Download 3. Gene Transcription

Chapter 10
Molecular Biology of the
Gene: (Molecular Genetics)
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
1. Nucleic Acid Structure
DNA and RNA are polymers of nucleotides
• DNA is a nucleic acid
– Made of long chains of nucleotide monomers
Sugar-phosphate backbone
Phosphate group
A
C
Nitrogenous base
A
Sugar
DNA nucleotide
C
Nitrogenous base
(A, G, C, or T)
Phosphate
group
O
H3C
O
T
T
O P
O
CH2
O–
G
C
HC
O
C
N
N
C
H
O
Thymine (T)
O
C H
G
H
C
HC
CH
H
Sugar
(deoxyribose)
T
T
DNA nucleotide
DNA polynucleotide
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• DNA has four kinds of nitrogenous bases
– A, T, C, and G
H
O
H3C
H
C
C
C
H
H
N
C
H
N
C
N
C
C
C
N
H
O
N
H
N
H
O
H
H
Thymine (T)
Cytosine (C)
Pyrimidines
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N
H
O
C
C
N
C
C
N
H
C
N
N
H
N
C
C
N
H
Adenine (A)
Guanine (G)
Purines
H
C
N
H
C
C
N
H
H
• RNA is also a nucleic acid
– But has a slightly different sugar
– And has U instead of T
Nitrogenous base
(A, G, C, or U)
O
Phosphate
group
H
O
O
P
C
C
N
C
C
H
O CH2
N
H
O
Uracil (U)
O–
O
C H
H C
H C
C H
O
OH
Sugar
(ribose)
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Key
Hydrogen atom
Carbon atom
Nitrogen atom
Oxygen atom
Phosphorus atom
DNA is a double-stranded helix
• James Watson and Francis Crick
– Worked out the three-dimensional structure of
DNA, based on work by Rosalind Franklin
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• The structure of DNA
– Consists of two polynucleotide strands
wrapped around each other in a double helix
Twist
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• Hydrogen bonds 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
Base
pair
T
C
G
C
C
G
A
T
T
O
O
P
–O
O
H2C
O
O
P
–O
O
H2C
G
T
O
OH
P
O
O
H2C
–O
A
O
O
–O P
O
H2C
A
A
T
A
Hydrogen bond
OH
O
O
A
T
CH2
O O–
P
O
O
O
CH2
O O–
O P
O
O
CH2
O
O–
P
O
O
O
CH2
O
O–
P
HO O
C
G
G
C
O
O
A
T
T
OH
G
A
Ribbon model
C
T
Partial chemical structure
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Computer model
2. DNA REPLICATION
DNA replication depends on specific base pairing
• DNA replication
– Starts with the separation of DNA strands
• Then enzymes use each strand as a template
– To assemble new nucleotides into
complementary strands
A
T
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
C
G
G
C
G
C
G
C
G
C
A
T
A
T
A
T
A
T
T
A
T
A
T
A
T
A
Parental molecule
of DNA
C
A
Nucleotides
Both parental strands serve
as templates
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Two identical daughter
molecules of DNA
• DNA replication is a complex process, which
occurs during “S” Phase of the eukaryotic cell
cycle.
– Due in part to the fact that some of the helical
DNA molecule must untwist
G C
A T
G
C
Replication Fork
C
G
A T
A
A
G
C
G
C
G
C
T
T
A
A
T A
T
A
G
G
T
A
C
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T
C
T
G
G
C G
C
C
C
A
A
T
G
T
T
A
T
A A
T
DNA replication: A closer look
• DNA replication
– Begins at specific sites on the double helix
Origin of replication
Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
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At the end of each replication bubble is a
replication fork, a Y-shaped region where
new DNA strands are elongating
Helicases are enzymes that untwist the
double helix at the replication forks
Single-strand binding protein binds to and
stabilizes single-stranded DNA until it can
be used as a template
Topoisomerase corrects “overwinding” ahead
of replication forks by breaking, swiveling,
and rejoining DNA strands
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• Each strand of the double helix
– Is oriented in the opposite direction
5 end
P
3 end
HO
5
4
3
2
2
1
A
T 1
5
P
3
4
P
C
G
P
P
G
C
P
P
T
OH
3 end
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A
P
5 end
• Using the enzyme DNA polymerase
– The cell synthesizes one daughter strand as a
continuous piece
• The other strand is synthesized as a series of short pieces
– Which are then connected by the enzyme DNA ligase
DNA polymerase
molecule
5
3
3
5
Daughter strand
synthesized
continuously
Parental DNA
3
5
5
3
DNA ligase
Overall direction of replication
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Daughter
strand
synthesized
in pieces
THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
• The DNA genotype is expressed as proteins,
which provide the molecular basis for phenotypic
traits
• The information constituting an organism’s
genotype
– Is carried in its sequence of its DNA bases
• A particular gene, a linear sequence of many
nucleotides
– Specifies a polypeptide
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3. Gene Transcription
• The DNA of the gene is transcribed into RNA
– Which is translated into the polypeptide
DNA
Transcription
RNA
Translation
Protein
Figure 10.6A
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Genetic information written in codons is
translated into amino acid sequences
• The “words” of the DNA “language”
– Are triplets of bases called codons
• The codons in a gene
– Specify the amino acid sequence of a
polypeptide
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DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
A A A C C G G C A A A A
Transcription
RNA
U U U G G C C G U U U U
Codon
Translation
Polypeptide
Amino acid
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The genetic code is the Rosetta stone of life
• Nearly all organisms
– Use exactly the same genetic code
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• An exercise in translating the genetic code
Strand to be transcribed
T
A
C
T
T
C
A
A
A
A
T
C
A
T
G
A
A
G
T
T
T
T
A
G
U
A
G
DNA
Transcription
A
U
G
A
A
G
U
U
U
RNA
Start
condon
Stop
condon
Translation
Polypeptide
Met
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Lys
Phe
Transcription produces genetic messages in the
form of RNA
• A close-up view of transcription
RNA nucleotides
RNA
polymerase
T C C A
A U
T
T
A
C C A
T A G G T
Direction of
transcription
Newly made RNA
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A
Template
Strand of DNA
• 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) peels away from the gene
– The DNA strands rejoin
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• Transcription of a gene
RNA polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
1 Initiation
2 Elongation
3 Termination
Completed RNA
Figure 10.9B
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Area shown
In Figure 10.9A
Growing
RNA
RNA
polymerase
Eukaryotic RNA is processed before leaving the
nucleus
• Noncoding segments called introns are spliced out
– And a cap and a tail are added to the ends
Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
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4. Translation: Gene to Protein
Transfer RNA molecules serve as interpreters
during translation
• Translation
– Takes place in the cytoplasm
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• A ribosome attaches to the mRNA
– And translates its message into a specific
polypeptide aided by transfer RNAs
(tRNAs)
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Anticodon
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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 acid
attachment site
Anticodon
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Ribosomes build polypeptides
• A ribosome consists of two subunits
– Each made up of proteins and a kind of
RNA called ribosomal RNA
tRNA
molecules
Growing
polypeptide
Large
subunit
mRNA
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Small
subunit
• The subunits of a ribosome
– Hold the tRNA and mRNA close together
during translation
tRNA-binding sites
Large
subunit
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
mRNAbinding site
Small
subunit
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mRNA
Codons
An initiation codon marks the start of an mRNA
message
Start of genetic message
End
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• mRNA, a specific tRNA, and the ribosome
subunits
– Assemble during initiation
Met
Met
Large
ribosomal
subunit
Initiator tRNA
P site
U
A C
A U G
U
A C
A U G
Start
codon
1
mRNA
A site
Small ribosomal
subunit
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2
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
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Each addition of an amino acid
– Occurs in a three-step elongation process
Amino
acid
Polypeptide
P site
A site
Anticodon
mRNA
Codons
1 Codon recognition
mRNA
movement
Stop
codon
2 Peptide bond
formation
New
Peptide
bond
3 Translocation
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• The mRNA moves a codon at a time
– And a tRNA with a complementary
anticodon pairs with each codon, adding its
amino acid to the peptide chain
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• Elongation continues
– Until a stop codon reaches the ribosome’s
A site, terminating translation
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Review: The flow of genetic information in the cell
is DNARNAprotein
• The sequence of codons in DNA, via the
sequence of codons
– Spells out the primary structure of a
polypeptide
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Summary of transcription and translation
DNA
Transcription
1 mRNA is transcribed
from a DNA template.
mRNA
RNA
polymerase
Amino acid
Translation
2 Each amino acid
attaches to its proper
tRNA with the help of a
specific enzyme and ATP.
Enzyme
ATP
tRNA
Anticodon
Large
ribosomal
subunit
Initiator
tRNA
Start Codon
mRNA
3 Initiation of
polypeptide synthesis
The mRNA, the first tRNA,
and the ribosomal
subunits come together.
Small
ribosomal
subunit
New peptide
bond forming
Growing
polypeptide
4 Elongation
A succession of tRNAs
add their amino acids to
the polypeptide chain as the
mRNA is moved through the
ribosome, one codon at a time.
Codons
mRNA
Polypeptide
5 Termination
The ribosome recognizes a
stop codon. The poly-peptide
is terminated and released.
Stop codon
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Mutations can change the meaning of genes
• Mutations are changes in the DNA base
sequence
– Caused by errors in DNA replication or
recombination, or by mutagens
Normal hemoglobin DNA
C
T
Mutant hemoglobin DNA
T
mRNA
A
T
G U
A
C
mRNA
G
A
A
Normal hemoglobin
Glu
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Sickle-cell hemoglobin
Val
1. Point Mutations
a. A gene mutation is a change that takes place within a
single gene. Point mutations are mutations that involved one
base pair and may include ... substitutions, insertions or
deletions of nucleotides.
The sentence below contains all three-letter words but has
experienced a mutation. Can you find the mutation?
THE IGR EDC ATA TET HEB IGB ADR AT
What kind of mutation did you find?
Now what does the sentence say?
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• Substituting, inserting, or deleting nucleotides
alters a gene
– With varying effects on the organism
Normal gene
A U G A A G U U U G G C G C A
mRNA
Met
Protein
Lys
Phe
Gly
Ala
Base substitution
A U G A A G U U U A G C G C A
Met
Lys
Phe
Ser
Ala
U Missing
Base deletion
A U G A A G U U G G C G C A U
Met
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Lys
Leu
Ala
His
MICROBIAL GENETICS
Viral DNA may become part of the host
chromosome
• Viruses
– Can be regarded as genes packaged in
protein
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• When phage DNA enters a lytic cycle inside a
bacterium
– It is replicated, transcribed, and translated
• The new viral DNA and protein molecules
– Then assemble into new phages, which
burst from the host cell
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• In the lysogenic cycle
– Phage DNA inserts into the host
chromosome and is passed on to
generations of daughter cells
• Much later
– It may initiate phage production
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• Phage reproductive cycles
Phage
1
Attaches
to cell
Bacterial
chromosome
Phage DNA
Cell lyses,
releasing phages
Phage injects DNA
7
2
Many cell
divisions
4
Lytic cycle
Lysogenic cycle
Phages assemble
Phage DNA
circularizes
3
Lysogenic bacterium reproduces
normally, replicating the prophage
at each cell division
Prophage
5
6
OR
New phage DNA and
proteins are synthesized
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Phage DNA inserts into the bacterial
chromosome by recombination
CONNECTION
Many viruses cause disease in animals
• Many viruses cause disease
– When they invade animal or plant cells
• Many, such as flu viruses
– Have RNA, rather than DNA, as their
Membranous
genetic material
envelope
RNA
Protein
coat
Glycoprotein spike
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• Some animal viruses
– Steal a bit of host cell membrane as a
protective envelope
– Can remain latent in the host’s body for
long periods
Glycoprotein spike
VIRUS
Protein coat
Envelope
Viral RNA
(genome)
Plasma membrane 1
of host cell
2
Viral RNA
(genome)
3
Entry
Uncoating
RNA synthesis
by viral enzyme
4 Protein
mRNA synthesis
New
viral proteins
5
RNA synthesis
(other strand)
Template
6 Assembly
Exit
7
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New viral
genome
CONNECTION
Plant viruses are serious agricultural pests
• Most plant viruses
– Have RNA genomes
– Enter their hosts via wounds in the plant’s
outer layers
Protein RNA
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CONNECTION
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Colorized TEM 370,000
Colorized TEM 50,000
Emerging viruses threaten human health
The AIDS virus makes DNA on an RNA template
• HIV, the AIDS virus
– Is a retrovirus
Envelope
Glycoprotein
Protein coat
RNA (two
identical strands)
Reverse transcriptase
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• Inside a cell, HIV uses its RNA as a template
for making DNA
– To insert into a host chromosome
Viral RNA
CYTOPLASM
1
RNA
strand
NUCLEUS
Chromosomal
DNA
2
Doublestranded
DNA
3
Provirus
DNA
4
Viral
RNA
and
proteins
5
RNA
6
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Bacteria can transfer DNA in three ways
• Bacteria can transfer genes from cell to cell by
one of three processes
– Transformation, transduction, or
conjugation
Mating bridge
DNA enters
cell
Fragment of DNA
from another
bacterial cell
Bacterial
chromosome
(DNA)
Phage
Phage
Fragment of
DNA from
another
bacterial cell
(former phage
host)
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Sex pili
Donor cell
(“male”)
Recipient cell
(“female”)
• Once new DNA gets into a bacterial cell
– Part of it may then integrate into the
recipient’s chromosome
Donated DNA
Recipient cell’s
chromosome
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Crossovers
Degraded DNA
Recombinant
chromosome
Bacterial plasmids can serve as carriers for gene
transfer
• Plasmids
– Are small circular DNA molecules separate
from the bacterial chromosome
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• Plasmids can serve as carriers
– For the transfer of genes
F factor (plasmid)
F factor
(integrated)
Male (donor) cell
Origin of F
replication
Bacterial
chromosome
F factor starts replication
and transfer of chromosome
Male (donor) cell
Bacterial chromosome
F factor starts replication
and transfer
Only part of the chromosome
transfers
Plasmid completes transfer
and circularizes
Plasmids
Recombination
can occur
Cell now male
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Colorized TEM 2,000
Recipient cell