© 2013 Pearson Education, Inc. Cell Cycle Defines changes from formation of cell until it reproduces Includes: –Interphase Cell grows and carries out functions

© 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


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


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


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


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


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


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


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


PLAYPLAY Animation: DNA Replication

DNA Replication


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


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


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


PLAYPLAY Animation: Mitosis

Cell Division


Prophase

• Chromosomes become visible, each with two chromatids joined at centromere

• Centrosomes separate and migrate toward opposite poles

• Mitotic spindles and asters form


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



Early ProphaseEarly mitoticspindle

Chromosomeconsisting of twosister chromatids

Centromere

Aster



Late Prophase

Kinetochore

Spindle pole Polar microtubuleFragmentsof nuclearenvelope

Kinetochoremicrotubule


Metaphase

• Centromeres of chromosomes aligned at equator

• Plane midway between poles called metaphase plate



Metaphase

Metaphaseplate

Spindle


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



Anaphase

Daughterchromosomes


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


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



Telophase CytokinesisNuclearenvelopeforming

Nucleolus forming Contractilering atcleavagefurrow


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



• 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



• "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


• 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


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


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



• Ribosomal RNA (rRNA)– Structural component of ribosomes that, along

with tRNA, helps translate message from mRNA



• Transfer RNAs (tRNAs)– Bind to amino acids and pair with bases of

codons of mRNA at ribosome to begin process of protein synthesis


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


Protein Synthesis

• Occurs in two steps– Transcription

• DNA information coded in mRNA

– Translation• mRNA decoded to assemble polypeptides


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


Transcription

• Three phases– Initiation

• RNA polymerase separates DNA strands

– Elongation• RNA polymerase adds complementary nucleotides

– Termination• Termination signal indicates "stop"


Processing of mRNA

• mRNA edited and processed before translation– Introns removed by spliceosomes– mRNA complex proteins associate to guide

export, ensure accuracy for translation


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


Figure 3.35 Overview of stages of transcription.RNA polymerase

Promoterregion


signal

DNA

Coding strand

Slide 2


Figure 3.35 Overview of stages of transcription.RNA polymerase

Promoterregion


signal

DNA



1

Coding strand

Slide 3




RNApolymerase

Unwindingof DNA


TemplatestrandmRNA



Rewinding of DNA

RNA nucleotides

RNA polymerase

Promoterregion


signal

DNA


mRNA transcript

2



1

Coding strand

Slide 4




RNApolymerase

Unwindingof DNA


TemplatestrandmRNA



Rewinding of DNA

RNA nucleotides

RNA polymerase

Promoterregion


signal

DNA


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



1

3

Coding strand

Slide 5


Translation

• Converts base sequence of nucleic acids into amino acid sequence of proteins

• Involves mRNAs, tRNAs, and rRNAs


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


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.


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


Sequence of Events in Translation

• Three phases that require ATP, protein factors, and enzymes– Initiation– Elongation– Termination


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


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


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


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


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


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)


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



Slide 2


Pre-mRNA



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



Start codon


Esite

Met

Psite

Asite

Met


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



Slide 3

Elongation. Amino acids areadded one at a time to thegrowing peptide chain via a processthat has three repeating steps.

2


Pro

tRNAanticodon


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



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



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



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



Slide 7


Stop codon

ACU

G GGU GACC

U

E P

GAC


3



Slide 8

Elongation. Amino acids are added one ata time to the growing peptide chain via aprocess that has three repeating steps.

2



Pre-mRNA

mRNA



Met

tRNA

The correct amino acid is attached to each species of tRNA by a synthetase enzyme. Ile

Pro

Leu

tRNAanticodon

Polypeptide

IlePro


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


Stop codon

PE


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


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


Aminoacyl-tRNAsynthetaseMet


Met

Newly made (andedited) mRNAleaves nucleus andtravels to a free orattached ribosomefor decoding.

Methionine(amino acid)


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


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


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





Rough ER cistern

ER signalsequence

mRNA

Cytosol

1

Ribosome





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





Rough ER cistern

ER signalsequence

mRNA

Cytosol


Growingpolypeptide

2


Sugargroup

Ribosome


3





Rough ER cistern

ER signalsequence

mRNA

Cytosol


Growingpolypeptide

2


Sugargroup

Releasedprotein


4

Ribosome


3




Once attached to the ER, the SRP isreleased and the growing polypeptidesnakes through the ER membrane poreinto the cistern.



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).


Rough ER cistern

Growingpolypeptide


Sugargroup

Releasedprotein

ER signalsequence

Ribosome

mRNA

CytosolTransport vesiclepinching off

Protein-coatedtransport vesicle

1 2

3

4

5


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


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


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


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


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


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


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


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


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


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

Documents

© 2013 Pearson Education, Inc. Cell Cycle Defines changes from formation of cell until it reproduces Includes: –Interphase Cell grows and carries out functions