Jun 15, 2015
8
6
Status
Pre-Dental
Hey guys,

I'm having a really hard time with all the questions that ask how many chromosomes are at __ phase. I've gone over mitosis and meiosis I/II so many times and no matter what I do I still get these questions wrong.

ie " If an organism has a diploid number of 18, how many chromosomes are visible at the end if mitotic prophase" (destroyer 2015 - Bio q 366).

The answer is 36 but I thought it was 18. As far as I know, during prophase the nucleoli disappears and chromatin condenses how are there double the # of chromosomes??

Thanks so much!
 
Aug 5, 2015
23
11
Status
Pre-Dental
Wait a minute.. Shouldn't the answer be 18? (you are correct @appleciderz)

2n=18 (n=9)
During interphase, DNA replication occurs in the synthesis (S) phase. At this point, the 18 single stranded chromosomes duplicate to produce 18 double stranded chromosomes. Note: To count the number of chromosomes, count the number of centromeres present. In this case, there are 18 chromosomes that contain two sister chromatids attached together by a centromere. The double stranded chromosomes don't split until anaphase where the number of chromosomes then becomes 36 (36 single stranded chromosomes). The chromosomes migrate to opposite sides of the cell and telophase followed with cytokinesis occurs. This produces two identical daughter cells with 18 chromosomes (single stranded).

Reply if I am mistaken, thanks!
 
Sep 27, 2015
27
10
Status
Pre-Dental
If 2n = 18 then after S phase you'll have 36 chromaTIDs not chromasomes - the chromasome # stays the same
 

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Hey guys,

I'm having a really hard time with all the questions that ask how many chromosomes are at __ phase. I've gone over mitosis and meiosis I/II so many times and no matter what I do I still get these questions wrong.

ie " If an organism has a diploid number of 18, how many chromosomes are visible at the end if mitotic prophase" (destroyer 2015 - Bio q 366).

The answer is 36 but I thought it was 18. As far as I know, during prophase the nucleoli disappears and chromatin condenses how are there double the # of chromosomes??

Thanks so much!
Destroyer is correct here. We are talking about Mitosis, therefore we are talking about Somatic cells, which are diploid. They must remain diploid after Mitosis. So, if the diploid number is 18, then replicated number, after S phase is 36.

The link in this thread points to Meiosis, not Mitosis.

Hope this helps.

Nancy
 
Jan 4, 2015
25
22
Status
Post Doc
Yes the destroyer's answer is right => at Prophase: Chromosomes = 18 & Chromatids = 36 (ie 2*18)

Look at it this way, if the "structure" is still bound by the centromere (forming an "X") then it is still considered 1 Chromosome (this is the part that causes the most confusion), however it has 2 Chromatids. Until they separate then the Chromosome number will equal the Chromatid number

So, S Phase, Prophase, Metaphase ------> you have 18 Chromosomes (still working with the same question) BUT 36 Chromatids

Until they separate (Anaphase) then Chromosome # = Chromatid #, both should be 36

If you're still confused, re-read the above with a pic of the Mitotic phases....it will stick in your head for sure!