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ChroChromosome Structure mosome Structure

What is cWhat is chromosohromosome?me?

What is cWhat is chromosohromosome?me?

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.

HistoryHistory

- 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 chromosomeCharacteristics 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).

Electron micrograph of one of the human chromosomes

Centromere

Structure of cStructure of chromosohromosomeme

Telomere

Chromosome arm

Secondary constriction or NOR (nucleolar organizing region)

Primary constriction or centromere

Telomere

Structure of Chromosome - TelomereStructure 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 - TelomereStructure of Chromosome - Telomere

Telomeric repeat organism

TTAGGG

TTAGGG

TTAGGG

TTAGGG

TTAGGG

GGGGTT

GGGGTT

GGGGTTTT

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 - TelomereStructure 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 - CentromeresStructure 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 - NORsStructure 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 - NORsStructure of Chromosome - NORs

Interphase nucleus of onion root tip through light microscope

nucleolus

Structure of Chromosome - NORsStructure of Chromosome - NORs

electron micrograph of interphase cell from bat pancreas

nucleolus