1. Chapter 17 AP Biology
From Gene to Protein

2. Central Dogma of Molecular Biology
DNA  RNA Protein

3. What is RNA? Contains the bases A, C, G, and U instead of T
single-stranded (often folds onto itself)
Three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA)

5. Protein Synthesis DNA  RNA  Protein
1. Transcription - DNA message is transcribed into mRNA and sent to ribosomes.
2. Translation - mRNA is translated into a Protein by a ribosome.
DNA  RNA  Protein

6. Overview – Eukaryotic Cell
Notice that transcription and translation occur in different places in a eukaryotic cell.

7. Overview – Prokaryotic Cell

8. The Triplet Code
Note that the bases of mRNA are complementary to the template strand of DNA
The bases of mRNA are read in groups of three – each group is called a codon

9. From mRNA to Amino Acids
The mRNA base triplets are called codons
mRNA is written in the 5’ to 3’ direction
The codons code for each of the 20 amino acids
The genetic code is redundant
More than one codon codes for each of the 20 amino acids

10. The Genetic Code
The three bases of an mRNA codon code for the 20 amino acids that are the subunits of proteins
The amino acids are designated by three letters

11. Universal Nature of the Genetic Code

12. Transcription Promoter
The DNA sequence at which RNA polymerase attaches is called the promoter
RNA polymerase adds RNA nucleotides to a growing RNA strand in the 5’  3’ direction

13. Transcription Unit
The entire stretch of DNA that is transcribed into an RNA molecule
A transcription unit may code for a polypeptide or an RNA (like tRNA or rRNA)
RNA Polymerase II makes mRNA

14. Terminator
The DNA sequence that signals the end of transcription is called the terminator

15. Stages of Transcription
1. Initiation
2. Elongation
3. Termination

16. Initiation of Transcription at a Eukaryotic Promoter

17. RNA Processing In eukaryotes, transcription results in pre-mRNA
Pre-mRNA undergoes processing to yield final mRNA which leaves the nucleus and goes to the ribosomes
In prokaryotes transcription results in mRNA
Processing of mRNA does not occur in prokaryotes
Transcription and translation can occur simultaneously

18. Processing of Pre-mRNA
Addition of a 5’ cap
A modified form of a guanine nucleotide
Addition of a poly-A tail
adenine (A) nucleotides are added
This is referred to as the poly-A tail
RNA splicing
Editing of the initial strand of mRNA (a cut and paste job)

19. Addition of 5’ cap and poly-A tail

20. 5’ cap and poly-A tail Both facilitate export of mRNA from the nucleus
Both help protect mRNA from degradation by enzymes
Both facilitate the attachment of mRNA to the ribosome

21. RNA Splicing Only takes place in eukaryotic cells
Large sections are spliced out; these are called introns
The sections that remain are called exons
“exons EXIT the nucleus”
These exons are spliced together by a spliceosome to form the mRNA that leaves the nucleus and travels to the ribosomes

22. RNA Splicing

23. snRNPs and Spliceosomes
Small nuclear RNAs (snRNA) and
small nuclear ribonucleoproteins (snRNPs) serve a catalytic function in the spliceosome
When RNA acts as a catalyst, it is called a ribozyme

24. Alternative RNA Splicing
Different regulatory proteins in different cells splice the pre-mRNA in different ways
This is called alternative gene splicing
This allows for different combinations of exons
This results in more than one polypeptide per gene
This explains why we have fewer genes in our genome than what was expected

25. Transcription Animation

26. 17.4 Translation
tRNA functions in transferring amino acids from the cytoplasm to a ribosome
rRNA complexes with proteins to form the two subunits that make up ribosomes

27. tRNA Each type of tRNA is specific for a particular amino acid
At one end tRNA loosely binds the amino acid and at the other end it has a nucleotide triplet called the anticodon
The anticodon allows it to pair specifically with a complementary codon on mRNA

28. Structure of tRNA

29. Basic Concept of Translation
Codons are the triplet nucleotides on mRNA
Anticodons are the triplet nucleotides on tRNA

30. Wobble
If one tRNA variety existed for each codon, there would need to be 61 tRNAs, there are only about 45, some can bind to more than codon
The rule for base pairing is not as strict between the third base of a codon and an anticodon, this relaxation of base-pairing rules is called wobble
Ex: a tRNA with the anticodon 3’-UCU-5’ can base pair with either the mRNA codon 5’-AGA-3’ or 5’-AGG-3’, both code for Arg

31. Three Stages of Translation
Initiation
Begins with the start codon AUG (always!)
Elongation
Codon recognition
Peptide bond formation (between 2 a.a.)
Termination
A stop codon is reached and translation stops

32. Initiation

33. Elongation

34. Termination

35. Translation Animation

36. Folding of the Polypeptide
Following release from the ribosome, the polypeptide then folds to its specific conformation
Chaperonins are the proteins that help with this folding process
The first 20 amino acids of the polypeptide serve as a signal peptide and act as a cellular zip code, directing the polypeptide to its final destination

37. Targeting Proteins to the ER

38. 17.5 Point Mutations
Mutations are alterations in the genetic material of the cell caused by mutagens
Point mutations are alterations of just 1 base pair in a gene
Base-pair substitution
Silent mutations – have no effect on the encoded protein
Missense mutations – change one amino acid to another; might still code for the correct amino acid
Nonsense mutations – change a regular amino acid codon into a stop codon
Insertions & deletions
Frameshift mutation – mRNA is read incorrectly

39. A Point Mutation Disorder

40. Base-Pair Substitution - Silent

41. Base-Pair Substitution - Missense

42. Base-Pair Substitution - Nonsense

43. Frameshift Mutation- Insertion

44. Frameshift Mutation - Deletion