cell reproduction or how cells make copies of themselves

Cell Reproductionor

How cells make copies of themselves

Also called

Cell Division

Cell Division

• Cell division consists of two phases:

• 1. the division of the nucleus, and

• 2. the division of the cytoplasm (cytokinesis)

2 Kinds of Nuclear Division

• 1. Mitosis – mitosis divides the nucleus so that both resulting new cells (daughter cells) are genetically identical…Same amount of

DNA

• 2. Meiosis – meiosis produces daughter cells that contain half the genetic

information… half the amount of DNA.

Before Division Begins

• Before a cell can successfully divide, the DNA must be packaged so it does not get

damaged. The “stringy” form of DNA (chromatin) is coiled into structures called

“chromosomes”.

Sister Chromatids

Each Chromosome is made up of two identical halves

called “sister chromatids” joined at the centromere.

Each Chromatid is a single, coiled DNA

molecule.

The point where two sister chromatids are

connected.

Organizing Chromatin into a Chromosome.

Diploid Cells(symbolized as 2n)

• Diploid cells (2n) have 2 copies of every chromosome, forming what is called a

“Homologous” chromosome pair.In Diploid cells, one pair of chromosomes comes

from the mother and the other pair comes from the father.

The diploid number for humans is 46, or 2n = 46,…. Or you could say…..Humans have 23

homologous pairs, or…. Humans have 92 chromatids.

Chromosomes

Chromosomes

The Life Cycle of a cell is called The “Cell Cycle”

The Cell Cycle consists of 5 Phases1.Interphase (part of the cell cycle, but not part of mitosis)

2.Prophase

3.Metaphase

4.Anaphase

5.Telophase

These 4 phases are known collectively as “Mitosis”

Interphase

90% of cell’s life is spent in interphase

During Interphase the cell grows, duplicates its chromosomes and performs its normal job.

Interphase has 3 stages The Events of Interphase

G1 stage - first gap - Cell grows and carries out regular biochemical functions.

S stage – synthesis - DNA is replicated or synthesized.

G2 stage - second gap - Cell completes preparations for division…..a cell can complete S, but fail to enter G2.

Plant Cells

Animal Cells

nuclear envelope

Mitosis(the splitting of the nucleus)

prophasemetaphaseanaphase telophase

Mitosis

• A cell that has grown in size and is about to

divide is called a “Mother Cell”.

• As a result of Mitosis and cytokinesis the

Mother cell splits into two genetically

identical “Daughter Cells”.

Mother

Cell

The Events of Prophase

• Nucleoli disappear.• Chromatin condenses into

the chromosomes.• Nuclear Envelope dissolves…

the nucleus comes apart • Centrioles (MTOC’s) separate

and move to opposite ends of the cell.

• Microtubules from each MTOC connect to a

specialized region of the centromere called the

kinetochore. This moves the chromosomes back and forth.• Mitotic spindle begins to

form.

Plant Cells

Animal Cells

Events of Metaphase

• Chromosomes line up at the equator of the cell…. called

the “metaphase plane”.• Centrioles arrive at opposite

ends of the cell.• Spindle apparatus fully

developed.• Metaphase ends when the

microtubules pull each chromosome apart into two chromatids. Once separated

from its sister chromatid, each chromatid is now called a chromosome. To count the number of chromosomes, at any time, count the number

of centromeres.metaphase plane

centriole

centriole

Plant Cells

Animal Cells

Events of Anaphase

• Anaphase begins when the chromosomes separate.

• Microtubules shorten as tubulin units uncouple, the chromosomes are pulled

away from each other toward opposite ends of

the cell.• Cell elongates; poles move

slightly further apart.• Anaphase ends when the

chromosomes reach their respective ends of the cell.

Chromosomes

Plant Cells

Animal Cells

Events of Telophase

• Chromosomes uncoil back to chromatin.

• Nuclear envelope reforms. The nucleus reforms in each newly formed cell.

• Nucleoli reappear.

• Spindle fibers disappear.

• Simultaneously Cytokinesis usually starts.

Plant Cells

Animal Cells

Animal Cell Cytokinesis • “Cleavage furrow” forms.

• Microfilaments contracts and divides the cytoplasm into two parts.

Plant Cell Cytokinesis

• Cell plate develops from Golgi vesicles.• New cell wall developed around the cell plate.

p

Cell Plate

Cytokinesis

Animal Cell - Mitosis

Plant Cell - Mitosis

Regulation of Cell Division

• Must be controlled.• Rate of cell division depends on the cell type.• Example: skin cells divide frequently

liver cells divide as needed brain cells rarely or never divideCells will stop dividing when the surrounding cell

density reaches a specific level….... this is called “Density-Dependent Inhibition”

• When density is high - no cell division.• When density is low - cells divide.

Cancer Cells• A tumor is a large mass of cells. Tumors form because

cells do not stop dividing. The

control mechanisms for cell division have

failed.

Metastasis

Why Cells Divide

• As cells grow in size they become less healthy.... If they don’t divide (split in two) they will die.

2 Reasons Why Cells Become Less Healthy

• 1. Surface area-to-Volume ratio becomes too small.

• 2. The nucleus is limited in regulating cell activities…. its genome-to-volume

ratio becomes too small.

Surface area-to-Volume Ratio

• When a cell grows, the volume of the cell increases faster than the surface area surrounding it.

• When we say “the surface area-to-volume ratio is large”, that means there is a large surface area relative to volume.

• When we say “the surface area-to-volume ratio is small”, that means the surface area is small relative to volume.

• When the surface-to-volume ratio is large, the cell can effectively react with the outside environment…..for example…. adequate

amounts of water and oxygen can move into the cell, and wastes can rapidly be eliminated.

• When the surface-to-volume ratio is small the cell is unable to exchange enough substances to service the cell. The cell dies.

Genome-to-Volume Ratio

• The DNA in a cell is referred to as its “genome” (all of its genes).• The genome controls all of the cell’s activities by producing

enzymes…. which trigger and control the cell’s necessary functions.• Because there is a finite amount of DNA, the amount of enzymes is

limited.• As the cell grows the volume increases, but the amount of DNA

remains the same…. In other words “the genome-to-volume ratio” decreases….. Eventually there is not enough DNA to regulate the

cell….. cell functions decrease and the cell dies.