Cell Growth and Cell Growth and DivisionDivision

Mitosis and MeiosisMitosis and Meiosis

Cell GrowthCell Growth

When an organism When an organism grows, the number grows, the number of of cellscells increase but increase but the the sizesize of each cell of each cell remains remains smallsmall..

Limits to cell growthLimits to cell growth1.1. DNA “overload”: The DNA “overload”: The largerlarger a cell becomes, a cell becomes,

the more the more demandsdemands the cell places on its the cell places on its DNA.DNA.

a.a.          DNA stores the information DNA stores the information that that controlscontrols how a cell functionshow a cell functions

b.b.        WWhen a cell is hen a cell is smallsmall, DNA can , DNA can meet the cells needsmeet the cells needs

c.c. When a cell is When a cell is largelarge, it still has , it still has only only oneone copy of copy of DNADNA so it is so it is more more difficult for the cell to perform its difficult for the cell to perform its function function

2.2. Exchanging materials: additionally, Exchanging materials: additionally, largelarge cells have more trouble cells have more trouble movingsubstances movingsubstances across the cell across the cell membranemembrane..

a.a.    If a cell is too If a cell is too largelarge, it is difficult , it is difficult to get enough to get enough oxygenoxygen and and

nutrients nutrients inin and and waste products waste products outout

b. This is why cells do NOT grow b. This is why cells do NOT grow much much largerlarger even if the organism does even if the organism does grow grow largelarge

Limits to cell growth

Division of the CellDivision of the Cell

1.1. Before a cell gets Before a cell gets too largetoo large, it , it will will dividedivide to form two to form two ““daughterdaughter” cells” cells

2.2. Before a cell Before a cell dividesdivides, it makes , it makes a a copycopy of its of its DNADNA for each for each daughter celldaughter cell

Cell DivisionCell Division1.1. Cell division in Cell division in eukaryoteseukaryotes is more complex is more complex

than in prokaryotes.than in prokaryotes.2.2. There are There are twotwo stages of eukaryotic cell stages of eukaryotic cell

divisiondivisiona.a. Mitosis: Division of the cell Mitosis: Division of the cell nucleusnucleusb.b. Cytokinesis: Division of the cell Cytokinesis: Division of the cell cytoplasmcytoplasm

3.3. UnicellularUnicellular organisms reproduce organisms reproduce asexuallyasexually by mitosisby mitosisa.a. The daughter cells are The daughter cells are identicalidentical to the to the

parents cellsparents cells 4.4. Mitosis is how a Mitosis is how a multicellularmulticellular organism organism

grows and developsgrows and develops

ChromosomesChromosomes1.1. ChromosomesChromosomes are made of are made of

condensed condensed chromatinchromatin..

2.2. Chromatin consists of Chromatin consists of DNADNA and and the the proteinsproteins it is wrapped around. it is wrapped around.

3.3. The cells of every organism have The cells of every organism have a specific number of a specific number of chromosomes (chromosomes (humanshumans have have 4646 chromosomes).chromosomes).

3.3. ChromosomesChromosomes are only visible during cell division, are only visible during cell division, when they are condensed. The rest of the time the when they are condensed. The rest of the time the chromatinchromatin is spread throughout the nucleus. is spread throughout the nucleus.

4.4. Before cell division, each chromosome is Before cell division, each chromosome is replicatedreplicated (meaning (meaning copiedcopied).).a.a. When a chromosome is replicated, it consists When a chromosome is replicated, it consists of two identical “sister” of two identical “sister” chromatidschromatids..b.b. When a cell divides the chromatids sWhen a cell divides the chromatids separateeparate, , and and one goes to each of the one goes to each of the two new cellstwo new cells..c. Sister chromatids are attached to c. Sister chromatids are attached to

each other at the spot called the each other at the spot called the centromerecentromere..

The Cell CycleThe Cell Cycle

• When a cell is NOT dividing, it is said to be When a cell is NOT dividing, it is said to be in in interphaseinterphase..

• The series of events that a cell goes The series of events that a cell goes through as it through as it grows and dividesgrows and divides is called is called the the cellcell cyclecycle..

Events of the cell cycleEvents of the cell cycleInterphase, when the cell is Interphase, when the cell is NOTNOT dividing, has dividing, has threethree

phases: G1, S, and G2.phases: G1, S, and G2.• 1.1.      G1G1 phase: period of activity in which cells do phase: period of activity in which cells do

most of their growing.most of their growing.– a.a.    Cells Cells increaseincrease in size in size– b.b.    Cells Cells synthesizesynthesize (make) new (make) new proteins and organellesproteins and organelles

• 2.2.      SS phase: DNA ( phase: DNA (chromosomeschromosomes) is ) is replicatedreplicated• 3.3. G2G2: organelles and molecules required: organelles and molecules required for for

cell divisioncell division are produced are produced

M phase is the phase of cell M phase is the phase of cell divisiondivision. . This includes:This includes:

– 1.1.MitosisMitosis, the division of the , the division of the cell cell nucleusnucleus, which is made up of four , which is made up of four segments including prophase, segments including prophase, metaphase, anaphase, and telophase.metaphase, anaphase, and telophase.

– 2.2.CytokinesisCytokinesis, or the division of , or the division of cytoplasmcytoplasm..

Stop at 1 minute

M phase

G2 phase

S phase

G1 phase

MitosisMitosis

There are There are fourfour phases in mitosis: phases in mitosis:

1.1.          ProphaseProphase

a.a. LongestLongest phase in mitosis (take 50- phase in mitosis (take 50-60% of total time mitosis requires)60% of total time mitosis requires)

b.b.          Chromosomes become Chromosomes become visiblevisible because they are condensedbecause they are condensed

c.c.        Centrioles become visible on Centrioles become visible on oppositeopposite sides of the nucleussides of the nucleusi.i. The The centriolescentrioles help organize the help organize the

spindle, a spindle, a structure made of microtubules structure made of microtubules that helps that helps separate the chromosomesseparate the chromosomes

ii.ii. Chromosomes attach to the spindle Chromosomes attach to the spindle fibers near the centromerefibers near the centromere

iii.iii.            Plant cellsPlant cells do not have centrioles but do do not have centrioles but do have have mitotic spindlesmitotic spindles

d.d.          NucleolusNucleolus disappearsdisappears

e.e.        Nuclear Nuclear envelopeenvelope breaks down breaks down

2.2. MetaphaseMetaphasea.a.          Chromosomes Chromosomes line up in theline up in the center center

of of the cellthe cellb.b. Microtubules connect to the Microtubules connect to the

centromerescentromeres

3.3. AnaphaseAnaphasea.a.         Centomeres Centomeres splitsplit and the sister and the sister

chromatids chromatids separateseparateb.b.          The sister chromatids become The sister chromatids become

individual individual chromosomeschromosomes

c.c.           Chromosomes Chromosomes movemove and and separate into separate into twotwo groups groups

nearnear the spindlethe spindled.d. Anaphase ends when the Anaphase ends when the

chromosomes stop movingchromosomes stop moving

4.4. TelophaseTelophase

a.a. Chromosomes change from being Chromosomes change from being condensedcondensed to dispersedto dispersed

b.b. A nuclear envelope forms around each A nuclear envelope forms around each cluster cluster of of chromosomeschromosomesc.c.      SpindleSpindle breaks apart breaks apartd.d. NucleolusNucleolus is is visiblevisible in each daughter nucleus in each daughter nucleus

Telophase in the midbodies of two daughter cells

Start at 1 minute

CytokinesisCytokinesis• Cytokinesis occurs within the Cytokinesis occurs within the cytoplasmcytoplasm of of oneone cell. cell.• Cell division is Cell division is completecomplete when the when the cytoplasmcytoplasm

divides (pinches together to create new cells).divides (pinches together to create new cells).• In In plantsplants, a structure called the cell , a structure called the cell plateplate forms forms

between the two daughter nuclei. The cell plate between the two daughter nuclei. The cell plate develops into develops into a cell membrane and cell walla cell membrane and cell wall..

• In In animalanimal cells, the cell membrane is cells, the cell membrane is drawn inward until the cytoplasm is drawn inward until the cytoplasm is pinchedpinched into into twotwo equal parts. Each part equal parts. Each part has a nucleus and cytoplasmic organelles.has a nucleus and cytoplasmic organelles.

CytokinesisCytokinesis

The cleavage of daughter cells is almost complete; this is visualized by microtubule staining

Centrioles

Chromatin

Interphase

Nuclear envelope

Cytokinesis

Nuclear envelope reforming

Telophase

Anaphase

Individual chromosomesMetaphase

Centriole

Spindle

CentrioleChromosomes (paired

chromatids)

Prophase

Centromere

Spindle forming

MeiosisMeiosis

ChromosomesChromosomes

• You have You have 2323 different different pairs of pairs of chromosomes, for a chromosomes, for a total of total of 4646 chromosomes. chromosomes.

• One chromosome in One chromosome in each pair came from each pair came from your your mothermother and one and one from your from your fatherfather..

Each chromosome in Each chromosome in a pair is said to be a pair is said to be homologoushomologous, , meaning that the meaning that the chromosome from chromosome from the father has a the father has a corresponding corresponding chromosome from chromosome from the mother.the mother.

• Cells that contain both sets of homologous Cells that contain both sets of homologous chromosomes are called chromosomes are called diploiddiploid. .

• All of your cells except the All of your cells except the sexsex cells (sperm and cells (sperm and eggs; also called eggs; also called gametesgametes) are diploid. Gametes ) are diploid. Gametes are are haploidhaploid, meaning they contain only one copy , meaning they contain only one copy of each chromosome.of each chromosome.

• When one sperm and one egg combine their When one sperm and one egg combine their DNADNA, , there are two versions of each chromosome.there are two versions of each chromosome.

haploid haploid

diploid

We use “N” to represent the We use “N” to represent the haploidhaploid number of number of chromosomes and “2N” to represent the chromosomes and “2N” to represent the diploiddiploid number of chromosomes. number of chromosomes.

– 1.1. For humans, the haploid number is For humans, the haploid number is 2323. We write this as N = 23. The . We write this as N = 23. The

diploid number is diploid number is 4646, which we , which we write write as 2N = 46.as 2N = 46.

– 2.2. In fruit flies, N = 4 and 2N = 8.In fruit flies, N = 4 and 2N = 8.

MeiosisMeiosis

Meiosis is the process that divides Meiosis is the process that divides oneone diploid diploid (2N) cell to form (2N) cell to form fourfour haploid (N) cells. haploid (N) cells.

This process is a This process is a reductionalreductional division because division because

the the number of chromosomes per cell are cut in half.number of chromosomes per cell are cut in half.

Meiosis is how Meiosis is how gametesgametes are formed. are formed.

There are two divisions that occur in meiosis: There are two divisions that occur in meiosis: Meiosis I and Meiosis II. Meiosis I and Meiosis II. – 1.1. Before meiosis I begins, cells go through Before meiosis I begins, cells go through

Interphase IInterphase I. . – 2.2. This involves DNA This involves DNA replicationreplication, forming a , forming a

duplicate copy of each chromosome.duplicate copy of each chromosome.– 3.3. Each chromosome is made of Each chromosome is made of twotwo sister sister

chromatids.chromatids.

cell

centriolesNucleus with duplicated DNA that is not condensed.

Meiosis I is similar to mitosis.Meiosis I is similar to mitosis.

a.a.          Prophase I:Prophase I:– i.i.            CentriolesCentrioles are visible on opposite are visible on opposite

sides of the nucleussides of the nucleus– ii.ii.            NucleolusNucleolus disappears disappears– iii.iii.            NuclearNuclear envelope breaks down envelope breaks down

centrioles

spindle

DNA condensed as chromosomes

iv.iv. Homologous chromosomes Homologous chromosomes pairpair1. When a pair of chromosomes aligns a 1. When a pair of chromosomes aligns a

tetradtetrad is formed. is formed.

2.2. When chromosomes form a tetrad, When chromosomes form a tetrad, they they exchangeexchange portions of their portions of their chromatidschromatids in a process called crossing-over.in a process called crossing-over.

3. 3. CCrossing-overrossing-over produces new produces new combinations combinations of DNA. of DNA.

tetrad

• b.b.          Metaphase I:Metaphase I:i.i.            Spindle fibers attach to the chromosomes at Spindle fibers attach to the chromosomes at

the the centromerecentromere

centrioles

spindle

chromosomes (DNA)

centromere

c.c.          Anaphase I:Anaphase I:i.i. Spindle fibers pull the Spindle fibers pull the homologoushomologous chromosomes toward opposite ends of the cell.chromosomes toward opposite ends of the cell.

ii.ii. Chromosomes move and Chromosomes move and separateseparate into two into two groups near the spindlegroups near the spindle

iii.iii. Anaphase ends when the chromosomes Anaphase ends when the chromosomes stop stop movingmoving

centrioles

spindle

chromosomes (DNA)

– d.d. Telophase ITelophase I• i.i.            Nuclear Nuclear membranesmembranes form around chromosomes form around chromosomes

• ii.ii.           Daughter Daughter nucleinuclei form form

e.e. CytokinesisCytokinesis

i.i. Cytoplasm dividesCytoplasm divides

centrioles

chromosomes (DNA)

Nucleus

Nucleus

Meiosis I produces two haploid (N) Meiosis I produces two haploid (N) daughter cells that have only one copy daughter cells that have only one copy of each chromosome. Each of each chromosome. Each chromosome is made of two sister chromosome is made of two sister chromatids.chromatids.

Meiosis IIMeiosis II

After Meiosis I, cells enter Meiosis II. After Meiosis I, cells enter Meiosis II.

No DNA replication occurs between No DNA replication occurs between Meiosis I and Meiosis II. Meiosis I and Meiosis II.

Meiosis II separates the sister Meiosis II separates the sister chromatids.chromatids.

a.a.          Prophase II:Prophase II:

i.i. CentriolesCentrioles are are visible on opposite visible on opposite sides of the nucleussides of the nucleus

ii.ii. Nucleolus Nucleolus disappearsdisappears

iii.iii. NuclearNuclear envelope envelope breaks downbreaks down

centrioles

chromosome

b.b.          Metaphase II: Metaphase II:

• i.i. Chromosomes Chromosomes alignalign like they like they

do do in mitosisin mitosis

• ii.ii. Chromosomes Chromosomes are are attachedattached to to the spindle at the spindle at

the the centromere.centromere.

centrioles

chromosome

spindle

c.c. Anaphase II:Anaphase II:

Sister Sister chromatidschromatids separate and separate and move towards move towards opposite ends opposite ends of the cell.of the cell.

centrioles

chromosome

spindle

d.d.          Telophase II: Telophase II: – i.i. Nuclear Nuclear

membranesmembranes form form around around chromosomeschromosomes

– ii. Daughter ii. Daughter nucleinuclei form form

e.e.          CytokinesisCytokinesis: : – i.i. Division of the Division of the

cytoplasm cytoplasm

f.f.          Meiosis II Meiosis II produces four produces four haploidhaploid (N) (N) daughter cellsdaughter cells

nucleus

chromosome

centrioles

Gamete formationGamete formation

In male animals, the haploid gametes are In male animals, the haploid gametes are called called spermsperm. .

In female animals, the haploid gametes In female animals, the haploid gametes are called are called eggseggs..

Comparing mitosis and meiosisComparing mitosis and meiosis

• Mitosis produces two genetically Mitosis produces two genetically identicalidentical diploid (2N) cells. Mitosis allows an diploid (2N) cells. Mitosis allows an organism’s body to grow and to replace cells. organism’s body to grow and to replace cells. In organisms that reproduce In organisms that reproduce asexuallyasexually, new , new organisms are produced by mitosis.organisms are produced by mitosis.

• Meiosis produces four genetically Meiosis produces four genetically differentdifferent haploid (N) cells. Meiosis produces gametes haploid (N) cells. Meiosis produces gametes for use in for use in sexualsexual reproduction. reproduction.