Cell Cycle and MitosisHonors Biology

Why is Cell Division Important?

• Unicellular organisms–Reproduce by cell division

increasing the population. 100 µm

(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).

Figure 12.2 A

Why Do Multicellular Organisms Depend on Cell Division?

–Development & Growth –Repair (ex: tissue renewal)

–Maintenance 20 µm200 µm

(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).

(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Cell Division (aka Mitosis)

Makes 2 genetically identical daughter cells from 1 parent cell

Before cells divide They duplicate their genetic

material ensures that each daughter cell receives an exact copy of the genetic material, DNA

What is the structure of a chromosome?• Where in a cell is the genetic material/chromosomes

located?– Nucleus

• Chromatin is an uncoiled mass of DNA and histone proteins– Exists in this form the majority of the time!

• Histones are proteins that help DNA condense• As a cell prepares to divide it coils up/condenses:

– We call this CHROMOSOMES (condensed DNA)

DNA Molecules

• DNA (in nucleus of eukaryotes) can be in 2 forms–Chromatin : DNA is not tightly

packed together (loosely coiled)• Occurs during interphase

–Chromosomes : tightly packed together (TIGHTLY coiled)• Occurs during mitosis (cell division)

Genes

• Segments of DNA (that make up the chromosome) are called genes

• A gene is a piece/segment of DNA that stores genetic information

What happens to chromosomes during cell division?

• What needs to be done to a chromosome before it can divide?– It must DUPLICATE! (DNA Replication)– After duplication each chromosome consists of 2 identically joined

copies Sister Chromatids– Sister Chromatids are held together by centromeres

Chromatin Chromatid

Sister Chromatids (condensed, duplicated chromosome)

Double Chromosome Structure

Kinetochore attaches to spindle fibers

Sister

Chromosomes

Every eukaryotic species has a characteristic, unique # of chromosomes in EACH cell nucleusEx: Humans have 46 chromosomes

# of chromosomes does NOT necessarily equal complexity

The Cell Cycle

• The mitotic phase alternates with interphase in the cell cycle– Interphasemitosisinterphasemitosis

What is Mitosis!?

• Mitosis is the process where cells divide to produce new cells

- Occurs in healing (Ex: if you cut yourself) • New cells are also produced as you grow

- Ex: Day-to-day life (new skin cells!)• ALL eukaryotic organisms produce new cells

through mitosis

Cell Cycle•Consists of 2 broad stages

– 1. Growing Stage called Interphase

– 2. Cell Division called Mitotic Phase (M Phase)

•The majority of the cell cycle (90%) is spent in Interphase

Phases of the Cell CycleINTERPHASE

G1

S(DNA synthesis)

G2

Cytoki

nesis

Mito

sis

MITOTIC(M) PHASE

Figure 12.5

Interphase can be divided into subphases

– G1 phase (GAP 1 phase)• cell grows in size• varies most in length from cell to cell

– S phase (synthesis phase)• DNA is copied (DNA replication)

–Single Double • Each chromosome is single• DNA replication occurs• Chromosomes have doubled each consisting of

two sister chromatids – G2 phase (GAP 2 phase)

• More growth and preparation (make proteins) for mitosis

Mitotic Phase

• After Interphase, Mitotic Phase begins–Two parts of M Phase:

1) Mitosis (division of the nucleus)

2) Cytokinesis (division of the cytoplasm)

M Phase

• Mitosis – the nucleus and duplicated chromosomes divide and create two identical daughter cells

• Cytokinesis – the process by which the cytoplasm is divided in two.– Cytokinesis usually begins before Mitosis is

completed.

Refresher…….

The Cell Cycle:

G1 phase: Growth

S phase: DNA replicationG2 phase: Preparation for cell division

M phase: Mitosis and Cytokinesis

Remember….InterphaseProphaseMetaphaseAnaphaseTelophaseCytokinesisI Passed My Accelerated Tough Class

IPMATC

Interphase

• Consists of G1, S, G2• Occurs BEFORE Mitosis begins• During S phase, the cell copies its DNA• Chromosomes appear as threadlike coils

– Made of Chromatin, a combination of DNA and protein molecules

• As the cell prepares to divide, its chromatin fibers condense, becoming the compact structure we call a chromosome.

• Chromosomes are copied (# doubles)

chromosome

Condensed, duplicated chromosome

Interphase

• Each chromosome has now been condensed and duplicated and consists of 2 sister chromatids

• The region where the two chromatids are joined tightly together is called the centromere.

Interphase: Animal Cell

Mitosis

• Continuous pathway (Early, Mid, & Late)

• Consists of 4 phases and cytokinesis– Prophase– Metaphase– Anaphase– Telophase

• Cytokinesis

Prophase (X’s)“Pack Together”

First phase of Mitosis:1. Chromatin becomes tightly coiled = chromosomes

– DNA “packs” together

2. Spindle Fibers (made by the centrioles) begins to form in the cytoplasm

3. Nuclear envelope breaks downLate: Nucleus and nucleolus disappear

Prophase:Centrioles

Spindles

Prophase:

1. DNA supercoils into chromosomes

2. Centrioles move

Metaphase (X’s)“Meet in the Middle”

Second phase of Mitosis:• Chromosomes attach to the spindle at the centromeres• Chromosomes line up in the middle of cell

– Called equatorial or metaphase plate • Spinder fibers pull and tug chromosomes to line up

Metaphase:

Third phase of Mitosis:1. Spindle pulls apart chromosomes2. SISTER CHROMATIDS separate

athe the centromere and begin moving to opposite ends (poles) of the cell

3. Each chromatid is now considered its own chromosome

Anaphase (V’s)“Adios and Away”

Anaphase:***Remember that each chromatid has the same DNA so each is now its own chromosome***

Telophase (V’s)“Two New Cells”

Fourth phase of Mitosis:1. Chromosomes reach end of

spindle2. Spindle breaks down

(disappear)3. Cleavage furrow begins to form4. Nuclear membrane begins to

reform5. 2 daughter nuclei 6. Chromosomes chromatin

Telophase:

Spindle fall apart

Cleavage furrow

Cytokinesis“Division of the Cytoplasm”

• Occurs in Late telophase• In animal cells

– a cleavage furrow forms, which pinches the cell in two.

• In plant cells– produce a cell plate at the middle of

the cell• At the end of cytokinesis, there are

two distinct IDENTICAL daughter cells.

Cytokinesis

• Final Phase of Cell Division/M Phase1. Cleavage furrow pinches all the way through 2. Result is two new cells3. 2 cells then enter Mitosis begins again!

- G1, S, G2 (Interphase)- PMAT & Cytokinesis

• Each new cell at the end of mitosis is DIPLOID

– has a full set of chromosomes

Cytokinesis: A Closer Look

In animal cellsCytokinesis

occurs by a process known as cleavage, forming a cleavage furrow

Cleavage furrow

Contractile ring of microfilaments Daughter cells

100 µm

(a) Cleavage of an animal cell (SEM)Figure 12.9 A

In plant cells, during cytokinesisA cell plate forms

Daughter cells

1 µmVesiclesforming cell plate

Wall of patent cell Cell plate New cell wall

(b) Cell plate formation in a plant cell (SEM)Figure 12.9 B

G2 OF INTERPHASE PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs)

Chromatin(duplicated)

Early mitoticspindle

Aster

CentromereFragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule

Nonkinetochoremicrotubules

Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate

Nucleolusforming

Cleavagefurrow

Nuclear envelopeforming

Figure 12.6

Mitosis in a plant cell

1 Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from.

Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment.

Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate.

Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten.

Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.

2 3 4 5

NucleusNucleolus

ChromosomeChromatinecondensing

Figure 12.10

Cell Cycle and Mitosis Animations

• http://www.ucopenaccess.org/courses/APBiologyI/course%20files/multimedia/lesson17/lessonp.html

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120073/bio14.swf::Mitosis%20and%20Cytokinesis

• http://www.sumanasinc.com/webcontent/animations/content/mitosis.html

• http://www.johnkyrk.com/mitosis.html