1. How Proteins are Produced
Protein Synthesis
How Proteins are Produced

2. An Analogy of Gene Expression
Transcribe (copy) a set of ingredients from the cookbook to make a recipe
Translate the ingredients into a dish

3. The Key Players….
DNA
RNA
Ribosomes
Amino acids

4. RNA- ribonucleic acid 2) Single Stranded instead of double
Structural Differences with DNA:
1) Ribose sugar instead of Deoxyribose
2) Single Stranded instead of double
3) Uracil instead of Thymine (GCAU)
Uracil
Thymine

5. Functional Differences:
1) Deliver instructions/blueprints from
DNA to ribosomes for protein-building

6. 3 different types of RNA - acts as a template for protein construction
Messenger RNA (mRNA)
- made from a DNA template. The sequence of bases in mRNA determines the sequence of amino acids in a protein. It carries the protein-building instructions out of the nucleus to the ribosomes
- acts as a template for protein construction
Transfer RNA (tRNA)
- translates mRNA code by attaching to the specific amino acid that is indicated in the code and bringing the amino acid to the ribosome to help build the required protein.
Ribosomal RNA (rRNA)
-structural component of a ribosome, involved in synthesis of ribosomes

7. mRNA

8. protein synthesis begins in the nucleus with the production of mRNA (Transcription)
mRNA code is translated into polypeptide (Translation)
DNA  RNA  protein
transcription translation
known as the CENTRAL DOGMA of MOLECULAR GENETICS

9. Part One: Transcription
process of converting DNA to messenger RNA
mRNA is made inside the nucleus and has the job of carrying the instructions from the DNA out into the cytoplasm. DNA never leaves the nucleus!
divided into three sequential processes: initiation, elongation and termination (Fig.2 page 243)

11. INITIATION How is mRNA made?
RNA polymerase binds to the DNA molecule in a region upstream of the gene to be transcribed – called promoter region
This region is rich in A’s
and T’s
A and T help together by two H-bonds – easier to break

12. ELONGATION
To build the chain of mRNA ( from the 5’  3’ end), RNA polymerase adds free-floating nucleotides that are complimentary to the strand of DNA that is being transcribed (template strand)
Coding strand
Nucleotides found in nucleoplasm
Template strand

13. TERMINATION
RNA polymerase recognizes the end of the gene when it reaches the terminator sequence
The newly synthesized mRNA detaches from the template DNA, and DNA zips back up.
mRNA takes the coded message to the ribosomes in the cytoplasm or attached to the ER

15. Posttranscriptional Modifications
Before exiting the nucleus, the mRNA strand
needs to be modified:
Capping - A 5’ cap is added– made of methyl guanosine – protects the mRNA from digestion by enzymes & helps in the initiation of translation.
Tailing - A poly-A tail (string of ~ 200 adenines) is added to the 3’ end, protects the strand from degradation

16. Removal of introns and joining of exons
Introns (noncoding regions) must be removed by splicesomes and the remaining exons (coding regions) must be joined
mRNA leaves nucleus, spliced-out introns stay and get degraded
- mRNA transcript is now ready to leave the nucleus
Splicesomes particles made of RNA and protein- cut out introns

17. How can we have over 120 000 proteins, and only about 25 000 genes?

19. DNA: 3’ T G G C A T G 5’ Question mRNA: 5’ A C C G U A C 3’
What would be the mRNA strand for the following DNA sequence?
DNA: 3’ T G G C A T G 5’
mRNA: 5’ A C C G U A C 3’

20. So far:
The order of bases in the DNA specifies the
order of bases in the mRNA
Now,
The order of bases in the mRNA specifies
the order of amino acids in the protein
A sequence of 3 mRNA bases is called a codon (codes for an amino acid)

21. Messenger RNA (mRNA) Primary structure of a protein
43 = 64 amino acids
41 = 4 amino acids
42 = 16 amino acids A U G C
mRNA
start
codon
codon 2
codon 3
codon 4
codon 5
codon 6
codon 7
codon 1
methionine
glycine
serine
isoleucine
alanine
stop
codon
protein
Primary structure of a protein
aa1
aa2
aa3
aa4
aa5
aa6
peptide bonds

22. Genetic Code U C A G UUU =phe UUC =phe UUA = leu UUG =leu UCU = ser
UCC = ser
UCA -= ser
UCG = ser
UAU = tyr
UAC = tyr
UAA = stop
UAG = stop
UGU = cys
UGC = cys
UGA = stop
UGG = trp
CUU = leu
CUC = leu
CUA = leu
CUG = leu
CCU = pro
CCC = pro
CCA = pro
CCG = pro
CAU = his
CAC = his
CAA = gln
CAG = gln
CGU = arg
CGC = arg
CGA = arg
CGG = arg
AUU = ile
AUC = ile
AUA = ile
AUG= met & start
ACU = thr
ACC = thr
ACA = thr
ACG = thr
AAU = asn
AAC = asn
AAA = lys
AAG = lys
AGU = ser
AGC = ser
AGA = arg
AGG = arg
GUU = val
GUC = val
GUA = val
GUG = val
GCU = ala
GCC = ala
GCA = ala
GCG = ala
GAU = asp
GAC = asp
GAA = glu
GAG = glu
GGU = gly
GGC = gly
GGA = gly
GGG = gly

23. Now that you have the mRNA (carrying the code), the
code needs to be translated (into a protein). This
process is called TRANSLATION
Uses the mRNA’s code to build proteins
Occurs in the cytoplasm on ribosomes
Need tRNA to transport amino acids to the ribosome
tRNA have anticodons that are complimentary to the mRNA codons
Bonds to corresponding mRNA sites on ribosome
**Note – the amino acid picked up matches the mRNA codon, not the anticodon

25. A closer look at ribosomes:

26. Transfer RNA (tRNA) U A C amino acid attachment site methionine
anticodon
amino acid
methionine

27. Translation – 3 Steps
Initiation – ribosome binds to mRNA. Start codon read (AUG). Special initiator tRNA brings in first amino acid- enters P-site
Elongation – another tRNA molecule with anticodon complementary to next mRNA codon enters A-site. Peptide bonds forms between adjacent amino acids. Initiator tRNA exits ribosome. Ribosome moves along the mRNA – one codon. tRNA that was previously in the A-site moves into the P-site (carrying it’s growing peptide chain), opening up the A-site for another tRNA. Process continues until stop codon is reached
Termination - stop codon reached. No tRNA for these codons. Release factor protein binds to stop codons causing polypeptide to be released from the ribosome. Ribosome disassembles.

29. Initiation G aa2 A U U A C aa1 A U G C U A C U U C G A codon hydrogen
2-tRNA G
aa2 A U
1-tRNA U A C
aa1
anticodon A U G C U A C U U C G A
hydrogen
bonds
codon
mRNA

30. Elongation G A aa3 peptide bond aa1 aa2 U A C G A U A U G C U A C U U
3-tRNA G A
aa3
peptide bond
aa1
aa2
1-tRNA
2-tRNA
anticodon U A C G A U A U G C U A C U U C G A
hydrogen
bonds
codon
mRNA

31. Ribosomes move over one codon
aa1
peptide bond
3-tRNA G A
aa3
aa2
1-tRNA U A C
(leaves)
2-tRNA G A U A U G C U A C U U C G A
mRNA
Ribosomes move over one codon

32. Ribosomes move over one codon
peptide bonds
4-tRNA G C U
aa4
aa1
aa2
aa3
2-tRNA G A U
(leaves)
3-tRNA G A A A U G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon

33. Ribosomes move over one codon
peptide bonds U G A
5-tRNA
aa5
aa1
aa2
aa3
aa4
3-tRNA G A A
4-tRNA G C U G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon

34. Termination aa5 aa4 aa3 primary structure of a protein aa2 aa1 A C U C
terminator
or stop
codon
200-tRNA A C U C A U G U U U A G
mRNA

36. Protein synthesis animation: