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Cromossomos Politeno
Os cromossomos politênicos são cromossomos gigantes em interfase com várias fitas de DNA colocadas lado a lado. Eles foram descobertos no ano de 1881…
Normalmente, quando uma célula diploide divide, ela primeiro duplica cada um dos seus cromossomas. Então a célula distribui uma cópia de cada cromossomo entre as duas células filhas, de modo que cada célula filha recebe um conjunto completo de cromossomos. Certas células sofrem múltiplos ciclos de duplicação cromossômica sem sofrer nenhuma divisão na célula.
Tais células, chamadas células poliploides, contêm conjuntos múltiplos de cada cromossoma. Nas células da glândula salivar de larvas de Drosophila, este processo é levado a um extremo. Aqui, centenas a milhares de cópias de cada cromossomo são geradas sem nenhuma divisão de célula.
Isso resulta na formação de cromossomos gigantes únicos, chamado de cromossomos politenos. Como milhares de sequências de DNA idênticas se colocam lado a lado como lápis em uma caixa, certas características na cromatina tornam-se visíveis. Estas características existem em cromossomas interfásicos ordinários, mas são difíceis de observar.
Quando visto sob um microscópio, um padrão alternado de faixas escuras e de interbandas mais claras pode ser visto nos cromossomas politenos. As bandas constituem 95%do DNA, enquanto as interbandas constituem 5%Onde a cromatina nas bandas é mais condensada e transcricionalmente inativa, a cromatina nas interbandas é menos condensada e transcricionalmente ativa. Enquanto as larvas de Drosophila progridem de um estágio de desenvolvimento a outro, faixas e interbandas específicas aumentam grandemente em diâmetro, produzindo estruturas chamadas puffs ou anéis de Balbiani.
Estes puffs surgem do decondensação da cromatina e são locais da transcrição ativa do DNA. A cromatina em puffs é arranjada em estruturas em loop, muito como aqueles observados em cromossomos plumosos.
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Q1: What are polytene chromosomes and how do they form?
Polytene chromosomes are giant interphase chromosomes formed when cells undergo multiple rounds of DNA replication without cell division. In Drosophila salivary gland cells, hundreds to thousands of identical DNA copies remain aligned side by side instead of segregating, creating these massive structures. This process produces unique chromosomes visible under a microscope with distinctive banding patterns.
Q2: Why do polytene chromosomes show visible banding patterns?
Polytene chromosomes display alternating dark bands and light interbands because thousands of identical DNA sequences align side by side, magnifying chromatin compaction differences. Bands constitute 95% of the DNA and contain condensed, transcriptionally inactive chromatin. Interbands make up 5% and contain less condensed, transcriptionally active chromatin, making these features easily observable under a microscope.
Q3: What is the relationship between polytene chromosome bands and gene activity?
In polytene chromosomes, bands represent transcriptionally inactive regions with condensed chromatin, while interbands are transcriptionally active regions with loosely organized chromatin. This relationship demonstrates how chromatin compaction directly affects gene expression. The visible banding pattern thus reflects the underlying transcriptional state of different chromosome regions.
Q4: What are puffs and Balbiani rings in polytene chromosomes?
Puffs are expanded structures that form when interband regions de-condense to allow active RNA synthesis during development. Balbiani rings are exceptionally large puffs found in salivary glands of certain species with highly decondensed chromatin supporting very high transcription rates. Both structures represent sites of intense gene activity and serve as excellent models for studying transcription.
Q5: How do polytene chromosomes differ from typical mitotically dividing cells?
Polytene chromosomes form in cells that lack cell division after DNA replication, causing chromatids to accumulate without segregating. Unlike mitotically dividing cells, polytene cells maintain an intact nuclear membrane and nucleolus throughout consecutive replication cycles. This results in thousands of aligned chromatids arranged side by side, creating the characteristic giant chromosome structure.
Q6: Where are polytene chromosomes found in organisms?
Polytene chromosomes are found in salivary glands, intestines, muscles, and other tissues of dipteran insects like Drosophila larvae. They also occur in certain organs of mammals and in synergids and antipodes of flowering plants. These chromosomes were first discovered in 1881 in salivary glands of Chironomus plumosus larvae, earning them the alternate name salivary gland chromosomes.
Q7: How do chromomere patterns vary in polytene chromosomes?
Chromomere patterns are species-specific, representing multiple compact dark bands formed by identical chromatin compaction in aligned homologous chromosomes. The number and size of chromomeres can change during an organism's lifetime as developmental stages progress. These variable patterns reflect dynamic changes in chromatin organization and gene activity across different developmental periods.