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:
- Polyploidy: the replicated chromosomes retain their individual identity.
- Polyteny: the replicated chromosomes remain in precise alignment forming "giant" chromosomes.
- various intermediate conditions between 1 and 2
Mitosis without cytokinesis produces a mass of cytoplasm with many nuclei (called a syncytium). Some examples include the free-nuclei stage in the embryonic development of flies like Drosophila and the plasmodial slime molds like Physarum polycephalum
Cells are polyploid if they contain more than two haploid (n) sets of chromosomes; that is, their chromosome number is some multiple of n greater than the 2n content of diploid cells. For example, triploid (3n) and tetraploid cell (4n) cells are polyploid. Polyploidy is usually limited to certain tissues in animals, such as liver cells, megakaryocytes; Megakaryocytes, from which platelets are made, may pass through as many as seven S phases producing a giant cell with a single nucleus containing 128n chromosomes (its fragmentation yields platelets), and giant trophoblast cells in the placenta. Polyploidy is very common in plants.
The most thoroughly-studied examples of polyteny are the giant chromosomes found in certain cells of larval flies.
Fig. 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.
The interbands contain regulatory elements — promoters and enhancers — that control the genes in the adjacent gray bands.
Polytene chromosomes also have regions — called "puffs" — that are swollen and appear to have a looser structure. The exact pattern of puffs differs in different types of cells (e.g., salivary gland vs. gut). They also differ with changing conditions in one type of cell. For example, giving the molting hormone ecdysone to an insect causes a predictable change in the puffing pattern. This is just what one would predict if the puffs represent regions of intense gene transcription.
Fig. 7.5.2 Puff in Chromosome courtesy of Claus Pelling, Max-Planck Institute of Biology, Tübingen.
The top autoradiograph shows the puff in chromosome IV of the tiny fly Chironomus tentans after giving ecdysone and radioactive uridine (an RNA precursor). The grains clustered over the puff show that it is a site of intense RNA synthesis. The bottom audiograph is taken when the antibiotic actinomycin D is given along with ecdysone, puffing is inhibited and little RNA synthesis occurs. Actinomycin D blocks access to DNA by RNA polymerase.
The pattern of puffing within a cell varies over time. For example, each time an insect larva prepares to molt, a definite, predictable sequence of puffing occurs.
Fig. 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.