meiosis Watch

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BLUREMI
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#1
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ok is this correct

at the start of meiosis, there are 23 pairs of chromosomes. (46 chromatids)

the 23 pairs of chromosomes replicate forming 46 pairs of cromosomes (92 chromotids)

The cromosomes pair up forming homologoes pairs of chromosomes, called bivalents. Each bivalent has two pairs chromosomes, and therfore 8 chromatids. There are 23 bivalents

The bivalents will exchange genetic material in meiosis one with homologoes chromosomes forming chiasmata and thus exhanging material, causeing genetic diversity.

The bivalents line at the center of the spindles, and the spindles fibres pull the bivalents apart. A set of chromosomes, goes to opposite ends of the cell. This produces two cells with 46 chromosomes in each cell. (23 pairs)

Each of these cell then do meotic division two. In which the chromosomes are pulled apart at the centromere and go to opposute ends of the cell, producing
4 cells with 23 chromosomes, no pairs

So at the start there is 2n, then 4n with formation of bivalents, then 2n after meitic division 1 then n after meiotic division 2. Am i right :confused:
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Golden Maverick
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(Original post by BLUREMI)
ok is this correct

at the start of meiosis, there are 23 pairs of chromosomes. (46 chromatids)

the 23 pairs of chromosomes replicate forming 46 pairs of cromosomes (92 chromotids)

The cromosomes pair up forming homologoes pairs of chromosomes, called bivalents. Each bivalent has two chromosomes, and therfore 4 chromatids. There are 23 bivalents

The bivalents will exchange genetic material in meiosis one with homologoes chromosomes forming chiasmata and thus exhanging material, causeing genetic diversity.

The bivalents line at the center of the spindles, and the spindles fibres pull the bivalents apart. A set of chromosomes, goes to opposite ends of the cell. This produces two cells with 46 chromosomes in each cell. (23 pairs)

Each of these cell then do meotic division two. In which the chromosomes are pulled apart at the centromere and go to opposute ends of the cell, producing
4 cells with 23 chromotids each. No chromosomes.

So at the start there is 2n, then 4n with formation of bivalents, then 2n after meitic division 1 then n after meiotic division 2. Am i right :confused:
Mostly right. You only have 4n though momentarily before the cell divides. Also you still say the 4 daughter cells contain chromosomes, just that they are haploid as opposed to diploid.

To summarise it further:
1 diploid cell (2n) -meiosis1->
2 diploid cells (2 x 2n) -meiosis2->
4 haploid cells (4 x n)
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BLUREMI
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(Original post by Golden Maverick)
Mostly right. You only have 4n though momentarily before the cell divides. Also you still say the 4 daughter cells contain chromosomes, just that they are haploid as opposed to diploid.

To summarise it further:
1 diploid cell (2n) -meiosis1->
2 diploid cells (2 x 2n) -meiosis2->
4 haploid cells (4 x n)
thanks so in meosis, chromosomes never break up i.e into chromatids (two chromatids make a chromosome right?). Only diploid chromosomes divide into haploid chromosomes.
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Golden Maverick
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(Original post by BLUREMI)
thanks so in meosis, chromosomes never break up i.e into chromatids (two chromatids make a chromosome right?). Only diploid chromosomes divide into haploid chromosomes.
It's really all in the naming of them - the chromatids do divide, but once they do they are called individual chromosomes. You don't need 2 chromatids to make a chromosome either.
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spin
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(Original post by Golden Maverick)
It's really all in the naming of them - the chromatids do divide, but once they do they are called individual chromosomes. You don't need 2 chromatids to make a chromosome either.
This is something I'm still failing to understand..

And when does the term 'sister chromatids' get used?
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BLUREMI
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(Original post by spin)
This is something I'm still failing to understand..

And when does the term 'sister chromatids' get used?
i thought it was sister chromosomes, when chromosomes replicate they pair up with there replicated chromosome

i really hope this doesnt come up in synoptics
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spin
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(Original post by BLUREMI)
i thought it was sister chromosomes, when chromosomes replicate they pair up with there replicated chromosome

i really hope this doesnt come up in synoptics
Oh, then what are bivalent pairs?
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corkskrew
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homologous pair = a bivalent = maternal and paternal chromosomes (a pair of X shapes either side of the equator in meiosis metaphase I)
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spin
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(Original post by corkskrew)
homologous pair = a bivalent = maternal and paternal chromosomes (a pair of X shapes either side of the equator in meiosis metaphase I)
Ah, thanks
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Revenged
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#10
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Yeah, so a bivalent could be called a chromosome even though it contains 2 chromatids, both of which are exact copies of the same chromosome. Therefore after mitosis the bivalent splits into 2 chromatids, both of which are exact copies of the same chromosomes in their daughter cells.

What i found confusing is that in meiosis 2 bivalents are shown whereas in mitosis there's only 1 even though both processes have their DNA replicated occuring during interphase.

The reason for this is that in mitosis, each cell needs an exact copy of both of the homologous chromosomes and so homologous chromosomes are not treated any differently than different chromosomes.

Whereas during meiosis homologous chromosomes are treated differently from all the other chromosomes which is why there are they are shown together during meiosis and not during mitosis.

(Maybe some of you found it obvious but it confused me!)
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BLUREMI
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(Original post by Revenged)
Yeah, so a bivalent could be called a chromosome even though it contains 2 chromatids, both of which are exact copies of the same chromosome. Therefore after mitosis the bivalent splits into 2 chromatids, both of which are exact copies of the same chromosomes in their daughter cells.

What i found confusing is that in meiosis 2 bivalents are shown whereas in mitosis there's only 1 even though both processes have their DNA replicated occuring during interphase.

The reason for this is that in mitosis, each cell needs an exact copy of both of the homologous chromosomes and so homologous chromosomes are not treated any differently than different chromosomes.

Whereas during meiosis homologous chromosomes are treated differently from all the other chromosomes which is why there are they are shown together during meiosis and not during mitosis.

(Maybe some of you found it obvious but it confused me!)
they pair up in meosis, so they can exvahnge material (chiasmata)

and a bivalent is 4 chromosomes. a pairs of two sister chromsomes :confused:
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Revenged
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Bivalent is 1 chromosome made up of 2 chromatids!
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Gaz031
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Ahh, the glorious ambiguity with the naming of single chromosomes, double choromosomes and chromatids.
Single chromosomes are also known as chromatids and contain 1 copy of each allele. Remember that humans have pairs of chromosomes and so have 2 copies of each allele.
Double chromosomes are made up of a pair of chromatids and contain 2 copies of each allele. Double chromosomes are only present in meiosis 1.
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Gaz031
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(Original post by Revenged)
Bivalent is 1 chromosome made up of 2 chromatids!
It is? I thought a bivalent was when a pair of double chromosomes (made up of 2 chromatids) pair up.
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Revenged
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(Original post by Gaz031)
Ahh, the glorious ambiguity with the naming of single chromosomes, double choromosomes and chromatids.
Single chromosomes are also known as chromatids and contain 1 copy of each allele. Remember that humans have pairs of chromosomes and so have 2 copies of each allele.
Double chromosomes are made up of a pair of chromatids and contain 2 copies of each allele. Double chromosomes are only present in meiosis 1.
There you go... That clarifies things!

Bivalent = Double chromosome
Chomatid = Single chromosome

Therefore, in meiosis 1 and in mitosis you have the double chromosome caused by DNA replication in interphase.

PS. I think maths makes you understand everything that no1 else does Gaz!
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BLUREMI
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(Original post by Gaz031)
It is? I thought a bivalent was when a pair of double chromosomes (made up of 2 chromatids) pair up.
yeh same here
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Juneau
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Yep bivalent is a pair of homologous chromosomes, not 1 chromosome.
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Revenged
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#18
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(Original post by Gaz031)
It is? I thought a bivalent was when a pair of double chromosomes (made up of 2 chromatids) pair up.
Nope... it's a pair of single chromosomes and you get a bivalent

Think about it...

A chromatid is a single chromosome and a bivalent is made up of two chromatids. Therefore a bivalent is a two single chromosomes, which you called a double chromosome.
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_poptart_
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#19
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god now Im confused lol!!
so a bivalent would have 2 chromatids on it yeh? And would these be made of identical DNA then :confused:
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Revenged
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#20
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(Original post by Juneau)
Yep bivalent is a pair of homologous chromosomes, not 1 chromosome.
Yeah, a bivalent is a double chromosome, soz, i was calling them both chromosomes

But it IS made up of 2 chromatids
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