1. Gene Expression : Transcription and Translation

2. How Are Different Types of Cells Created and Maintained?
By differential gene expression.
The same genetic information is in all 100 trillion cells of any one person. Different cells use the same blueprint in different ways.
How?
In essence, the control of gene expression occurs by regulating the flow of information from DNA to protein.

3. The “Central Dogma” of Molecular Genetics
Transcription
Translation
DNA
RNA
Protein
Trait
RNA processing

4. Review DNA RNA Proteins Made up of 4 different nucleotides
Made up of 20 different amino acids

5. Gene Expression in Prokaryotes vs. Eukaryotes
Transcription and translation are separated in time and space
Transcription and translation may occur simultaneously

6. One Gene - One Polypeptide Hypothesis
Theory: one gene codes for one polypeptide
Some proteins are composed of a number of polypeptide chains. In this theory each chain has its own gene.
However, eukaryotic genes are much more complex and this is not always the case!
Some genes control the expression of other genes
Some genes code for RNA which do not produce polypeptides

7. The Genetic Code
Problem: How do only 4 different nucleotides code for the 20 amino acids that make up proteins?
Solution: Each group of 3 nucleotides codes for a different amino acid. These 3 nuclotide units are called codons. 1 codon codes for 1 amino acid.
DNA
RNA
Amino Acid C G
Valine A U T A

8. Reading Frame
The 3-nucleotide units (codons) must be read in the correct reading frame
Start codons determine the reading frame

9. The Genetic Code Dictionary
There are multiple codons for each amino acid
AUG is always the start codon
UAA, UGA, and UAG are stop codons

10. Characteristics of the Genetic Code
The genetic code is degenerate
More than one codon codes for an amino acid
The genetic code is universal (almost)
Almost all organisms on Earth use the same code.

11. Transcription
DNA is used to make a strand of RNA called the primary transcript (pre-mRNA)
The pre-mRNA is further processed to create the finished mRNA
mRNA exits the nucleus to be translated

12. DNA has two strands
1) Anti-sense strand (Template strand) (Runs 3’ 5’) – the strand of DNA that is transcribed. Has the complementary genetic code of the mRNA. (Runs 3’ 5’)
 2) Sense strand (coding strand) (runs 5’  3’) – the strand of DNA that is not transcribed. It has the same genetic code as the mRNA (except U instead of T).

13. Transcription
3 main steps
1. Initiation
2. Elongation
3. Termination

14. Transcription Initiation
RNA polymerase binds to DNA at a region called the promoter
RNA polymerase unwinds the DNA and adds nucleotides in the 5’ → 3’ direction

15. Transcription Elongation
RNA polymerase moves along the DNA strand, adding 60 nucleotides/sec
DNA strands rejoin after polymerase passes by

16. Transcription Termination
Polymerase stops when it reaches a DNA sequence called the terminator
The mRNA has been completely transcribed
In eukaryotes, this is pre-mRNA and must be further processed

17. mRNA Processing
In eukaryotes, pre-mRNA must be further processed to mRNA before it leaves the nucleus
Guanine is added to 5’ end, forming the 5’ cap
100’s of adenines are added to 3’ end, forming the poly-A tail
Non-coding regions of RNA are spliced out

18. Intron (non-coding sequences) are cut out by spliceosomes
Intron (non-coding sequences) are cut out by spliceosomes. Leaving only Exons (Coding sequences) making up the mRNA that leaves the nucleus.
Alternative splicing patterns means one gene can make more than one protein

19. mRNA Splicing

20. Translation The process in which mRNA is used to make proteins
Occurs in the cytoplasm using ribosomes
Requires tRNA (transfer RNA) bound to an amino acid
3 steps: initiation, elongation, termination

21. Structure of tRNA A clover-shaped RNA molecule
Bottom loop has an anti-codon complementary to the mRNA codon
3’ end has an aa attachment site with the sequence “ACC” (CCA read from 5’ 3’)

22. Amino acid attachment RNA is made in the nucleus
Amino acids float free in the cytoplasm
Aminoacyl-tRNA synthase joins each amino acid to the appropriate tRNA

23. Ribosomes 2 subunits Composed of proteins and rRNA
3 tRNA binding sites

24. Translation: Initiation
mRNA, tRNA and small ribosomal subunit bind with the P site at the start codon (AUG = Met)
Large subunit binds using energy from GTP

25. Translation: Elongation
mRNA is read 3 nucleotides at a time (Codons)
tRNA brings corresponding amino acid into the A site of the ribosome

26. Transition Elongation
Ribosome catalyses dehydration synthesis reaction between aa’s in P site and A site forming a peptide bond between aa’s
Growing polypetpide now attached to tRNA in A site
Ribosome moves forward one codon
Free tRNA in P site exits out the back of ribosome (out of E site)
tRNA (with polypeptide) moves into P site

29. Translation: Termination
Elongation continues until reaching a stop codon
Release factor binds and hydrolyzes the bond between the last tRNA and its a.a., freeing the new polypeptide chain

30. Polyribosomes
Many ribosomes may simultaneously translate from a single mRNA

31. Gene Expression: Overview