ukm'mychoong

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http//www.ukm.my/choong
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Chromosome Structure
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What is chromosome?
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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.
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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).

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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 (its named after in
  Greek chroma is colour, soma is body).

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Centromere
Electron micrograph of one of the human
chromosomes
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Structure of chromosome
Secondary constriction or NOR (nucleolar
organizing region)
Telomere
Primary constriction or centromere
Chromosome arm
Telomere
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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.

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Structure of Chromosome - Telomere
Telomeric repeat
organism
TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG GGGGTT GGGGTT G
GGGTTTT 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)
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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.

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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.

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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.

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Structure of Chromosome - NORs
Interphase nucleus of onion root tip through
light microscope
nucleolus
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Structure of Chromosome - NORs
electron micrograph of interphase cell from bat
pancreas
nucleolus