Cell Division

Mitosis and Meiosis

Cell Cycle

• Encompasses the time between the creation of a new cell and that cell’s division.

• Cell Division: the splitting of one cell into two.– The process that makes growth and reproduction

possible for any organism. – Each division different depended upon if the cell is

eukaryotic or prokaryotic

Continue• Two major phases– 1- Interphase: Preparation for cell division. • Three phases:

– G1 phase (Growth 1): Cell growth – Cell organelles are formed within the cell.

– S phase (Synthesis): DNA is synthesized– G2 phase (Growth 2): Second period of cell growth, during which the

cell prepares for the division.

Example: Some cells, including many nerve cells, are programmed never divide. These cells are said to be in G0 or resting phase.

– 2- Mitosis: Division of Nucleus• Four phases of mitosis (Prophase, Metaphase, Anaphase,

Telophase)• Cytokinesis: division of cytoplasm and cell membrane

Cell Cycle Control• All of the cell cycle are controlled by

checkpoints• There are three checkpoints:

Checkpoints Occurs at Details

G1 The end of the phase

If conditions are not suitable for replication, the cell will not proceed to S phase but will instead enter a resting phase G0

G2 The end of the phase

If conditions are not suitable, transition to the M phase will be delayed.If DNA is damaged, cell division will be delayed to allow time for DNA repair

M Between metaphase and anaphase stages of mitosis

If the chromosomes are aligned properly and ready for division, the cell will proceed from metaphase to anaphase, during which it will divide. If the chromosome are not aligned properly, the anaphase stage will be delayed

Continue…• Triggers at each checkpoint assess the cell’s

readiness to proceed to the next stage.• Checkpoints makes sure proper number of

chromosomes and type of chromosomes & organelles

Example of Checkpoint• Example: Malignant cancer are deadly, in part,

because they undergo unregulated cell division, which enables them to spread rapidly throughout the body. Scientists have discovered one reason behind this uncontrolled growth: a defective p53 gene. Proteins produced by the p53 gene assess the cell’s DNA for damage at the G1 checkpoint. If the DNA is intact, cell division proceeds. If the DNA is damaged, however, the p53 proteins halt cell division until the DNA is repaired or the cell is destroyed. If the p53 gene itself has been damaged, as in the case of cells that are cancerous the G1 checkpoint will fail and a malignant cancer cell may develop.

Other Cell Division Controls

• Density-dependent inhibition: When a certain density of cells is reached, growth of the cells will slow or stop because there are not enough raw materials for the growth and survival of more cells.– Example: Cancer cells can lose this inhibition and

grow out of control.• Growth Factors: Some cells will not divide if

certain factors are absent.

Continue with Other Cell Division Control

• Cyclins: is a protein that acccumlates during G1, S, G2, of the cell cycle

• Protein Kinase: is a protein that control other proteins through the addition of phosphate groups.

Chromosomes

• In eukaryotic cells, DNA and associated proteins are wrapped together in packages called chromosomes.

• DNA in eukaryotic cells is wrapped around the proteins to form a complex called chromatin

• Throughout the cell’s life, the chromatin becomes is loosely packed within the nucleus. – Chromatin can not been seen by humans.– Think a rubber band ball.

Continue…• During cell division, however, the chromatin becomes

highly condensed and folds up to form condensed chromosomes. (This is when we can see it).

• DNA is always replicated, or copied before becoming condensed .

• The x shape associated with chromosomes actually represents a replicated chromosome consisting of two identical sister chromatids joined at the centromere

Example: Prokaryotes do not have chromosomes. Prokaryotic DNA exist in a single loop

Chromosome Number

• Refers to the number of chromosomes within each cell of an organism.

• Most animals possess two nonidentical version of every chromosome. These are known as homologous chromosomes.

• Homologous chromosomes have the same size, shape, and function but may have slightly different versions of most genes, the basic unit of hereditary information.

Continue…• Cells with two sets of every chromosomes

between their homologous chromosomes are diploid (2n), while cells with one set of every chromosome are haploid (1n)– Diploid: Somatic (Body) Cells– Haploid: Gamete (Sex) Cells

Human Chromosome Number• Humans has 46 chromosomes or 23 pairs• 2n indicates diploid – 2n= 46 (2 sets of 23 chromosomes)

• 1n indicates haploid– 1n=23 (1 set of 23 chromosomes)• Egg and sperms are haploid

• The union of sperm and egg that occurs during fertilization restores the chromosomes number of the resulting embryo to 2n = 46

Mitosis

• Is the method of eukaryotic cell division that produces two genetically identical cells.

• All cells in an organisms, except for sperm and eggs, are produced by the process of mitosis.

• Mitosis progresses along five stages: Prophase, Metaphase, Anaphase, Telophase, and Cytokinesis

Prophase• Duplicated chromosomes condense and become visible

as distinct sister chromatids.• Nuclear envelope breaks down• Centromeres move toward the poles of the cell.• The mitotic spindle, which is made of microtubules

attaches to a specialized structure called the kinetochore, located at the centromere of each replicated chromosomes.

Metaphase

• Replicated chromosomes align at the equator, or metaphase plate, of the cell.

• M&Ms (Metaphase Middle)

Anaphase• The sister chromatids separate and are moved

toward opposite poles of the cell by the spindle.

• As this happens, the cell begins to elongate toward the poles.

Telophase

• Mitotic spindle breaks down.• Nuclear envelope forms at each end of the

cell, and the chromosomes within begin to unfold into chromatin.

Cytokinesis• The cytoplasm and organelles are evenly divided

between the two new cells during cytokinesis, completing the process of cells division.

• Plants and animals cells differ in cytokinesis– Plants, a cell plate is formed as vesicles containing cell

membrane materials fuse together along the equator of the cell.

– Animals, a ring of microfilaments contracts in the center of the elongated cell, producing a cleavage furrow that eventually pinches off the two cells.

Example of cytokinesis

Animal Cell

Plant Cell

Cell Cycle

Binary Fission• Occurs in prokaryotes because have a single

double-stranded loop of DNA.• Occurs in four steps– 1. DNA is replicated– 2. Cell doubles in size– 3. Cell membrane grows into the center of the cell,

between the two circles of DNA, dividing the cell in two.

– 4. Two cell seperate, and a cell wall forms around each new cell.

Meiosis“Me” likes Sex (Cells)

• The method of cell division that takes place in sexually reproducing organisms specifically for the creation of gametes– sperm and egg cells.

• Production four haploid cells, each genetically different

• Meiosis requires two rounds of cell division.

Continue…• Meiosis I: Homologous pairs of each chromosome

join and might exchange genetic material. The homologous chromosomes are pulled to opposite poles in the cell, at which point the cell separates, resulting in two cells. Each cell contains half the chromosome number of the original diploid cell. Each chromosome remains in the duplicated state and is made up of two sister chromatids

Continue…

• Meiosis II: The second stage of meiosis follows similar steps as mitosis in the creation of two more cells. Chromosomes do not replicate between Meiosis I and Meiosis II.– The result is four haploid cells genetically different

from one another.

Prophase I• The most important events in prophase I are synapsis

and crossing over– Synapsis: occurs when the two homologous

chromosomes condense and combine to form complexes called tetrads

• Crossing over is the exchange of genetic material that takes place between these homologous chromosomes along several junctions known as chiasmata (place where crossing over occurs)

Metaphase I

• The tetrads align along the metaphase plate of the cell.

Anaphase I• The homologous chromosome of each tetrad and

are pulled toward opposite poles of the cell by the spindle.

• The side of the cell toward which a homologous chromosome is pulled a random, depending only on the orientation of the tetrad.

• The independent assortment of chromosome for each cell is result of this random mix of chromosomes derived from that organism’s parent

Telophase I

• Identical to telophase in mitosis.• The cell continues to elongate, and the mitotic

spindle breaks down.• A new nuclear envelope forms at each end of

the cell the chromosomes within unfold into chromatin

Example

• Crossing over and the independent assortment of chromosomes during meiosis are two forces that help to produce genetic variation . By independent assortment alone, a single human can produce more than 8 million genetically different gametes. When crossing over is also considered, the possible number of genetically different is nearly limitless.

Cytokinesis I

• Cytokinesis is very similar to mitosis divide cytoplasm and organelles.

• Two genetically different haploid cells have been produced.

• Each chromosome is still in the duplicated state and is made up of two sister chromatids.

• Because crossing over during prophase I, the sister chromatids are no longer identical.

Meiosis II

• Meiosis II occurs right after Cytokinesis I• There is no Interphase (therefore No DNA

Replication)

Prophase II

• Chromosome condense within haploid cell condense, and the spindle attaches to the kinetochore of each chromosome.

• The nuclear envelope breaks down and the centrosomes move toward the poles of the cell.

Metaphase II

• Chromosomes align along the center of the metaphase plate

Anaphase II

• The sister chromatids separate and are moved toward opposite poles of the cell by the spindle.

• The cell begins to elongate toward the poles

Telophase II

• The cell continues to elongate and the mitotic spindle breaks down.

• A new nuclear envelope forms at each end of the cell and the chromosomes within may unfold into chromatin.

Cytokinesis II

• The cytoplasm and organelles are divided between the two cells, completing the process of cell division.

• By the end of this stage, four genetically different haploid cells have been produced.

Sex Cells (Gametes)

• Meiosis produces four genetically different haploid cells.– Males haploid cells are sperms– All four sperm can be used in sexual reproduction.– Females haploid cells are 1 egg and 3 polar bodies– Only the 1 egg can be used in sexual reproduction– The three polar bodies will be recycled back into

the body.

Example• The process of meiosis results in four genetically different

haploid cells. In animals, these haploid cells develop into gametes, a sperm in males and an egg in females. Fertilization is the process by which a sperm and egg fuse together. The resulting zygote is diploid, with half the chromosomes coming from the mother and other half coming from the father. The processes of meiosis and fertilization both account for the genetic variation found in animals of the same species. Meiosis is responsible for creating gametes whose genetic material varies from that of the parent. Fertilization then combines the genetic material of the two parents to produce the genetic material of the offspring

Life Cycles• Life Cycle is the sequence of events that make up

the reproductive cycle of an organisms.• Alternation of generations: Plants sometimes

exist as a diploid organism and other times as a haploid cell.– Two haploid gametes combine to form a diploid

zygote, which divides mitotically to produce.• Sporophyte: undergoes under meiosis to produce a haploid

spore• Gametophytes: Mitotic division leads to production of

haploid multicellular organisms.– Produces haploid gametes, which form diploid zygotes

Diagram of Alternation of Generation

Human Life Cycle

• The only haploid cells present in this life cycle are gametes formed during meiosis.

• Two haploid gametes combine during fertilization to produce a diploid zygote.

• Mitotic division then leads to formation of the diploid multicellular organisms.

• Meiotic division later produces haploid gametes.

Example of Human Life Cycle

Life Cycle of a Fungi• Fungi are haploid organisms with the zygote being

the only diploid form.• Like humans, the gametes for fungi are haploid

(n), and fertilization yield a diploid zygote. • Instead of dividing by mitosis, the zygote divides

by meiosis to form a haploid organisms.• Gametes are formed by mitosis, not meiosis—the

organism is already haploid, before forming the gametes.

Life Cycle of a Fungi

• We will discuss more about the life cycles as we get into the individual kingdoms.