Repair methods 5 - End joining

Repairpairedchains.bmp
When the chains pair-up, key goes with lock; hook with eye.

 

 

 

 

 

 

 

Let's imagine that both of the paired chains have been broken - alpha radiation can cause this type of damage.

Repairpairedchainstemplate1.bmp
Both strands have been broken.

 

 

 

 

 

 

 

If a second copy of the instruction manual is not available then we cannot use template repairs.  In this case, the cell must join the ends together ...

Repairpairedchainsendjoin1.bmp
There are now two missing keys.

 

 

 

 

 

 

 

 

And then fill in the missing code using the information available.  In this case two locks will match with two keys.  Adding the two keys reconstructs the original instruction manual correctly.

Repairpairedchains.bmp
When the chains pair-up, key goes with lock; hook with eye.

 

 

 

 

 

 

 

The problem is that this repair method is prone to error.  Imagine what would happen if the original chains looked like this ...

Repairpairedchainsendjoin3.bmp
The repair has gone wrong.

 

 

 

 

 

 

 

Double strand damage might then produce ...

Repairpairedchainsendjoin2.bmp
This double strand break can be repaired in two ways.

 

 

 

 

 

 

 

The cell might get this right.  Filling in the missing links gives us ...

Repairpairedchainsendjoin4b.bmp
Joining the ends gives the right result.

 

And filling in the missing code gives the correct result.

 

Repairpairedchainsendjoin3.bmp
The repair has gone wrong.

 

But joining up the ends might happen to produce this:

Repairpairedchainsendjoin5.bmp
Direct end-joining has got it wrong.

 

 

 

 

 

 

 

 

Definitely not the right result.

There are literally billions of bases in the human encyclopaedia. Surely one or two mistakes can’t hurt? Surely the cell would realise the mistake or maybe the code now translates into gibberish? Link on the link to find out how small changes can build-up into a big problem.