I’ve been writing up my talks and am surprised how long it has taken me to get through the material. For now, I thought I’d address some common questions:
- what is a chromosome vs. a sister chromatid?
- what n value do you use? (especially during meiosis)
When we see pictures of chromosomes, they often look like an X (as seen in the thumbnail for these chromosome posts). In my posts, I often draw them as a straight line. Are these both chromosomes? Yes.
I’m sorry for the confusion but both the single and double purple lines 1 are chromosomes. This gets extra confusing when we are taking chromosomes through mitosis and meiosis – how many chromosomes are at each stage?
NOTE: We won’t go over meiosis and mitosis yet but my advice is that you don’t worry so much about compartmentalizing all the information but rather think about what is happening. Cell division and gamete creation is a fluid process so the snapshots of details don’t give the best understanding. An analogy would be trying to analyze a runner by the snapshots of footstrike then heel raised = you can get lots of detail but the feeling is gone. So we’ll go after the major concepts and then add any needed details at certain spots.
Number of N
Some people like to measure the amount of chromosome material, using ‘n’. When an organism is has one chromosome complement, it has just 1n and is considered haploid. This is like human eggs and sperm = there is only one set of the 23 chromosomes. When egg and sperm combine in conception, the zygote is 2n or diploid. This is most of our cells. I don’t worry about 2n, 4n, or 1n as I go through meiosis because it just confuses me. Again, just figure out the concept and you’ll be able to answer the questions re: amount of genetic material.
Now that we’re not worrying about n, this becomes less of a worry and I’ll tell you my cheat. I count something as a chromosome when there is a centromere. So in the diagram above, each structure is one chromosome because there is one centromere.
If you want more information, a chromatid is one of those lines in the X-structure. The sister chromatids are identical copies of the same DNA that are joined at the centromere. This chromosome splits into two chromosomes with their own centromeres at the end of meiosis or cell division.
I hope I’ve helped your worries on these areas of confusion – either you aren’t concerned about the question or better yet, you understand the concepts
- changed from strand on May 5, 2013 to decrease confusion from the word ‘strand’. ↩