What is the difference in the genetic material present in the nucleus :
i)at the time of cell division
ii) when the cell is not dividing
Answers
Answer:
Chromosomes
Cells package their DNA not only to protect it, but also to regulate which genes are accessed and when. Cellular genes are therefore similar to valuable files stored in a file cabinet — but in this case, the cabinet's drawers are constantly opening and closing; various files are continually being located, pulled, and copied; and the original files are always returned to the correct location.
Of course, just as file drawers help conserve space in an office, DNA packaging helps conserve space in cells. Packaging is the reason why the approximately two meters of human DNA can fit into a cell that is only a few micrometers wide. But how, exactly, is DNA compacted to fit within eukaryotic and prokaryotic cells? And what mechanisms do cells use to access this highly compacted genetic material?
What Are Chromosomes?
Cellular DNA is never bare and unaccompanied by other proteins. Rather, it always forms a complex with various protein partners that help package it into such a tiny space. This DNA-protein complex is called chromatin, wherein the mass of protein and nucleic acid is nearly equal. Within cells, chromatin usually folds into characteristic formations called chromosomes. Each chromosome contains a single double-stranded piece of DNA along with the aforementioned packaging proteins.
A circular cell-cycle diagram shows the degree to which chromatin is condensed inside a cell during the five stages of mitosis. Each stage is labeled and numbered beside an illustration of a cell. At the center of each cell is a nucleus containing chromatin. The illustration for stages 1 (interphase), 2 (prophase), and 3 (metaphase) show only a single cell. Stage 4 (anaphase) shows a cell in the process of dividing: two distinct cell shapes with two developing nuclei are shown. In stage 5 (telophase), two separate cells are shown, each with its own nuclei and chromatin.
Figure 1: Chromatin condensation changes during the cell cycle.
During interphase (1), chromatin is in its least condensed state and appears loosely distributed throughout the nucleus. Chromatin condensation begins during prophase (2) and chromosomes become visible. Chromosomes remain condensed throughout the various stages of mitosis (2-5).
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Figure Detail
Eukaryotes typically possess multiple pairs of linear chromosomes, all of which are contained in the cellular nucleus, and these chromosomes have characteristic and changeable forms. During cell division, for example, they become more tightly packed, and their condensed form can be visualized with a light microscope. This condensed form is approximately 10,000 times shorter than the linear DNA strand would be if it was devoid of proteins and pulled taut. However, when eukaryotic cells are not dividing — a stage called interphase — the chromatin within their chromosomes is less tightly packed. This looser configuration is important because it permits transcription to take place (Figure 1, Figure 2).
In contrast to eukaryotes, the DNA in prokaryotic cells is generally present in a single circular chromosome that is located in the cytoplasm. (Recall that prokaryotic cells do not possess a nucleus.) Prokaryotic chromosomes are less condensed than their eukaryotic counterparts and don't have easily identified features when viewed under a light microscope.
Two photomicrographs and an illustration show DNA during interphase and mitosis. On the left-hand side are two greyscale photomicrographs of fluorescently labeled DNA in mouse cells during interphase and mitosis. On the right-hand side are illustrations of a cell in interphase and a cell in mitosis. The pericentric heterochromatin is labeled in the illustrations.
Figure 2: A the appearance of DNA during interphase versus mitosis.
During interphase, the cell's DNA is not condensed and is loosely distributed. A stain for heterochromatin (which indicates the position of chromosomes) shows this broad distribution of chromatin in a mouse cell (upper left). The same stain also shows the organized, aligned structure of the chromosomes during mitosis. Scale bars = 10 microns.
© 2004 Nature Publishing Group Maison, C. & Almouzni, G. HP1 and the dynamics of heterochromatin maintenance. Nature Reviews Molecular Cell Biology 5, 296-305 (2004). All rights reserved. View Terms of Use
Figure Detail
How Are Eukaryotic Chromosomes Structured?
A greyscale electron micrograph shows chromatin in an extended form, which resembles beads on a string. The beads are nucleosomes, and the string is DNA. Each nucleosome looks like a small black circle. Many circles are visible on alternating sides of a long string of DNA. Two nucleosomes are indicated with black arrows.