Confusing terminology in Meiosis: Homologous chromosomes and Bivalent?

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jo7777
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Report Thread starter 7 years ago
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Hi so I'm just trying to make sense of meiosis, and I'm not sure I completely understand what these words mean. Do you think you could correct me on any which are wrong?

Chromosome: Two chromatids attached to each other, forming an X. You have one chromosome from your mum and one from your dad.

Homologous Pair: Each chromosome is copied. This means you have two of each chromosome so you have XX which are both exactly the same.

Bivalent: When a homologous pair XX attach to each other they form a bivalent and can then cross over.

So surely after interphase you have a homologous pair of each chromosome, so actually you have 4n? In my book it says after one division you have 2n, but it also says that after interphase you have 2n? Im stumped. Any help would be much appreciated
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KiwiMonkey96
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(Original post by jo7777)
Hi so I'm just trying to make sense of meiosis, and I'm not sure I completely understand what these words mean. Do you think you could correct me on any which are wrong?

Chromosome: Two chromatids attached to each other, forming an X. You have one chromosome from your mum and one from your dad.

Homologous Pair: Each chromosome is copied. This means you have two of each chromosome so you have XX which are both exactly the same.

Bivalent: When a homologous pair XX attach to each other they form a bivalent and can then cross over.

So surely after interphase you have a homologous pair of each chromosome, so actually you have 4n? In my book it says after one division you have 2n, but it also says that after interphase you have 2n? Im stumped. Any help would be much appreciated
A chromosome is only made up of two chromatids during DNA replication, a chromosome is just a very long strand of DNA that is wound around many histone proteins and highly coiled to give a compact shape.
Homologous chromosomes are just two of the same chromosome, eg. Chromosome 1 and another chromosome 1.
Bivalent is the 2 homologous chromosomes that have already joined or paired in a synapsis at prophase 1 of meiosis 1.
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JacobAlevels
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(Original post by jo7777)
Hi so I'm just trying to make sense of meiosis, and I'm not sure I completely understand what these words mean. Do you think you could correct me on any which are wrong?

Chromosome: Two chromatids attached to each other, forming an X. You have one chromosome from your mum and one from your dad.

Homologous Pair: Each chromosome is copied. This means you have two of each chromosome so you have XX which are both exactly the same.

Bivalent: When a homologous pair XX attach to each other they form a bivalent and can then cross over.

So surely after interphase you have a homologous pair of each chromosome, so actually you have 4n? In my book it says after one division you have 2n, but it also says that after interphase you have 2n? Im stumped. Any help would be much appreciated
A cell actually is 2n after Interphase.

The important point to note is: before Interphase, each chromosome is not made up of 2 chromatids, it's just one single strand of DNA. So you have 46 chromosomes and 46 chromatids (chromosomes=chromatids at this stage).

After DNA replication, each chromosome is made up 2 complementary CHROMATIDS. So you still have 46 chromosomes but 92 chromatids. That's still 2n.

It'll be clearer if you look at the number of CHROMOSOMES vs CHROMATIDS during mitosis:

Prophase 46 vs 92
Metaphase 46 vs 92
Anaphase 92 vs 92
Telophase 92 vs 92

In Prophase the number of chromosomes is 46, meaning 2n.
At Anaphase what happens is that the CHROMOSOMES get broken into the CHROMATIDS that they are made up of.
After Telophase, 46 CHROMATIDS go to the mother cell and 46 to the daughter cell.
These chromatids then literally just turn into chromosomes, meaning now they're called chromosomes. Like at the start, they're just single strands of DNA. So the cell is 2n. It's not ready for replication though, because it's got just 46 chromatids.
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JacobAlevels
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Just noticed your definitions are a bit wrong, so to make them more precise:

(Original post by jo7777)
Chromosome: Two chromatids attached to each other, forming an X. You have one chromosome from your mum and one from your dad.
A chromosome is simply a length of DNA. It's not always made up of two chromatids. It's only made up of two chromatids AFTER DNA replication.

Homologous Pair: Each chromosome is copied. This means you have two of each chromosome so you have XX which are both exactly the same.
Cells ALWAYS have pairs of homologous chromosomes (before and after DNA replication).
Homologous chromosomes are chromosomes that have the same structure and the same genes (not necessarily the same ALLELES) at the same loci. In one pair of homologous chromosomes, one chromosome is from the mum and one from the dad.
We've all got 23 pairs of homologous chromosomes, so 46 chromosomes in total.

Bivalent: When a homologous pair XX attach to each other they form a bivalent and can then cross over.
'Bivalent' is just a name for homologous chromosomes that PAIR UP (NOT attach) during mitosis. They only 'attach' after they've already paired up, i.e. after they're already a bivalent. 'Attaching' is more correctly called 'crossing over'.
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jo7777
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(Original post by JacobAlevels)
Just noticed your definitions are a bit wrong, so to make them more precise:



A chromosome is simply a length of DNA. It's not always made up of two chromatids. It's only made up of two chromatids AFTER DNA replication.



Cells ALWAYS have pairs of homologous chromosomes (before and after DNA replication).
Homologous chromosomes are chromosomes that have the same structure and the same genes (not necessarily the same ALLELES) at the same loci. In one pair of homologous chromosomes, one chromosome is from the mum and one from the dad.
We've all got 23 pairs of homologous chromosomes, so 46 chromosomes in total.


'Bivalent' is just a name for homologous chromosomes that PAIR UP (NOT attach) during mitosis. They only 'attach' after they've already paired up, i.e. after they're already a bivalent. 'Attaching' is more correctly called 'crossing over'.
Thank you for that wonderful explanation!! Just what I needed +rep
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