1. Unit 7: Molecular Genetics
7.3 Protein Synthesis: Transcription

2. Protein Synthesis Overview
A gene’s instructions for making proteins are coded in the sequence of nucleotides.
Making a protein happens in two steps: Transcription and Translation.
Both of these processes use the nucleic acid RNA.

3. Ribonucleic Acid (RNA)
A nucleic acid that is made up of linked nucleotides
Differs from DNA in 3 ways:
RNA is a single strand of nucleotides, not two strands
RNA contains the 5 carbon sugar called Ribose, instead of deoxyribose
RNA has the nitrogen bases A, C, and G and Uracil (U) instead of thymine (T).
A bonds with U
C bonds with G
U is considered a pyrimidine

4. Transfer of DNA to RNA: Transcription
Transcription (mRNA Synthesis): the process where DNA is transferred from the gene to an RNA molecule.
Takes place in the nucleus

5. Steps of Transcription:
RNA polymerase binds to the “start” codon: 3 nucleotides in a specific sequence
Ex: AUG
RNA polymerase then unwinds and separates the two strands of the double helix, exposing the DNA nucleotides on each strand.
RNA polymerase adds and then links complementary nucleotides as it “reads” the gene.
Transcription follows the same base-pairing rules as DNA but U pairs with A instead of T.

6. Steps of Transcription:
RNA polymerase continues until it reaches a “stop” codon (UAA, UGA, UAG)
DNA closes up by reforming Hydrogen bonds after the RNA polymerase adds nucleotides to the strand.
Many stands of RNA are made at one time from a single double helix of DNA.