How do correct DNA strands get to attach at the centromere?

[vibhuav]

I have a basic question about DNA replication. As I understand, the double helix DNA replicates, and two double helix DNA strands are created. I understand that part. Now the question is, are these two DNA strands completely separated and floating about in the nucleus?

If so, the next question is, how do the correct DNA pairs join together at the centromere to begin the mitosis process? How do each DNA strand know its exact counterpart to pair with?

If they are not completely separated after DNA replication, how and where on the DNA strand are they attached?

Thanks a lot. BTW, I am an engineer by profession with a with a curiosity in biology.

 

[vibhuav]

I think I found the answer. Let me know if this is right (or wrong)...

The attachment of the two sister DNA is via the protein globules of the chromatins, and not via the DNA per-se. Initially at the centromere, but all along the length during mitosis.

Secondly, even though the textbooks show independently existing DNA, in reality, the DNA is always wound on protein globules and exist as chromatins. The chromatins can unfold to enable replication and transcription, but the protein globules and DNA always exist together. Also, after DNA replication, even though the two DNA strands are separate, they are still held together by the protein globules; so the DNA strands are never “freely floating” in the nucleus. The two sister chromatids are always attached and the question of having to pair the correct DNA pairs does not come; they are attached during construction itself.

 

[mishrashubham]

Yes you are right, after replication the sister chromatids don't separate but are held together, which is called a cohesion complex. The primary structure involved is a protein complex called cohesin, which forms ring like structures to hold the strands together (not 'protein globules').

Here are some schematics to help you visualise

http://www.google.co.in/url?sa=i&so..._YmkQR4psbGGog3CnyXO5SMg&ust=1359734896785449

http://www.google.co.in/url?sa=i&so...BfUTMFZsxY4pU_5SNSmDr2Rw&ust=1359734872722097

 

[vibhuav]

Yes you are right, after replication the sister chromatids don't separate but are held together, which is called a cohesion complex. The primary structure involved is a protein complex called cohesion, which forms ring like structures to hold the strands together (not 'protein globules').

Thanks a lot!

 

[atyy]

mishrashubham's first link in #3 indicates that DNA is replicated during S phase. Here's some details about that.

During DNA replication the DNA is opened up bit by bit - this is called the replication fork.
http://www.ncbi.nlm.nih.gov/books/NBK26826/
http://www.ncbi.nlm.nih.gov/books/NBK21751/

Here are some reviews about what happens as the replication fork passes the centromere. Links to free versions of some of these are at the top right of the page.

http://www.nature.com/nrg/journal/v9/n12/fig_tab/nrg2466_F3.html
http://www.ncbi.nlm.nih.gov/pubmed/19002142
http://www.ncbi.nlm.nih.gov/pubmed/23095988
http://www.ncbi.nlm.nih.gov/pubmed/21467140

 

[vibhuav]

How do homologues pair up correctly to form the bivalents? The best answer I found on the internet is that biologists don’t know yet. Any better answer?

 

[atyy]

http://genesdev.cshlp.org/content/11/20/2600.short
http://jcs.biologists.org/content/124/12/1955.abstract

Looks not so well understood as you say. This is during meiosis (not mitosis). One interesting possibility seems to be the "telomere bouquet". From the second article:

"Before homologous chromosomes recombine and form a bivalent, they must find each other within the cell nucleus. In most organisms, the initiation of homologous pairing occurs at numerous sites along chromosomes by a mechanism that still remains unclear. These early interactions are then stabilized only at sites where there is good flanking homology between chromosomes. In many organisms, this sorting and stabilizing process appears to be promoted by a meiosis-specific organization of chromosomes called the ‘bouquet configuration’, which is initiated by a clustering of telomeres on the inner nuclear envelope. The bouquet appears to facilitate homologous recognition and alignment by concentrating chromosomes within a limited region of the nuclear volume, thus enabling chromosome movements that promote the identification of homologs, perhaps by the DNA DSB repair process (Box 3) (Hiraoka, 1998; Scherthan, 2001; Harper et al., 2004)."