1. TRANSCRIPTION
Sections 5.2 & 5.3

2. Recall… Protein synthesis occurs in two distinct stages:
Transcription, in the nucleus
Product: mRNA
Translation, in the cytoplasm
Product: polypeptide

3. Transcription
The process of copying DNA’s instructions for building a protein.
The instructions are copied into a complementary mRNA molecule.

4. Overview
Three stages:
Initiation
Elongation
Termination

5. Stage
Events
Initiation
RNA polymerase binds to the DNA and unwinds it
Elongation
RNA polymerase creates a complementary mRNA strand. It adds on ribonucleotides in the 5’  3’ direction
Termination
Translation ceases, and the mRNA is released

6. Terminology The strand used as the template is the “template strand”

7. The strand not used as the template is the “coding strand.”
The mRNA is identical to the coding strand except for one difference... uracil in RNA is complementary to adenine in DNA
coding strand

8. The details of eukaryotic transcription
Section 5.3
The details of eukaryotic transcription

9. Initiation
RNA polymerase binds to a promoter region located upstream of the gene
The promoter region is high in A-T base pairs
Only two H-bonds; easy to break
RNA polymerase unzips the double strands in order to expose the bases

10. Initiation
Binding to promoter
Unzipping the DNA

11. Elongation RNA polymerase builds an mRNA strand
Incorporates ribonucleotides that are complementary to the template DNA
Catalyzes formation of phosphodiester bonds
A primer is not required

12. Elongation

13. Transcription (like replication) occurs in the 5’  3’ direction
(ribonucleotides are added on to the 3’ end of the nascent strand)
The promoter does not get transcribed

14. Termination A terminator sequence is located at the end of the gene
Eukaryotes vs. prokaryotes: different sequences
RNA polymerase
recognizes the terminator, and
releases the mRNA strand

15. Termination

16. After transcription… The mRNA will exit the nucleus
Transcription of mRNA  protein will occur in the cytoplasm
BUT
Before it exits, the mRNA is modified

17. Post-transcriptional modifications
“Primary transcript” – the newly-built mRNA
Must be modified before it exits the nucleus.
Why?
remove non-coding regions
protect the mRNA from cytoplasmic nucleases and phosphatases
(enzymes that can degrade the RNA)

18. Modifications Splicing Add 5’ cap
Add poly-A tail addition (“polyadenylation”)

19. Splicing removal of non-coding regions (introns) in the transcript
spliceosomes (a) excise the introns, and (b) ligate the exons
spliced introns are degraded in the nucleus

20. 5’ cap is added poly-A tail is added 7-methyl guanosine
200 adenine nucleotides
Catalyzed by poly-A polymerase

22. primary transcript (modification)  mRNA TRANSCRIPT
Following modification, the primary transcript is now called the “mRNA transcript”
primary transcript (modification)  mRNA TRANSCRIPT
It is ready to leave the nucleus!!

23. Transcription: Quality control
no proofreading mechanisms (unlike replication)
more errors in transcription than replication
but more copies of mRNA in a cell than DNA  errors are less detrimental
Backup mechanism: redundancy of the genetic code used in translation
*more on this later on…

24. Homework
Pg. 249 #1-9