1. How are proteins made. (Ch 17) How is Gene Expression Controlled
How are proteins made? (Ch 17) How is Gene Expression Controlled? (Ch 18)
Eukaryotes
Prokaryotes
Video Link Cold Spring
Central dogma molecular biology of gene link

2. Landmarks in Developing Model of Protein Synthesis
Beadle-Tatum - Gene codes for a protein
(1941)
Note: brown area indicates growth of
Bread mold

3. SYNTHESIS OF PROTEINS TAKES PLACE IN RIBOSOMES (made of RNA)
Figure Ribosomes
SYNTHESIS OF PROTEINS TAKES PLACE IN RIBOSOMES (made of RNA)

4. Central Dogma of Molecular Biology (Crick – mid 1950’s)
Flow of genetic information
DNA → RNA → Protein
Dogma – in religion, idea cannot be challenged
Crick’s defn: Idea for which no definitive experimental evidence exists
Note: mRNA not discovered until early 1960’s; first models incorrectly hypothesized that unique ribosomes of RNA were synthesized for each protein

5. RNA tie club
Very informal club of approximately 20 scientists (included Watson, Crick and CU atomic physicist George Gamow) working on protein synthesis problem; Purpose was to allow informal discussions and brainstorming; Crick predicted existence t-RNA before it was actually discovered and group predicted triplet codon. Confusion over concept of mRNA and different roles of RNA molecules took almost a decade to solve.

6. Table 17.1 Types of RNA in a Eukaryotic Cell

7. Extra Credit hints!
3 Types of RNA directly involved in protein synthesis
Ribosomal RNA (rRNA)
(85 % of RNA in cell)
– found in site of protein synthesis (ribsomes) associated with bound protein factors; composition identical within an organism;
t-RNA (10% of RNA in cell)– found in cytosol with a specific
amino acid attached; composition very similar within organism
mRNA (5%) – very short lived; composition varies tremendously within organism; complementary to gene (DNA sequence) being expressed

8. Figure Overview: the roles of transcription and translation in the flow of genetic information (Layer 1)

9. Figure Overview: the roles of transcription and translation in the flow of genetic information (Layer 2)

10. Figure Overview: the roles of transcription and translation in the flow of genetic information (Layer 3)

11. Figure Overview: the roles of transcription and translation in the flow of genetic information (Layer 4)

12. Link to Prokaryotes vs. Eukaryotes
Figure Overview: the roles of transcription and translation in the flow of genetic information (Layer 5)
Link to Prokaryotes vs. Eukaryotes

13. Prokaryotes vs. Eukaryotes Questions
1) What is the key difference between a prokaryotic cell vs. a eukaryotic cell?
ANS: Eukaryotic cells have nucleus, prokaryotes cells do not.
2) State two key differences in protein synthesis between prokaryotes and eukaryotes.
i) prokaryotes have no nucleus, therefore simultaneous transcription and translation.
ii) eukaryotes – transcription in nucleus; process mRNA (i.e., splicing, adding caps and tails) in nucleus; translation occurs separately in cytosol
III) Prokaryotic messages may be polycistronic (multiple) genes with related functions coded for on a single mRNA

14. Genetic Code Is Universal
In this picture a gene from a firefly is being expressed in a tobacco plant
The firefly’s DNA was read by the plant
Genetic code is essentially the same for all organisms

15. NOTE: PIG IS GLOWING FLUORESCING BLUE/GREEN
Fig. 17-6b
(b) Pig expressing a
jellyfish gene

16. Fig. 17-4
DNA template strand for mRNA is complementary to gene and is called “antisense”
Gene 2
DNA
molecule
Gene 1
Gene is located on “sense” strand (IB exam) mRNA thus has same sequence as gene. (IB exam)
Gene 3
DNA
template
strand
TRANSCRIPTION
mRNA
Codon
TRANSLATION
Protein
Amino acid

17. How many RNA bases code for 1 amino acid?
21 amino acids must be coded for
Link to triplet code
Genetic code is triplet code – 3 bases code for an amino acid (4 bases = 43 or 64 possible codons)
Codon – a group of 3 RNA bases that code for an amino acid

18. Figure 17.4 The dictionary of the genetic code
Genetic Code is Redundant but Unambiguous
Unambiguous- a particular codon only codes for 1 amino acid or start or stop
1 Start Codon (AUG)
3 Stop Codons
UAA UAG UGA
61 codons for amino acids
Redundant – often more than one codon for each amino acid
Redundant codons are often share first two bases but differ in third (“wobble” effect)

19. What protein sequence would be expected from the 3’ to 5’ DNA sequence TAC GCC GCA ACT?
mRNA:
AUG CGG CGU UGA
Protein:
Met (start) Arg Arg Stop =
Met-Arg-Arg