DNA repair mechanism

[Paul C. Anagnostopoulos]

This just in from the Weizmann Institute (well, a couple of years ago, anyway):

http://www.sciencedaily.com/releases/2002/10/021025065902.htm

Rehovot, Israel -- October 24, 2002 -- Twenty thousand hits per day -- that's the average dose of damage sustained by the genes within each cell of our body. How are innumerable mutations avoided? In a study published in the October issue of Molecular Cell, Weizmann Institute researchers have proved the existence of a vital repair mechanism used by cells to correct this damage and showed that it is responsible for about 85% of what are termed "last-resort" repairs.
...
The other last-resort repair system was hypothesized by scientists in the 1960's, yet was never proved until the current study. This system, which relies on the help of "sister chromosomes," enables the cell to repair genetic damage without the risk of creating mutations. (During the process of cell division, each chromosome -- the structure in the nucleus that contains DNA -- gives rise to two identical "sister" chromosomes. These move on to the two separate cells created from the dividing cell.)

Except that the two chromosomes are not identical. Is this some science writer spacing out?

~~ Paul

 

[Anders]

This just in from the Weizmann Institute:

http://www.sciencedaily.com/releases/2002/10/021025065902.htm


Except that the two chromosomes are not identical. Is this some science writer spacing out?

~~ Paul

Well, the chromosomes are perhaps not identical but the usually contain the same genes, so they can borrow from each other. It's called homologous recombination and takes usually place between sequences in the same chromosome.

But in this article they talk about bacteria, and bacteria has somewhat different repair mechanisms.

Anyway, molecular biology is very complicated so I'm happy as long as journalists even try to write about it. I say: more molecular biology for the people!

 

[Dr. Imago]

Except that the two chromosomes are not identical. Is this some science writer spacing out?

Paul,

Each individual chromosome in the pair is not identical, but the copy of the individual chromosome that goes into the new cell is. Make sense?

Example: You have a cell from the lining of the colon. These cells 'turn-over' about every 2-3 days. In an individual colon cell, there are (for simplicity sake) 23 pairs of chromosomes. When that cell divides, each of those 23 pairs are copied and the "new" cells contain an identical replica of the parent cell.

Let's take chromosome 15. There is a chromosome 15 from the father (15F) and one from the mother (15M). We consider them "pairs" because they are complementary from each individual parent. But, in reality, they don't exist coupled in pairs in the cell. So, chromosome 15F will be copied during division into 15Fa and15Fb. Likewise, chromosome 15M, will do the same 15Ma and 15Mb.

In the two new cells, you will have full copies - cell 'a' containing 15Ma and 15Fa, and cell 'b' will contain 15Mb and 15Fb. Each are exact copies of the original cell... barring any repication errors during the process.

-TT

 

[Paul C. Anagnostopoulos]

understand that ThirdTwin, but I'm not sure what it has to do with this repair mechanism. As far as I can tell, this is an in-cell repair mechanism having nothing to do with mitosis. If chromosome 15F is damaged, taking the equivalent sequence from 15M might result in a sort of hybrid gene that has portions of both alleles. Or does the mechanism copy a complete gene somehow? Is that what you're suggesting, Anders?

~~ Paul

 

[Anders]

understand that ThirdTwin, but I'm not sure what it has to do with this repair mechanism. As far as I can tell, this is an in-cell repair mechanism having nothing to do with mitosis. If chromosome 15F is damaged, taking the equivalent sequence from 15M might result in a sort of hybrid gene that has portions of both alleles. Or does the mechanism copy a complete gene somehow? Is that what you're suggesting, Anders?

~~ Paul

Well, is a very complicated process, and I’m not sure I remember correctly, but it goes something like this:

Yes, the DNA strands can copy genes from other strands or from different portions of the same strand. Hybrid gene is perhaps the wrong wording because genes are nearly always shuffled and rearranged, because of errors, viruses, or because of signals. For example the immunoglobulin genes are quite few, but these few genes are expressed as billions of proteins.

Anyways, I might come back to this. It’s a fascinating subject but as I said, very complicated.