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Chapter 12: DNA & RNA
What do you already know about DNA?
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12.1 Contributors to the Genetic Code
1. Griffith and Transformation– Worked with bacteria causing pneumonia– Two Strains
1. S – strain (smooth) – DEADLY
2. R – strain (rough) - HARMLESS
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12.1. Contributors to the Genetic Code
1. Griffith Experiment
1. The Experiment
• Mouse + R = Life
• Mouse + S = Death
• Mouse + heat-killed S = Life
• Mouse + heat-killed S and R = Death
Transformation: changing one strain of bacteria into another using genes. Pointed to some type of “transforming” factor.
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12.1. Contributors to the Genetic Code1. Griffith
• Conclusion: “something” transformed the living R-strain (harmless) into the S-strain (deadly) = Transformation
2. Oswald Avery – repeated Griffith’s work
• Destroyed all the organic compounds in heat killed bacteria except DNA: Result = transformation occurred.
• Destroyed all the organic compounds and DNA: Result = transformation did not occur.
• Conclusion: DNA was the transforming factor that caused the change in the R-strain
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12.1 Contributors to the Genetic Code
3. Alfred Hershey & Martha Chase• Question: Are genes made of DNA or Proteins• What they know: viruses use other organisms to
reproduce
Phage attachesto bacterial cell.
Phage injects DNA. Phage DNA directs hostcell to make more phageDNA and protein parts.New phages assemble.
Cell lyses and releases new phages.
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12.1. Contributors to the Genetic Code3. Alfred Hershey and Martha Chase
• Experiment• They tagged the virus DNA with blue radioactive
phosphorous
• They tagged the protein coat with radioactive sulfur
Conclusion: Virus only injects DNA
(DNA is the genetic material)
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Bacteriophage Images
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12.1 Three important functions of DNA
1. Store genetic information – stores genes
2. Copy information – copy genes prior to cell division
3. Transmit the information – pass genetic information along to next generation
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12.2 Structure of DNA• DNA = Deoxyribonucleic Acid• A nucleotide is composed of:
1. Sugar (deoxyribose)2. Phosphate group3. Nitrogenous Base
• A nucleotide is the monomer of a DNA strand (polynucleotide):
DNA polynucleotide
A
C
T
G
T
Sugar-phosphate backbone
Phosphate group
Nitrogenous base
SugarA
C
T
G
T
Phosphategroup
O
O–
OO P CH2
H3C C
C
C
CN
C
N
H
H
O
O
C
O
O
H
C H H
H
C
H
Nitrogenous base(A, G, C, or T)
Thymine (T)
Sugar(deoxyribose)
DNA nucleotide
DNA nucleotide
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12.2 Structure of DNA
Nitrogenous Bases1. Purines – Adenine & Guanine (two rings in
structure)2. Pyrimidines – Cytosine & Thymine (one ring)
CC
C
CC
C
O
N
C
H
H
ONH
H3C
H H
H
H
N
N
N
H
OC
H HN
H C
N
N N
N
C
CC
C
H
H
N
N
H
C
CN
C HN
CN
H C
O
H
H
Thymine (T) Cytosine (C) Adenine (A) Guanine (G)
Purines – two ring structurePyrimidines – one ring structure
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12.2 Structure of DNA
DNA is a double-stranded helixJames Watson and Francis Crick
• Worked out the three-dimensional structure of DNA, based on work (photos taken using x-ray crystallography) by Rosalind Franklin
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12.2 Structure of DNA
The structure of DNA• Consists of two polynucleotide strands wrapped
around each other in a double helix (twisted ladder)
Twist
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12.2 Structure of DNA
Hydrogen bonds (weak) between bases• Hold the strands together
Each base pairs with a complementary partner• A with T, and G with C
G C
T A
A T
G
G
C
C
A T
GC
T A
T A
A T
A T
G C
A T
O
O
OH–O
P
OO
–OPO
OO
P– O
– O OP
OO
O
OH
H2C
H2C
H2C
H2C
O
O
O
O
O
O
O
O
PO–
O–
O–
O–
OH
HO
O
O
O
P
P
P
O
O
O
O
O
O
O
O
T A
G C
C G
A T
CH2
CH2
CH2
CH2
Hydrogen bond
Basepair
Ribbon model Partial chemical structure Computer model
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Chromosome structure
• Chromatin = DNA that is tightly packed around proteins called histones
- during cell division, chromatin form packed chromosomesChromosome
Supercoils
Coils
Nucleosome
Histones
DNA
double
helix
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12-3 DNA Replication
When does DNA replicate?– DNA must copy before cell division (mitosis)
How does it replicate?
1. DNA is separated
2. Nucleotides are added according to base pairing rules, using DNA polymerase (enzyme).A T
C G
G C
A T
T A
A T
C G
G C
A T
T A
A T
C G
G C
A T
T A
A T
C G
G C
A
T
A T
C G
AC
T
A
Parental moleculeof DNA
Both parental strands serve as templates
Two identical daughtermolecules of DNA
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12-3 DNA Replication
DNA replication is semi-conservative
1. The parent strand gives rise to two daughter strands.
2. Each daughter strand is composed of one half the parent (old strand) and one half new.
Parental strandOrigin of replication
Two daughter DNA molecules
Daughter strand
Bubble
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12.3 DNA Replication
DNA replication is a complex process:• The helical DNA molecule must untwist• Each strand of the double helix is oriented in the
opposite direction (antiparallel)
P
P
P
P
P
P
P
P
HO
OH
A
C
G
T
T
C
G
A
2134
5
15 4
32
5 end 3 end
3 end 5 end
G C
A TG C
A TC G
AGA
CG
CGCG
TAG
C
TAT
AA
TT
A
CG
CG
CG
TA
G
C
T
A
TA
AT
TA
TCT
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DNA ReplicationReplication = process of copying DNA
- occurs during S phase of Interphase
- process:
1. DNA is separated into two strands by an enzyme
2. Free nucleotides are added by DNA polymerase according to base pairing rule
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DNA Replication
Nitrogenous bases
Growth
Growth
Replication fork
DNA polymerase
New strand
Original strand DNA
polymerase
Replication fork
Original strand
New strand
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Chapter 13: Protein Synthesis
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Central Dogma of Cell Biology• DNA codes for DNA = REPLICATION
• DNA codes for RNA = TRANSCRIPTION
• RNA codes for protein = TRANSLATION
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– The DNA of the gene is transcribed into RNA• Which is translated into protein
• The flow of genetic information from DNA to RNA to Protein is called the CENTRAL DOGMA
DNA
Transcription
RNA
Protein
Translation
Chapter 13 Protein Synthesis - Overview
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FLOW IS FROM DNA TO RNA TO PROTEIN
• Genes on DNA are expressed through proteins, which provide the molecular basis for inherited traits
• A particular gene, is a linear sequence of many nucleotides
– Specifies a polypeptide (long protein made of amino acids)
Chapter 13 Protein Synthesis (Overview)
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13-1 Messenger (mRNA)
1. Monomer: nucleotide2. Parts of a mRNA Nucleotide
• Ribose Sugar• Phosphate• Nitrogenous Base
3. Three main differences between mRNA and DNA
• Ribose instead of deoxyribose• mRNA is generally single stranded• mRNA has uracil in place of thymine (U instead of
T)
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13.1 RNA4. Three Types of RNA
• Messenger RNA (mRNA) – carries copies of genes (DNA) to the rest of the cell.
• Ribosomal RNA (rRNA) – make up the ribosomes.
• Transfer RNA (tRNA) – transfers the amino acids to the ribosomes as specified by the mRNA
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from to to make up
also called which functions to also called also called which functions towhich functions to
can be
RNA
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13.1 TRANSCRIPTION: The process of making mRNA from DNA
– Why do you need this process?• Location of DNA?
Nucleus
• Location of Ribosome? Cytoplasm
– mRNA takes code from DNA in the nucleus to the cytoplasm
T A C T T C A A A A T C
A T G A A G T T T T A G
A U G A A G U U U U A G
Transcription
Translation
RNA
DNA
Met Lys PhePolypeptide
Startcondon
Stopcondon
Strand to be transcribed
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13.1 Transcription produces genetic messages in the form of mRNA
– During transcription, segments of DNA serve as templates to produce complementary RNA molecules.
RNADNA
RNApolymerase
Adenine (DNA and RNA)Cytosine (DNA and RNA)Guanine(DNA and RNA)Thymine (DNA only)Uracil (RNA only)
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Transcription – Transcription requires an enzyme,
known as RNA polymerase, that is similar to DNA polymerase.
– RNA polymerase binds to DNA during transcription and separates the DNA strands.
RNADNA
RNApolymerase
Adenine (DNA and RNA)Cytosine (DNA and RNA)Guanine(DNA and RNA)Thymine (DNA only)Uracil (RNA only)
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Promoters
– RNA polymerase binds only to promoters, regions of DNA that have specific base sequences.
– Promoters are signals in the DNA molecule that show RNA polymerase exactly where to begin making RNA.
– Similar signals in DNA cause transcription to stop when a new RNA molecule is completed.
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13.1 In the nucleus, the DNA helix unzips• And RNA nucleotides line up along one strand of the
DNA, following the base pairing rules
– As the single-stranded messenger RNA (mRNA) is released away from the gene
• The DNA strands rejoin
RNApolymerase
RNA nucleotides
Direction of transcription
Template Strand of DNA
Newly made RNA
TC
A T C C A A TT
GG
CC
AATTGGAT
G
U
C A U C C AA
U
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13.1 Eukaryotic mRNA is processed before leaving the nucleus
– Noncoding segments called introns are spliced out leaving only the coding exons
• A 5’ cap and a poly A tail are added to the ends of mRNA
• Cap and tail protect mRNA
Exon Intron Exon Intron Exon
DNA
Cap TranscriptionAddition of cap and tail
RNAtranscript with capand tail
Introns removedTail
Exons spliced together
mRNA
Coding sequence Nucleus
Cytoplasm
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The Genetic Code– Proteins are made by joining amino acids
together into long chains, called polypeptides.
– As many as 20 different amino acids are commonly found in polypeptides.
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The Genetic Code– The specific amino acids in a
polypeptide, and the order in which they are joined, determine the properties of different proteins.
– The sequence of amino acids influences the shape of the protein, which in turn determines its function.
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The Genetic Code– RNA contains four different bases: adenine,
cytosine, guanine, and uracil.
– These bases form a “language,” or genetic code, Each three-letter “word” in mRNA is known as a codon.
– A codon consists of three consecutive bases that specify a single amino acid to be added to the polypeptide chain.
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How to Read Codons – Because
there are four different bases in RNA, there are 64 possible three-base codons (4 × 4 × 4 = 64) in the genetic code.
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How to Read Codons – Most amino acids
can be specified by more than one codon.
– For example, six different codons—UUA, UUG, CUU, CUC, CUA, and CUG—specify leucine. But only one codon—UGG—specifies the amino acid tryptophan.
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Start and Stop Codons – The methionine
codon AUG serves as the initiation, or “start,” codon for protein synthesis.
– Following the start codon, mRNA is read, three bases at a time, until it reaches one of three different “stop” codons, which end translation.
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DNA:CCGTCATGTTCGCGCTACAAATGAAATGAGGCAGTACAAGCGCGATGTACTTTACT
mRNA:
Polypeptide:
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13-2 Protein Synthesis - Translation
• Translation is defined as going from mRNA to protein– tRNA which have amino acids attached are
going to the ribosome.• What are amino acids? monomers of proteins
• Does the order of amino acids matter? Yes, they must be in order for the protein to fold correctly.
– How does the correct tRNA (with amino acid attached) bind to the mRNA? The tRNA contains an anticodon which matches up with the mRNA sequence (codon).
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– A ribosome attaches to the mRNA and translates its message into a specific polypeptide aided by transfer RNAs (tRNAs)
• tRNAs can be represented in several ways
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
AnticodonAnticodon
Amino acid attachment site
Transfer RNA (tRNA) molecules serve as interpreters during translation
– Translation
• Takes place in the cytoplasm
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– Each tRNA molecule • Is a folded molecule bearing a base triplet
called an anticodon on one end
– A specific amino acid • Is attached to the other end
Amino acidattachment site
Anticodon
13.2 Translation
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Ribosomes build polypeptides (proteins)– A ribosome consists of two subunits
• Each made up of proteins and a kind of RNA called ribosomal RNA
tRNAmolecules
mRNA Small subunit
Growingpolypeptide
Largesubunit
13.2 Translation
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– The subunits of a ribosome• Hold the tRNA and mRNA close together
during translation
Largesubunit
mRNA-binding site
Smallsubunit
tRNA-binding sites
Growing polypeptide
Next amino acid to be added to polypeptide
mRNA
tRNA
Codons
13.2 Translation
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– An initiation codon marks the start of an mRNA message
– mRNA, a specific tRNA, and the ribosome subunits assemble during initiation
Met Met
Initiator tRNA
1 2mRNA Small ribosomal
subunit
Startcodon
Large ribosomalsubunit
A siteU A CAU C
A U G A U G
P site
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Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation
– Once initiation is complete amino acids are added one by one to the first amino acid
– The mRNA moves a codon at a time• A tRNA with a complementary anticodon pairs with
each codon, adding its amino acid to the peptide chain
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– Each addition of an amino acid• Occurs in a three-step elongation process
Polypeptide
P site
mRNA Codons
mRNAmovement
Stopcodon
NewPeptidebond
Anticodon
Aminoacid
A site
Figure 10.14
1 Codon recognition
2 Peptide bondformation
3 Translocation
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13.3 Mutations• Mutations – heritable changes in genetic information
(changes to the DNA sequence)• Two types - gene and chromosomal mutations• Mutations can be caused by chemical or physical agents
(mutagens)– Chemical – pesticides, tobacco smoke, environmental pollutants– Physical – X-rays and ultraviolet light
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13.3 Mutations• Gene mutations
– Point Mutation: mutations that affect a single nucleotide– Frameshift mutation: shift the reading frame of the
genetic message.• Can change the entire protein so it doesn’t work• Gene Mutations Explained
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13.3 Mutations
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13.3 Chromosomal Mutations• Chromosomal mutation: mutation
that changes the number or structure of chromosomes.
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13.3 Chromosomal Mutations
• Types of chromosomal mutations:
– Deletion: The loss of all or part of a chromosome
– Duplication: A segment is repeated
– Inversion: part of the chromosome is reverse from its usual direction.
– Translocation: one chromosome breaks off an attaches to another chromosome.