Can someone please explain the meiosis II?

[Doowally]

Can someone please explain the meiosis II?
I don't understand how it's possible to get 4 haploid gametes out of one.
So there's 23 chromosomes in one. It divides into two which both have 23 chromosomes cause before it divides it doubles everything. Am I right? But the second time it divides it doesn't double and magically there's four 23 chromosome gametes? Where's the logic? If two 23 chromosome gametes divide into two then there's supposed to be four 16,5 chromosome gametes.

[Brodo]

Here is how it works.
1)Your inital cell will have 46 chromosomes (23 from mom, and 23 from dad)

2)These 46 chromosomes will duplicate into double stranded chromosomes (all 46 were initially single chromatids however they have replicated and become double stranded)
-There are still only 46 chromosomes!

3) In Meosis 1 the chromosomes will align not in a linear line like mitosis but instead in pairs
-one pair will be more towards the "left" the other to the "right" of the cell
-These are homologous pairs (ie code for same traits, one is from mom, the other from dad)

4)Cell divides and you get two cells. Each cell now has 23 double stranded chromosomes.

5)Each of these two cells will undergo the follow events :
-Align in a strand line (NO pairs)
-"Pull" single chromatids to opposite poles of cell
- Division of cell

6) You are done!

To summarize

1cell (46SS chromosomes) --> 1cell (46DS chromosomes) --> 2cells (23DS chromosomes) ---> 4cells (23SS Chrom)

[Doowally]

In my book it says there's 23 chromosomes in sex cells and they divide without the mum and dad cell ever getting together so...it has to happen with 23 chromosomes. ?

[JackBean]

Brodo: The DNA is double-stranded all the time!!!

Doowally: you have it messed up. The sex cells originate from meiosis, THEY are the PRODUCT of the meiosis, but they do not undergo meiosis, they actually do not divide at all, while they fuse together to form the diploid cells again.

So, first you have somatic cell, which is diploid, undergoes meiosis and thus goes haploid (first the cell has 23 pairs of monochromatid chromosomes, which replicate and become dichromatid, undergoes one division resulting in 23 pairs of monochromatid chromosomes which immediately divide again resulting in cells with only 23 single chromosomes). This haploid cell is the sex cell which mate with another sex cell and becomes diploid again

[Doowally]

Ooohhh...so the sex cells aren't any different at all?
Okay at first what I understood that mitosis is the division of somatic cells and meiosis is the division of the sex cells.
And, JakcBean, what you're saying is that at first there is the somatic cell which divides into to two 46 chromosome cells and when those two divide it's meiosis and the four cells stay 23 chromosome ones?

[JackBean]

I do not exactly understand, what are you saying.

At first, meiosis is how the sex cells ORIGINATE, but they do not divide by meiosis (i.e. first is meiosis, then the sex cell).
Second - probably yes. First the cell contains 46 chromosomes (23 pairs) with only one chromatid. Then the DNA is replicated and the chromosomes contain two chromatides (still 23 pairs AKA 46 chromosomes). This divides in meiosis I into cells with 26 one-chromatid chromosomes and these further divide into sex cells with 23 chromosomes.


And BTW these 46 and 23 apply only for human, OK?

[Brodo]

[quote="JackBean"]Brodo: The DNA is double-stranded all the time!!!

I never said that the DNA is single stranded....
I said double stranded chromosome (as in two chromatids) a total of 4 strands of DNA

[canalon]

Brodo: The DNA is double-stranded all the time!!!

I never said that the DNA is single stranded....
I said double stranded chromosome (as in two chromatids) a total of 4 strands of DNA

I guess it is more because of your poor choice of abbreviation in the last line: DS and SS
Sound more like double and single stranded than double and single chromatid...

[JackBean]

2)These 46 chromosomes will duplicate into double stranded chromosomes (all 46 were initially single chromatids however they have replicated and become double stranded)

[Doowally]

Okay. I've read the thing so many times in my book and I'm still confused.

What kind of cell starts off the meiosis? Is it a sex cell or a somatic cell? How many chromosomes are there at the beginning of meiosis? At the very start?

[Tashib]

The only thing that I know is the following ones:
Meiosis occurs in two stages: (1). When homologous chromosomes separate (the mother and father chromosomes separate) after the chiasmata formation. and (2) Chromatids separate to form 4 daughter cells.

[Francol]

Brodo: The DNA is double-stranded all the time!!!

Hm i thought also it isn't huh, here is something to read through.. Thanks!