1. Genetics Unit I-Part C Transcription
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2. I. General Info
-Process in which double-stranded DNA will be used to make a single-stranded RNA molecule called messenger RNA (mRNA)
Transcription is, chemically and enzymatically, very similar to DNA replication, but there are some important differences:
-RNA is made of ribonucleotides (rather than deoxyribonuleotides)

3. I. General Info
RNA polymerase catalyzes the reaction (does not need a primer)
The synthesized RNA does not remain base-paired to the template DNA strand
Transcription selectively copies only certain parts of the genome and makes one to several hundred, or even thousand, copies of any given section of the genome.

4. I. General Info

5. I. General Info Catalyzed by a class of enzymes called RNA polymerases
3 different RNA pol’s in eukaryotes:
-RNA pol I- makes ribosomal RNA
-RNA pol II- makes mRNA
-RNA pol III- makes tRNA
The shape of the enzyme resembles a crab claw

6. Cont. -The RNA pol enzyme is made of 2 sub-units: a. Sigma factor
b. Core-enzyme
-Transcription is a 4 step process:
a. binding of enzyme to DNA
b. Initiation
c. Elongation
d. Termination
-The sigma factor handles step a
-The core enzyme handles steps b, c, d

7. II. Detailed Process A. Binding of enzyme
- Sigma factor binds at sequence of DNA known as the promoter
- Promoter is located before or “upstream” from where transcription will begin
promoter
upstream
downstream
20-30 bases
SS DNA
site of initiation

8. Binding of Enzyme Cont. Promoter is always 20-30 bases upstream
Promoter is recognized by sigma factor by a unique sequence of bases – “TATA” called a TATA box
Transcription begins at the 1st A or G after base #20 from the promoter
ex.
TATA box
1st base transcribed
ACGTATATTGCGCGCTTTTTTTAGTCCCTA

9. Detailed Process Cont. B. Initiation
controlled by the core enzyme, the loosely bound enzyme binds more tightly as bases are added
The sigma factor falls off
C. Elongation
The core factor adds bases in the 5' 3' direction as the double helix unwinds
As elongation continues, promotion and initiation may begin again with a new enzyme
This mean multiple mRNAs may be made simultaneously

10. Cont.
D. Termination
Transcription continues until a sequence rich in A bases is reached
The RNA pol reacts with the A bases and falls off
E. Other considerations
Certain antibiotics work by stopping transcription
Rifamycin- prevents initiation
Actinomycin- prevents elongation

11. Diagram

12. III. Post-transcriptional Events
Prokaryotes
The mRNA made is polycistronic, containing the messages of several genes
These genes are then cut up
B. Eukaryotes
- The RNA produced is termed hnRNA- at this point it is mostly junk

13. Eukaryotes Cont.
Enzymes in the nucleus will degrade the junk to produce the sequences that will become mRNA
Although it was believed euk. mRNA had junk at both ends with coding regions in the middle, it instead was found that the genes are split into coding and non-coding regions
Non-coding regions (introns) are junk
Coding regions are exons

14. Eukaryotes Cont. Introns are spliced out in the following manner:
Sequences of complementary bases are found at each end –termed “splice junctions”
A small RNA-protein molecule (a “snurp”) binds to the splice junction, causing a looping out and removal of the intron
Enzyme called ribozymes then bind exons together
Snurps and ribozymes together make up the spliceosome complex

15. Diagram
exon
intron
snurp
ribozyme

16. Cont.
The piece of mRNA must now be processed further to protect it from enzymes in the cytoplasm
The following will be added to the mRNA:
1) Methyl Cap
-The first base is modified by adding methyl groups (-CH3) to the ribose sugar
-A molecule of GTP is added

17. Cont.
2) Poly-A Tail
-An enzyme will add 200 A bases to the 3' end of the molecule
Final Product
CH3
GTP 3'
AAA
mRNA 5'