Chp 7 DNA Structure and Gene Function
Herpes virus particles and cold sore blister.
1
DNA is a Double-Stranded Helix
✶ DNA stores
the
information that
cells use to
produce proteins
✶ DNA
has two
strands in a
double helix
✶ Base
sequence
on both strands is
complimentary
– A pairs with T, and
G pairs with C
Hydrogen bonds Ribbon model
Partial chemical structure
Space-fill model
DNA molecule shown in different models
2
✶ RNA is a single nucleic acid chain,
– has ribose sugar instead of deoxyribose in DNA
– has the base uracil (U) instead of thymine
3
DNA Replication Uses Base Pairing
✶ DNA replication
- semi-conservative model
– Parental strands separate
– Each strand used to build a complimentary strand
– New DNA has 1 old strand, 1 new strand
Figure 10.4A DNA replication
4
5
DNA Replication Starts at Many Sites
✶ Begins
at origins of replication
✶ Replication bubbles form and then merge
Replication bubbles during DNA replication
6
Two Key Enzymes in
DNA Replication
DNA polymerase
molecule
3ʹ′
5ʹ′
Daughter strand
synthesized
continuously
Parental DNA
5ʹ′
3ʹ′
Daughter
strand
synthesized
in pieces
✶ DNA
polymerase –
links nucleotides into
a chain, moving in
3’ to 5’ direction
ligase
- links small
fragments into a
chain
3ʹ′
5ʹ′
✶ DNA
5ʹ′
3ʹ′
DNA ligase
Overall direction of replication
Figure 10.5C Enzymes of DNA replication
7
Clicker Question #1
What is the main function of DNA?
A. To encode proteins
B. To produce ATP
C. To speed up cell reactions
D. To provide structural support to
the cell
E. All of these
Protein Synthesis Starts with DNA
✶ Gene
– a sequence of
DNA that directs the
synthesis of a specific
protein
✶ Protein
synthesis
occurs in two stages
– DNA gene
used to
make RNA copy
– RNA copy
used to
make a polypeptide
Protein synthesis in eukaryotic cells
9
Brownie Analogy for Protein Production
Making brownies is a simple
analogy to protein production.
Transcription Uses DNA to Create RNA
Let’s first look at how a
cell produces an RNA
copy of a gene.
Transcription occurs
in the nucleus.
Transcription Has Three Steps
a. Initiation
RNA polymerase enzyme
DNA
DNA template strand
Promoter
b. Elongation
DNA
RNA polymerase
G
G
C
C
T
G
DNA
G
G
C
C
U
G
RNA
C
C
G
G
A
C
RNA
c. Termination
RNA polymerase
DNA
Terminator
RNA
Transcription has
three steps
- Initiation
- Elongation
- Termination
DNA
template
strand
We will look at
these steps one
at a time
Initiation
Initiation
RNA polymerase enzyme
DNA
Promoter
DNA template strand
• RNA polymerase
binds to the promoter,
which is the
beginning of the gene
• Enzymes (not shown)
unzip the DNA
• The DNA template
strand encodes the
RNA molecule
Elongation
RNA polymerase moves
along the template
strand, making an RNA
copy
Elongation
RNA polymerase
DNA
RNA
Notice that the RNA
molecule is
complementary to the
DNA template strand
Termination
Termination
RNA polymerase
DNA
Terminator
RNA
• RNA polymerase
reaches the
terminator, which
is the end of the
gene
• RNA, DNA, and
RNA polymerase
separate
• DNA becomes a
double helix again
Clicker Question #2
If the DNA template strand has the
following sequence, what would be the
nucleotide sequence of the
complementary RNA molecule produced
in transcription?
Template strand: AGTCTT
A. AGTCTT
B. AGUCUU
C. TCAGAA
D. TCUGUU
E. UCAGAA
Translation Builds the Protein
Now let’s look at how a
ribosome uses RNA to
produce a protein.
Codons – A Set of Three Nucleotides
DNA
DNA template strand
TRANSCRIPTION
T
T
C
A
G
T
C
A
G
A
A
G
U
C
A
G
U
C
mRNA
Codon
Codon
Codon
Lysine
Serine
Valine
TRANSLATION
Protein
Polypeptide (amino acid sequence)
A codon is a three-nucleotide sequence that encodes
one amino acid
The Genetic Code: MRNA->Amino Acid
The genetic code shows
which mRNA codons
correspond to which amino
acids
A A G U C A G U C
mRNA
Codon
Codon
Codon
Lysine
Serine
Valine
TRANSLATION
Protein
Polypeptide (amino acid sequence)
The Genetic Code
U
U
UUU
UUC
UUA
UUG
Leucine (Leu; L)
CUU
CUC
CUA
A
Phenylalanine (Phe; F)
Leucine (Leu; L)
A
UCU
UAU
UCC
UCA
Serine (Ser; S)
UAC
UGA
Stop
A
UGG
Tryptophan (Trp; W)
G
CCU
CAU
CCC
CCA
Proline (Pro; P)
CAC
CAA
AAU
ACC
ACA
Proline (Pro; P)
AAC
AAA
AUG Start Methionine (Met; M) ACG
AAG
GCU
GAU
GUA
GUG
Valine (Val; V)
C
Stop
ACU
GUC
UGC
U
Cysteine (Cys; C)
Stop
AUU
G GUU
UGU
UAG
CAG
Isoleucine (Ile; I)
Tyrosine (Tyr; Y)
UAA
CCG
AUC
G
UCG
CUG
AUA
C
GCC
GCA
GCG
Proline (Pro; P)
GAC
GAA
GAG
Histidine (His; H)
Glutamine (Gln; Q)
Asparagine (Asn; N)
Lysine (Lys; K)
Aspartic acid (Asp; D)
Glutamic acid (Glu; E)
CGU
CGC
CGA
U
Arginine (Arg; R)
CGG
AGU
AGC
AGA
AGG
GGA
GGG
A
G
Serine (Ser; S)
Arginine (Arg; R)
GGU
GGC
C
U
C
A
G
U
Glysine (Gly; G)
C
A
G
Third letter of codon
First letter of codon
C
Second letter of codon
Transfer RNA are the “Translators”
Transfer RNA (tRNA)
molecules translate
the genetic code.
tRNA binds to an mRNA
codon here, at the
anticodon…
and binds to the
corresponding amino acid
here.
Section 7.4
Figure 7.8
Translation Builds the Protein
Translation also occurs
in three steps:
-Initiation
-Elongation
-Termination
All of these steps
happen at ribosomes.
Initiation
• Small ribosomal
subunit binds to
mRNA
• Large ribosomal
subunit binds
• First tRNA
molecule binds
Initiation
• tRNA
complementary
base pairs to mRNA
• tRNA already
carries an amino
acid (Met).
Elongation
• The second tRNA
enters the
ribosome next to
the first tRNA
• Amino acids
covalently bond
Covalent bond
Elongation
• The first tRNA
leaves
• The ribosome
moves to the right,
and a third tRNA
comes in
Elongation
• But, notice that the
amino acids
remain bonded
together!
• This process
continues and the
protein grows
Linked amino acids
Termination
• The ribosome
reaches the stop
codon
• A release factor
binds
• The polypeptide
detaches from the
mRNA and folds
into a functional
protein
Translation is efficient when multiple ribosomes
attach to an mRNA molecule simultaneously.
mRNA
Ribosome
Polypeptide
SEM (false color) 50 nm
Clicker Question #3
Look at the image below. Which
ribosome has been on the mRNA the
longest?
A. The one on the far right.
B. The one of the far left.
Polypeptide (purple)
Ribosome (green)
Mastering Concepts
What are the steps of translation?
Where in the cell does translation
occur?
Flow of Genetic Info in Living Cells:
DNA → RNA → Protein
– Which
represents translation?
– DNA → RNA, or RNA → protein
– Where
does the info for making a protein originate?
– DNA, or RNA
– Which
has a linear sequence of codons?
– rRNA, mRNA, or tRNA
– Which
directly produces observable traits?
– DNA, RNA, or protein
32
Mutations Change DNA
✶ Mutation
= any change
in a cell’s DNA sequence
✶ – Can be caused by errors
in DNA replication, or
– By mutagens (highenergy radiation, or
chemicals)
A mutation in one gene
causes a fly to develop legs
where its antenna should be!
33
A point mutation changes one, or a few base pairs
in a gene
The table to the
left uses
sentences to
show a few
examples of
point mutations
• Remember that codons are sequences of three
nucleotides
• Each word in the sentences above represents one
codon
“Frameshift”
mutations affect
multiple codons.
Insertion of one
nucleotide
changes every
codon after the
insertion.
Sickle Cell Anemia
Normal red blood cells
G G A C T C C T T
C C U G A G G A A
No aggregation
of hemoglobin
molecules
SEM 6 µm
Pro
Glu
Glu
Sickled red blood cells
Abnormal
G G A C A C C T T aggregation
C C U G U G G A A of hemoglobin
molecules
Pro
Val
Glu
SEM 6 µm
A single base
substitution in a
hemoglobin gene
causes blood
cells to form
abnormally,
leading to sickle
cell disease
Types of mutations and their effects
But mutations are not
always harmful!
Mutations create
different versions of
alleles, which are alternative
versions of the same gene.
Genetic variation is important
for evolution.
Plant breeders even induce
mutations to create new
varieties of plants.
Mastering Concepts
1. Describe the components of DNA and its
three-dimensional structure
2. What is the relationship between a gene and
a protein?
3. What are the steps of translation?
4. Where in the cell does translation occur?
5. What are the types of mutations, and how
does each alter the encoded protein?