Where RNA Comes From: A Transcript!

Remember DNA replication? The DNA molecule unwinds, then each strand serves as template for a new complementary strand of DNA. RNA transcription is very similar.

The DNA molecule unwinds, and an enzyme called RNA polymerase creates a complementary strand of RNA, using one of the strands of DNA as a template. Guanine (on DNA) is paired with cytosine (on RNA).

Cytosine (on DNA) is paired with guanine (on RNA). Thymine on DNA is paired with adenine on RNA. And adenine (on DNA) is paired with uracil (on RNA).

The two strands (one DNA and one new RNA) do NOT then bond together to form a ladder. Remember: RNA is a single-stranded molecule. The new RNA molecule is released, and the DNA strand rejoins with its complementary partner.

One last thing to point out: in replication, the entire DNA molecule (the entire chromosome) is copied over. In transcription, only a portion of the DNA molecule is used to make RNA (is transcribed). Remember that much of the DNA in human cells does NOT carry instructions for protein synthesis, and that the portions that DO carry instructions for protein synthesis are called genes.

So we only need to transcribe the genes. Furthermore, we only need to transcribe some of the genes - the ones that correspond to proteins needed by the cell at that time.

Here's a summary of the differences between DNA replication and RAN transcription: