CELLULAR CELLULAR DIVISIONDIVISION

Lecture 4Genetics

Dr. Heba Al-Fares

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Cell Reproduction For any cell to reproduce successfully,

three fundamental events must take place:(1)its genetic information must be copied,(2)the copies of genetic information must be

separated from one another, and (3)the cell must divide. All cellular reproduction includes these

three events, but the processes that lead to these events differ in prokaryotic (bacteria) (bacteria) and eukaryotic cells (protists, (protists, fungi, plants, & animals)fungi, plants, & animals).

Cell DivisionCell Division

All cells are derived fromAll cells are derived from pre-existingpre-existing cellscells

New cells are produced for New cells are produced for growthgrowth and to replace and to replace damaged or old cells.damaged or old cells.

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Types of Cell Types of Cell ReproductionReproductionAsexual reproductionAsexual reproduction involves involves

a a single cellsingle cell dividing to make dividing to make 2 2 new, identical daughter cellsnew, identical daughter cellsMitosis & binary fission Mitosis & binary fission are examples are examples

of asexual reproductionof asexual reproduction

Sexual reproductionSexual reproduction involves involves two cellstwo cells (egg & sperm) joining (egg & sperm) joining to make a to make a new cell (zygote)new cell (zygote) that is that is NOT identical to the NOT identical to the original cellsoriginal cellsMeiosis Meiosis is an exampleis an example

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Prokaryotic Cell Reproduction• When prokaryotic cells reproduce, the circular

chromosome of the bacterium is replicated. • The two resulting identical copies are attached

to the plasma membrane, which grows and gradually separates the two chromosomes.

• Finally, a new cell wall forms between the two chromosomes, producing two cells, each with an identical copy of the chromosome.

• Under optimal conditions, some bacterial cells divide every 20 minutes. At this rate, a single bacterial cell could produce a billion descendants in a mere 10 hours.

Prokaryotic ChromosomeProkaryotic Chromosome

The DNA of The DNA of prokaryotes prokaryotes (bacteria) is (bacteria) is one, circular one, circular chromosomechromosome attached to attached to the inside of the inside of the cell the cell membranemembrane

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Cell Division in Cell Division in ProkaryotesProkaryotes Prokaryotes such as Prokaryotes such as

bacteriabacteria divide into 2 divide into 2 identical cells by the identical cells by the process of process of binary fissionbinary fission

Binary fission: the Binary fission: the physical process whereby physical process whereby a bacterial cell divides a bacterial cell divides into two daughter cellsinto two daughter cells

Single chromosome Single chromosome makes a copy of itselfmakes a copy of itself

Cell wall forms between Cell wall forms between the chromosomes the chromosomes dividing the celldividing the cell by the growth of a transverse septum

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Parent cellParent cell

22 identical daughter cellsidentical daughter cells

Chromosome Chromosome doublesdoubles

Cell splitsCell splits

Eukaryotic ChromosomesEukaryotic Chromosomes All All eukaryoticeukaryotic cells store genetic information in cells store genetic information in

chromosomeschromosomes Most eukaryotes have between Most eukaryotes have between 10 and 50 chromosomes10 and 50 chromosomes in in

their body cells, their body cells, potatoes have 48 chromosomes, fruit flies have 8, and humans have 46.

There appears to be no special significance between the complexity of an organism and its number of chromosomes per cell.

Each chromosome in one set has a corresponding chromosome in the other set, together constituting a homologous pair.

The two chromosomes of a homologous pair are usually alike in structure and size, and each carries genetic information for the same set of hereditary characteristics. (An exception is the sex chromosomes,

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cell division• The process of cell division is necessary

for life to function. • In this process, one cell, the parent cell,

divides into two daughter cells. • This is called the cell cycle and has four

distinct steps: a G1 (or gap-1) phase, an S phase, a G2 (or gap-2) phase, and mitosis.

Five Phases of the Cell Five Phases of the Cell CycleCycle

• GG11 - - a cell grows in size and prepares to replicate (or copy) the chromosomes.

• S – S – is the synthesizing phase, when the DNA molecule is replicated, doubling it.

GG22 - secondary growth phase - secondary growth phasecollectively these 3 stages are called collectively these 3 stages are called

interphaseinterphase• M – mitosis; M – mitosis; is the stage in which the

cell actually divides into two daughter cells, each with its own copy of the DNA molecule

C - cytokinesisC - cytokinesis10

Cell CycleCell Cycle

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Interphase - GInterphase - G11 Stage Stage 11stst growth stage growth stage after cell division after cell division Cells Cells maturemature by making more cytoplasm & by making more cytoplasm &

organellesorganelles Cell carries on its Cell carries on its normal metabolic activitiesnormal metabolic activities There is a critical point in the cell cycle, termed the G1/S

checkpoint, in G1; after this checkpoint has been passed, the cell is committed to divide.

Before reaching the G1/S checkpoint, cells may exit from the active cell cycle in response to regulatory signals and pass into a nondividing phase called G0, which is a stable state during which cells usually maintain a constant size.

They can remain in G0 for an extended period of time, even indefinitely, or they can re-enter G1 and the active cell cycle.

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Interphase – S StageInterphase – S Stage SynthesisSynthesis stage stage DNADNA is copied or is copied or replicatedreplicated If DNA synthesis is blocked (with drugs or by a mutation), the cell will

not be able to undergo mitosis. Before S phase, each chromosome is composed of one chromatid;

following S phase, each chromosome is composed of two chromatids

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Two Two identical identical copies of copies of

DNADNA

Original DNAOriginal DNA

Replication is the process of making replicas or copies (DNA synthesis)

Interphase – GInterphase – G22 Stage Stage

22ndnd Growth Growth Stage StageOccurs Occurs after DNA has been after DNA has been

copiedcopiedAll cell All cell structures needed for structures needed for

divisiondivision are made (e.g. are made (e.g. centrioles)centrioles)

Both Both organelles & proteinsorganelles & proteins are are synthesizedsynthesized

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• Centrioles is cytoplasmic bodies contain in special region in the centrosome.

• Function:• The centrioles, small organelles that will produce

spindle fibers needed to allow the cell to divide, also move to the opposite ends of the cell during this phase.- Spindle fiber occurs during early stage of mitosis and

meiosis composed of arrays of microtubules these fibers plays an important role in the movement of chromosome during cell division

- microtubules polymers of alpha and beta subunits of the protein tubulin.

• Centrosome: cellular structure from which microtubules originate.

What’s Happening in What’s Happening in InterphaseInterphase??

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What the cell looks likeWhat the cell looks like

Animal Cell

What’s occurring

Sketch the Cell CycleSketch the Cell Cycle

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Daughter Daughter CellsCells

DNA CopiedDNA Copied

Cells Cells MatureMature

Cells prepare for Cells prepare for DivisionDivision

Cell Divides into Identical Cell Divides into Identical cellscells

MitosisMitosisDivision of the Division of the

nucleusnucleusAlso called Also called

karyokinesiskaryokinesisOnly occurs in Only occurs in

eukaryoteseukaryotesHas Has fourfour stages stagesDoesn’t occur Doesn’t occur

in some cells in some cells such as such as brain brain cellscells

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Four Mitotic StagesFour Mitotic Stages

ProProphasephaseMetaMetaphasephaseAnaAnaphasephaseTeloTelophasephase

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Early ProphaseEarly ProphaseChromatinChromatin in nucleus condenses to in nucleus condenses to

form form visible chromosomes through a visible chromosomes through a process of DNA condensation and its process of DNA condensation and its associated histone protein in to a associated histone protein in to a series of coiling. series of coiling.

Mitotic spindle Mitotic spindle forms from fibers in forms from fibers in cytoskeleton or cytoskeleton or centrioles (animal)centrioles (animal)

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ChromosomesChromosomes

NucleolusNucleolus CytoplasmCytoplasm

Nuclear MembraneNuclear Membrane

Late ProphaseLate Prophase Nuclear membrane & nucleolusNuclear membrane & nucleolus are broken downare broken down ChromosomesChromosomes continue condensing & are continue condensing & are clearly clearly

visiblevisible Centromere replicated Centromere replicated during the formation of during the formation of

sister chromatids sister chromatids Multiproten complex called Multiproten complex called kinetochoreskinetochores attach attach

to each sister centromereto each sister centromere of each chromosomeof each chromosome Spindle fiberSpindle fiber consists of microtubules which are consists of microtubules which are

a chain of protein called tubulin a chain of protein called tubulin finishes forming finishes forming between the between the poles of the cell and poles of the cell and attach to one attach to one sister chromatids from one side and to the other sister chromatids from one side and to the other sister chromatids in the opposite side.sister chromatids in the opposite side.

The The kinetochoreskinetochores act as the sites for act as the sites for attachment attachment of microtubulesof microtubules

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Late ProphaseLate Prophase

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Nucleus & Nucleolus have disintegratedNucleus & Nucleolus have disintegrated

ChromosomesChromosomes

Review of ProphaseReview of Prophase

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What the cell looks likeWhat the cell looks like

What’s happeningWhat’s happening

Three Types of Spindle Three Types of Spindle FibersFibersThe mitotic The mitotic spindlespindle forms from forms from

the the microtubulesmicrotubules in plantsin plants and and centrioles in animal cellscentrioles in animal cells1.1. Polar fibers Polar fibers extend from one pole of extend from one pole of

the cell to the opposite polethe cell to the opposite pole2.2. Kinetochore fibersKinetochore fibers extend from the extend from the

pole to the centromere of the pole to the centromere of the chromosome to which they attachchromosome to which they attach

3.3. AstersAsters are short fibers radiating from are short fibers radiating from centrioles important for positioning centrioles important for positioning spindle apparatus within cell.spindle apparatus within cell.

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Sketch The SpindleSketch The Spindle

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MetaphaseMetaphaseChromosomes, attached to the Chromosomes, attached to the kinetochore kinetochore

fibersfibers, move to the center of the cell, move to the center of the cellSpindle fibers apply tension to the chromosomes,

causing them to line up in the center (the the equator)equator) of the cell.

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Pole of Pole of the Cellthe Cell

Equator of CellEquator of Cell

ChromosomeChromosomes at Equators at Equator

AsterAster

Review of MetaphaseReview of Metaphase

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What the cell looks likeWhat the cell looks like

What’s What’s occurringoccurring

AnaphaseAnaphase Occurs Occurs rapidlyrapidly• The spindle fibers begin

to shorten, pulling the Sister chromatidsSister chromatids apart and toward opposite ends of the cell by by kinetochore fiberskinetochore fibers

As each sister As each sister chromatid moves; its chromatid moves; its two arms two arms appear to appear to trail its centromeres; trail its centromeres; a set of a set of V-shaped V-shaped structures with the structures with the points of the Vs points of the Vs directed to the polesdirected to the poles

• The chromatids are now two separate daughter chromosomes.

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Anaphase ReviewAnaphase Review

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What the What the cell looks cell looks

likelike

What’s What’s occurringoccurring

Removal of the tublin subunits frommicrotubules at the kinetochore and perhaps molecular motors, are responsible for the pole-ward movement of chromosomes duringanaphase.Microtubules pull sister Microtubules pull sister chromatids to opposite poles; the chromatids to opposite poles; the microtubules depolymerize microtubules depolymerize (release free tubulin) at (release free tubulin) at kinetochores resulting in kinetochores resulting in shortening of microtubules shortening of microtubules

TelophaseTelophaseThe daughter chromosomes (Sister Sister

chromatids) chromatids) arrive at the opposite poles of the cell

The spindle fibers disappear (disassembles)disassembles).

A new nuclear envelope also forms around the two sets of daughter chromosomes.

Chromosomes are no longer visible under a light microscope during telophase.

NucleolusNucleolus reappears reappearsCYTOKINESISCYTOKINESIS occurs occursChromosomes reappear as Chromosomes reappear as chromatinchromatin

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Comparison of Anaphase & Comparison of Anaphase & TelophaseTelophase

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CytokinesisCytokinesisThe cytoplasm of the parent cell is

divided into two daughter cells, each of which contains one set of identical chromosomes inside their new nucleus

Means division of the cytoplasmMeans division of the cytoplasmDivisionDivision of cell into two, identical of cell into two, identical

halves called halves called daughter cellsdaughter cellsIn plant cells, cell plate In plant cells, cell plate forms at the forms at the

equator to divide cellequator to divide cellIn animal cells, In animal cells, cleavage furrow cleavage furrow

forms to split cellforms to split cell

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CytokinesisCytokinesis

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Cleavage Cleavage furrow in furrow in

animal cellanimal cell

Cell plate in Cell plate in plant cellplant cell

Daughter Cells of MitosisDaughter Cells of MitosisHave the Have the same number of same number of

chromosomes as each otherchromosomes as each other and as the parent celland as the parent cell from from which they were formedwhich they were formed

BBut ut smaller than parent cellsmaller than parent cellMust Must grow in sizegrow in size to become to become

mature cells mature cells (G(G11 of Interphase) of Interphase)

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Eukaryotic Cell DivisionEukaryotic Cell Division

Used for Used for growth and growth and repairrepair

Produce two new cells Produce two new cells identical to the original identical to the original cellcell

Cells are Cells are diploid (2n)diploid (2n)

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Chromosomes during Chromosomes during Metaphase of mitosisMetaphase of mitosis

ProphaseProphase MetaphaseMetaphase AnaphaseAnaphaseTelophaseTelophase

CytokinesisCytokinesis

Uncontrolled Mitosis If mitosis is not If mitosis is not

controlledcontrolled, , unlimited cell unlimited cell division occurs division occurs causing cancerous causing cancerous tumorstumors

OncogenesOncogenes are are special proteins special proteins that increase the chance increase the chance that a normal cell that a normal cell develops into adevelops into a tumor cell tumor cell

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Cancer cellsCancer cells