The primary reason chromosomes/DNA duplicate themselves is to ensure that when a cell divides, each new daughter cell receives an identical set of chromosomes. This process is crucial for maintaining genetic continuity and is part of the cell cycle, specifically during the S phase (synthesis phase).
Here are the reasons you mentioned, clarified:
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A cell needs to make a copy of its DNA so there is a second set of DNA to go into the new cell that's created during cell division.
True. When a cell prepares to divide (either through mitosis or meiosis), it must duplicate its DNA so that each resulting daughter cell has a complete set of chromosomes. -
A cell needs two sets of DNA in order to do its job properly.
Partially true, but context-dependent. In somatic (body) cells, having a complete set of DNA is crucial for the cell's functionality and maintenance. However, during certain stages of the life cycle (like meiosis), specific processes like reduction division occur to create gametes with a half set of chromosomes, which is essential for sexual reproduction. -
A cell makes new DNA when the old DNA gets damaged.
Not accurate as a primary purpose. While cells have repair mechanisms for damaged DNA and can synthesize new DNA strands during repair processes, the process of DNA duplication is primarily to prepare for cell division, not as a direct response to damage.
In summary, the main purpose of DNA duplication is to ensure that each new cell has the necessary genetic information following cell division.
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