4 Why DNA evolved from RNA

Question 1

1 - what did RNA originally evolve to use

Answer 1

Uracil and cytosine

Question 2

2. Why is cytosine a poor choice in evolutionary terms

Answer 2

because it decomposes slowly to give uracil, but there’s no way back in evolution

This means the sequence is degraded over time, but RNA is pretty unstable anyway, so maybe the RNA world could live with this

Question 3

3. When does this degradation become a bigger problem

Answer 3

Once DNA evolves as a stable long-lived repository of the sequence

Question 4

4. Why is cytosine to uracil conversion a problem

Answer 4

Because it corrupts the sequence, and the cell doesn’t know which are the ‘real’ Us in the sequence, and which are those that have arisen from decomposition of C

Question 5

5. What did DNA evolve to do to combat this

Answer 5

Use T in place of U (T is just 5-methyl U, and base-pairs just the same as U)

This is essentially like labelling the real Us using a methyl group

Question 6

6. So, any actual Us that appear must have come from Cs…

Answer 6

The cell can recognise them and remove them, using the enzyme uracil glycosylase; ultimately they are replaced with Cs again (because they will be opposite a G in the double helix).

Ts are not recognised by the enzyme.

Question 7

FULL TIMELINE

Answer 7

1. RNA evolved originally to use uracil and cytosine
2. Cytosine is actually a poor choice in evolutionary terms, because it decomposes slowly to give uracil, but there’s no way back in evolution…
3. This means the sequence is degraded over time, but RNA is pretty unstable anyway, so maybe the RNA world could live with this…
4. This becomes a bigger problem once DNA evolves as a stable long-lived repository of the sequence
5. C –> U conversion is a problem, because it corrupts the sequence, and the cell doesn’t know which are the ‘real’ Us in the sequence, and which are those that have arisen from decomposition of C
6. So, DNA evolves to use T in place of U (T is just 5-methyl U, and base-pairs just the same as U)
7. This is essentially like labelling the real Us using a methyl group
8. So, any actual Us that appear must have come from Cs, and the cell can recognise them and remove them, using the enzyme uracil glycosylase; ultimately they are replaced with Cs again (because they will be opposite a G in the double helix). Ts are not recognised by the enzyme.