Transcript DNA structure

DNA structure
 Remember Ch 3 we learned the building block to the
macromolecules of life
 Question
 What are the building blocks of nucleic acids?
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DNA structure
 Remember Ch 3 we learned the building blocks to the
macromolecules of life
 Question
• What are the building blocks of nucleic acids?
• Nucleotides!
Copyright © 2009 Pearson Education, Inc.
10.2 DNA and RNA are polymers of nucleotides
– The monomer unit of
DNA and RNA is the
nucleotide, containing
– Nitrogenous base
– 5-carbon sugar
– Phosphate group
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10.2 DNA and RNA are polymers of nucleotides
– The monomer unit of
DNA and RNA is the
nucleotide, containing
– Nitrogenous base
– 5-carbon sugar
– Phosphate group
Copyright © 2009 Pearson Education, Inc.
10.3 DNA is a double-stranded helix
 James D. Watson and Francis Crick deduced the
secondary structure of DNA, with X-ray
crystallography data from Rosalind Franklin and
Maurice Wilkins
– Specific pairs of bases give the helix a uniform
shape
– A pairs with T, forming two hydrogen bonds
– G pairs with C, forming three hydrogen bonds
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Watson and Crick in 1953 with
their model of the DNA
double helix
3-D structure of DNA
 Licorice
• Phosphate group
• Toothpicks
• 5-carbon sugar
• Marshmallows
• Nitrogenous bases
•
Green = Adenine
•
Yellow =Guanine
•
Pink = Thymine
•
Orange = Cytosine
Replication of DNA is a zipping action
 DNA replication follows a
semiconservative model
Parental
molecule
of DNA
– The two DNA strands separate
– Each strand is used as a pattern to produce a
complementary strand, using specific base pairing
Nucleotides
Parental
molecule
of DNA
Both parental
strands serve
as templates
– Each new DNA helix has one old strand with one new
strand
Nucleotides
Parental
molecule
of DNA
Both parental
strands serve
as templates
Two identical
daughter
molecules of DNA
Origin of replication
Parental strand
Daughter strand
Bubble
DNA replication occurs in many sections simultaneously
Two daughter DNA molecules
DNA polymerase
molecule
5
3
3
5
Daughter strand
synthesized
continuously
Parental DNA
3
5
5
3
DNA ligase
Overall direction of replication
Daughter
strand
synthesized
in pieces
Now we know how to get more
DNA
Where do RNA and proteins come
into play?
Novelty Gene
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THE FLOW OF GENETIC
INFORMATION FROM DNA TO
RNA TO PROTEIN
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From Gene to Protein
 What is a gene?
 Give some examples of genes.
 What is a protein?
 Give some examples of proteins.
From Gene to Protein
 Most of us have a protein enzyme that can synthesize
melanin
 the main pigment that gives color to our skin and hair

Albino people make a defective version of this protein enzyme
 They are unable to make melanin and they have very pale skin and
hair
From Gene to Protein
 The instructions for making a protein are provided
by a gene, which is a specific segment of a DNA
molecule.
 Each gene contains a specific sequence of nucleotides.
This sequence of nucleotides specifies which sequence of
amino acids should be joined together to form the
protein.
 The sequence of amino acids in the protein determines
the structure and function of the protein.
 For example, the defective enzyme in albinos has a
different amino acid sequence than the normal enzyme
for synthesizing melanin.
From Gene to Protein
 A gene directs the synthesis of a protein by
a two-step process.


Transcription-DNA is copied into a
messenger RNA (mRNA) molecule. The
sequence of nucleotides in the gene
determines the sequence of nucleotides in
the mRNA.
Translation - mRNA is used by ribosomes to insert the correct amino
acids in the correct sequence to form the protein coded for by that gene.
The sequence of nucleotides in the mRNA determines the sequence of
amino acids in the protein.
From Gene to Protein
The following table summarizes the basic characteristics of transcription and translation.
Original
Molecule which Location where this
message or
is synthesized takes place
instructions in:
Transcription
Translation
Nucleotide
sequence in
gene in DNA in
chromosome
Nucleus
From Gene to Protein
The following table summarizes the basic characteristics of transcription and translation.
Transcription
Translation
Original
Molecule which
message or
is synthesized
instructions in:
mRNA
Nucleotide
sequence in
gene in DNA in
chromosome
Nucleotide
Protein
sequence in
mRNA
Location where this
takes place
Nucleus
Cytoplasm/ribosomes
Synthesis of Hemoglobin
 What is hemoglobin?
 We will begin with the process of transcription to
make hemoglobin.
 Remember, transcription is the process that makes
messenger RNA (mRNA)
 RNA is different from DNA in that it is single stranded
and has the 5-carbon sugar ribose
RNA polymerase separates the two DNA strands and
elongates the mRNA
 When the mRNA is synthesized, RNA nucleotides are added one at a
time by RNA polymerase, and each RNA nucleotide is matched to
the corresponding DNA nucleotide in the gene. The base-pairing rule
is very similar to the base-pairing rule in the DNA double helix, but
what is the one difference?
Complementary nucleotides Complementary nucleotides
for base-pairing
for base-pairing
between two
between DNA and RNA
strands of DNA
G (guanine) pairs with C
(cytosine).
G pairs with C.
A (adenine) pairs with T
(thymine).
A in DNA pairs with U (uracil) in
RNA.
T in DNA pairs with A in RNA.
RNA polymerase separates the two DNA strands and
elongates the mRNA
RNA polymerase can add up to 50 nucleotides per second
Transcription modeling
DNA nucleotide
G
C
T
A
Complementary
nucleotide in RNA
Transcription modeling
 Notice that the process of transcription is similar to the
process of DNA replication. What are some similarities
between transcription and DNA replication?
 What are some of the differences?
DNA replication
The whole chromosome is
replicated.
DNA is made.
DNA is double-stranded.
Transcription
___________________is
transcribed.
mRNA is made.
mRNA is _____________ stranded.
DNA polymerase is the enzyme
which carries out DNA replication.
_____ polymerase is the enzyme
which carries out transcription.
T = thymine is used in DNA,
so A pairs with T in DNA.
T = thymine is replaced
by ___ = uracil in RNA,
so A in DNA pairs with ___ in
mRNA.
Summary
 To summarize what you have learned, explain how a gene
directs the synthesis of an mRNA molecule. Include in your
explanation the words and phrases: base-pairing rule,
complementary nucleotides, cytoplasm, DNA, gene,
messenger RNA, nucleotide, nucleus, and RNA polymerase.
Translation
 In the process of
translation, the sequence
of nucleotides in
messenger RNA (mRNA)
determines the sequence
of amino acids in a
protein. The figure
below shows an example
of how transcription is
followed by translation.
Translation
 Codon –
In translation, three
nucleotides in an mRNA
molecule codes for one
amino acid in a protein
 Each codon specifies a
particular amino acid.
 For example, the first
codon shown, CGU,
instructs the ribosome to
put the amino acid arg
(arginine) as the first amino
acid in this protein.
mRNA codon
ACU
CAU
CCU
CUG
GAG
GUG
Amino acid
Threonine (Thr)
Histidine (His)
Proline (Pro)
Leucine (Leu)
Glutamic acid
(Glu)
Valine (Val)
10.8 The genetic code is the Rosetta stone of life
– Redundant: More than one codon for some amino
acids
– Unambiguous: Any codon for one amino acid does
not code for any other amino acid
– Nearly universal
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Third base
First base
Second base
How does translation actually take place?
– Transfer RNA (tRNA) molecules match an
amino acid to its corresponding mRNA codon
– tRNA structure allows it to convert one language to
the other (the nucleic acid language to protein
language)
– An amino acid attachment site allows each tRNA to carry
a specific amino acid
– An anticodon on tRNA allows the tRNA to bind to a
specific mRNA codon, complementary in sequence
– A pairs with U, G pairs with C
How does translation actually take place?
 There are multiple different types of tRNA. Each type of
tRNA molecule can bind to one specific type of amino acid
on one end. On the other end, the tRNA molecule has
three nucleotides that form an anti-codon.
 The three nucleotides in the tRNA anti-codon are
complementary to the three nucleotides in the mRNA
codon for that specific type of amino acid.
How does translation actually take place?
 Where are the anticodons and codons in this picture?
How does translation actually take place?
Amino acid
Threonine (Thr)
Histidine (His)
Proline (Pro)
Leucine (Leu)
Glutamic acid
(Glu)
Valine (Val)
mRNA codon
ACU
CAU
CCU
CUG
GAG
GUG
Anti-codon in
tRNA molecule
that carries this
amino acid
UGA
How does translation actually take place?
Amino acid
Threonine (Thr)
Histidine (His)
Proline (Pro)
Leucine (Leu)
Glutamic acid
(Glu)
Valine (Val)
mRNA codon
ACU
CAU
CCU
CUG
GAG
Anti-codon in
tRNA molecule
that carries this
amino acid
UGA
GUA
GGA
GAC
CUC
GUG
CAC
Translation occurs on a ribosome
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Next amino acid
to be added to
Polypeptide- A site
Growing
Polypeptide
- P site
tRNA
mRNA
Codons
A – site think add site
P – site think protein site
10.13 An initiation codon marks the start of an
mRNA message
 Initiation brings together the components needed
to begin RNA synthesis
 Initiation occurs in two steps
1. mRNA binds to a small ribosomal subunit, and the
first tRNA binds to mRNA at the start codon
2. A large ribosomal subunit joins the small subunit,
allowing the ribosome to function
– The first tRNA occupies the P site, which will hold the
growing peptide chain
– The A site is available to receive the next tRNA
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Large
ribosomal
subunit
Initiator tRNA
P site
1
mRNA
Start
codon
Small ribosomal
subunit
2
A site
10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon
terminates translation
 Elongation is the addition of amino acids to the
polypeptide chain
 Each cycle of elongation has three steps
1. Codon recognition: next tRNA binds to the mRNA at
the A site
2. Peptide bond formation: joining of the new amino
acid to the chain
– Amino acids on the tRNA at the P site are attached by a
covalent bond to the amino acid on the tRNA at the A site
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10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon
terminates translation
3. Translocation: tRNA is released from the P site and
the ribosome moves tRNA from the A site into the P
site
 Elongation continues until the ribosome reaches a
stop codon
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Amino
acid
Polypeptide
A site
P site
Anticodon
mRNA
Codons
1 Codon recognition
Amino
acid
Polypeptide
A site
P site
Anticodon
mRNA
Codons
1 Codon recognition
2 Peptide bond
formation
Amino
acid
Polypeptide
A site
P site
Anticodon
mRNA
Codons
1 Codon recognition
2 Peptide bond
formation
New
peptide
bond
3 Translocation
Amino
acid
Polypeptide
A site
P site
Anticodon
mRNA
Codons
1 Codon recognition
mRNA
movement
Stop
codon
2 Peptide bond
formation
New
peptide
bond
3 Translocation
Modeling Translation
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Check your comprehension
1.
What is the function of mRNA?
2.
What is the function of tRNA?
3.
Describe one similarity in the structure of mRNA and tRNA.
4.
Describe one difference between the structure of mRNA and tRNA.
5.
The proteins in biological organisms include 20 different kinds of amino acids.
What is the minimum number of different types of tRNA molecules that must
exist in the cell? 41 = ? 42 = ?
43 = ?
6.
Explain why it makes sense to use the word translation to describe protein
synthesis.
7.
Explain why it would not make sense to use the word translation to describe
mRNA synthesis.
8.
What part of translation depends on the same base-pairing rule that is used in
transcription and DNA replication?
9.
You have modeled how ribosomes carry out translation. Why is it appropriate
to say that the function of ribosomes is protein synthesis?
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10.16 Mutations can change the meaning of genes
 A mutation is a change in the nucleotide
sequence of DNA
– Base substitutions: replacement of one nucleotide
with another
– Effect depends on whether there is an amino acid change
that alters the function of the protein
– Deletions or insertions
– Alter the reading frame of the mRNA, so that nucleotides
are grouped into different codons
– Lead to significant changes in amino acid sequence
downstream of mutation
– Cause a nonfunctional polypeptide to be produced
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10.16 Mutations can change the meaning of genes
 Mutations can be
– Spontaneous: due to errors in DNA replication or
recombination
– Inherited
– Induced by mutagens
– High-energy radiation
– Chemicals
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Sickle Cell Anemia
Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
Normal gene
mRNA
Protein
Met
Lys
Phe
Gly
Ala
Lys
Phe
Ser
Ala
Base substitution
Met
Base deletion
Met
Missing
Lys
Leu
Ala
His