[Help!!]# of chromosomes and Chromatids

[sminj85]

Hi all!
I have a quick question which confuses me all the time!!!
In my knowledge, I know that for 1 chromosome, there are 2 chromatids.
However, I see sometimes, for example, in haploid, # of chromosome and # of chromatids are the same....

How do you count them in mitosis and meiosis? I understand that mitosis ends up with 2n but meiosis ends up with n. BUT I do not know how to apply this into actual number.

Can someone clearly explain about this??
It would be great if someone list all the phases in both mitosis and meiosis and show a change in number of chromosome and chromatid..

Thank you!!

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[osimsDDS]

This also confused me when i started studying, but once you define the terms and think about the 2 cell divisions its easy....

First of all define homologous chromosomes and sister chromatids:

The difference is that homologous chromosomes have different gene variations or alleles at different positions meaning they are identical but not at the same position on the chromosome.

Then sister chromatids are EXACTLY alike with position and their alleles.

Therefore:

Mitosis: Sister Chromatids, NOOOO Homologous chromosomes (because crossing over doesnt happen).

Meiosis: Homologous chromosomes cross over in prophase I where they exchange genetic information. Think about this, if it were sister chromatids that were exactly the same then interchanging genetic information via crossing over wouldnt produce any variation it would end up with the same exact genes. Then after telophase I and cytokinesis the cell goes through another separation of chromosomes where now they are chromatids separating.

 

[Zerconia2921]

To add to the post:
In meosis you have a haploid number of chromosomes example the egg. The egg has a total of 23 chromosomes and also 23 chromatides.

In mitosis you have a diploid number of chromosomes example the zygote. The zygote has a total of 46 chromosomes and 92 chromatides.

 

[OMFS08]

In mitosis one chromosome refers to a single strand of DNA but in meiosis chromosome refers to a double stranded DNA so it has Sister chromatids but in mitosis its just single stranded.

 

[bigstix808]

check this out...its an older post..
http://forums.studentdoctor.net/showthread.php?t=501694

and here is a pic i used to try and explain kind of what you are asking...