Human Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 21
DNA Biology
and Technology
Lecture Outline
Part 2
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1
21.1 DNA and RNA Structure and Function
Comparing DNA and RNA
• Similarities
– They are nucleic
acids.
– They are made of
___________.
– The have sugarphosphate
backbones.
– They are found in
the _________.
• Differences
– DNA is _________
________ while RNA
is ______________.
– DNA has T while
RNA has U.
– RNA is also found in
the ___________ as
well as the nucleus
while DNA is not.
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21.2 Gene Expression
Proteins: A review
•
Proteins are composed of subunits called _______
_______
•
The sequence of amino acids determines the
shape of the protein.
•
They are synthesized at the ____________.
•
Proteins are important for diverse functions in the
body including hormones, enzymes, and transport.
•
They can _________, causing a loss of function.
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21.2 Gene Expression
2 steps of gene expression
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nontemplate strand
1.
2.
____________ –
DNA is read to
make a mRNA in
the nucleus
____________ –
mRNA is read to
make a protein in
the cytoplasm
5′
DNA
3′
A
G
C
G
A
C
C
C
C
T
C
G
C
T
G
G
G
G
3′
5′
template strand
transcription
innucleus
5′
mRNA
translation
at ribosome
3′
A
G
C
codon 1
G
A
codon 2
O
polypeptide
C
C
C
codon 3
O
O
N C C N C C N C C
R1
Figure 21.5 Summary of gene expression.
C
R2
R3
Serine Aspartic acid Proline
4
21.2 Gene Expression
The genetic code
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•
•
•
It is made of 4 kinds
of bases.
Bases act as a code
for __________ used
in translation.
Every 3 bases of the
mRNA is called a
________; a typical
codon specifies a
particular _________
in translation.
First
Base
U
C
A
G
Figure 21.6 The genetic code.
Second Base
Third
Base
U
C
A
G
UUU
phenylalanine
UCU
serine
UAU
tyrosine
UGU
cysteine
U
UUC
phenylalanine
UCC
serine
UAC
tyrosine
UGC
cysteine
C
UUA
leucine
UCA
serine
UAA
stop
UGA
stop
A
UUG
leucine
UCG
serine
UAG
stop
UGG
tryptophan
G
CUU
leucine
CCU
proline
CAU
histidine
CGU
arginine
U
CUC
leucine
CCC
proline
CAC
histidine
CGC
arginine
C
CUA
leucine
CCA
proline
CAA
glutamine
CGA
arginine
A
CUG
leucine
CCG
proline
CAG
glutamine
CGG
arginine
G
AUU
isoleucine
AUC
isoleucine
ACU
threonine
ACC
threonine
AAU
asparagine
AAC
asparagine
AGU
serine
AGC
serine
U
AUA
isoleucine
AUG (start)
methionine
ACA
threonine
AAA
lysine
AGA
arginine
A
ACG
threonine
AAG
lysine
AGG
arginine
G
GUU
valine
GCU
alanine
GAU
aspartic acid
GGU
glycine
U
GUC
valine
GCC
alanine
GAC
aspartic acid
GGC
glycine
C
GUA
valine
GCA
alanine
GAA
glutamic acid
GGA
glycine
A
GUG
valine
GCG
alanine
GAG
glutamic acid
GGG
glycine
G
C
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21.2 Gene Expression
1. Transcription
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5′
3′
•
mRNA is made from
a DNA template.
C
A
A
•
template
strand
mRNA is _________
before leaving the
nucleus.
3′
A
C
T
G
C
•
DNA
template
strand
mRNA moves to the
___________ to be
read.
mRNA
transcript
C
A
T
A
A
Figure 21.7 Transcription of DNA into mRNA.
C
direction of
polymerase
movement
T
RNA
polymerase
5′
T
T
5′
3′
to RNA processing
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21.2 Gene Expression
Processing of mRNA after transcription
Modifications of mRNA
•
One end of the RNA is capped.
•
_________ are removed.
•
A ___________ is added.
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21.2 Gene Expression
Processing of mRNA after transcription
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exon
exon
exon
DNA
intron
intron
transcription
exon
exon
exon
pre-mRNA
5′
intron
exon
3′
intron
exon
exon
5′
3′
intron
cap
intron
poly-A tail
spliceosome
exon
exon
exon
5′
3′
poly-A tail
cap
pre-mRNA
splicing
intron RNA
mRNA
5′
3′
cap
poly-Atail
nuclear pore
in nuclear envelope
nucleus
cytoplasm
Figure 21.8 mRNA processing.
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21.2 Gene Expression
2. Translation
3 steps
1. Initiation: mRNA binds to the small ribosomal
subunit and causes the 2 ribosomal units to
associate
2. ___________: polypeptide lengthens
• tRNA picks up an amino acid.
• tRNA has an anticodon that is
complementary to the codon on the mRNA.
• tRNA anticodon binds to the codon and
drops off an amino acid to the growing
polypeptide.
3. Termination: a ______________ on the mRNA
causes the ribosome to fall off the mRNA
9
21.2 Gene Expression
Visualizing the 3 steps of translation
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met
polypeptide bond
ser
thr
met
ala
amino acid
ser
tryp
polypeptide
ala
val
tryp
asp
val
asp
C U G
G A C A C C
U A
C A U C U G
G U A G A C
2b. Second stage of elongation. The
ribosome has moved to the right
and the tRNA polypeptide at the P site
is now longer by one amino acid. One
tRNA is outgoing and another tRNA
is incoming.
2a. Elongation occurs in two stages.
First stage of elongation.
tRNA-polypeptide is at the P site
and a tRNA amino acid is at the
A site. The polypeptide will be
transferred to the tRNA-amino acid.
free polypeptide
met
ser
large ribosomal
subunit
ala
tryp
P site
A site
met
small
ribosomal
subunit
val
asp
thr
U A C
A U G
Figure 21.10 Formation of
the polypeptide during
translation.
1. Initiation. The small ribosomal subunit,
the mRNA, the first tRNA amino acid,
and the large ribosomal subunit
come together.
3. Termination. When the ribosome
reaches a stop codon, all participants
separate and the polypeptide is
released.
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21.2 Gene Expression
Overview of transcription and translation
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TRANSCRIPTION
1. DNA in nucleus
serves as a template
for mRNA.
Translation
DNA
3. mRNA moves into
cytoplasm and becomes
associated with ribosomes.
2. mRNA is processed
before leaving the nucleus.
introns
primary
mRNA
exons
mature
mRNA
mRNA
nuclear pore
large and small
ribosomal subunits
amino
acids
peptide
tRNA
UG G U U U
A C C A A A G
anticodon
4. tRNAs with
anticodons carry
amino acids
to mRNA.
6. Polypeptide synthesis
takes place one amino
acid at a time.
ribosome
codon
5. Anticodon–codon
complementary
base pairing occurs.
Figure 21.12 Summary of transcription and translation.
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