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Hello Shikha,
Before we answer your question, let’s review the steps followed by cells when they duplicate their DNA and divide, known as the cell division cycle or cell cycle. There are four stages in the eukaroytic cell cycle: Gap 1 (G1), DNA synthesis (S), Gap 2 (G2), and Mitosis (M). Most cells spend the majority of their time in interphase, which consists of the G1, S, and G2 phases. During interphase, a cell grows, duplicates its chromosomal DNA, and prepares to divide. As its name suggests, DNA replication (i.e., synthesis) occurs during the S phase. During mitosis, a cell divides and splits its entire contents (including its chromosomes) between two daughter cells.
As you can imagine, the cell cycle is a tightly regulated process. There are checkpoints throughout the cell cycle to ensure that everything is in order at the end of each stage before the cell proceeds to the next stage. As you might imagine, a failure to properly complete one stage of the cell cycle before jumping into the next could have drastic consequences for a cell!
Although mitosis occupies only a small fraction of the entire cell cycle, it is an extremely important stage because it involves attaching the chromosomes to the mitotic spindle and precisely distributing one copy of each and every chromosome into the two resulting daughter cells. Mitosis can be further subdivided into five phases: prophase, metaphase, anaphase, telophase, and cytokinesis. If any of the proteins involved in mitosis malfunction, or if the chromosomes do not segregate correctly, the result could be failed cell division or, at the other extreme, uncontrolled cell growth (which can lead to cancer!).
As we mentioned, DNA replication occurs during the S phase of the cell cycle, before the cell enters the M phase and divides. You might be surprised to read that although the chromosomal DNA must be unwound during S phase to permit access by the DNA replication machinery, it becomes highly condensed and compacted as cells enter mitosis. Indeed, mitotic chromosomes are often hundreds to thousands times more condensed than interphase chromatin. As a result, individual interphase chromosomes are often not visible whereas mitotic chromosomes are clearly observed under the microscope. Why are chromosomes condensed during mitosis? Chromosome condensation plays a key role in the segregation of chromosomes between two daughter cells when a cell divides.
Now, let’s return to your specific question. You’d like to understand how cell metabolism occurs when DNA is unwound during S phase. Indeed, DNA replication typically does not occur at the same time as RNA transcription. So, how do cells produce the RNA they need to make metabolic proteins when DNA replication is happening? The quick answer is that cells often produce a stockpile of metabolic enzymes capable of supporting metabolism even when the chromosomal DNA is being replicated. Furthermore, the DNA is unwound at replication forks for only a brief moment in time before a complementary daughter strand of DNA is synthesized. Thus, the transcription of a given gene will only experience a momentary pause during DNA replication, which should not negatively impact the cell’s metabolic functions. As you can see, cell division is carefully regulated so that cells can maintain proper levels of metabolic activity while accurately segregating their DNA when they divide!
Follow these links to learn more about the cell division cycle, chromosome condensation, and DNA replication, follow these links:
http://www.nature.com/scitable/definition/mitosis-cell-division-47
http://www.nature.com/scitable/topicpage/mitosis-and-nbsp-cell-division-205
http://www.nature.com/scitable/course-content/essentials-of-genetics-8/105
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4∂=A3169
http://www.nature.com/scitable/topicpage/Major-Molecular-Events-of-DNA-Replication-413
http://www.nature.com/scitable/topicpage/DNA-Packaging-Nucleosomes-and-Chromatin-310
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Nature Education
Sep 17, 2010 09:28AM