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March 29, 2024, 01:48:40 am

Author Topic: Number of chromosome after meiosis I  (Read 1908 times)  Share 

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NN900

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Number of chromosome after meiosis I
« on: October 05, 2021, 04:34:12 pm »
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Hello people  :)
I'm just confused with the answers to these two questions "After meiosis I, are there any changes in chromosome number? Explain." and "After meiosis II, how many chromosomes do the daughter cells have?" (this is the case of human chromosome = 46 in total)
Some people say that after meiosis I, number of chromosome will be haploid which is 23 chromosomes, but some people say it should be diploid after meiosis.
So, can someone please explain the answers to these two questions? 

Kind regards.                                                                                                         

sam777

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Re: Number of chromosome after meiosis I
« Reply #1 on: October 05, 2021, 09:20:45 pm »
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First, an important thing to note is that chromosomes are counted by the number of functional centromere. Hence a duplicated chromosome is still counted as one chromosome rather than counting by sister chromatids.

Basically, before meiosis, is replication. And before replication, the number of chromosomes is the diploid number where each chromosome hasn't been duplicated yet (so its just 1 strand/chromatid). Therefore in humans, there are 46 chromosomes and 46 chromatids.

After replication, there are still 46 chromosomes (diploid number) but there are 92 chromatids.

After Meiosis I (the first division) the number of chromosomes would be halved, as the homologous chromosomes are separated from each other within their pair. Hence there would be 23 chromosomes in each daughter cell (the haploid number), although there are 46 chromatids.

After Meiosis II (the second division) the number of chromosomes would be halved again, as the sister chromatids are separated. Hence there would still be 23 chromosomes in each of the 4 daughter cells (the haploid number), but there are now 23 chromatids. There is only one chromosome of each type at this stage (e.g. one chromosome 21) and each is unduplicated.

Therefore: Before replication = 2n (46 chromosomes, 46 chromatids) --> After replication = 2n (46 chromosomes, 92 chromatids) --> After Meiosis I = n (23 chromosomes, 46 chromatids) --> After Meiosis II = n (23 chromosomes, 23 chromatids) --> After fusion of gametes (sperm fertilises ovum) = 2n (46 chromosomes, 46 chromatids)
« Last Edit: October 05, 2021, 09:22:51 pm by sam777 »

NN900

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Re: Number of chromosome after meiosis I
« Reply #2 on: October 06, 2021, 10:20:09 am »
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First, an important thing to note is that chromosomes are counted by the number of functional centromere. Hence a duplicated chromosome is still counted as one chromosome rather than counting by sister chromatids.

Basically, before meiosis, is replication. And before replication, the number of chromosomes is the diploid number where each chromosome hasn't been duplicated yet (so its just 1 strand/chromatid). Therefore in humans, there are 46 chromosomes and 46 chromatids.

After replication, there are still 46 chromosomes (diploid number) but there are 92 chromatids.

After Meiosis I (the first division) the number of chromosomes would be halved, as the homologous chromosomes are separated from each other within their pair. Hence there would be 23 chromosomes in each daughter cell (the haploid number), although there are 46 chromatids.

After Meiosis II (the second division) the number of chromosomes would be halved again, as the sister chromatids are separated. Hence there would still be 23 chromosomes in each of the 4 daughter cells (the haploid number), but there are now 23 chromatids. There is only one chromosome of each type at this stage (e.g. one chromosome 21) and each is unduplicated.

Therefore: Before replication = 2n (46 chromosomes, 46 chromatids) --> After replication = 2n (46 chromosomes, 92 chromatids) --> After Meiosis I = n (23 chromosomes, 46 chromatids) --> After Meiosis II = n (23 chromosomes, 23 chromatids) --> After fusion of gametes (sperm fertilises ovum) = 2n (46 chromosomes, 46 chromatids)

THANK YOU SO MUCH for your explanation. It makes a lot of sense now.  :)
Have a great day!!!

sam777

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Re: Number of chromosome after meiosis I
« Reply #3 on: October 06, 2021, 02:33:32 pm »
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THANK YOU SO MUCH for your explanation. It makes a lot of sense now.  :)
Have a great day!!!
No worries. Happy that I managed to help. Let me know if you have any other questions :)