An Analogy of Gene Expression
Transcribe (copy) a set of ingredients
from the cookbook to make a recipe
Translate the ingredients into a
dish
RNA- ribonucleic acid
Structural Differences with DNA: 1) Ribose sugar instead of Deoxyribose 2) Single Stranded instead of double 3) Uracil instead of Thymine (GCAU)
Uracil
Thymine
Functional Differences:
1) Deliver instructions/blueprints from DNA to ribosomes for protein-building
3 different types of RNA
1.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 construction2.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.
3.Ribosomal RNA (rRNA) -structural component of a ribosome, involved in
synthesis of ribosomes
• 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
• 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)
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
INITIATION
• 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)
ELONGATION
Template strand
Coding strand
• 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
TERMINATION
Posttranscriptional ModificationsBefore exiting the nucleus, the mRNA strand
needs to be modified:
• Capping - A 5’ cap is added– made of 7-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
• 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
• 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’
• 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)
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methionine glycine serine isoleucine glycine alanine stopcodon
proteinprotein
A U G G G C U C C A U C G G C G C A U A AmRNAmRNA
startcodon
Primary structure of a proteinPrimary structure of a protein
aa1 aa2 aa3 aa4 aa5 aa6
peptide bonds
codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1
41 = 4 amino acids42 = 16 amino acids
43 = 64 amino acids
Genetic Code
U C A G
U 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 = stopUGG = trp
U
C
A
G
C 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
U
C
A
G
A 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
U
C
A
G
G 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
U
C
A
G
Now that you have the mRNA (carrying the code), thecode needs to be translated (into a protein). Thisprocess 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
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amino acidamino acidattachment siteattachment site
U A C
anticodonanticodon
amino acidamino acidmethionine
• 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.
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mRNAmRNA
A U G C U A C U U C G
1-tRNA 2-tRNA
U A C G
aa1 aa2
A UA
anticodon
hydrogenbonds codon
peptide bond
3-tRNA
G A A
aa3
ElongationElongation
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mRNAmRNA
A U G C U A C U U C G
1-tRNA
2-tRNA
U A C
G
aa1
aa2
A UA
peptide bond
3-tRNA
G A A
aa3
Ribosomes move over one codon
(leaves)
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mRNAmRNA
A U G C U A C U U C G
2-tRNA
G
aa1aa2
A U
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
(leaves)
Ribosomes move over one codon
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mRNAmRNA
G C U A C U U C G
aa1aa2
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
U G A
5-tRNA
aa5
Ribosomes move over one codon
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mRNAmRNA
A C A U G U
aa1
aa2
U
primaryprimarystructurestructureof a proteinof a protein
aa3
200-tRNA
aa4
U A G
aa5
C U
aa200
aa199
terminatorterminator or stopor stop codoncodon
TerminationTermination