Question

Reference here: http://www.stmary.ws/highschool/science/apbio/Heredity/heredity%20images/meiosis-big.gif Short question: After meiotic division 1 (DNA replication and recombination) are there more chromosomes in total in the single cell than there were before? (right after male and female chromosomes had joined in a single cell after fertilization)

Answer 1

Continued/specified: After 23 chromosomes from the male have joined to be in the same cell with those of 23 from the female 'DNA replication and recombination' happens. For there is the word replication ( which necessary for forming synapsis -> genetic variety) I assume there is replication aka duplication of the number of chromosomes so we would end up with 92 chromosomes in the same single cell. Now naturally the cell divides so that we would only have 46 chromosomes in each of the two cells. This should be the end of meiosis 1. Secondly in meiotic division 2, as I understand, one of these new cells divides such that there are 23 chromatids in each of its daughter cells. Is this right? Because I can't really find explanations of meiosis in humans where there would be step-by-step chromosome numbers included and this diagram i found is confusing.

Answer 2

No, you dont end up with 92 chromosomes after DNA replication
as referred in your picture, you see how before replication is single stranded (chromatids)
after replication, it is double stranded

so there is still only 46 chromosomes, but it is really did duplicated

Answer 3

You may be confused that chromosomes must be in that "X shape", but it does not have to be
chromatids can be chromosome by itself too

Answer 4

Oh so - in first picture we have 23 + 23 single strands. They replicate so that we get two complete chromosomes which regularly duplicate into body-daughter cells like regularly outside meiosis. However since we now have two separate single cells with 23 double stranded chromosomes, these by themselves cannot start a mitosis yet. That is why instead of yet having a division where spindles rip to two different cells sister chromatids, we have one more division before that in first picture of Meiotic division 2. There each of the double stranded chromosomes divide to form daughter cells which each have just a single stranded chromosome, which can then duplicate their genetic material to form identical cells to what they are. However, as you can see from the last part of the picture with 4 different colored cells something has happened. Due to the Meiosis 1's Recombination process we have acquired more genetic variance among the later mitosis-performing cells seen in the last part of the picture; there are different alleles in different cells even though they have the same 'genes'. Returning to the original point we have 4 cells in the end which each will then divide by the regular mitosis means, which I was after. If you think I got it right, I'd like to say: Thank you so much for helping me! Although if there's a mistake in my thinking, please point it out. Thanks : )

Answer 5

sorry: to fix the first part: in first picture we have 23 + 23 single strands. They replicate and recombinate so that we get two double-stranded chromosomes with variance in genetic material - that is why the two pairs of single strands from male and female don't just join by their okazaki fragment locations and start mitosis. Now due to this extra step we now have two separate single cells with 23 double stranded chromosomes, these by themselves cannot start a mitosis yet.

Answer 6

so not they dont start mitosis yet, is they dont start mitosis
remember what is being copied in meiosis is sex cells [sperm , egg]
mitosis is for somatic cells, not for sex cells

Answer 7

In fourth picture (with humans) how many chromosomes are there and are they single or double stranded, and same with picture 5. That is the unclear part.

Answer 8

srry i need to eat dinner now.. i will reply back after

Answer 9

ok back
#4 : 23 double stranded
#5 : 23 single stranded

u see before is 46 double stranded, and it spilts, so now becomes 23 double stranded in #4

Answer 10

But if a human cell divides by mitosis it has 46 single stranded chromosomes which each then duplicate so we have 46 X's in a cell before it divides with spindles pulling the chromatids to opposite ends so each of the new cells have the 46 chromosomes humans have right? So I'm not getting why would we end up with 23 single stranded ones : /

Answer 11

23 is only for meiosis
23 for egg, 23 for sperm, they combine to form a zygote
so a zygote has 46 chromosomes
then ur body cells there will undergo mitosis, starting with 46

Answer 12

That does not respond to how the 4 cells at the end of Meotic division 2 we supposedly have 23 single stranded chromosomes per cell. That gives each cell only half of the genetic material required for normal mitosis-dividing body cells to function. So are there a normal number of chromosomes in the last picture (6)? If so, why? If not, why again? Thanks in advance for taking the time

Answer 13

Oh got it. I had misunderstood one fundamental thing. K thanks.