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Sexual vs. Asexual Reproduction

MEIOSIS

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Asexual ReproductionAsexual Reproduction

The new organism has a single parent, The new organism has a single parent, the purpose is to create genetically the purpose is to create genetically identical individuals rapidly.identical individuals rapidly.

Mitosis is an example of asexual Mitosis is an example of asexual reproduction, specifically binary fission reproduction, specifically binary fission (dividing in half after replicating DNA).(dividing in half after replicating DNA).

Other types are budding and vegetative Other types are budding and vegetative reproduction.reproduction.

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Sexual ReproductionSexual Reproduction Two cells from different parents Two cells from different parents

unite to form the first cell of the unite to form the first cell of the new individual, increase genetic new individual, increase genetic variability.variability.

Examples of organisms that reproduce sexuallyExamples of organisms that reproduce sexually HumansHumans PlantsPlants DolphinsDolphins SnakesSnakes

So how does sexual reproduction occur?So how does sexual reproduction occur?

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MEIOSISMEIOSIS

The second type of cell The second type of cell divisiondivision

Takes place in the gonads Takes place in the gonads (ovaries, testes) to produce (ovaries, testes) to produce gametesgametes (sperm, eggs) (sperm, eggs) Gametes have half of the chromosomes Gametes have half of the chromosomes of the of the

all other body cells (called all other body cells (called somatic cells)somatic cells) For humans: somatic cells – 46 For humans: somatic cells – 46

chromosomes; gametes – 23 chromosomes; gametes – 23 chromosomeschromosomes

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MEIOSISMEIOSIS

Somatic cellsSomatic cells are called are called diploiddiploid cells cells (2(2nn); gametes are called ); gametes are called haploidhaploid cells cells ((nn))

Meiosis ensures that new organisms Meiosis ensures that new organisms have the correct number of have the correct number of chromosomeschromosomes

Upon fertilization, the normal Upon fertilization, the normal chromosome number (diploid) is chromosome number (diploid) is restoredrestored

The The zygotezygote formed by fertilization formed by fertilization divides by mitosis and grows to be a divides by mitosis and grows to be a new organism. new organism.

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LIFE CYCLELIFE CYCLE

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MEIOSISMEIOSIS

Consists of two stages, meiosis Consists of two stages, meiosis I and meiosis III and meiosis II

In diploid cells chromosomes In diploid cells chromosomes occur as occur as homologous homologous chromosomeschromosomes Chromosomes that have similar genesChromosomes that have similar genes

Meiosis I begins with Meiosis I begins with homologous chromosomes homologous chromosomes composed of 2 chromatids composed of 2 chromatids (tetrads) Why?(tetrads) Why?

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HOMOLOGOUS HOMOLOGOUS CHROMOSOMESCHROMOSOMES

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PROPHASE IPROPHASE I What is happening? What is happening?

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PROPHASE I PROPHASE I

Chromatin coils and thickens into Chromatin coils and thickens into chromosomeschromosomes

Nucleoli disappear and the nuclear Nucleoli disappear and the nuclear membrane is disassembledmembrane is disassembled

Spindle fibers form from centriolesSpindle fibers form from centrioles Homologous chromosomes Homologous chromosomes synapsesynapse

(pair up) with one another forming a (pair up) with one another forming a tetradtetrad

Crossing-overCrossing-over may occur – may occur – exchange of DNAexchange of DNA

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METAPHASE IMETAPHASE I What is happening? What is happening?

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METAPHASE IMETAPHASE I

Tetrad moves to the equatorial plane Tetrad moves to the equatorial plane (middle)(middle)

Chromosomes become attached to Chromosomes become attached to the spindle fibers at the centromerethe spindle fibers at the centromere

Homologous chromosomes line up Homologous chromosomes line up randomly in a process known as randomly in a process known as independent assortmentindependent assortment

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ANAPHASE IANAPHASE IWhat is happening?What is happening?

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ANAPHASE IANAPHASE I

Chromosome number is reduced from Chromosome number is reduced from diploid to diploid to haploid when homologous haploid when homologous chromosomes split and move to chromosomes split and move to opposite poles – called segregationopposite poles – called segregation

Each chromosome is independently Each chromosome is independently attached to a spindle at the centromereattached to a spindle at the centromere

The centromeres do not split – The centromeres do not split – each each chromosomes still consists of two chromosomes still consists of two chromatidschromatids

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MEIOSIS IMEIOSIS I What is happening? What is happening?

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TELOPHASE ITELOPHASE I

Chromosomes uncoil and become long, Chromosomes uncoil and become long, thin threadsthin threads

The nuclear membrane reforms around The nuclear membrane reforms around the chromosomesthe chromosomes

Nucleoli reappearNucleoli reappear Cytokinesis divides the cell into two Cytokinesis divides the cell into two

daughter cells.daughter cells. Each daughter cell has one member of the Each daughter cell has one member of the

original homologous chromosome pairoriginal homologous chromosome pair

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MEIOSIS IIMEIOSIS II

The two daughter cells formed in The two daughter cells formed in meiosis I go through another divisionmeiosis I go through another division

Events are similar to those in mitosisEvents are similar to those in mitosis Chromosomes ARE NOT Chromosomes ARE NOT

duplicated before this stageduplicated before this stage

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PROPHASE IIPROPHASE II What is happening? What is happening?

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PROPHASE IIPROPHASE II

Chromosomes recoilChromosomes recoil Nuclear membrane disintegratesNuclear membrane disintegrates Nucleoli disappearNucleoli disappear Spindle fibers reformSpindle fibers reform Crossing over, segregation, and Crossing over, segregation, and

independent assortment do not independent assortment do not occur during meiosis IIoccur during meiosis II

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METAPHASE IIMETAPHASE II What is happening? What is happening?

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METAPHASE IIMETAPHASE II

Chromosomes are attached to the Chromosomes are attached to the spindle by their centromere spindle by their centromere

The chromosomes move to the The chromosomes move to the equatorial plane (middle)equatorial plane (middle)

Pairs of chromosomes are not Pairs of chromosomes are not attached, therefore, each attached, therefore, each chromosome moves as a separate chromosome moves as a separate unitunit

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ANAPHASE IIANAPHASE II What is happening? What is happening?

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ANAPHASE IIANAPHASE II

The centromere of each chromosome The centromere of each chromosome divides as the chromatids (daughter divides as the chromatids (daughter chromosomes) move to the opposite chromosomes) move to the opposite poles (poles (like mitosis)like mitosis)

No segregation or independent No segregation or independent assortment at this stage – very assortment at this stage – very similar to mitosissimilar to mitosis

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MEIOSISMEIOSIS What is happening? What is happening?

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TELOPHASE IITELOPHASE II Cytokinesis occursCytokinesis occurs Nuclear membrane reappears and Nuclear membrane reappears and

spindle fibers disappearspindle fibers disappear Chromosomes uncoil, nuclei re-formChromosomes uncoil, nuclei re-form Four gametes are formed (sperm or Four gametes are formed (sperm or

eggs)eggs) In humans and other organisms only In humans and other organisms only

one functional egg is produced. The one functional egg is produced. The other three (polar bodies) disintegrate.other three (polar bodies) disintegrate.

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SOURCES OF VARIATIONSOURCES OF VARIATION

Genetic variation is influenced by:Genetic variation is influenced by: MutationsMutations Crossing-overCrossing-over SegregationSegregation Independent assortmentIndependent assortment Fertilization Fertilization

Mutations increase variation by the Mutations increase variation by the production of different proteins.production of different proteins. May or may not be beneficialMay or may not be beneficial

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GENETIC VARIATIONGENETIC VARIATION

Crossing-over, which occurs during Crossing-over, which occurs during prophase I, causes genetic prophase I, causes genetic information to be exchanged information to be exchanged between chromosomesbetween chromosomes

Can occur at any point on a Can occur at any point on a chromosome, resulting in new chromosome, resulting in new chromosomal combinations chromosomal combinations Can explain why a child can show a Can explain why a child can show a

mixture of family characteristicsmixture of family characteristics

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CROSSING OVERCROSSING OVER

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GENETIC VARIATIONGENETIC VARIATION

Segregation involves the separation and Segregation involves the separation and movement of homologous chromosomes to the movement of homologous chromosomes to the poles poles Occurs during anaphase IOccurs during anaphase I

Causes genes to be separated from each Causes genes to be separated from each other so that they have an equal chance of other so that they have an equal chance of being transmitted to the next generation.being transmitted to the next generation.

Independent assortment refers to how Independent assortment refers to how homologous pairs line up during homologous pairs line up during metaphase Imetaphase I Effects of this arrangement increases as the Effects of this arrangement increases as the

number of chromosome pairs increasenumber of chromosome pairs increase

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GENETIC VARIATIONGENETIC VARIATION Effects of independent assortment Effects of independent assortment

can be explained using a can be explained using a mathematical equation:mathematical equation:22nn,, where where nn is the number of pairs of is the number of pairs of

chromosomes.chromosomes. If n=3, 23=8 possibilitiesIf n=3, 23=8 possibilities If n=23, 223 = 8,388,608 possibilities!If n=23, 223 = 8,388,608 possibilities!

More than 8 million kinds of sperm or More than 8 million kinds of sperm or egg cells are possible resulting from egg cells are possible resulting from independent assortment alone! independent assortment alone!

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FERTILIZATIONFERTILIZATION

As a result of all genetic As a result of all genetic variation mechanisms, the variation mechanisms, the number of different types of number of different types of possible offspring is large.possible offspring is large.

Because of these, every human Because of these, every human being is genetically unique.being is genetically unique.Except for identical twins Except for identical twins

(identical DNA)(identical DNA)

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NONDISJUNCTIONNONDISJUNCTION

Normally during meiosis, diploid cells Normally during meiosis, diploid cells separate and create haploid cellsseparate and create haploid cells

Occasionally, a pair of homologous Occasionally, a pair of homologous chromosomes remain attached and chromosomes remain attached and end up on the same gameteend up on the same gameteKnown as nondisjunctionKnown as nondisjunctionUsually causes cell deathUsually causes cell death

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NONDISJUNCTIONNONDISJUNCTION

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NONDISJUNCTIONNONDISJUNCTION Some cells can surviveSome cells can survive If a sperm or egg carrying the wrong If a sperm or egg carrying the wrong

number of chromosomes fertilizes number of chromosomes fertilizes another gamete, the offspring will another gamete, the offspring will show abnormalitiesshow abnormalities

One example is Down syndromeOne example is Down syndromeAffected individuals carry an extra Affected individuals carry an extra

chromosome 21. chromosome 21.

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SEX DETERMINATIONSEX DETERMINATION In many organisms sex-determining genes In many organisms sex-determining genes

are located on specific chromosomes are located on specific chromosomes called called sex chromosomessex chromosomes. . In humans, males have XY and females have In humans, males have XY and females have

XXXX All other chromosomes are known as All other chromosomes are known as

autosomal chromosomesautosomal chromosomes. . Genes that determine male characteristics Genes that determine male characteristics

are located on the Y chromosome.are located on the Y chromosome. X chromosomes have genes necessary for X chromosomes have genes necessary for

the survival of both males and femalesthe survival of both males and females

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Mitosis vs. MeiosisMitosis vs. Meiosis

•Two divisions of nucleus•One division of nucleus

•Produces 4 daughter cells•Produces 2 daughter cells

•Haploid cells – n•Diploid cells – 2n

•Occurs in Gametes•Occurs in body cells

•MeiosisMeiosis•MitosisMitosis