Meiosis

MeiosisMeiosisChapter 12Chapter 12

GeneticsGenetics

““Like begets like”Like begets like” Heredity:Heredity:

The continuity of traits from one generation to The continuity of traits from one generation to the nextthe next

This is why organisms resemble their parentsThis is why organisms resemble their parents

Variation:Variation: Offspring exhibit individuality, different Offspring exhibit individuality, different

somewhat from their parents and their siblingssomewhat from their parents and their siblings

GenesGenes

Gene:Gene: A hereditary unit that codes for genetic informationA hereditary unit that codes for genetic information A segment of DNAA segment of DNA

Genome:Genome: The tens of thousands of genes we inherit from our parents The tens of thousands of genes we inherit from our parents

make up our genomemake up our genome All of genes put together make up our GENOMEAll of genes put together make up our GENOME

The DNA of a eukaryotic cell is subdivided into The DNA of a eukaryotic cell is subdivided into chromosomes, located in the nucleus of every cellchromosomes, located in the nucleus of every cell

A gene’s specific location along the length of a A gene’s specific location along the length of a chromosome is called the gene’s chromosome is called the gene’s locuslocus Like an addressLike an address

Asexual ReproductionAsexual Reproduction A single individual is A single individual is

the sole parent and the sole parent and passes copies of all its passes copies of all its genes on to its genes on to its offspringoffspring

Since the offspring has Since the offspring has the same genetic info the same genetic info as its parent, it is as its parent, it is known as a known as a cloneclone

The only genetic The only genetic variation is due to variation is due to mutationsmutations

Sexual ReproductionSexual Reproduction

Usually results in greater variationUsually results in greater variation Two parents give rise to offspring that have Two parents give rise to offspring that have

unique combinations of genes inherited from unique combinations of genes inherited from both parents. both parents.

Offspring of sexual reproduction vary Offspring of sexual reproduction vary genetically from their siblings and both genetically from their siblings and both parents.parents.

HOW??HOW?? MEIOSIS!! MEIOSIS!!

The Human Life CycleThe Human Life Cycle

Gametes vs. Somatic CellsGametes vs. Somatic Cells Gametes: Sperm or ova cellsGametes: Sperm or ova cells Somatic Cells: Any other cell besides sperm Somatic Cells: Any other cell besides sperm

or ova (muscle, nerve, etc.)or ova (muscle, nerve, etc.)

Every human somatic cell contains 46 Every human somatic cell contains 46 chromosomeschromosomes

Human gametes contain 23 chromosomesHuman gametes contain 23 chromosomes

ChromosomesChromosomes A chromosome is a long coiled-up strand of DNAA chromosome is a long coiled-up strand of DNA Humans have 23 pairs of homologous Humans have 23 pairs of homologous

chromosomes in each somatic cellchromosomes in each somatic cell Homologous Chromosomes:Homologous Chromosomes:

Chromosomes that have the same length, centromere Chromosomes that have the same length, centromere position, and staining patternposition, and staining pattern

Carry genes controlling the same inherited characteristics (ie. Carry genes controlling the same inherited characteristics (ie. Eye color, attached ear lobes)Eye color, attached ear lobes)

One chromosome of each pair comes from each parentOne chromosome of each pair comes from each parent

Only exception…Only exception… Male sex chromosomes (XY) Male sex chromosomes (XY) Female sex chromosomes (XX) Female sex chromosomes (XX)

Haploid vs. Diploid CellsHaploid vs. Diploid Cells Haploid cells contain a single set of Haploid cells contain a single set of

chromosomeschromosomes Abbreviated nAbbreviated n In humans, there are 23 chromosomes in a haploid cellIn humans, there are 23 chromosomes in a haploid cell Gametes are the only haploid cellsGametes are the only haploid cells Produced by meiosisProduced by meiosis

Diploid cells contain a double set of Diploid cells contain a double set of chromosomeschromosomes Abbreviated 2nAbbreviated 2n In humans, there are 46 chromosomes in a diploid cellIn humans, there are 46 chromosomes in a diploid cell All cells other than gametes are diploid cellsAll cells other than gametes are diploid cells Produced by mitosisProduced by mitosis

MeiosisMeiosis

Big Picture:Big Picture: Replication of chromosomes (DNA) followed Replication of chromosomes (DNA) followed

by by twotwo consecutive cell divisions consecutive cell divisions Results in 4 daughter cells, each with only Results in 4 daughter cells, each with only

half as many chromosomes as the parenthalf as many chromosomes as the parent

Meiosis I: Meiosis I: Separating Homologous ChromosomesSeparating Homologous Chromosomes

Interphase I:Interphase I: Chromosome (DNA) Chromosome (DNA)

replication occursreplication occurs

13-07a-InterphaseI.mov

Meiosis I: Meiosis I: Separating Homologous ChromosomesSeparating Homologous Chromosomes

Prophase I:Prophase I: Chromosomes begin to condense (you Chromosomes begin to condense (you

can see them!)can see them!) Synapsis occurs:Synapsis occurs:

Homologous chromosomes come together (4 Homologous chromosomes come together (4 sister chromatids total)sister chromatids total)

ChiasmataChiasmata Chromatids cross over at various places, Chromatids cross over at various places,

known as chiasmataknown as chiasmata An exchange of genetic information occursAn exchange of genetic information occurs

Nucleoli and nuclear envelope disappearNucleoli and nuclear envelope disappear Spindle microtubules begin to formSpindle microtubules begin to form

13-07b-ProphaseI.mov

Meiosis I: Meiosis I: Separating Homologous ChromosomesSeparating Homologous Chromosomes

Metaphase I:Metaphase I: Chromosomes are Chromosomes are

arranged on the arranged on the metaphase plate in metaphase plate in homologous pairshomologous pairs

13-07c-MetaphaseI.mov

Meiosis I: Meiosis I: Separating Homologous ChromosomesSeparating Homologous Chromosomes

Anaphase I:Anaphase I: The spindle fibers The spindle fibers

move the move the chromosomes toward chromosomes toward the polesthe poles

Each homologous Each homologous chromosome of a chromosome of a tetrad goes to opposite tetrad goes to opposite polespoles

13-07d-AnaphaseI.mov

Meiosis I: Meiosis I: Separating Homologous ChromosomesSeparating Homologous Chromosomes

Telophase I:Telophase I: The chromosomes reach The chromosomes reach

the poles of the cellthe poles of the cell Cytokinesis occursCytokinesis occurs

Animals: cleavage furrowAnimals: cleavage furrow Plants: cell platePlants: cell plate

13-07e-TelophaseICytokin.mov

Meiosis II: Meiosis II: Separating Sister ChromatidsSeparating Sister Chromatids

Prophase II:Prophase II: Spindle apparatus re-forms (in some species)Spindle apparatus re-forms (in some species) Chromosomes move towards middleChromosomes move towards middle

Metaphase II:Metaphase II: Chromosomes line up on the metaphase plateChromosomes line up on the metaphase plate

Anaphase II:Anaphase II: Centromeres of sister chromatids separateCentromeres of sister chromatids separate Sister chromatids move toward opposite poles of Sister chromatids move toward opposite poles of

the cellthe cell Telophase II/Cytokinesis:Telophase II/Cytokinesis:

Nuclei form at opposite poles of the cellNuclei form at opposite poles of the cell Cytokinesis occursCytokinesis occurs 4 haploid daughter cells4 haploid daughter cells

13-07f-MeiosisIICytokin.mov

Mitosis vs. MeiosisMitosis vs. Meiosis

Three key differences that occur during Meiosis:Three key differences that occur during Meiosis:1. Prophase I1. Prophase I

Crossing over of nonsister chromosomes occurs, resulting in Crossing over of nonsister chromosomes occurs, resulting in the exchange of genetic informationthe exchange of genetic information

2. Metaphase I2. Metaphase I Homologous pairs of chromosomes line up at the metaphase Homologous pairs of chromosomes line up at the metaphase

plate, instead of individual chromsomesplate, instead of individual chromsomes

3. Anaphase I3. Anaphase I Sister chromatids of a chromosome remain attached at the Sister chromatids of a chromosome remain attached at the

centromere and go to opposite polescentromere and go to opposite poles

Crossing Over Results in Genetic VariationCrossing Over Results in Genetic Variation

Random FertilizationRandom Fertilization

Random nature of fertilization adds to the Random nature of fertilization adds to the genetic variation arising from meiosisgenetic variation arising from meiosis

A zygote is made up of an ovum and a A zygote is made up of an ovum and a sperm cellsperm cell 8 million different possibilities for sperm8 million different possibilities for sperm 8 million different possibilities for ovum8 million different possibilities for ovum 64 trillion different diploid possibilities64 trillion different diploid possibilities