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I'm having doubts about whether the sister chromatids are exactly the same or not. I know that they are the arms that carry the information genes (the alleles), but I don't know if they are exactly the same and / or in what these arms differ, since in a partial exercise they ask me the following:

Explain briefly why the sister chromatids are not exactly the same and the homologous chromosomes are different. To answer, consider the origin of both structures.

Nor do I understand quite well when the statement clarifies "take into account the origin of both structures."

Thank you very much for the help!

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    $\begingroup$ Well, what is the origin of the sister chromatids? In other words, by what process does a cell go from one chromosome consisting of one DNA molecule to one chromosome consisting of two DNA molecules (two chromatids)? $\endgroup$ – canadianer May 10 '18 at 18:53
  • $\begingroup$ Then the sister chromatids would be formed from the DNA replication? $\endgroup$ – Osrion May 10 '18 at 19:51
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    $\begingroup$ Yes exactly. Now, is DNA replication perfect or are errors introduced? $\endgroup$ – canadianer May 10 '18 at 19:53
  • $\begingroup$ I suppose it is perfect or is it possible that there are errors? $\endgroup$ – Osrion May 10 '18 at 20:58
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A new chromatid is synthesized by copying the other chromatid. As such two sister chromatids are (quasi) perfectly identical.

Why did I say quasi? Because there are sometimes "errors" when replicating DNA. These "errors" are called mutations. As far as I know, most mutations occur during meiosis though and only very few occur during mitosis. So, to a pretty good approximation, yes sister chromatids are identical.

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  • $\begingroup$ Ohh ok, with this I understood what canadianer means. Thank you very much for the explanation! $\endgroup$ – Osrion May 11 '18 at 15:13

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