Discussion of all aspects of cellular structure, physiology and communication.
Tue Sep 23, 2008 6:40 pm
I have looked many places over much time for this. It seems so elementary but .... Where needed, I'm using human cells for dicussion. It is written that if a cell is diploid, then it has 2 sets of chromosomes in the nucleus. In humans this is 2 * 23 = 46 chromosomes.
1. Are these 46 chromosomes aware of the existence of a 'mate' wanderding around the nucleus? Are the pairs in any way attached?
2. I think I read that the 2n chromosomes come one from a male parent and one from a female parent. True for mitoically (sic?) created cells?
2. During Interphase, each of the 46 chromosomes replicates its DNA. So for a limited period of time there are 4n chromosomes floating around?
3. If the sister chromatids are pictured in an "X" formation, is it reasonable to think of a chromosome as a stick figure? (a picture's worth a thousand words, huh).
4. How do the homologous sister chromatids pair up? Is there any relation between the homologous pairs and the matching 2n chromosomes?
I found some postings here that pretty much answered what has been bothering me. Perhaps my beginning plant botany and many web sites are just too elementary but some of this stuff is rather blythly glossed over.
Wed Sep 24, 2008 2:18 am
2. well you have 23 from the mom and 23 from the dad. In a mitotic division nothing changes, so yes. In the case of meiosis, you are one of the parents. Let's assume you're in bed with your girlfriend. the spermatogonia divide to form sperm cells, and 23 of your 46 chromosomes are randomly selected to get into each sperm. In this case, the 23 are the sperm on the father's side, and the other 23 in the future baby's cells will come from your girlfriend.
2(why do you have two 2s?). Not really, because the two sister chromatids are still attached at the centromere. the cell does have 4n amount of DNA though...
4. sister chromatids do not come together, they are together. When DNA gets replicated, each chromosome doubles its DNA quantity, resulting in a "X" shaped chromosome made out of two sister chromatids.
Yes, there is a realtion between homologous pairs and 2n - there are 2 chromosomes(a homologous pair) for each one of the n. Hence, 2n.
Wed Sep 24, 2008 3:44 pm
I forgot this isn't Word. Or overlooked the numbering problem during proofreading.
2nd 2. There are,then, 46 chromosomes, 46 pairs of sister chromatids, and 92 chromotids.
4. I asked the wrong question. How do the homologous chromosomes pair up (mate/join)? How do they recognize that chromosome with whom to pair? If there are different genes on different chromosomes, I assume that this pairing would have to have 2 chromosomes with the same set of genes. Is this the "1" and the "2" of the 2n (one from the mother side, one from the father side)? Thus each pair of homologous chromosomes have a total of 4 sister chromatids, 2 on each of the homologous chromosomes. How the chromosomes pair 'correctly' is my biggest stumbling point.
Andrew thanks. Some of what I wrote above is obvious even to me but it helps me to keep my thoughts together.
Fri Sep 26, 2008 1:24 pm
2nd 2. Yes
4. the pairing is directed by DNA sequence. When the homologous chromosomes pair together, one sister chromatid of each pairs with one sister chromatid of the other. This can be either way, because the sister chromatids of a duplicated chromosome have formed by DNA replication, and are thus identical(they are "twin" sisters)
Fri Sep 26, 2008 9:54 pm
In other words, when you write "When the homologous chromosomes pair together..." it is because there is 'something' in the DNA sequences of the 2 chromosomes that directs (your word)/propels/draws/attracts the two (homologous) chromosomes (each with their 2 chromatids) towards one-another? Do we know what it is? Some low level pheromone-object, chemical,...?
Thank you so much, again.