Question

Why are there no Pol I mutants that completely lack $5\text{'}\to 3\text{'}$ exonuclease activity?

Short answer

There are no Pol I mutants that completely lack$5\text{'}\to 3\text{'}$ exonuclease activity because that would be lethal for the organism.

Step-by-step solution

5'→3'Exonuclease activity.

RNA primers are needed for the initiation of DNA replication and synthesis. In the leading strand, only one RNA primer is required, but many RNA primers are required to create Okazaki fragments for the lagging strand. Later, RNA primer is replaced by DNA fragments.The $5\text{'}\to 3\text{'}$Exonuclease activity helps in the removal of RNA primers during replication.

Lack of 5'→3' Exonuclease activity.

If the $5\text{'}\to 3\text{'}$exonuclease activity is absent, the RNA primers cannot be removed from the DNA strand, causing the synthesized DNA to be non-functional. The process of DNA synthesis will not occur in the absence of $5\text{'}\to 3\text{'}$exonuclease activity, making it lethal for the organism. Hence, the Pol I mutants that completely lack $5\text{'}\to 3\text{'}$ exonuclease activity cannot exist.