IB Biology Sub-topic D1.2 Notes

In Topic A1.2, you learned that nucleic acids code for protein production. Since DNA is located in the nucleus and proteins are produced outside the nucleus, the genetic code needs to be transported out of the nucleus. This done by messenger RNA (mRNA), which is produced by the process of transcription:

1. RNA polymerase unwinds the double helix and separates the strands.
2. RNA polymerase links nucleotides to one of the pre-existing strands via complementary base pairing (uracil instead of thymine with adenine).
3. The mRNA strand separates from the DNA and the DNA pairs up again and twists back into a double helix.

Note that using single DNA strands fixed in the nucleus to nucleosomes as templates provides a very stable base from which to perform transcription. However, the base sequence cannot change so in cells that do not divide, these sequences cannot change throughout the entire life cycle of the cell.