Confusing Destroyer Bio Problem

[mrdeez]

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#280 in 2016 destroyer

Consider somatic cell undergoing mitosis. At anaphase the chromosome number is:

a) N
b) 2N
c) 4N
d) 6N
e) 3N

The answer is 4N according to destroyer.

I understand what happens during the phases but my definition of ploidy must be wrong? I thought the ploidy number refers to the amount of DIFFERENT genetic material within the cell. When DNA is copied, the ploidy number stays the same (2N) but there are two COPIES (2C). Since no new genetic material is introduced, the ploidy number is still 2N. Am I losing my mind? Someone please explain this.

 

[deleted816661]

You are correct that typically n represents genetic amount and c represents chromosome material. In this case, I believe they just used the wrong variable widely used. The answer would be 4C.

 

[w4nt2baDDS]

In human mitosis we create 2 cells with 46 chromosomes each [(2 sets of 23 chromosomes) * 2], each new cell is diploid (2N) thus we have 2N*2=4N. Before cytokinesis occurs you have 1 mitotic cell with 2 sets of (23*2) chromosomes on opposite polls of the cell, thus you have 4N. Also, at this point there are no sister chromatids, each sister chromatid split to become a chromosome in its own right. The number of C is the same as the number of N during anaphase.

Just realize that during metaphase there are 46 chromosomes (each with 2 sister chromatids) lined up the middle of a cell, which splits to make 92 chromosomes, with 46 going to each new cell and of these 46 they each have a paired chromosome.. which is why we're diploid.

Later on there is interphase which has S phase within it. This is when the DNA will be duplicated again and regenerate the other half of the centromere. Thus allowing the cycle to repeat.

 

[DAT Destroyer]

#280 in 2016 destroyer

Consider somatic cell undergoing mitosis. At anaphase the chromosome number is:

a) N
b) 2N
c) 4N
d) 6N
e) 3N

The answer is 4N according to destroyer.

I understand what happens during the phases but my definition of ploidy must be wrong? I thought the ploidy number refers to the amount of DIFFERENT genetic material within the cell. When DNA is copied, the ploidy number stays the same (2N) but there are two COPIES (2C). Since no new genetic material is introduced, the ploidy number is still 2N. Am I losing my mind? Someone please explain this.

You thinking about it wrong. Do not make it complicated. Somatic cells are diploid, means 2N. During S phase, DNA is replicated. So before telophase, the cell has 4N number of chromosomes. That means 2N duplicated.
That is what the question wanted.
Hope this helps.

 

[mrdeez]

In human mitosis we create 2 cells with 46 chromosomes each [(2 sets of 23 chromosomes) * 2], each new cell is diploid (2N) thus we have 2N*2=4N. Before cytokinesis occurs you have 1 mitotic cell with 2 sets of (23*2) chromosomes on opposite polls of the cell, thus you have 4N. Also, at this point there are no sister chromatids, each sister chromatid split to become a chromosome in its own right. The number of C is the same as the number of N during anaphase.

Just realize that during metaphase there are 46 chromosomes (each with 2 sister chromatids) lined up the middle of a cell, which splits to make 92 chromosomes, with 46 going to each new cell and of these 46 they each have a paired chromosome.. which is why we're diploid.

Later on there is interphase which has S phase within it. This is when the DNA will be duplicated again and regenerate the other half of the centromere. Thus allowing the cycle to repeat.

Thanks so much for this picture. So just to be clear, is n and ploidy referring to the number of centromeres and is independent of what the genetic information is? For example I was thinking about this in terms of 1 set from dad 👎 and 1 set from mom 👎 so that makes 2n and is why we are diploid. If its just the amount of centromeres then your drawing makes way more sense. I don't know why this is tripping me up this much.

 

[w4nt2baDDS]

Thanks so much for this picture. So just to be clear, is n and ploidy referring to the number of centromeres and is independent of what the genetic information is? For example I was thinking about this in terms of 1 set from dad 👎 and 1 set from mom 👎 so that makes 2n and is why we are diploid. If its just the amount of centromeres then your drawing makes way more sense. I don't know why this is tripping me up this much.

ploidy (N) is referring to the amount of sets of chromsomes an organism has. In diploid, you get a set from dad (1N) and a set from mom (1N) like you said. In our case N represents 23 chromosomes, thus we have 46 total chromosomes and 46 centromeres. When a cell enters s-phase, all 46 chromosomes duplicate but each chromosome remains adhered to a central centromere, thus forming 46 chromosomes that each look like the letter X, with each side of the X being a sister chromatid. At this point, the cell now has 4C the DNA material (92 sister chromatids) but because there are still only 46 centromeres in the cell, the cell is still 2N. Now, all 46 of these X looking chromosomes line up in a straight line on the equator of the cell during metaphase. The spindle fibers pull these 92 sister chromatids apart during anaphase and each side gets 46 sister chromatids, but each of these sister chromatids now has its own centromere (like the image shows) and thus there are now 46 chromosomes (not sister chromatids) on each side of the cell, this makes a total of 92 chromosomes in the cell or in other words 4N. Once cytokineses occurs, each cell has 46 chromosomes but no sister chromatids. It is not until s-phase that the cell will copy it's DNA and each linear chromosome will get a sister-chromatid which gives the chromosomes an X looking appearance.

# of centromeres = # of chromosomes --- the size of the centromere doesn't matter
sets of chromosomes = ploidy (N) --- in humans each set represents 23 chromosomes

 

[mrdeez]

ploidy (N) is referring to the amount of sets of chromsomes an organism has. In diploid, you get a set from dad (1N) and a set from mom (1N) like you said. In our case N represents 23 chromosomes, thus we have 46 total chromosomes and 46 centromeres. When a cell enters s-phase, all 46 chromosomes duplicate but each chromosome remains adhered to a central centromere, thus forming 46 chromosomes that each look like the letter X, with each side of the X being a sister chromatid. At this point, the cell now has 4C the DNA material (92 sister chromatids) but because there are still only 46 centromeres in the cell, the cell is still 2N. Now, all 46 of these X looking chromosomes line up in a straight line on the equator of the cell during metaphase. The spindle fibers pull these 92 sister chromatids apart during anaphase and each side gets 46 sister chromatids, but each of these sister chromatids now has its own centromere (like the image shows) and thus there are now 46 chromosomes (not sister chromatids) on each side of the cell, this makes a total of 92 chromosomes in the cell or in other words 4N. Once cytokineses occurs, each cell has 46 chromosomes but no sister chromatids. It is not until s-phase that the cell will copy it's DNA and each linear chromosome will get a sister-chromatid which gives the chromosomes an X looking appearance.

# of centromeres = # of chromosomes --- the size of the centromere doesn't matter
sets of chromosomes = ploidy (N) --- in humans each set represents 23 chromosomes

I have been through so many books and websites looking for exactly this explanation. Thanks so much. The centromere splitting and each becoming a chromosome on its own just before cytokinesis was what was confusing me. You saved me a bunch of time and energy, thanks again.

 

[fan2342]

ploidy (N) is referring to the amount of sets of chromsomes an organism has. In diploid, you get a set from dad (1N) and a set from mom (1N) like you said. In our case N represents 23 chromosomes, thus we have 46 total chromosomes and 46 centromeres. When a cell enters s-phase, all 46 chromosomes duplicate but each chromosome remains adhered to a central centromere, thus forming 46 chromosomes that each look like the letter X, with each side of the X being a sister chromatid. At this point, the cell now has 4C the DNA material (92 sister chromatids) but because there are still only 46 centromeres in the cell, the cell is still 2N. Now, all 46 of these X looking chromosomes line up in a straight line on the equator of the cell during metaphase. The spindle fibers pull these 92 sister chromatids apart during anaphase and each side gets 46 sister chromatids, but each of these sister chromatids now has its own centromere (like the image shows) and thus there are now 46 chromosomes (not sister chromatids) on each side of the cell, this makes a total of 92 chromosomes in the cell or in other words 4N. Once cytokineses occurs, each cell has 46 chromosomes but no sister chromatids. It is not until s-phase that the cell will copy it's DNA and each linear chromosome will get a sister-chromatid which gives the chromosomes an X looking appearance.

# of centromeres = # of chromosomes --- the size of the centromere doesn't matter
sets of chromosomes = ploidy (N) --- in humans each set represents 23 chromosomes

All of the above explanation were great especially this one. I am confident for these questions now. Thanks