Why are giant chromosomes needed?
In classical cytology, it is generally accepted that during interphase (the period between divisions), chromosomes are despiralized and not visible under a light microscope. However, there are exceptions—giant chromosomes. Due to their special structure and manifold increased volume of genetic material, they remain clearly distinguishable even outside of mitosis or meiosis.
In biology and medicine, two main types of such structures are distinguished, each having its own unique formation mechanism and strict localization in certain tissues.
1. Polytene chromosomes
This type of giant chromosome is formed as a result of a specific cell cycle disruption, which has been evolutionarily fixed for the needs of certain tissues.
Mechanism of formation The basis for the formation of polytene chromosomes is multiple reduplication (doubling) of DNA, which is not followed by cell division. As a result, chromatin threads do not diverge, but remain closely pressed to each other, forming thick bundles. Example: In cells of the Drosophila (Drosophila) larva, such a bundle can contain up to 1024 undivided strands.
Localization Normally, polytene chromosomes are found in the salivary glands of dipteran insects (flies and mosquitoes).
Structural features (transverse banding) If you look at a polytene chromosome under a microscope, it will look like a barcode due to the alternation of dark and light areas:
- Dark bands (disks): these are areas of densely packed heterochromatin.
- Light areas (interband spaces): this is less condensed euchromatin.
2. Lampbrush chromosomes
These structures got their unusual name from their external similarity to brushes used for cleaning kerosene lamps. Despite the low degree of chromatin condensation, they are clearly visible under a light microscope due to their huge size.
Cause of occurrence and structure The formation of lampbrush chromosomes is directly related to the colossal functional activity of the cell—enhanced RNA synthesis. Structurally, they represent long axes of chromatin from which giant lateral loops extend. Numerous newly formed RNA molecules attach to these loops.
Time and place of appearance
- Localization: Oocytes (female germ cells) of amphibians, reptiles, birds, and mammals.
- Time of appearance: They form strictly at the dictyotene stage (which is part of meiotic diplotene) during egg maturation.
P-D-M: Polytene — Diptera (salivary glands) — Multiple reduplication. "Lampbrush" chromosomes clean oocytes (found only in vertebrate female germ cells).
Class and exam questions
Why do polytene chromosomes exhibit transverse striation?
The striation arises due to the alternation of heterochromatin regions (which appear as dark bands) and euchromatin (light interband regions).
As a result of what process do polytene chromosomes form?
They are formed due to multiple rounds of DNA molecule reduplication not accompanied by subsequent cell division. The strands remain in a single bundle.
At which stage of meiosis do lampbrush chromosomes appear?
They are visualized during meiotic diplotene (at a specific stage called dictyotene) during oocyte formation.
What else to study on this topic
- Functional differences between euchromatin and heterochromatin
- Features of the dictyotene stage in mammalian oogenesis
- Relationship between RNA synthesis and decondensation of chromatin loops
- Mechanisms of cell division arrest during DNA reduplication
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