MEIOSIS AND MEIOSIS AND SEXUAL SEXUAL

REPRODUCTION REPRODUCTION

MEIOSIS: FORMATION OF MEIOSIS: FORMATION OF HAPLOID CELLSHAPLOID CELLS

SOME ORGANISMS SOME ORGANISMS REPRODUCE BY JOINING REPRODUCE BY JOINING GAMETESGAMETES TO FORM THE FIRST TO FORM THE FIRST CELL OF A NEW INDIVIDUAL CELL OF A NEW INDIVIDUAL

GAMETES ARE GAMETES ARE HAPLOIDHAPLOID THEY THEY CONTAIN CONTAIN ONEONE SET OF SET OF CHROMOSOMESCHROMOSOMES

MEIOSIS IS A FORM OF CELL DIVISION THAT

HALVES THE NUMBER OF CHROMOSOMES WHEN FORMING SPECIALIZED REPRODUCTIVE CELLS SUCH AS GAMETES OR

SPORES.

BEFORE MEIOSIS BEGINS BEFORE MEIOSIS BEGINS THE DNA IN THE THE DNA IN THE ORIGINAL CELL IS ORIGINAL CELL IS REPLICATED. REPLICATED. THEREFORE, THEREFORE, MEIOSIS STARTS OUT MEIOSIS STARTS OUT WITH WITH HOMOLOGOUS HOMOLOGOUS CHROMOSOMES.CHROMOSOMES.

*** PREFIX HOMO MEANS THE SAME*** PREFIX HOMO MEANS THE SAME

STEP 1: PROPHASE 1STEP 1: PROPHASE 1

THE CHROMSOMES CONDENSE AND THE NUCLEAR ENVELOPE BREAKS DOWN. HOMOLOGOUS CHROMOSOMES PAIR ALONG THEIR LENGTH.

CROSSING OVER OCCURS WHEN PORTIONS OF A CHROMATID ON ONE HOMOLGOUS CHROMOSOMES ARE BROKEN AND EXCHANGED WITH THE CORRESPONDING CHROMATID PORTIONS OF THE OTHER HOMOLOGOUS CHROMOSOMES.

Step 2: Metaphase 1Step 2: Metaphase 1 The pairs of The pairs of

homologoushomologous chromosomes chromosomes

are are movedmoved by by the spindle to the spindle to the the equatorequator of of the cell and the cell and remain remain together.together.

Step 3: Anaphase 1Step 3: Anaphase 1 The homologous The homologous

chromosomes chromosomes separate and each separate and each pair are pulled to pair are pulled to oppositeopposite poles of poles of the cell. Each the cell. Each chromosomes is still chromosomes is still composed of composed of 22 chromatidschromatids. The . The geneticgenetic materialmaterial has has been been recombinedrecombined. .

Step 4: Telophase 1Step 4: Telophase 1 Individual Individual

chromosomes gather chromosomes gather at each of the poles, at each of the poles, the cytoplasm the cytoplasm divides (divides (Cytokinesis)Cytokinesis) forming forming 2 new cells2 new cells containingcontaining 11 chromosome from chromosome from each pair of each pair of homologous homologous chromosomes. chromosomes.

Step 5: Prophase IIStep 5: Prophase II

A new spindle A new spindle forms around the forms around the chromosomeschromosomes

Step 6: Metaphase IIStep 6: Metaphase II

The chromosomes line up along the equator and are attached at the centromeres to the spindle fibers.

Step 7: Anaphase IIStep 7: Anaphase II

The centromeres divide and the chromatids (now called chromosomes) move to opposite poles of the cell.

Step 8: Telophase IIStep 8: Telophase II A nuclear A nuclear

envelope envelope forms around forms around each set of each set of chromosomes. chromosomes. The cells The cells undergo undergo Cytokinesis.Cytokinesis. Results: Results: 4 4 haploid cells.haploid cells.

Meiosis and Genetic Variation Meiosis and Genetic Variation

MeiosisMeiosis is an is an important important process that process that allows for the allows for the rapidrapid generation generation of of new genetic new genetic combinations. combinations.

Mechanisms contributing to Mechanisms contributing to Genetic Variations Genetic Variations

1. Independent assortment: 1. Independent assortment: each each of the 23 pairs of chromosomes of the 23 pairs of chromosomes segregates (separately) segregates (separately) independentlyindependently

2. Crossing over: 2. Crossing over: DNA exchangeDNA exchange adds even more recombinations adds even more recombinations

3. Random Fertilization: 3. Random Fertilization: random random joining of two gametesjoining of two gametes

Importance of Genetic Importance of Genetic Variation Variation

Meiosis and the joining of gametes are Meiosis and the joining of gametes are essential to essential to evolution.evolution.

The The pacepace of evolution is sped up by of evolution is sped up by genetic genetic recombinationrecombination

The combination of genes from two The combination of genes from two organisms results in a organisms results in a 33rdrd type not identical to type not identical to either parent.either parent.

Natural SelectionNatural Selection does not always favor does not always favor genetic change. This means it may favor genetic change. This means it may favor existing combinations of genes existing combinations of genes slowingslowing the the pace of evolution.pace of evolution.

Meiosis and Gamete Formation Meiosis and Gamete Formation

The fundamental events of meiosis The fundamental events of meiosis occur in occur in all sexually reproducing all sexually reproducing organismsorganisms, but may vary in , but may vary in timingtiming and and structuresstructures associated with associated with gamete formation. gamete formation.

MeiosisMeiosis is the primary event in the is the primary event in the formation of gametes-formation of gametes-gametogenesis.gametogenesis.

Meiosis in MalesMeiosis in Males The process by which The process by which

sperm are produced sperm are produced in male animals is in male animals is called called spermatogenesisspermatogenesis and and occurs in the occurs in the testestestes. . 4 cells4 cells change in change in form, develop a tail form, develop a tail to become male to become male gametes called gametes called sperm.sperm.

Meiosis in FemalesMeiosis in Females The process by which The process by which

gametes are produced in gametes are produced in female animals is called female animals is called oogenesisoogenesis and occurs in and occurs in the the ovariesovaries. The cytoplasm . The cytoplasm divides divides unequallyunequally with one with one cell getting nearly all of the cell getting nearly all of the cytoplasm and will cytoplasm and will ultimately give rise to an ultimately give rise to an eggegg cell or an cell or an ovumovum with a with a large storehouse of large storehouse of nutrients to nourish young nutrients to nourish young after fertilization. The other after fertilization. The other cell is small and called a cell is small and called a polar bodypolar body; its offspring will ; its offspring will not survivenot survive. .

Sexual and Asexual Sexual and Asexual ReproductionReproduction

Reproduction, the process of producing Reproduction, the process of producing offspring, can be offspring, can be asexualasexual or or sexual sexual

AsexualAsexual ReproductionReproduction a single parent a single parent passes passes copiescopies of all its genetic genes to of all its genetic genes to each of its offspring, a each of its offspring, a cloneclone; no fusion ; no fusion of of haploidhaploid cells such as cells such as gametes.gametes.

• Example: Example: prokaryotesprokaryotes

In contrast, in In contrast, in sexual sexual reproductionreproduction two two parents each form parents each form reproductive cells that reproductive cells that have have halfhalf the number of the number of chromosomes.chromosomes.

Therefore both parents Therefore both parents contribute contribute genetic genetic materialmaterial to the to the offspring, however the offspring, however the offspring are not offspring are not exactlyexactly like either parent they like either parent they have similar traits. have similar traits.

Types of Asexual ReproductionTypes of Asexual Reproduction1.1. FissionFission – separation of a parent into two or – separation of a parent into two or

more individuals of about more individuals of about equal sizeequal size Examples: Examples: amoebasamoebas

2.2. MulticellularMulticellular eukaryotes undergo eukaryotes undergo fragmentationfragmentation, a type of reproduction in which , a type of reproduction in which the body breaks into several pieces; some are the body breaks into several pieces; some are all later develop into complete adults when all later develop into complete adults when missing parts are missing parts are regrown. regrown.

3.3. BuddingBudding, is where new individuals split off , is where new individuals split off from existing ones; the from existing ones; the budsbuds may break from may break from the parent and become an the parent and become an independentindependent organism, or it may remain organism, or it may remain attachedattached to the to the parent. parent. Example: Example: hydrahydra

Genetic Diversity Genetic Diversity Asexual ReproductionAsexual Reproduction is the is the simplestsimplest and and

most primitive method of reproduction. most primitive method of reproduction.

In a In a stablestable environment, environment, manymany offspring offspring can be produced in a short period of can be produced in a short period of time time withoutwithout using energy to produce using energy to produce gametes gametes or to find a mate. or to find a mate.

A A disadvantagedisadvantage is the changing is the changing environment because a population of environment because a population of organisms may not be able to organisms may not be able to adapt to a adapt to a new environmentnew environment

Sexual ReproductionSexual Reproduction provides a powerful provides a powerful means of quickly making different means of quickly making different combinationscombinations of genes among of genes among individuals. individuals.

Evolution of Sexual ReproductionEvolution of Sexual Reproduction

Only Only diploiddiploid cells can repair certain kinds cells can repair certain kinds of chromosomes of chromosomes damagedamage, such as breaks , such as breaks

in both strands of in both strands of DNA.DNA.

The process of The process of meiosismeiosis and the pairing of and the pairing of homologoushomologous chromosomes may have chromosomes may have allowed early allowed early protistanprotistan cells to cells to repair repair damaged damaged DNA. DNA.

Sexual Life Cycles in Sexual Life Cycles in EukaryotesEukaryotes

1.1. The entire life span in the life of an The entire life span in the life of an organism from one generation to organism from one generation to the next is called a the next is called a life cycle.life cycle.

2.2. Eukaryotes that undergo sexual Eukaryotes that undergo sexual reproduction can have one of three reproduction can have one of three types of sexual life cycles: types of sexual life cycles: Haploid, Haploid, Diploid or alternation of generation Diploid or alternation of generation

Facts for the Haploid Life Facts for the Haploid Life CycleCycle

1.1. SimplestSimplest of sexual of sexual life cycleslife cycles

2.2. Haploid cells Haploid cells occupy the major occupy the major portions of portions of meiosismeiosis immediately after immediately after it is formed; it is formed; creating new creating new haploid cells.haploid cells.

3.3. Fusion – Fusion – gametes gametes fuse to produce a fuse to produce a zygotezygote

Facts for the Haploid Life Facts for the Haploid Life CycleCycle

4. Damaged is repaired during meiosis

5. Two homologous chromosomes line up side by side in preparation for crossing over

6. Found in many protists, some fungi and algae

Facts for the Diploid Life Cycle Facts for the Diploid Life Cycle

1. Adult individuals are diploid

2.Inherit chromosomes from each parent

3.A reproductive cell undergoes meiosis to produce gametes.

4.Sperm + egg = zygote (fertilization)

Facts for the Diploid Life CycleFacts for the Diploid Life Cycle

5. A 5. A singlesingle diploid diploid cell eventually cell eventually gives gives riserise to all of to all of the cells of the the cells of the adult.adult.

6. The diploid 6. The diploid individual that individual that develops from the develops from the zygotezygote occupies occupies the the major portionmajor portion of the diploid of the diploid life life cyclecycle..

Facts on Alternation of Generations Facts on Alternation of Generations

1.1. PlantsPlants, , algaealgae, and , and some some protistsprotists have a have a life cycle that regularly life cycle that regularly alternates alternates between a between a haploid haploid phase and a phase and a diploid diploid phase. phase.

2.2. In plants, the In plants, the diploid diploid phase in the life cycle phase in the life cycle that produces spores that produces spores is called is called sporophyte. sporophyte.

Facts on Alternation of GenerationsFacts on Alternation of Generations

3. A 3. A sporespore is a haploid is a haploid reproductive cell reproductive cell produced by produced by meiosismeiosis that that is capable of developing is capable of developing into an adult without into an adult without fusing with another cell. fusing with another cell.

4. In the life cycle of a 4. In the life cycle of a plant, the plant, the gametophytegametophyte is is the haploid phase that the haploid phase that produces gametes by produces gametes by mitosis.mitosis.

Facts on Alternation of GenerationsFacts on Alternation of Generations

5. In 5. In mossmoss, haploid spores , haploid spores develop in a capsule at develop in a capsule at the tip of the sporophyte the tip of the sporophyte “stalk” and when the lid “stalk” and when the lid of the of the capsulecapsule pops off, pops off, the spores the spores scatter.scatter.

6. The 6. The male gametophytesmale gametophytes release sperm which release sperm which swim through a film of swim through a film of moisturemoisture to the egg in to the egg in the the female gametophyte.female gametophyte.

SUMMARYSUMMARY

ALL THREE INVOLVE AN ALTERNATION ALL THREE INVOLVE AN ALTERNATION OF HAPLIOD AND DIPLOID PHASES OF HAPLIOD AND DIPLOID PHASES

THE THREE TYPES OF SEXUAL LIFE THE THREE TYPES OF SEXUAL LIFE CYCLES DIFFER FROM EACH OTHER CYCLES DIFFER FROM EACH OTHER ONLY IN WHICH PHASES BECOME ONLY IN WHICH PHASES BECOME MULTICELLULARMULTICELLULAR