1. From Gene to Protein
Chapter 17 - Campbell

2. What do genes code for? How does DNA code for cells & bodies?
how are cells and bodies made from the instructions in DNA
All the traits
of the body
DNA
proteins

3. transcription and translation
The “Central Dogma”
Flow of genetic information in a cell
How do we move information from DNA to proteins?
transcription
translation
DNA
RNA
protein
trait
To get from the chemical language of DNA to the chemical language of proteins requires 2 major stages:
transcription and translation
replication

4. DNA RNA RNA Monomers = nucleotides Phosphate Ribose sugar
Nitrogen Bases
uracil instead of thymine
U bonds with A
C bonds with G
single stranded
transcription
DNA
RNA

5. Allowed to travel from nucleus to cytoplasm
Compare DNA and RNA
DNA
RNA
Shape
Double helix
2 strands
Single strand
Sugar
Deoxyribose
Ribose
Bases
A, T, C, and G
A, U, C and G
Location
Only in the nucleus
Allowed to travel from nucleus to cytoplasm

6. Types of RNA Ribosomal RNA (rRNA) Transfer RNA (tRNA)
Major component of ribosomes
Transfer RNA (tRNA)
Folded upon itself
Carries the amino acids to the mRNA
Messenger RNA (mRNA)
Sequence of nucleotides that determines the primary sequence of the polypeptide
Made in the nucleus from the DNA: transcription
snRNA (small-nuclear “snurps”)
Forms the “spliceosomes” which are used to cut out introns from pre-mRNA
siRNA (small-interfering)
targets specific mRNA and prohibits it from being expressed

7. Protein Synthesis: From gene to proteinaa a
nucleus
cytoplasm
transcription
translation
DNA
mRNA
protein
ribosome
trait

8. Which gene is read on the DNA?
Promoter region
binding site before beginning of gene
Generally referred to as a TATA box because it is a repeating sequence of T and A
binding site for RNA polymerase & transcription factors
Enhancer region
binding site far upstream of gene
Speeds up process

9. Transcription Factors
transcription factors bind to promoter region of DNA
proteins
can be activated by hormones (cell signaling)
turn on or off transcription
triggers the binding of RNA polymerase to DNA

10. Transcription: DNA to mRNA
Takes place in the nucleus
A section of DNA is unzipped
RNA polymerase lays down nucleotides 5’ to 3’ direction.
The mRNA then leaves the nucleus through the nuclear pores and enters the cytoplasm

11. Coding strand = this is the protein needed or “sense strand”
Template strand = this is the “anti-sense strand”

12. Eukaryotic genes have untranscribed regions!
mRNA must be modified before it leaves the nucleus
exons = the real gene
expressed / coding DNA
introns = non-coded section
in-between sequence
Spliceosomes cut out introns with ribozyme
introns come out!
intron = noncoding (inbetween) sequence
eukaryotic DNA
exon = coding (expressed) sequence

13. Starting to get hard to define a gene!
Alternative splicing
Same piece of DNA can be read many different
Not all the exons may make it to the final product
Intron presence can determine which exons stay or go
Increases efficiency and flexibility of cell
snRNA’s have big role in alternative splicing
Starting to get hard to define a gene!

14. Final mRNA processing…
Need to protect mRNA on its trip from nucleus to cytoplasm (enzymes in cytoplasm attack mRNA)
protect the ends of the molecule
add 5 GTP cap
add poly-A tail
longer tail, mRNA lasts longer
eukaryotic RNA is about 10% of eukaryotic gene. A
3' poly-A tail
mRNA 5'
5' cap 3' G P
A’s

15. The Transcriptional unit
enhancer
1000+b
translation
start
translation
stop
exons
20-30b
transcriptional unit (gene)
RNA
polymerase 3'
TAC
ACT 5'
TATA
DNA
transcription
start
introns
transcription
stop
promoter
DNA
pre-mRNA 5' 3'
mature mRNA 5' 3'
GTP
AAAAAAAA

16. Genetic Code
Genetic code is based on sets of 3 nucleotides …called CODONS!
Read from the mRNA
64 different possible combinations exist
Only 20 amino acids commonly exist in the human body
Some codons code for the same amino acids (degenerate or redundant)
Sequence of codons determines the sequence of the polypeptide
Code is “almost” universal…same for all organisms (evolutionary heritage)

17. The Code You don’t need to memorize the codons (except for AUG)
Start codon
AUG
methionine
Stop codons
UGA, UAA, UAG
Strong evidence for a single origin in evolutionary theory.

18. mRNA codes for proteins in triplets
TACGCACATTTACGTACGCGG
DNA
codon
AUGCGUGUAAAUGCAUGCGCC
mRNA ?
MetArgValAsnAlaCysAla
protein

19. How is the code “translated?”
Process of reading mRNA and creating a protein chain from the code.

20. Ribosomes: Site of Protein Synthesis
Facilitate coupling of tRNA anticodon to mRNA codon
Structure
ribosomal RNA (rRNA) & proteins
2 subunits
large
small E P A

21. Ribosomes: 3 binding sites
A site (aminoacyl-tRNA site)
holds tRNA carrying next amino acid to be added to chain
P site (peptidyl-tRNA site)
holds tRNA carrying growing polypeptide chain
E site (exit site)
Empty tRNA leaves ribosome from exit site
Met 5' U A C A U G

22. Transfer RNA Found in cytoplasm Carries amino acids to ribosome
Contains an “anticodon” of nitrogen bases
Anticodons use complementary bond with codons
Less tRNA’s than codons, so one tRNA may bind with more than one codon.
Supports the degenerate code
“Wobble” hypothesis: anticodon with U in third position can bind to A or G

23. Translation: mRNA to Protein
In the cytoplasm ribosomes attach to the mRNA
Ribosome covers 3 codons at a time
Initiation - The tRNA carrying an amino acid comes into P-site and bonds by base pairing its anti-codon with the mRNA start codon (what is the start codon?)
Elongation – The second tRNA then comes into A-site and bonds to codon of mRNA
The two amino acids joined with peptide bond
Termination – ribosome continues reading mRNA until a STOP codon is reached (doesn’t code for anything)
McGraw Hill Animations

24. Building a polypeptide1 2 3
Initiation
mRNA, ribosome subunits, initiator tRNA come together
Elongation
adding amino acids based on codons
Termination
STOP codon = Release factor
Good Overview animation
Leu
Val
release
factor
Ser
Met
Met
Met
Met
Leu
Leu
Leu
Ala
Trp
tRNA C A G C U A C 5' U A C G A C U A C G A C G A C 5' A 5' U A A U G C U G A U A U G C U G A A U A U G C U G A A U 5' A A U
mRNA A U G C U G 3' 3' 3' 3' A C C U G G U A A E P A 3'

25. Can you tell the story? exon intron 5' GTP cap poly-A tail
RNA polymerase
DNA
Can you tell the story?
amino acids
exon
intron
tRNA
pre-mRNA
5' GTP cap
mature mRNA
poly-A tail
large ribosomal subunit 3'
polypeptide 5'
tRNA
small ribosomal subunit E P A
ribosome

26. Prokaryote vs. Eukaryote Differences
Prokaryotes
DNA in cytoplasm
circular chromosome
naked DNA
no introns
No splicing
Promoter & terminator sequence
Smaller ribosomes
Eukaryotes
DNA in nucleus
linear chromosomes
DNA wound on histone proteins
introns and exons
TATA box promoter
Transcription factors present
Walter Gilbert hypothesis:
Maybe exons are functional units and introns make it easier for them to recombine, so as to produce new proteins with new properties through new combinations of domains.
Introns give a large area for cutting genes and joining together the pieces without damaging the coding region of the gene…. patching genes together does not have to be so precise.

27. Protein Synthesis in Prokaryotes
Transcription & translation are simultaneous in bacteria
Both occur in cytoplasm
no mRNA editing
ribosomes read mRNA as it is being transcribed