Chromosome

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The two ends of a chromosome are known as telomeres.

It required for the replication and stability of the chromosome.

When telomeres are damaged or removed due to chromosome breakage, the damaged chromosome ends can readily fuse or unite with broken ends of other chromosome.

Thus it is generally accepted that structural integrity and individuality of chromosomes is maintained due to telomeres.

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McClintock noticed that if two chromosomes were broken in a cell, the end of one could attach to the other and vice versa.

What she never observed was the attachment of the broken end to the end of an unbroken chromosome.

Thus the ends of broken chromosomes are sticky, whereas the normal end is not sticky, suggesting the ends of chromosomes have unique features.

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Telomere Repeat Sequences

until recently, little was known about molecular structure of telomeres. However, during the last few years, telomeres have been isolated and characterized from several sp.

Species Repeat Sequence

Arabidopsis TTTAGGG

Human TTAGGG

Oxytricha TTTTGGGG

Slime Mold TAGGG

Tetrahymena TTGGGG

Trypanosome TAGGG

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The telomeres of this organism end in the sequence 5'-TTGGGG-3'.

The telomerase adds a series of 5'-TTGGGG-3' repeats to the ends of the lagging strand.

A hairpin occurs when unusual base pairs between guanine residues in the repeat form.

Finally, the hairpin is removed at the 5'-TTGGGG-3' repeat.

Thus the end of the chromosome is faithfully replicated.

Tetrahymena - protozoa organism.

RNA Primer - Short stretches of ribonucleotides (RNA substrates) found on the lagging strand during DNA replication. Helps initiate lagging strand replication

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Staining and Banding chromosome

Staining procedures have been developed in the past two decades and these techniques help to study the karyotype in plants and animals.

Feulgen Staining:

Cells are subjected to a mild hydrolysis in 1N HCl at 600C for 10 minutes.

This treatment produces a free aldehyde group in deoxyribose molecules.

When Schiff’s reagent (basic fuschin bleached with sulfurous acid) to give a deep pink colour.

Ribose of RNA will not form an aldehyde under these conditions, and the reaction is thus specific for DNA

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2. Q banding:

The Q bands are the fluorescent bands observed after quinacrine mustard staining and observation with UV light.

The distal ends of each chromatid are not stained by this technique.