Cytogenetics

This is the book I bought.. Haha.. bangga wei!!
Hmm, explored some parts of the book, esp the parts about polytene chromosomes and Lampbrush chromosomes. Both polytene chromosome and Lampbrush chromosome are specialised, and adaptive chromosome.


The photo below shows a set of polytene chromosomes (giant chromosome) in the nucleus of the larval stages of Drosophila (flies). The nucleus is found in the salivary glands of the larvae. There are 4 chromosomes in a set of polytene chromosome.



"Polytene" means 'many strands'.
The chromosomes in the nucleus come together(somatic synapsis) and undergo repeated rounds of DNA replication, up to 9 times of DNA replication! There is no cell division and therefore no daugther nucleus is produced. The process is called endoreplication. But the centromeric regions of the chromosomes do not endoreplicate very well and therefore all the centromeres bundle together in a mass called chromocenter.

From the photo you can observe the alternating dark and light bands. The bands contain most of the mass of DNA and intensively stained using appropriate reagents (eg. aceto-orcein). The regions between the bands are called interbands and they are stained more lightly. The bands and interbands are of unequal length.
Each chromosome is like a large number of parallel fibers running longitudinally. The fibers are more condensed in the bands, and less condensed in the interbands. Probably each fiber represents a single C, ie haploid chromosomes. The degree of polyteny is the number of the haploid chromosomes contained in a giant chromosome.
Rearrangement like deletions, inversions or duplications of chromosomes will cause the alterations of the order of bands.

Besides the alternating bands, you can see the light coloured mass at the right hand side of the photo, next to the dark spot. It is called 'puff' or 'Balbiani rings'. These are the sites of gene expression. At the puff region, the chromosome fibers unwind and become diffused from their usual state of packing in the band. The formation of puff is due to gene activation, directly by hormone, or indirectly by the products of earlier puffs.
You might wonder what kind of gene expression is it at the puff sites. It is the transcription of DNA to produce messenger RNA(mRNA). In order for transcription to take place, the bands need to expands and relax its structure. Therefore, the formation of puffs can be seen as the result of transcription. A single active gene is enough to generate a puff.

Interesting?? I find it amzing!!


Another cekap chromosome -- Lampbrush chromosome, its formed during extended meiosis(diplotene phase) in amphibian oocyte.



The lampbrush chromosomes are meiotic bivalents. Each of the bivalent consists of 2 pairs of sister chromatids, held at the chiasmata. Sister chromatids are chromatids held at the centromere of the chromosome. Chiasmata are the points of contact between paired chromatids during meiosis, resulting in a cross-shaped configuration and representing the cytological manifestation of crossing over. Lateral loops extrude from the chromomeres at certain positions along the chromosomes. Chromomeres are beads of densely stained granules.

The loops represent the chromosomal material extended from its more compact organisation in the chromomeres. These loops are the sites of gene expression(transcription unit)-that is the transcription of DNA to produce mRNA. They are the extruded segments of DNA where transcription takes place. The chromosomes will revert back to their usual compact size later during meiosis.
The loops are surrounded by a matrix of ribonucleoprotein(RNP). Newly transcribed RNA chains are found in the matrix.


Both of these specialised chromosomes have adaptive features to facilitate gene expression.



Hmm, wonder why suddenly I so semangat to post this kind of thing. Hey, I spent the whole afternoon doing this la..