THE CELL CYCLEMitosis and Meiosis

The Cell Cycle• Series of events that take place in a cell

leading to its division and duplication.• In cells without a nucleus (prokaryotic), the

cell cycle occurs via a process termed binary fission.

• In cells with a nucleus (eukaryotic), the cell cycle can be divided in two periods; Interphase and mitotic phase.

The Cell Cycle• Vital process by which a single-celled

fertilized egg develops into a mature organism.

• Process by which a single-celled fertilized egg develops into a mature organism.

• Process by which hairs, skin, blood cells and some internal organ renewed.

The Cell Cycle

• Four phases of Cell Cycle• G1 Phase• S Phase/Synthesis• G2 Phase• M Phase/Mitosis

• After the cell division, each of the daughter cell begins the interphase of a new cycle.

The Cell Cycle

The Cell Cycle

• Mitosis• The process by which a eukaryotic cell separates

the chromosomes in its cell nucleus into two identical sets in two nuclei.

• Followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components.

The Cell Cycle

• Mitosis and Cytokinesis together define the mitotic (M) phase of the cell cycle- the division of the mother cell into two daughter cells, genetically identical to each other and their parent cell.

Mitosis• Interphase• Prophase• Metaphase• Anaphase• Telophase • Cytokinesis

Interphase (occurs before mitosis begins)

• Chromosomes are copied (# doubles)• Chromosomes appear as threadlike coils

(chromatin) at the start, but each chromosome and its copy(sister chromosome) change to sister chromatids at end of this phase

Interphase

Nucleus

Cytoplasm

CELL MEMBRANE

InterphaseAnimal Cell Plant Cell

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

Prophase (1st step in Mitosis)

• Mitosis begins (cell begins to divide)• Centrioles (or poles) appear and begin to

move to opposite end of the cell. • Spindle fibers form between the poles.

Prophase

Spindle fibers

Sister chromatidsCentrioles

ProphaseAnimal Cell Plant Cell

Spindle fibers

Centrioles

Metaphase 2nd step in Mitosis• Chromatids (or pairs of chromosomes)

attach to the spindle fibers.

Spindle fibers

Centrioles

MetaphaseAnimal Cell Plant Cell

Anaphase (3rd step in Mitosis)• Chromatids (or pairs of chromosomes)

separate and begin to move to opposite ends of the cell.

Spindle fibers

Centrioles

Anaphase

Animal Cell Plant Cell

Telophase (4th step in Mitosis)

• Two new nuclei form. • Chromosomes appear as chromatin (threads

rather than rods).• Mitosis ends.

NucleiNuclei

Chromatin

TelophaseAnimal Cell Plant Cell

Cytokinesisoccurs after mitosis

• Cell membrane moves inward to create two

daughter cells – each with its own nucleus with identical chromosomes.

Significance

• Development and growth• Cell replacement• Regeneration• Asexual Reproduction

Animal Mitosis -- ReviewInterphase

                                                            

Prophase

                                                            

Metaphase

                                                            

Anaphase

                                                            

Telophase

                                                            

Cytokinesis

                                                            

Plant Mitosis -- ReviewInterphase

                                                            

Prophase

                                                            

Metaphase

                                                            

Anaphase

                                                            

Telophase

                                                            

Cytokinesis

                                                            

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The Cell Cycle

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Cell Division

• The process by which a parent cell divides into two or more daughter cells.

• Usually occurs as part of a larger cell cycle.

• Has 2 types: a. Vegetative Division (mitosis)b. Reductive Division (meiosis)

Why do you share some but not all characters of each parent?

MEIOSIS“The source of distinction”

Meiosis

• Come from the Greek meaning “decrease”• Forms the basis for sexual reproduction• Form of nuclear division by which a diploid

parent produces four haploid daughter cells.

• Has two stages: Meiosis I and Meiosis II• Occurs for both plants and animals.

Meiosis does two things :1. Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells.

2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives.

This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.

Genetic diversity is important for the evolution of populations and species.

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

STAGES OF MEIOSIS

Meiosis I : Separates Homologous Chromosomes

• Interphase–Each of the chromosomes replicate–The result is two genetically identical

sister chromatids which remain attached at their centromeres

Prophase I• This is a crucial phase for mitosis.• During this phase each pair of chromatids

don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad.

• Extremely IMPORTANT!!! It is during this phase that crossing over can occur.

• Crossing Over is the exchange of segments during synapsis.

Metaphase I

• The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles.– Still in homologous pairs

Anaphase I• The spindle guides the movement of the

chromosomes toward the poles– Sister chromatids remain attached– Move as a unit towards the same pole

• The homologous chromosome moves toward the opposite pole– Contrasts mitosis – chromosomes appear as

individuals instead of pairs (meiosis)

Telophase I• This is the end of the first meiotic cell

division. • The cytoplasm divides, forming two new

daughter cells. • Each of the newly formed cells has half the

number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division

Cytokinesis• Occurs simultaneously with telophase I

– Forms 2 daughter cells• Plant cells – cell plate• Animal cells – cleavage furrows• NO FURTHER REPLICATION OF

GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

Meiosis II : Separates sister chromatids• Proceeds similar to mitosis• THERE IS NO INTERPHASE II !

Prophase II

• Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

Metaphase II

• The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

Anaphase II

• The centromeres of sister chromatids finally separate

• The sister chromatids of each pair move toward opposite poles– Now individual chromosomes

Telophase II and Cytokinesis

• Nuclei form at opposite poles of the cell and cytokinesis occurs

• After completion of cytokinesis there are four daughter cells – All are haploid (n)

MITOSIS vs. MEIOSIS

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis

Mitosis The first (and distinguishing) division of meiosis

Significance

• Facilitates stable sexual reproduction.• Produces genetic variety in gametes that

produces offspring.• Allows a species to maintain stability

under environmental changes.