1. Gene Expression: From Gene to Protein
Chapter 17

2. YOU MUST KNOW
How RNA and DNA are similar and different, and how this defines their roles
The differences between replication, transcription, and translation, and the role of DNA and RNA in each process
How eukaryotic cells modify RNA after transcription
How genetic material is translated into polypeptides
How mutations can change the amino acid sequence of a protein and be able to predict how a mutation can result in changes in gene expression

3. Polymer of nucleotides
DNA vs. RNA
DNA
RNA
Polymer of nuleotides
Polymer of nucleotides
Deoxyribose sugar
Ribose sugar
Double stranded
Single stranded
Bases = A, T, G, C
Bases = A, U, G, C

5. Genes – sections of DNA that code for proteins
Transcription
Genes – sections of DNA that code for proteins
Only 1 strand is transcribed
Sense strand – NOT transcribed - carries same sequence as resulting mRNA (but with Ts instead of Us)
Anti-sense strand – Transcribed - complimentary to sense strand and to mRNA

6. 3 segments of the gene
1. Promoter – short sequence of bases that is not transcribed, but indicates where transcription should start
a. RNA Polymerase – unzips DNA and attaches at promoter, creating a transcription bubble
b. Usually indicated by TATA box

7. 2. Transcription unit – section of the DNA that codes for RNA a
2. Transcription unit – section of the DNA that codes for RNA a. Can code for mRNA for a polypeptide or rRNA or tRNA

8. 3. Terminator – sequence of nucleotides that cause the mRNA to detach from the DNA

9. 3 Stages of Transcription
Initiation
a. In prokaryotes, RNA polymerase simply binds to promoter
b. Eukaryotes – use RNA polymerase II, requires transcription factors (proteins to help it bind to promoter) – together they form the transcription initiation complex

10. a. Free-floating RNA nucleotides pair up with complimentary DNA
Elongation
a. Free-floating RNA nucleotides pair up with complimentary DNA
b. New nucleotides always add on to the 3’ end (build 5’ 3’)
c. RNA Polymerase bonds RNA nucleotides together

11. Termination
At termination sequence, RNA is released, polymerase detaches, sense and antisense strand rejoin

12. Post-transcription modification
1. Addition of 5’ cap and poly-A tail – protect mRNA from degradation by enzymes and help mRNA attach to ribosome

13. RNA splicing – removal of introns and fusing of exons by spliceosomes (complex of proteins and snurps) a. Snurps (small nuclear RNA/snRNA) – RNA that works like an enzyme to catalyze the reactions b. Alternative splicing means 1 gene can code for more than one polypeptide (20,000 genes, 100,000 proteins in humans)

14. Translation
Types of RNA involved
RNA Type
Function
mRNA – messenger
Complimentary to a gene, carries message (genetic code) from DNA in nucleus to ribosomes in cytoplasm
rRNA – ribosomal
Component of the ribosomes (along with proteins)
tRNA – transfer
Brings amino acids to the ribosome to build the polypeptide

15. Forms a t-shape because part of it are complimentary with itself
tRNA
Forms a t-shape because part of it are complimentary with itself
Sequence of anticodon determines which amino acid is attached

16. Structure of the ribosome
Consists of a large and a small subunit, each made of rRNA (2/3) and proteins (1/3)
rRNA constructed in nucleolus and exits nucleus through nuclear pores
mRNA will fit in between large and small subunits

17. 3 sites A – holds tRNA carrying next amino acid P – holds tRNA carrying polypeptide E – site where deactivated tRNA exits ribosome

18. The Genetic Code
Bases are read 3 at a time
DNA – triplets
mRNA – codons
tRNA - anticodons
Each codon codes for a specific amino acid, but there can be more than one codon for an amino acid
Universal – same in all species

19. Initiation
Small subunit attaches to mRNA and moves along it until it finds start codon (AUG)
Activated tRNA hydrogen bonds to codon – anti-codon is UAC, amino acid is methionine

20. Large subunit joins small subunit/mRNA to form translation initiation complex
Initiation factors (proteins that help the process ) join the complex – this requires GTP (energy!)

21. Elongation
tRNAs bring in amino acids 1 at a time
Elongation factors – proteins that assist tRNA binding at A site
Bond holding amino acid to tRNA in P site breaks and peptide bond forms between that amino acid and the amino acid on the tRNA in the A site

22. Translocation – tRNA in P site, now deactivated, moves to E site and tRNA in A site moves to P site
Deactivated tRNA leaves ribosome (goes to be reactivated)

23. Ribosome moves along mRNA in the 5’ 3’ direction, new activated tRNAs move into the A site, translocation happens repeatedly

24. Stop codon on mRNA moves into A site
Termination
Stop codon on mRNA moves into A site
Release factor – protein fills the A site – has no amino acid attached
Catalyzes hydrolysis between tRNA in the P site and its amino acid (which is holding the polypeptide chain)
Polypeptide is released
and ribosome
subunits break apart

25. Polypeptide to Protein
Polypeptides fold and associate with other molecules to gain secondary, tertiary, and quarternary structure
Signal peptide – sequence of about 20 amino acids and the leading end, helps direct proteins to their destination

26. Point Mutations – alteration of 1 nucleotide
Nucleotide – pair substitution – one nucleotide replaced with a different one
Silent mutations – mutation still codes for same amino acid
Missense mutation – mutation codes for a different amino acid
Nonsense mutation – change amino acid to a stop

27. Insertions and deletions – bases added or taken out
Unless 3 bases are added or deleted, it results in a frameshift mutation-shifts the reading frame for every codon after the mutation

28. Substances or forces that interact with DNA and cause mutations
Mutagens
Substances or forces that interact with DNA and cause mutations
X-rays, radiation, chemicals