7.10
Cromosomi politenici
I cromosomi politenici sono cromosomi interfasici giganti con diversi filamenti di DNA affiancati. Furono scoperti nell'anno 1881 da Balbiani nelle gh…
Tipicamente, una cellula diploide prima di dividersi, duplica ciascuno dei suoi cromosomi. La cellula distribuisce, in seguito, una copia di ogni cromosoma alle due cellule figlie, in modo che ogni cellula derivata riceva un insieme completo di cromosomi. Alcune cellule subiscono cicli multipli di duplicazione del cromosoma senza subire alcuna divisione cellulare.
Tali cellule, chiamate cellule poliploidi, contengono serie multiple di ogni cromosoma. Nelle cellule delle ghiandole salivari delle larve di Drosophila, questo processo è reso estremo:qui, centinaia e migliaia di copie di ogni cromosoma sono generate senza alcuna divisione cellulare, comportando la formazione di cromosomi giganti unici, chiamati cromosomi politeni;migliaia di sequenze di DNA identiche sono affiancate una ad una, come pastelli in una scatola, rendendo visibili alcune caratteristiche della cromatina. Caratteristiche che esistono nei cromosomi interfase ordinari, ma sono difficili da osservare.
Visto al microscopio, nei cromosomi politeni si può vedere un pattern alternato di bande scure e di interbande più chiare. Le bande costituiscono il 95%del DNA, mentre le interbande costituiscono il 5%Dove la cromatina nelle bande è più condensata e trascrizionalmente inattiva, la cromatina nelle interbande è meno condensata e trascrizionalmente attiva. Mentre le larve di Drosophila avanzano da una fase di sviluppo ad un'altra, bande specifiche e interbande aumentano notevolmente nel diametro, producendo le strutture chiamate puffs Le puffs derivano dalla decondensazione della cromatina e sono siti di trascrizione attiva del DNA.
La cromatina nei puffs è situata in strutture ad anello, come quelle osservate nei cromosomi a spazzola.
View the full transcript and gain access to JoVE Core videos
Q1: What are polytene chromosomes and where are they found?
Polytene chromosomes are giant chromosomes formed through multiple rounds of DNA replication without cell division, resulting in many parallel DNA strands aligned together. They are primarily found in the salivary glands of dipteran larvae, such as Drosophila, and in other larval tissues. Their large size makes them ideal for cytological study under light microscopy.
Q2: How do polytene chromosomes form through endoreplication?
Polytene chromosomes form through endoreplication, a process where DNA replicates multiple times without intervening mitosis. The replicated DNA strands remain attached and aligned in parallel, creating a thick, banded structure. This amplification can produce chromosomes with hundreds or thousands of DNA copies, dramatically increasing their visibility and allowing detailed analysis of banding patterns.
Q3: What do the bands and interbands on polytene chromosomes represent?
Bands on polytene chromosomes represent regions of tightly packed chromatin containing genes, while interbands are less condensed regions between them. The banding pattern reflects differences in chromatin packaging and gene density. These visible bands correspond to specific genetic loci, allowing researchers to map genes and study their organization along the chromosome.
Q4: Why are polytene chromosomes useful for studying gene expression?
Polytene chromosomes are valuable for gene expression studies because their large size and visible banding patterns allow direct observation of active genes. Puffs, or swollen regions on polytene chromosomes, indicate sites of active transcription where genes are being expressed. This makes them excellent models for understanding tissue-specific gene expression and developmental gene regulation.
Q5: How do polytene chromosomes differ from normal chromosomes in structure?
Polytene chromosomes contain many more DNA copies than normal chromosomes due to endoreplication, making them significantly larger and more visible under light microscopy. While normal chromosomes exist as single or paired copies, polytene chromosomes can contain hundreds of aligned DNA strands. This structural difference enables detailed cytological analysis impossible with standard chromosomes.
Q6: What can polytene chromosome puffs reveal about cellular activity?
Polytene chromosome puffs are expanded regions that form at sites of intense gene transcription and protein synthesis. These puffs indicate which genes are actively expressed in specific tissues or developmental stages. By observing puff formation and regression, researchers can track changes in gene activity during development and in response to environmental stimuli.
Q7: How do polytene chromosomes relate to heterochromatin and euchromatin organization?
Polytene chromosomes display distinct heterochromatin and euchromatin regions visible as bands and interbands. Heterochromatin appears as dark, condensed bands with few active genes, while euchromatin appears as lighter interbands with active transcription. This clear visual distinction makes polytene chromosomes excellent models for studying constitutive heterochromatin and facultative heterochromatin organization.