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DNA structure
Copyright © 2009 Pearson Education, Inc.
Remember Ch 3 we learned the building block to the macromolecules of life
Question
What are the building blocks of nucleic acids?
DNA structure
Copyright © 2009 Pearson Education, Inc.
Remember Ch 3 we learned the building blocks to the macromolecules of life
Question
• What are the building blocks of nucleic acids?• Nucleotides!
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.
10.2 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
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– 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
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
Parentalmoleculeof DNA
DNA replication follows a semiconservative model
Parentalmoleculeof DNA
Nucleotides
Both parentalstrands serveas templates
– The two DNA strands separate
– Each strand is used as a pattern to produce a complementary strand, using specific base pairing
Parentalmoleculeof DNA
Nucleotides
Both parentalstrands serveas templates
Two identicaldaughter
molecules of DNA
– Each new DNA helix has one old strand with one new strand
Origin of replication Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
DNA replication occurs in many sections simultaneously
Parental DNA
35
DNA polymerasemolecule
DNA ligase
35
Overall direction of replication
Daughter strandsynthesizedcontinuously
35
35
Daughter strandsynthesizedin 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
Original message or instructions in:
Molecule which is synthesized
Location where this takes place
Transcription Nucleotide sequence in gene in DNA in chromosome
Nucleus
Translation
The following table summarizes the basic characteristics of transcription and translation.
From Gene to Protein
Original message or instructions in:
Molecule which is synthesized
Location where this takes place
Transcription Nucleotide sequence in gene in DNA in chromosome
mRNA Nucleus
TranslationNucleotide sequence in mRNA
Protein Cytoplasm/ribosomes
The following table summarizes the basic characteristics of transcription and translation.
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 for base-pairing between two strands of DNA
Complementary nucleotides for base-pairing between DNA and RNA
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 Complementary nucleotide in RNA
G C T A
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 Transcription
The whole chromosome is replicated.
___________________is transcribed.
DNA is made.DNA is double-stranded.
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 Amino acidACU Threonine (Thr)CAU Histidine (His)CCU Proline (Pro)CUG Leucine (Leu)GAG Glutamic acid
(Glu)GUG 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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Fir
st
ba
se
Th
ird
ba
se
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 mRNA codon Anti-codon in tRNA molecule that carries this amino acid
Threonine (Thr) ACU UGAHistidine (His) CAUProline (Pro) CCULeucine (Leu) CUGGlutamic acid (Glu)
GAG
Valine (Val) GUG
How does translation actually take place?
Amino acid mRNA codon Anti-codon in tRNA molecule that carries this amino acid
Threonine (Thr) ACU UGAHistidine (His) CAU GUAProline (Pro) CCU GGALeucine (Leu) CUG GACGlutamic acid (Glu)
GAG CUC
Valine (Val) GUG CAC
Translation occurs on a ribosome
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mRNA
Next amino acidto be added toPolypeptide- A site
GrowingPolypeptide - P site
Codons
tRNA
A – site think add siteP – 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 steps1.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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Small ribosomalsubunit
Startcodon
P site
mRNA
A site
Large ribosomalsubunit
Initiator tRNA
Met Met
21
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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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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10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation
Polypeptide
A site
1 Codon recognitionCodons
Aminoacid
Anticodon
P site
mRNA
Polypeptide
A site
1 Codon recognitionCodons
Aminoacid
Anticodon
P site
mRNA
2 Peptide bondformation
Polypeptide
A site
1 Codon recognitionCodons
Aminoacid
Anticodon
P site
mRNA
2 Peptide bondformation
3 Translocation
Newpeptidebond
Polypeptide
A site
1 Codon recognitionCodons
Aminoacid
Anticodon
P site
mRNA
2 Peptide bondformation
3 Translocation
Newpeptidebond
Stopcodon
mRNAmovement
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Modeling Translation
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Check your comprehension1. 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?
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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Normal hemoglobin DNA Mutant hemoglobin DNA
Sickle-cell hemoglobinNormal hemoglobin
mRNAmRNA
ValGlu
Sickle Cell Anemia
Normal gene
Protein
Base substitution
Base deletion Missing
mRNA
Met Lys Phe Ser Ala
Met Lys Phe Gly Ala
Met Lys Leu Ala His
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