Why is it important that the hydrogen bonds that bind the nitrogenous bases (AT and CG) together be weak?

Most enzyme names end in the

What is the most abundant vertebrate on earth?

Which of the following is a reason why early living organisms on earth could not have survived on the surface? 1. the lack of an ozone layer 2. the lack of oxygen in the atmosphere 3. the fact that these organisms were single-celled