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7.5: Endoreplication

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    4849
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    Endoreplication is the replication of DNA during the S phase of the cell cycle without the subsequent completion of mitosis and/or cytokinesis. Endoreplication is also known as endoreduplication. Endoreplication occurs in certain types of cells in both animals and plants. There are several variations:

    • replication of DNA with completion of mitosis, but no cytokinesis.
    • repeated replication of DNA without forming new nuclei in telophase. This can result in:
      1. Polyploidy: the replicated chromosomes retain their individual identity.
      2. Polyteny: the replicated chromosomes remain in precise alignment forming "giant" chromosomes.
      3. various intermediate conditions between 1 and 2
    alt
    Figure 7.5.1 Polytene Chromosomes courtesy of B. P. Kaufmann

    The photomicrograph shows the polytene chromosomes in a salivary gland cell of a Drosophila melanogaster larva. Such chromosomes are found in other large, active cells as well.

    • Each of Drosophila's 4 pairs of chromosomes has undergone 10 rounds of DNA replication.
    • The maternal and paternal homologs — as well as all their duplicates — are aligned in exact register with each other.
    • So each chromosome consists of a cable containing 2048 identical strands of DNA.
    • These are so large that they can be seen during interphase; even with a low-power light microscope.

    Function of polyteny

    The probable answer: gene amplification. Having multiple copies of genes permits a high level of gene expression; that is, abundant transcription and translation to produce the gene products. This would account for polyteny being associated with large, metabolically active cells (like salivary glands). Polytene chromosomes are subdivided into some 5,000 dense bands separated by light interbands. The bands are further subdivided into:

    • dark bands of heterochromatin where the DNA is tightly compacted and there is little gene transcription;
    • gray bands of euchromatin where the DNA is more loosely compacted and there is active gene transcription.
    alt
    Figure 7.5.3 Changes in puffing patterns of Chromosomes courtesy of Dr. Michael Ashburner, University of Cambridge

    These eight photomicrographs () show the changes in the puffing pattern of equivalent segments of chromosome 3 in Drosophila melanogaster over the course of some 20 hours of normal development.

    Note that during this period, when the larvae were preparing to pupate, certain puffs formed, regressed, and formed again. However, the order in which they did often differed. For example, in the larva, band 62E becomes active before 63E (c, d, and e), but when pupation begins, the reverse is true (g, h).

    In general, early puffs reflect the activation of genes encoding transcription factors. These proteins then bind to the promoters of other genes, turning them on and causing a puff to appear at their loci.


    This page titled 7.5: Endoreplication is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by John W. Kimball via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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