© 2013 pearson education, inc. cell cycle defines changes from formation of cell until it...
TRANSCRIPT
© 2013 Pearson Education, Inc.
Cell Cycle
• Defines changes from formation of cell until it reproduces
• Includes:– Interphase
• Cell grows and carries out functions
– Cell division (mitotic phase)• Divides into two cells
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Interphase
• Period from cell formation to cell division
• Nuclear material called chromatin
• Three subphases:– G1 (gap 1)—vigorous growth and metabolism
• Cells that permanently cease dividing said to be in G0 phase
– S (synthetic)—DNA replication occurs
– G2 (gap 2)—preparation for division
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Figure 3.31 The cell cycle.
G1 checkpoint(restriction point)
SGrowth and DNA
synthesis
G1
Growth
G2
Growth and finalpreparations for
divisionM
Prophase
Metaphase
An
aph
ase
Telo
ph
aseC
ytokinesisG2 checkpoint
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (1 of 6)
InterphaseCentrosomes (eachhas 2 centrioles)
Plasmamembrane
Nucleolus
Nuclearenvelope
Chromatin
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DNA Replication
• Prior to division cell makes copy of DNA• DNA helices separated into replication
bubbles with replication forks at each end– Each strand acts as template for
complementary strand
• DNA polymerase begins adding nucleotides at RNA primer
• DNA polymerase continues from primer– Synthesizes one leading, one lagging strand
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DNA Replication
• DNA polymerase only works in one direction– Leading strand synthesized continuously– Lagging strand synthesized discontinuously
into segments– DNA ligase splices short segments of
discontinuous strand together
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DNA Replication
• End result: two identical DNA molecules formed from original– During mitotic cell division one complete copy
given to new cell; one retained in original cell
• Process is called semiconservative replication– Each DNA composed of one old and one new
strand
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Figure 3.32 Replication of DNA: summary.
ChromosomeFree nucleotides DNA polymerase Old (parental) strand acts as a
template for synthesis of newstrand
Leadingstrand
Replicationbubble
Laggingstrand
Replicationfork
Enzymes unwindthe double helix andexpose the bases
OldDNA
DNApolymerase
Old (template)strand
Two new strands (leadingand lagging) synthesizedin opposite directions
Adenine Thymine Cytosine Guanine
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PLAYPLAY Animation: DNA Replication
DNA Replication
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Cell Division
• Meiosis - cell division producing gametes• Mitotic cell division - produces clones
– Essential for body growth and tissue repair– Occurs continuously in some cells• Skin; intestinal lining
– None in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle• Repairs with fibrous tissue
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Events Of Cell Division
• Mitosis—division of nucleus– Four stages ensure each cell receives copy of
replicated DNA• Prophase• Metaphase• Anaphase• Telophase
– Cytokinesis—division of cytoplasm-by cleavage furrow
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Figure 3.31 The cell cycle.
G1 checkpoint(restriction point)
SGrowth and DNA
synthesis
G1
Growth
G2
Growth and finalpreparations for
divisionM
Prophase
Metaphase
An
aph
ase
Telo
ph
aseC
ytokinesisG2 checkpoint
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PLAYPLAY Animation: Mitosis
Cell Division
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Prophase
• Chromosomes become visible, each with two chromatids joined at centromere
• Centrosomes separate and migrate toward opposite poles
• Mitotic spindles and asters form
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Prophase
• Nuclear envelope fragments
• Kinetochore microtubules attach to kinetochore of centromeres and draw them toward equator of cell
• Polar microtubules assist in forcing poles apart
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (2 of 6)
Early ProphaseEarly mitoticspindle
Chromosomeconsisting of twosister chromatids
Centromere
Aster
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (3 of 6)
Late Prophase
Kinetochore
Spindle pole Polar microtubuleFragmentsof nuclearenvelope
Kinetochoremicrotubule
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Metaphase
• Centromeres of chromosomes aligned at equator
• Plane midway between poles called metaphase plate
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (4 of 6)
Metaphase
Metaphaseplate
Spindle
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Anaphase
• Shortest phase
• Centromeres of chromosomes split simultaneously—each chromatid becomes a chromosome
• Chromosomes (V shaped) pulled toward poles by motor proteins of kinetochores
• Polar microtubules continue forcing poles apart
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (5 of 6)
Anaphase
Daughterchromosomes
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Telophase
• Begins when chromosome movement stops
• Two sets of chromosomes uncoil to form chromatin
• New nuclear membrane forms around each chromatin mass
• Nucleoli reappear
• Spindle disappears
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Cytokinesis
• Begins during late anaphase
• Ring of actin microfilaments contracts to form cleavage furrow
• Two daughter cells pinched apart, each containing nucleus identical to original
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (6 of 6)
Telophase CytokinesisNuclearenvelopeforming
Nucleolus forming Contractilering atcleavagefurrow
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Control of Cell Division
• "Go" signals:– Critical volume of cell when area of
membrane inadequate for exchange– Chemicals (e.g., growth factors, hormones)– Availability of space–contact inhibition
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Control of Cell Division
• To replicate DNA and enter mitosis requires– Cyclins–regulatory proteins
• Accumulate during interphase
– Cdks (Cyclin-dependent kinases)–bind to cyclins activated
• Enzyme cascades prepare cell for division
– Cyclins destroyed after mitotic cell division
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Control of Cell Division
• "Go" signals– G1 checkpoints (restriction point) most
important• If doesn't pass G0–no further division
– Late in G2 MPF (M-phase promoting factor) required to enter M phase
• "Other Controls" signals– Repressor genes inhibit cell division
• E.g., P53 gene
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• DNA is master blueprint for protein synthesis
• Gene - segment of DNA with blueprint for one polypeptide
• Triplets (three sequential DNA nitrogen bases) form genetic library– Bases in DNA are A, G, T, and C– Each triplet specifies coding for number, kind,
and order of amino acids in polypeptide
PLAYPLAY Animation: DNA and RNA
Protein Synthesis
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Protein Synthesis
• Genes composed of exons and introns– Exons code for amino acids– Introns–noncoding segments
• Role of RNA– DNA decoding mechanism and messenger– Three types–all formed on DNA in nucleus
• Messenger RNA (mRNA); ribosomal RNA (rRNA); transfer RNA (tRNA)
• RNA differs from DNA– Uracil is substituted for thymine
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Roles of the Three Main Types of RNA
• Messenger RNA (mRNA)– Carries instructions for building a polypeptide,
from gene in DNA to ribosomes in cytoplasm
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Roles of the Three Main Types of RNA
• Ribosomal RNA (rRNA)– Structural component of ribosomes that, along
with tRNA, helps translate message from mRNA
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Roles of the Three Main Types of RNA
• Transfer RNAs (tRNAs)– Bind to amino acids and pair with bases of
codons of mRNA at ribosome to begin process of protein synthesis
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Figure 3.34 Simplified scheme of information flow from the DNA gene to mRNA to protein structure during transcription and translation.
RNA Processing Pre-mRNA
DNATranscription
Polypeptide
Ribosome
Nuclearpores
Translation
Nuclearenvelope
mRNA
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Protein Synthesis
• Occurs in two steps– Transcription
• DNA information coded in mRNA
– Translation• mRNA decoded to assemble polypeptides
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Transcription
• Transfers DNA gene base sequence to complementary base sequence of mRNA
• Transcription factors–gene activators– Loosen histones from DNA in area to be
transcribed– Bind to promoter-DNA sequence specifying
start site of gene on template strand– Mediate binding of RNA polymerase (enzyme
synthesizing mRNA) to promoter
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Transcription
• Three phases– Initiation
• RNA polymerase separates DNA strands
– Elongation• RNA polymerase adds complementary nucleotides
– Termination• Termination signal indicates "stop"
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Processing of mRNA
• mRNA edited and processed before translation– Introns removed by spliceosomes– mRNA complex proteins associate to guide
export, ensure accuracy for translation
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Figure 3.35 Overview of stages of transcription.
The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid.
RNApolymerase
Unwindingof DNA
Coding strand of DNA
TemplatestrandmRNA
DNA-RNA hybrid region
Direction oftranscription
Rewinding of DNA
RNA nucleotides
RNA polymerase
Promoterregion
Template strand Termination
signal
DNACoding strand
mRNA Template strand
mRNA transcript
Completed mRNA transcript
RNA polymerase
Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed mRNA transcript are released.
2
Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand.
Elongation: As the RNA polymerase moves along the template strand, elongating the mRNA transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it.
1
3
Slide 1
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Figure 3.35 Overview of stages of transcription.RNA polymerase
Promoterregion
Template strand Termination
signal
DNA
Coding strand
Slide 2
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Figure 3.35 Overview of stages of transcription.RNA polymerase
Promoterregion
Template strand Termination
signal
DNA
mRNA Template strand
Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand.
1
Coding strand
Slide 3
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Figure 3.35 Overview of stages of transcription.
The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid.
RNApolymerase
Unwindingof DNA
Coding strand of DNA
TemplatestrandmRNA
DNA-RNA hybrid region
Direction oftranscription
Rewinding of DNA
RNA nucleotides
RNA polymerase
Promoterregion
Template strand Termination
signal
DNA
mRNA Template strand
mRNA transcript
2
Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand.
Elongation: As the RNA polymerase moves along the template strand, elongating the mRNA transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it.
1
Coding strand
Slide 4
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Figure 3.35 Overview of stages of transcription.
The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid.
RNApolymerase
Unwindingof DNA
Coding strand of DNA
TemplatestrandmRNA
DNA-RNA hybrid region
Direction oftranscription
Rewinding of DNA
RNA nucleotides
RNA polymerase
Promoterregion
Template strand Termination
signal
DNA
mRNA Template strand
mRNA transcript
Completed mRNA transcript
RNA polymerase
Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed mRNA transcript are released.
2
Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand.
Elongation: As the RNA polymerase moves along the template strand, elongating the mRNA transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it.
1
3
Coding strand
Slide 5
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Translation
• Converts base sequence of nucleic acids into amino acid sequence of proteins
• Involves mRNAs, tRNAs, and rRNAs
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Genetic Code
• Each three-base sequence on DNA (triplet) represented by codon– Codon—complementary three-base
sequence on mRNA– Some amino acids represented by more than
one codon
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G
U
C
A
U
C
GA
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
TH
IRD
BA
SE
FIR
ST B
ASE
SECOND BASE
GGG
GGU
GGA
GGC
lle
Phe
GUG
GUU
GUA
GUC
Trp
GCG
GCU
GCA
GCC
Stop
Thr
Stop
GAG
GAU
GAA
GAC
Tyr Cys
Stop
UGG
UGU
UGA
UGC
UUG
UUU
UUA
UUC
UCG
UCU
UCA
UCC
UAG
UAU
UAA
UAC
Leu
Ser
Lys
Asn Ser
CGG
CGU
CGA
CGCLeu
CUG
CUU
CUA
CUC
CCG
CCU
CCA
CCCPro
CAG
CAU
CAA
CAC
Gln
His
Arg
Met orStart
Glu
AGG
AGU
AGA
AGC
AUG
AUU
AUA
AUC
ACG
ACU
ACA
ACC
AAG
AAU
AAA
AAC
Arg
Val
Asp
GlyAla
Figure 3.36 The genetic code.
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Role of tRNA
• 45 different types• Binds specific amino acid at one end
(stem) • Anticodon at other end (head) binds
mRNA codon at ribosome by hydrogen bonds– E.g., if codon = AUA, anticodon = UAU
• Ribosome coordinates coupling of mRNA and tRNA; contains three sites– Aminoacyl site; peptidyl site; exit site
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Sequence of Events in Translation
• Three phases that require ATP, protein factors, and enzymes– Initiation– Elongation– Termination
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Translation: Initiation
• Small ribosomal subunit binds to initiator tRNA and mRNA to be decoded; scans for start codon
• Large and small ribosomal units attach, forming functional ribosome
• At end of initiation– tRNA in P site; A site vacant
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Translation: Elongation
• Three steps– Codon recognition
• tRNA binds complementary codon in A site
– Peptide bond formation• Amino acid of tRNA in P site bonded to amino acid
of tRNA in A site
– Translocation• tRNAs move one position–A P; P E
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Translation: Elongation
• New amino acids added by other tRNAs as ribosome moves along mRNA
• Initial portion of mRNA can be "read" by additional ribosomes– Polyribosome
• multiple ribosome-mRNA complex
– Produces multiple copies of same protein
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Figure 3.38 Polyribosome arrays.
Growing polypeptides Completedpolypeptide
Incomingribosomalsubunits
PolyribosomeStart of mRNA End of
mRNA
Each polyribosome consists of one strand of mRNAbeing read by several ribosomes simultaneously. In thisdiagram, the mRNA is moving to the left and the “oldest”functional ribosome is farthest to the right.
This transmission electron micrograph shows a large polyribosome (400,0003).
Ribosomes
mRNA
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Translation: Termination
• When stop codon (UGA, UAA, UAG) enters A site– Signals end of translation– Protein release factor binds to stop codon
water added to chain release of polypeptide chain; separation of ribosome subunits; degradation of mRNA
– Protein processed into functional 3-D structure
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Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 1
Elongation. Amino acids are added one ata time to the growing peptide chain via aprocess that has three repeating steps.
2
Amino acidcorrespondingto anticodon
Templatestrand of DNA
Pre-mRNA
mRNA
Nucleus (siteof transcription)
Amino acidcorrespondingto anticodon
Met
tRNA
The correct amino acid is attached to each species of tRNA by a synthetase enzyme. Ile
Pro
Leu
tRNAanticodon
Polypeptide
IlePro
ComplementarymRNA codon
Leu
New peptidebond
ReleasedtRNA
ProLeu
Ile
APE
APE
Peptide bondformation. The growingpolypeptide bound to thetRNA at the P site istransferred to the aminoacid carried by the tRNAin the A site, and a newpeptide bond is formed.
2b
2c Translocation. As theentire ribosome translocates, itshifts by one codon along themRNA:• The unloaded tRNA in the P site is moved to the E site and then released.• The tRNA in the A site moves to the P site.• The next codon to be translated is now in the empty A site ready for step 2a again.
Direction ofribosome movement
PolypeptideRelease factor
Stop codon
PE
Termination. When a stop codon (UGA,UAA, or UAG) arrives at the A site, elongationends. Release of the newly made polypeptideis triggered by a release factor and theribosomal subunits separate, releasing themRNA.
3
Codon recognition.The anticodon of anincoming tRNA binds withthe complementary mRNAcodon (A to U and C to G)in the A site of theribosome.
2a
Smallribosomalsubunit
Start codon
Asite
Psite
Esite
Initiation. Initiation occurswhen four components combine:• A small ribosomal subunit• An initiator tRNA that carries the amino acid methionine• The mRNA• A large ribosomal subunit Once this is accomplished, the nextphase, elongation, begins.
1
Initiator tRNAbearing anticodon
Aminoacyl-tRNAsynthetaseMet
Cytosol (siteof translation)
Met
Newly made (andedited) mRNAleaves nucleus andtravels to a free orattached ribosomefor decoding.
Methionine(amino acid)
Largeribosomalsubunit
U A C
UA
C
C
C
C
U
A
U
U
U
A
A
G
G
APEGGC
GGC
GAU
GAUGAUACC CUA
ACCGCU CUC
ACUGGG UGACCU
GAUACC CUA
GAU
GGC
GAC
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Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 2
Templatestrand of DNA
Pre-mRNA
Nucleus (siteof transcription)
Amino acidcorrespondingto anticodon
Met
The correct amino acid is attached to each species of tRNA by a synthetase enzyme.
Aminoacyl-tRNAsynthetase
Methionine(amino acid)Newly made
(and edited)mRNA leavesnucleus andtravels to a freeor attachedribosome fordecoding.
tRNA
Initiator tRNAbearing anticodon
Largeribosomalsubunit
Start codon
Smallribosomalsubunit
Esite
Met
Psite
Asite
Met
Cytosol (siteof translation)
Initiation. Initiation occurswhen four components combine:• A small ribosomal subunit• An initiator tRNA that carries the amino acid methionine• The mRNA• A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins.
1U A C
UA
C
C
C
UA
U
U
UA
A
G
G
C
mRNA
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Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 3
Elongation. Amino acids areadded one at a time to thegrowing peptide chain via a processthat has three repeating steps.
2
Amino acidcorrespondingto anticodon
Pro
tRNAanticodon
ComplementarymRNA codon
Codon recognition.The anticodon of anincoming tRNA binds withthe complementary mRNAcodon (A to U and C to G) inthe A site of the ribosome.
lle
Leu
APEGGC
GAUACC CUA
GAU
2a
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Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 4
Polypeptide
New peptidebond
IlePro
Leu
GAUA CC CUAGGGC AU
E P A
Peptide bondformation. The growingpolypeptide bound to thetRNA at the P site istransferred to the amino acid carried by the tRNA in the A site, and a new peptide bondis formed.
2b
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Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 5
ReleasedtRNA
E P AG AU
C CG CUA CUC
GGC LeuProIle
Translocation. As the entire ribosome translocates, it shifts by one codon along the mRNA:• The unloaded tRNA in the P site is moved to the E site and then released.• The tRNA in the A site moves to the P site.• The next codon to be translated is now in the empty A site ready for step 2a again.
Direction of ribosome movement
2c
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Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 6
Release factor
Stop codon
ACU
G GGU GACC
U
E P
GAC
Termination. When a stop codon (UGA,UAA, or UAG) arrives at the A site, elongation ends. Release of the newly made polypeptide is triggered by a release factor and the ribosomal subunits separate, releasing the mRNA.
3
© 2013 Pearson Education, Inc.
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 7
PolypeptideRelease factor
Stop codon
ACU
G GGU GACC
U
E P
GAC
Termination. When a stop codon (UGA,UAA, or UAG) arrives at the A site, elongationends. Release of the newly made polypeptideis triggered by a release factor and theribosomal subunits separate, releasing themRNA.
3
© 2013 Pearson Education, Inc.
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in theribosome to form a particular polypeptide.
Slide 8
Elongation. Amino acids are added one ata time to the growing peptide chain via aprocess that has three repeating steps.
2
Amino acidcorrespondingto anticodon
Templatestrand of DNA
Pre-mRNA
mRNA
Nucleus (siteof transcription)
Amino acidcorrespondingto anticodon
Met
tRNA
The correct amino acid is attached to each species of tRNA by a synthetase enzyme. Ile
Pro
Leu
tRNAanticodon
Polypeptide
IlePro
ComplementarymRNA codon
Leu
New peptidebond
ReleasedtRNA
ProLeu
Ile
APE
APE
Peptide bondformation. The growingpolypeptide bound to thetRNA at the P site istransferred to the aminoacid carried by the tRNAin the A site, and a newpeptide bond is formed.
2b
2c Translocation. As theentire ribosome translocates, itshifts by one codon along themRNA:• The unloaded tRNA in the P site is moved to the E site and then released.• The tRNA in the A site moves to the P site.• The next codon to be translated is now in the empty A site ready for step 2a again.
Direction ofribosome movement
PolypeptideRelease factor
Stop codon
PE
Termination. When a stop codon (UGA,UAA, or UAG) arrives at the A site, elongationends. Release of the newly made polypeptideis triggered by a release factor and theribosomal subunits separate, releasing themRNA.
3
Codon recognition.The anticodon of anincoming tRNA binds withthe complementary mRNAcodon (A to U and C to G)in the A site of theribosome.
2a
Smallribosomalsubunit
Start codon
Asite
Psite
Esite
Initiation. Initiation occurswhen four components combine:• A small ribosomal subunit• An initiator tRNA that carries the amino acid methionine• The mRNA• A large ribosomal subunit Once this is accomplished, the nextphase, elongation, begins.
1
Initiator tRNAbearing anticodon
Aminoacyl-tRNAsynthetaseMet
Cytosol (siteof translation)
Met
Newly made (andedited) mRNAleaves nucleus andtravels to a free orattached ribosomefor decoding.
Methionine(amino acid)
Largeribosomalsubunit
U A C
UA
C
C
C
C
U
A
U
U
U
A
A
G
G
APEGGC
GGC
GAU
GAUGAUACC CUA
ACCGCU CUC
ACUGGG UGACCU
GAUACC CUA
GAU
GGC
GAC
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Role of Rough ER in Protein Synthesis
• mRNA–ribosome complex directed to rough ER by signal-recognition particle (SRP)
• Forming protein enters ER
• Sugar groups may be added to protein, and its shape may be altered
• Protein enclosed in vesicle for transport to Golgi apparatus
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Figure 3.39 Rough ER processing of proteins. Slide 1
The SRP directs themRNA-ribosome complex to therough ER. There the SRP binds toa receptor site.
Once attached to the ER, the SRP isreleased and the growing polypeptidesnakes through the ER membrane poreinto the cistern.
An enzyme clips off the signal sequence. As protein synthesiscontinues, sugar groups may beadded to the protein.
In this example, the completed proteinis released from the ribosome and foldsinto its 3-D conformation, a process aidedby molecular chaperones.
The protein is enclosed within aprotein coated transport vesicle. Thetransport vesicles make their way tothe Golgi apparatus, where furtherprocessing of the proteins occurs(see Figure 3.19).
Signalrecognitionparticle(SRP) Receptor site
Rough ER cistern
Growingpolypeptide
Signalsequenceremoved
Sugargroup
Releasedprotein
ER signalsequence
Ribosome
mRNA
CytosolTransport vesiclepinching off
Protein-coatedtransport vesicle
1 2
3
4
5
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Figure 3.39 Rough ER processing of proteins. Slide 2
The SRP directs themRNA-ribosome complex to therough ER. There the SRP binds toa receptor site.
Signalrecognitionparticle(SRP) Receptor site
Rough ER cistern
ER signalsequence
mRNA
Cytosol
1
Ribosome
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Figure 3.39 Rough ER processing of proteins. Slide 3
The SRP directs themRNA-ribosome complex to therough ER. There the SRP binds toa receptor site.
Signalrecognitionparticle(SRP) Receptor site
Rough ER cistern
ER signalsequence
mRNA
Cytosol
1 U Once attached to the ER, the SRP isreleased and the growing polypeptidesnakes through the ER membrane poreinto the cistern.
Growingpolypeptide
2
Ribosome
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Figure 3.39 Rough ER processing of proteins. Slide 4
The SRP directs themRNA-ribosome complex to therough ER. There the SRP binds toa receptor site.
Signalrecognitionparticle(SRP) Receptor site
Rough ER cistern
ER signalsequence
mRNA
Cytosol
1 U Once attached to the ER, the SRP isreleased and the growing polypeptidesnakes through the ER membrane poreinto the cistern.
Growingpolypeptide
2
Signalsequenceremoved
Sugargroup
Ribosome
An enzyme clips off the signal sequence. As protein synthesiscontinues, sugar groups may beadded to the protein.
3
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Figure 3.39 Rough ER processing of proteins. Slide 5
The SRP directs themRNA-ribosome complex to therough ER. There the SRP binds toa receptor site.
Signalrecognitionparticle(SRP) Receptor site
Rough ER cistern
ER signalsequence
mRNA
Cytosol
1 U Once attached to the ER, the SRP isreleased and the growing polypeptidesnakes through the ER membrane poreinto the cistern.
Growingpolypeptide
2
Signalsequenceremoved
Sugargroup
Releasedprotein
In this example, the completed proteinis released from the ribosome and foldsinto its 3-D conformation, a process aidedby molecular chaperones.
4
Ribosome
An enzyme clips off the signal sequence. As protein synthesiscontinues, sugar groups may beadded to the protein.
3
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Figure 3.39 Rough ER processing of proteins. Slide 6
The SRP directs themRNA-ribosome complex to therough ER. There the SRP binds toa receptor site.
Once attached to the ER, the SRP isreleased and the growing polypeptidesnakes through the ER membrane poreinto the cistern.
An enzyme clips off the signal sequence. As protein synthesiscontinues, sugar groups may beadded to the protein.
In this example, the completed proteinis released from the ribosome and foldsinto its 3-D conformation, a process aidedby molecular chaperones.
The protein is enclosed within aprotein coated transport vesicle. Thetransport vesicles make their way tothe Golgi apparatus, where furtherprocessing of the proteins occurs(see Figure 3.19).
Signalrecognitionparticle(SRP) Receptor site
Rough ER cistern
Growingpolypeptide
Signalsequenceremoved
Sugargroup
Releasedprotein
ER signalsequence
Ribosome
mRNA
CytosolTransport vesiclepinching off
Protein-coatedtransport vesicle
1 2
3
4
5
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Summary: From DNA to Proteins
• Complementary base pairing directs transfer of genetic information in DNA into amino acid sequence of protein– DNA triplets mRNA codons– Complementary base pairing of mRNA
codons with tRNA anticodons ensures correct amino acid sequence
– Anticodon sequence identical to DNA sequence except uracil substituted for thymine
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Figure 3.40 Information transfer from DNA to RNA to polypeptide.
DNA: DNA base sequence(triplets) of the gene codesfor synthesis of aParticular polypeptide chain
DNAmolecule
Gene 1
Gene 2
Codons
Triplets
Anticodon
tRNA
Stop;detachStart
translation
mRNA: Base sequence(codons) of the transcribed mRNA
tRNA: Consecutive base sequences of tRNAanticodons recognize themRNA codons calling forthe amino acids theytransport
Polypeptide: Amino acidsequence of thepolypeptide chain
Gene 4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
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Other Roles of DNA
• Intron ("junk") regions of DNA code for other types of RNA:– Antisense RNA
• Prevents protein-coding RNA from being translated
– MicroRNA• Small RNAs that silence mRNAs made by certain
exons
– Riboswitches• Folded RNAs that act as switches regulating
protein synthesis in response to environmental conditions
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Cytosolic Protein Degradation
• Autophagy– Cytoplasmic bits and nonfunctional organelles
put into autophagosomes; degraded by lysosomes
• Ubiquitins – Tag damaged or unneeded soluble proteins in
cytosol– Digested by soluble enzymes or proteasomes
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Extracellular Materials
• Body fluids-interstitial fluid, blood plasma, and cerebrospinal fluid
• Cellular secretions-intestinal and gastric fluids, saliva, mucus, and serous fluids
• Extracellular matrix–most abundant extracellular material– Jellylike mesh of proteins and
polysaccharides secreted by cells; acts as "glue" to hold cells together
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Developmental Aspects of Cells
• All cells of body contain same DNA but cells not identical
• Chemical signals in embryo channel cells into specific developmental pathways by turning some genes on and others off
• Development of specific and distinctive features in cells called cell differentiation
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Apoptosis and Modified Rates of Cell Division
• During development more cells than needed produced (e.g., in nervous system)
• Eliminated later by programmed cell death (apoptosis)– Mitochondrial membranes leak chemicals that
activate caspases DNA, cytoskeleton degradation cell death
– Dead cell shrinks and is phagocytized
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Apoptosis and Modified Rates of Cell Division
• Organs well formed and functional before birth
• Cell division in adults to replace short-lived cells and repair wounds
• Hyperplasia increases cell numbers when needed
• Atrophy (decreased size) results from loss of stimulation or use
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Theories of Cell Aging
• Wear and tear theory-Little chemical insults and free radicals have cumulative effects
• Mitochondrial theory of aging–free radicals in mitochondria diminish energy production
• Immune system disorders-autoimmune responses; progressive weakening of immune response; C-reactive protein of acute inflammation causes cell aging
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Theories of Cell Aging
• Most widely accepted theory– Genetic theory-cessation of mitosis and cell
aging programmed into genes• Telomeres (strings of nucleotides protecting ends
of chromosomes) may determine number of times a cell can divide
• Telomerase lengthens telomeres– Found in germ cells; ~ absent in adult cells