Logic probe circuit; what is the use of some of his components?

I found this schematic on the Web and I built the circuit but,due to space constraints, I removed some components and it work well.

I've not mounted the two diodes \$D_3\$ and \$D_4\$ and the capacitors.
For the logic gates I used a 74LS00.

The questions:

1. What is the purpose of \$D_3\$ and \$D_4\$?
   To protect the supply in case of fault in case I connect the probe at high voltage?
2. And what is the use of the two capacitors?

Thanks in advance.