In my GEN BIO 1 course we just did cell division. I breezed through the chapter since it was a nice review from AP Biology. All of the sudden, when I got to lecture, though, I was slammed with confusion.

 

On my professors powerpoint, for mitosis she had labeled Interphase as going from 2n--->4n. This made no sense to me. Isn't the DNA replicated in S phase, and the number of chromosomes remains the same? So for instance, if this was a human cell and there were 46 chromosomes, wouldn't this indicate that there are now 92 chromosomes (4n)? 92 chromatids, yes, but chromosomes? Seems weird. For prophase all the way to telophase she has "4n" underneath each phase picture, until the cells are ready for cytokinesis in which it says "2n"

 

Again, for meiosis, she did the same thing.

 

I am very confused on this...I never knew that the cells could ever have 4n. The whole class seemed to be confused by this as well. Any clarification would be great!

Sure. You start with your normal diploid cells (2n). These are not necessarily identical chromosomes, it is one from dad and one from mom. Then, the DNA in them replicates. So now you have two copies of what you had before, 2*2n = 4n. Now you have two copies from dad and two copies from mom, 4n. Making a copy means you have two copies. Eventually these need to end up in two cells with once again one copy from mom and one copy from dad (2n, mitosis), or in four cells (n, meiosis).

your prof should be referring to the amount of genetic material instead of chromosomes.

Yeah I always thought that n referred to chromosomes and it seemed incorrect to refer to the DNA duplication as 4n, so I was getting thrown off. Thanks.

Edited October 3, 201015 yr by Marconis

No, it's the number of copies of chromosomes (homologous or exact copy). Unless the cell has defects, this should be the same as the number of (homologous) copies of chromosome 1.

Thanks fellas