If you have not already done so, download Avogadro for free and watch the tutorials if necessary.
Build a double DNA fragment with Avogadro (sequence Build → Insert → DNA / RNA) according to the ACGTACGTAGCTATCG sequence. Rotate the molecule in order to make the double helix structure clearly visible. Then click on an atom so that one DNA strand is selected and delete it. Watch the tutorial.
On the same Avogadro page, build an RNA fragment with the same base sequence, putting uracil in place of thymine. If necessary, rotate the two molecules so that the structures are visible in a similar way.
Can you fit them together?
Now you can have fun building your own DNA sequence and read how mRNA works in COVID-19 vaccines (go to the sheet).