Sentence Correction

I'm a little confused in Run on sentences. Please consider the below example-:

1) The university has three sections, each of which specialises in a particular business area.

2) The university has three sections, all of them specialise in a particular business area.

Second is a run on sentence whereas the first is not. Can anyone help me explain why?

Isn't the phrase after comma in both the sentences acts as modifier?

1) The university has three sections, each of which specialises in a particular business area.
==> here, "each..." is a modifier.. describing the sections..

2) The university has three sections, all of them specialise in a particular business area.
==> here, "all of them" is a clause.. We cannot seperate two clauses by comma...
So, Incorrect

theCodeToGMAT wrote:1) The university has three sections, each of which specialises in a particular business area.
==> here, "each..." is a modifier.. describing the sections..

2) The university has three sections, all of them specialise in a particular business area.
==> here, "all of them" is a clause.. We cannot seperate two clauses by comma...
So, Incorrect

What if instead of 'each of which' there is 'each of them'??

charan13387 wrote:

theCodeToGMAT wrote:1) The university has three sections, each of which specialises in a particular business area.
==> here, "each..." is a modifier.. describing the sections..

2) The university has three sections, all of them specialise in a particular business area.
==> here, "all of them" is a clause.. We cannot seperate two clauses by comma...
So, Incorrect

What if instead of 'each of which' there is 'each of them'??

Then the sentence will become run-on. The issue is between 'Which' and 'Them' not between 'Each' and 'All'.
Experts, please correct me if I am wrong.