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Chemistry 20
Chapter 18
Gene Expression &
Protein Synthesis
Functions of DNA
1. It reproduces itself (Replication)
Chapter 17
2. It supplied the information to make up RNA, proteins, and enzymes.
Central dogma
Central dogma of molecular biology:
Information contained in DNA molecules is expressed (encoded) to the
production of proteins.
Gene expression: activation of a gene to produce a specific protein.
Gene expression
Transcription: synthesis of mRNA (messenger RNA)
Translation
DNA
replication
RNA
replication
DNA Transcription mRNA
Reverse
transcription
Revers
e transcriptase
Translation
protein
Transcription
Genetic information is copied from a gene in DNA to make a mRNA.
Begins when the section of a DNA that contains the gene to be copied unwinds.
Polymerase enzyme identifies a starting point to begin mRNA synthesis.
Transcription
C is paired with G, T pairs with A
But A pairs with U (not T).
Polymerase enzyme moves along the unwound DNA,
forming bonds between the bases.
RNA Polymerase
Section of bases on DNA:
Complementary base sequence in mRNA:
-G–A–A–C–T-C–U–U–G–A-
Translation
mRNA (as a carrier molecule) moves out of the nucleus and goes to ribosomes.
tRNA converts the information into amino acids.
Amino acids are placed in the proper sequence.
Proteins are synthesized.
Gene expression
Overall function of RAN’s in the cell: facilitate the task of synthesizing protein.
Genetic code
Genetic code: language that relates the series of nucletides in mRNA
to the amino acids specified.
• The sequence of nucleotides in the mRNA determines the amino
acid order for the protein.
• Every three bases (triplet) along the mRNA makes up a codon.
• Each codon specifies a particular amino acid.
• Codons are present for all 20 amino acids.
Genetic code
5'
U
C
U
UUU
UUC
UUA
UUG
CUU
CUC
CUA
CUG
AU U
AU C
A
AU A
AU G
GU U
G GU C
GU A
GU G
Phe
Phe
Leu
Leu
Leu
Leu
Leu
Leu
Ile
Ile
Ile
Met*
Val
Val
Val
Val
C
UCU
UCC
UCA
UCG
Ser
Ser
Ser
Ser
A
U AU
U AC
U AA
U AG
CAU
CAC
CAA
CAG
Tyr
Tyr
Stop
Stop
His
His
Gln
Gln
G
U GU
U GC
U GA
U GG
CGU
CGC
CGA
CGG
Cys
Cys
S top
Trp
Arg
Arg
Arg
Arg
CCU
CCC
CCA
CCG
Pro
Pro
Pro
Pro
ACU
ACC
ACA
ACG
GCU
GCC
GCA
GCG
Thr
Thr
Thr
Thr
Ala
Ala
Ala
Ala
AAU
AAC
AAA
AAG
GAU
GAC
GAA
GAG
As n
As n
Lys
Lys
A sp
A sp
Glu
Glu
A GU
A GC
A GA
A GG
GGU
GGC
GGA
GGG
Ser
Ser
Arg
Arg
Gly
Gly
Gly
Gly
3'
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
*AUG s ign als tran slation initiation as w ell as codin g for Met
Genetic code
• 64 condons are possible from the triplet combination of A, G, C, and U.
• UGA, UAA, and UAG, are stop signals.
(code for termination of protein synthesis).
• AUG has two roles:
1. Signals the start of the proteins synthesis (at the beginning of an mRNA).
2. Specifies the amino acid methionine (Met) (in the middle of an mRNA).
tRNA (transfer RNA)
tRNA translates the codons into specific amino acids.
Serine
Anticodon loop
A G U
Codon on mRNA
U C A
Transcription
Translation
Protein synthesis
• mRNA attaches to smaller subunit of a ribosome.
• tRNA molecules bring amino acids to the mRNA.
• Peptide bonds form between an amino acid and the end of the
growing peptide chain.
• The ribosome moves along mRNA until the end of the codon
(translocation).
• The polypeptide chain is released from the ribosome and
becomes an active protein.
Sometimes several ribosomes (polysome) translate the same strand of mRNA
at the same time to produce several peptide chains.
Termination
5'
U
C
A
G
3'
C
A
G
3'
UCU
Ser
U GU Cys
UUU
Phe
U AU Tyr
U
Ribosome
encounters
a
stop
condon.
UCU Ser
Phe
U AU
UCCCys
Ser U U AC Tyr
U GC Cys
UUCTyr
Phe U GU
C
UCC
Ser
U
GC
Phe
UCACys
Ser C U AA Stop
UUATyr
Leu
U U AC
U GA S top A
UCA Ser
U AA
Stop
Leu
Ser A U AG Stop
UCGS top
UUG
Leu U GA
U GG Trp
G
UCG Ser
U AG Stop
Leu
U GG Trp
G
CUU Leu
CCU Pro
CAU His
CGU Arg
U
His
Leu
CCU Pro
CAU
CUC Leu CGU
CCCArg
Pro U CAC His
CGC Arg
C
No
tRNA
to
complement
the
termination
codon.
HisLeu CGC
Leu
CCC Pro C CAC
CUA
CCAArg
Pro C CAA Gln
CGA Arg
A
GlnLeu CGA
Leu
CCA Pro
CAA
CUG
CCGArg
Pro A CAG Gln
CGG Arg
G
Leu
CCG Pro
CAG Gln
CGG Arg
G
AU U Ile
AAU As n
U
ACU Thr
A GU Ser
An
enzyme
releases
the
complete
polypeptide
chain
from
the
ribosome.
AsIle
n
Ile
AAU
Ser
U AAC As n
ACU Thr
A GU
C
AU
C
ACC
Thr
A GC Ser
A
AsIle
n
Ile
ACC Thr
AAC
A GC
A
AU A
ACASer
Thr C AAA Lys
A GA Arg
Ile
ACA Thr
AAA
LysMet*A GA
Arg
G
AU G
ACG
Thr A AAG Lys
A GG Arg
G
Met* ACG Thr
AAG Lys
A GG Arg
GU U Val
GCU Ala
GAU A sp
GGU Gly
U
Val
GCU Ala
GAU
A sp
GGU
Gly
Val
Ala Ustructure
sp
Gly
GU
C
GCC
GAC A(active
GGC
C
Amino acids
form
the
three-dimensional
protein).
G
Val
Ala
A
sp
Gly
GCC
GAC
GU A Val GGC
GCA Ala C GAA Glu
GGA Gly
A
Val
GCA Ala
GluVal GGA
Gly
GAA
GU G
GCG
Ala A GAG Glu
GGG Gly
G
Val
GCG Ala
GGG Gly
G
GAG Glu
*AUG s ign als tran slation initiation as w ell as codin g for Met
ign als tran slation initiation as w ell as codin g for Met
Mutation
A heritable change in DNA nucleotide sequence.
It changes the sequence of amino acids (structure and function of proteins).
Enzyme cannot catalyze.
X rays, Overexpose to sun (UV light), Chemicals (mutagens), or Viruses
Effect of Mutation
Somatic cell (nonreproductive cell):
Altered DNA will be limited to that cell and its daughter cells.
Cancer
Germ cell (reproductive cell like egg or sperm):
All new DNA will contain the same default.
Genetic diseases
Type of Mutations
Substitution Mutation
Most common
Replacement of one base in the coding strand of DNA with another.
Different amino acid
Frameshift Mutation
A base is added to or deleted from the normal order of bases in DNA.
All the triplets shift over by one base.
Different sequence of amino acids