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Chapters 12 and 13
DNA and Protein
Synthesis
PowerPoint Lectures for
Biology: Concepts and Connections, Fifth Edition
– Campbell, Reece, Taylor, and Simon
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Discovery of the Role of DNA
A. 1928 - Frederick Griffith discovers transformation
in bacteria :
* discovered that “something” was able to transform
harmless (non – virulent) bacteria into harmful (virulent)
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Discovery of the Role of DNA (cont’d)
B. 1944 -Oswald Avery and
colleagues show that DNA
can transform bacteria
C. 1952 - Alfred Hershey
and Martha Chase
use bacteriophage to
confirm that DNA
(not protein) is the
genetic material
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
animation
Discovery of the Role of DNA (cont’d)
D. 1953 - James Watson and Francis Crick propose a structural
model for the DNA molecule
Based On:
1. X-Ray crystallography images
prepared by Maurice Wilkins
and Rosalind Franklin
2. Chargaff’s Rule:
# of Adenines = # of Thymines
# Guanines = # of Cytosines
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
DNA and RNA are Polymers of Nucleotides
• Both are nucleic acids made of long chains of nucleotide
monomers
• A nucleotide (building block of a nucleic acid)
has 3 parts:
1. A phosphate (PO4-)
group that is
negatively charged
2. A 5-Carbon sugar
(deoxyribose in DNA
or ribose in RNA)
3. A nitrogen-containing
base
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
DNA (deoxyribonucleic acid) bases:
Thymine, Cytosine, Adenine, and Guanine
pyrimidines
Pyrimidines: single ring bases
Purines: double ring bases
Complimentary bonding pattern:
• Adenine + Thymine
(share 2 hydrogen bonds)
• Cytosine + Guanine
(share 3 hydrogen bonds)
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
purines
RNA: ribonucleic acid
Similar to DNA except:
• Sugar in RNA = ribose
• Base “uracil” instead of thymine
• Single stranded
Figure 10.2C, D
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
The Structure of DNA
• Two polynucleotide strands wrapped around each other in a
double helix
• A sugar-phosphate backbone
• Steps made of hydrogen-bound bases (A=T, C G)
Twist
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
DNA REPLICATION:
1. Helicase enzyme splits
H bonds between bases…
unzips a portion of DNA
2. DNA polymerase binds to
each strand
3. DNA polymerase adds
complimentary nucleotides to
each parent strand
4. Replication continues until
both parent strands are copied
5. DNA polymerase “proof-reads”
molecule for mistakes
replication fork: location where DNA helix is still together.
Next place to be unzipped
replication bubble: location where DNA is being copied
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
A Structural Problem with DNA Replication
•
Each strand of the double helix is
oriented in the opposite direction
(“anti-parallel”)
•
“prime” #’s refer to carbons in the sugar
•
At one end, the 3’ carbon has an (OH)
and at the opposite, a 5’ carbon has the
PO4-
•
Why does this matter?
DNA polymerase can only add
nucleotides to the 3’ end. A daughter
strand can only grow from 5’  3’
•
Therefore, only one daughter strand is
made continuously (leading strand)
•
The other strand (lagging strand) is
made in a series of short pieces
(Okazaki fragments), later connected
by DNA ligase
•
Bioflix movie
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Replication in Living Cells: Prokaryote v. Eukaryote
Prokaryote:
* one single loop of DNA
* regulatory proteins bind to a
starting point on the 1 chromosome
* bi-directional replication until all is copied
Eukaryote:
• much more DNA to copy
• replication begins at many points,
bi-directional replication
• after replication is finished, replicated
chromatin threads condense into
chromosomes
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
When DNA can repair mistakes and when it can’t
DNA enzymes work like a spell checker
•
Cut out wrong sequences
•
Undamaged strand is template
•
Only 2 or 3 stable changes per year
Mutations: some severe, others are not
•
Inheritable changes occur in gametogenesis
•
Now the “wrong” sequences are copied
•
–
Ex: cystic fibrosis (CF): a deletion of 3 nucleotides in a certain gene
–
Ex: sickle cell anemia: one nucleotide substitution in the hemoglobin gene
Mutagen: a mutation causing substance (can break DNA)
–
Ex: x-rays, radioactivity, nicotine
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Protein Synthesis: the transfer of information from:
DNA  RNA  Proteins “gene expression”:
A gene is a linear sequence of many nucleotides. 3 Types:
1. Structural genes: have info to make proteins
2. Regulatory genes: code for proteins which are on/off switches for other genes
3. Genes that code for tRNA, rRNA, histones
DNA
vs.
• double stranded
• A T C G
• deoxyribose sugar
RNA
•
•
•
•
single stranded
A U C G
ribose sugar
3 types of RNA:
•messenger, transfer, ribosomal
mRNA (messenger): copies DNA’s message in nucleus  brings it to cytoplasm
tRNA (transfer): carries amino acids to mRNA so protein can be made
rRNA (ribosomal): major part of the ribosome. Helps link amino acids from
tRNA’s together  assemble protein
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Protein Synthesis is Two Steps:
1. Transcription:
The DNA of the
gene is
transcribed into
mRNA
2. Translation:
decoding the
mRNA and
assembling the
protein
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
1. Transcription: Eukaryote
• DNA sequence (message for protein) is
transcribed by mRNA
• Only one strand (bottom; non-coding
strand) is needed as a template
Steps:
1.
2.
3.
4.
5.
6.
7.
RNA polymerase splits H bonds in DNA section
RNA polymerase attaches to a promoter region
on DNA to begin transcription
RNA polymerase travels along bottom strand of
DNA. RNA nucleotides join in a complimentary
pattern (A=U, C=G)
A termination signal reached, transcription over
mRNA strip detaches from DNA, DNA helix
closes up
mRNA is processed: Introns are cut out, Exons
are glued together, cap and tail are added.
Mature mRNA leaves nucleus through pores 
cytoplasm for next step (translation)
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
2. Translation: the synthesis of proteins using
mRNA, tRNA and ribosomes
• The Genetic Code: the language in which instructions for
proteins are written in the nucleotide sequences
• Each triplet of mRNA nucleotides is a “codon” because it will
“code” for 1 amino acid
• Ex: AUG GUC CCU AAU CCU
Met – Val – Pro – Asn – Pro
• Original strand of DNA (coding strand):
ATG GTC CCT AAT CCT
– Only difference: U is substituted for T
• Use the Genetic Code chart to “decode” mRNA message
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
The Genetic Code is the Rosetta Stone of Life
•
Nearly all organisms use
exactly the same genetic code
• More than one codon for most
amino acids = degenerate
nature…a change (mutation) in
gene does not always mean a
different amino acid.
• what does CAU code for? ACU?
UAU? GCC?
HIStidine…THReonine…TYRosine…ALAnine
• how many codons for Leu?
• what is special about AUG and
it’s amino acid, Methionine?
• what is special about UAA,
UAG, and UGA?
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
An exercise in translating the genetic code:
Step 1: fill in
corresponding DNA
bases to dark blue
strand (non-coding)
Step 2: Transcribe
the dark blue strand
into mRNA (pink)
A
T
G
A
Coding strand
(gene we want
copied)
Step 3: Translate
the codons into
correct amino acids
(use chart)
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
A
G
T
T
T
T
transcription
translation
A
G
An exercise in translating the genetic code: answers
Step 1: fill in
corresponding DNA
bases to dark blue
strand (non-coding)
Step 2: Transcribe
the dark blue strand
into mRNA (pink)
Step 3: Translate
the codons into
correct amino acids
(use chart)
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
How Does Translation Happen?
Need: tRNAs and ribosomes (rRNA)
tRNA: single stranded RNA, folded up
* 2 parts: anticodon and aa attachment site
Ribosome: 2 protein subunits and
ribosomal RNA
* allows aa’s to attach by making peptide bonds
* travels along mRNA strip, tRNA’s join and bring
correct amino acids
Confused?
Watch demo on board and videos
animation
bioflix animation
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Put It All Together:
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Mutations can change the message of genes
Mutations:
• changes in DNA
base sequence
• caused by errors
in DNA replication,
recombination, or by
mutagens
• substituting, inserting,
or deleting nucleotides
also alters a gene
“point mutation”…may or may
not alter amino acid sequence
“frame-shift mutation”…most
devastating to protein structure
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Chromosomal Mutations:
•
affect chromosome structure and shape
•
can occur during crossing over (prophase 1)
• 4 types: deletion, duplication, inversion, translocation
Genetic Mutations: Harmful of helpful?
• can be both
• depends on environment
Harmful Examples:
a) defective proteins from defective genes can disrupt
normal cell function (ex: regulation of cell cycle  cancer)
b) sickle cell anemia (one point mutation in hemoglobin
gene). Cells form sickle shape, can’t carry oxygen, can
block blood vessels, painful.
Helpful Examples:
a) new proteins made by mutations can give an advantage
in a certain environment
ex: antibiotic resistant bacteria, peppered moths
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Please make the following changes:
On multiple choice section 2:
#7 “A”
Cross out: “the growing molecule”
+
“C”
PUT IN: “the parent strand”
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings