a.sehgal
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Right i read on wikipedia that the first stage of meiosis, meiosis 1 is the reductional stage where the chromosome number is halved from 2n to n. Does this mean that the prophase stage has been missed out i.e the DNA has not been replicated.

Then meiosis 2 would be the exact same as mitosis, where the DNA number is doubled and then it splits into four daughter cells from the inital two daughter cells?

am i correct here ... so if it asks on an exam question that what is the number of chromosomes after meiosis 1 i put n ... and then n again after meiosis two

what i dont understand is why my teacher was going on about bivalents in meiosis 1 ... as how can there be when the dna has not replicated?

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[stephanie]
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(Original post by a.sehgal)
Right i read on wikipedia that the first stage of meiosis, meiosis 1 is the reductional stage where the chromosome number is halved from 2n to n. Does this mean that the prophase stage has been missed out i.e the DNA has not been replicated.

Then meiosis 2 would be the exact same as mitosis, where the DNA number is doubled and then it splits into four daughter cells from the inital two daughter cells?

am i correct here ... so if it asks on an exam question that what is the number of chromosomes after meiosis 1 i put n ... and then n again after meiosis two

what i dont understand is why my teacher was going on about bivalents in meiosis 1 ... as how can there be when the dna has not replicated?

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Firstly, you have to realise that a chromosome is one strand and a chromatid is two strands (when the chromosome has replicated). It took me ages to grasp this, lol. I'm pretty sure it's correct, because I checked it with my teachers.
This is how I understand it:
-The diploid parental cell is 2n (i.e. 46 chromosomes). There is one pair of homologous chromosomes.
-Each chromosome replicates, forming two pair of chromatids (=bivalent). So, the total number of chromosomes is 92 (although this number is rarely mentioned - it's probably best to say 46 replicated chromosome, I think)
-MEIOSIS I - chromatids move to opposite poles. Two cells are formed, each with 46 chromosomes.
-MEIOSIS II - chromosomes move to opposite poles. Four cells are formed. These are haploid gametes - each cell is n, or 23 chromosomes.

So, the number of chromosomes after Meiosis I is 46 (2n) and after Meiosis II is 23 (n). This must be true because haploid gametes must be formed
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MGIL
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YO it's this basically:


Replicates from 46 of these |
to 46 of these X
then in M1 divides into 23 of these X (where homologous chromosomes are separated)
then M2 23 of these |


woo
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MGIL
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(Original post by [stephanie])
Firstly, you have to realise that a chromosome is one strand and a chromatid is two strands (when the chromosome has replicated). It took me ages to grasp this, lol. I'm pretty sure it's correct, because I checked it with my teachers.
This is how I understand it:
-The diploid parental cell is 2n (i.e. 46 chromosomes). There is one pair of homologous chromosomes.
-Each chromosome replicates, forming two pair of chromatids (=bivalent). So, the total number of chromosomes is 92 (although this number is rarely mentioned - it's probably best to say 46 replicated chromosome, I think)
-MEIOSIS I - chromatids move to opposite poles. Two cells are formed, each with 46 chromosomes.
-MEIOSIS II - chromosomes move to opposite poles. Four cells are formed. These are haploid gametes - each cell is n, or 23 chromosomes.

So, the number of chromosomes after Meiosis I is 46 (2n) and after Meiosis II is 23 (n). This must be true because haploid gametes must be formed
I really think everything you said about chromatids and chromosomes should be swapped

but hey what do I know
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[stephanie]
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(Original post by MGIL)
I really think everything you said about chromatids and chromosomes should be swapped

but hey what do I know
Hmmm....I got my information from my teachers and a mark scheme which clearly said chromatids move to opposite poles at Meiosis I. But I'm still unsure because there's a lot of conflicting information!
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MGIL
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(Original post by [stephanie])
Hmmm....I got my information from my teachers and a mark scheme which clearly said chromatids move to opposite poles at Meiosis I. But I'm still unsure because there's a lot of conflicting information!

oh yes in mitosis they do

i was thinking in meiosis isn't it the chromosomes that move in M1 then the chromatids in M2?
or did you say that?
sorry
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Woody.
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(Original post by [stephanie])
Hmmm....I got my information from my teachers and a mark scheme which clearly said chromatids move to opposite poles at Meiosis I. But I'm still unsure because there's a lot of conflicting information!
That's mitosis. In meiosis one of the homologous cromosomes move to the opposite poles of the cell.
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[stephanie]
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(Original post by Woody.)
That's mitosis. In meiosis one of the homologous cromosomes move to the opposite poles of the cell.
The mark scheme says: Metaphase II, independent assortment of chromatids (i.e. 'X' as opposed to '|'). And I thought that, in all the pictures of meiosis, you see with four daughter cells at the end which all contain '|'. These cells are haploid, so '|' must be chromosomes :confused:
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a.sehgal
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yeh meiosis 1 = chromosomes
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MGIL
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(Original post by [stephanie])
The mark scheme says: Metaphase II, independent assortment of chromatids (i.e. 'X' as opposed to '|'). And I thought that, in all the pictures of meiosis, you see with four daughter cells at the end which all contain '|'. These cells are haploid, so '|' must be chromosomes :confused:

that's meiosis 2
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Woody.
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(Original post by [stephanie])
The mark scheme says: Metaphase II, independent assortment of chromatids (i.e. 'X' as opposed to '|'). And I thought that, in all the pictures of meiosis, you see with four daughter cells at the end which all contain '|'. These cells are haploid, so '|' must be chromosomes :confused:
Let's say a diploid number is 6.
6 of |
Replicate to X (still 6 chromosomes)
X line up on metaphase plate in this style:
X X
X X
X X
etc.

These X move to opposite poles of the cell
The cell divides
So you have two cells with X X X in them. These cells are haploid, but each chromosome contains two chromatids

The X separate and move to the opposite poles of the cell

You end up with 4 cells containing ||| each.
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a.sehgal
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im so confused with meiosis
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[stephanie]
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(Original post by Woody.)
Let's say a diploid number is 6.
6 of |
Replicate to X (still 6 chromosomes)
X line up on metaphase plate in this style:
X X
X X
X X
etc.

These X move to opposite poles of the cell
The cell divides
So you have two cells with X X X in them. These cells are haploid, but each chromosome contains two chromatids

The X separate and move to the opposite poles of the cell

You end up with 4 cells containing ||| each.
I think I understand what you're saying, but where would you label chromosome and chromatids in the process? Meiosis I - independent assortment of chromosomes (or bivalents would be better to use here, I suppose) and Meiosis II - independent assortment of chromatids?
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Woody.
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(Original post by [stephanie])
I think I understand what you're saying, but where would you label chromosome and chromatids in the process? Meiosis I - independent assortment of chromosomes (or bivalents would be better to use here, I suppose) and Meiosis II - independent assortment of chromatids?

The part labelled '1' is a chromatid. This is the only time you ever call anything a chromatid. A chromatid is only a chromatid when it is with another chromatid making up a chromosome.

When the chromosome separates TWO chromosomes are formed. The strands that WERE chromatids are now called chromosomes.

Meiosis II is definitely not independent assortment of chromatids. It is assortment of chromosomes, 23 X.

a.sehgal, what are you still confused with?
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[stephanie]
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(Original post by Woody.)

The part labelled '1' is a chromatid. This is the only time you ever call anything a chromatid. A chromatid is only a chromatid when it is with another chromatid making up a chromosome.

When the chromosome separates TWO chromosomes are formed. The strands that WERE chromatids are now called chromosomes.

Meiosis II is definitely not independent assortment of chromatids. It is assortment of chromosomes, 23 X.

a.sehgal, what are you still confused with?
Thanks
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Ejit
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(Original post by [stephanie])
Thanks
and you're supposed to be doing Pharmacology? :confused:

OP, DNA doesn't replicate during prophase.
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[stephanie]
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(Original post by Ejit)
and you're supposed to be doing Pharmacology? :confused:

OP, DNA doesn't replicate during prophase.
Well I will be ... hopefully
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GonazGirl
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Sorry for jumping onthis thread, but meiosis really does confuse me.
I really dont get the bit where

"6 of |
Replicate to X (still 6 chromosomes)"

is this like the replication in mitosis?
how is it still 6 chromosomes?
Sorry if im being dim, i just can't seem to get this.
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MGIL
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(Original post by Ejit)
and you're supposed to be doing Pharmacology? :confused:

OP, DNA doesn't replicate during prophase.


why so harsh?
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MGIL
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(Original post by GonazGirl)
Sorry for jumping onthis thread, but meiosis really does confuse me.
I really dont get the bit where

"6 of |
Replicate to X (still 6 chromosomes)"

is this like the replication in mitosis?
how is it still 6 chromosomes?
Sorry if im being dim, i just can't seem to get this.

Argh so many people don't get it!


Ok for the gazillionath time


| is a chromosome

| replicates to X during Interphase/s phase.

You now have 46 of X instead of what you originally had which was 46 of these |

What you originally had -46 of these |-were also called chromosomes despite only having one little |
And also after replication you STILL HAVE 46 CHROMOSOMES-X-called the same thing!

With the X type of chromosomes you call both the two sticks in the X a chromatid-so the X consists of two chromatids.

OK what next-mitosis or meiosis?

Talk about meiosis

After the first meiotic division you get 23 of X in each cell
so two cells with 23 X in each

After the second meitoic division you get the chromatids in X separating to give 4 cells of | with 23 of | in each
Again although you have | it is still called a chromosome
Thus after both meiotic division the cells are haploid
However the examiners wants you to understand that it is after the first meitotic division that the cells become haploid.


Does that make sense?!?!!!
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