MeiosisMeiosis

Formation of Sex Cells to Formation of Sex Cells to Transfer Genetic Information Transfer Genetic Information to the Next Generationto the Next Generation

Mitosis and Meiosis in the Mitosis and Meiosis in the Human Life CycleHuman Life Cycle

Nuclear division reducing the chromosome

number, leading to sperm or eggs

Diploid number of chromosomes,

two full sets

Haploid number of chromosomes,

one full set

Nuclear division retaining the original chromosome number

Mitosis and MeiosisMitosis and Meiosis

2 Genetically-Identical2 Genetically-IdenticalHuman Cells each withHuman Cells each with46 chromosomes 46 chromosomes (DIPLOID) (DIPLOID)

4 Genetically-Different 4 Genetically-Different Human Cells each withHuman Cells each with23 chromosomes23 chromosomes (HAPLOID)(HAPLOID)

Human Cell with 46 Human Cell with 46 chromosomes (DIPLOID) chromosomes (DIPLOID)

Human Cell with 46 Human Cell with 46 chromosomes (DIPLOID) chromosomes (DIPLOID)

InterphaseInterphaseMitosisMitosisCytokinesisCytokinesis

InterphaseInterphaseMeiosisMeiosisCytokinesisCytokinesis

Homologous Chromosome PairsHomologous Chromosome Pairs

• Diploid cells have Diploid cells have pairs of pairs of chromosomeschromosomes

• Each homologous Each homologous chromosome pair is chromosome pair is genetically matched genetically matched so that the genes are so that the genes are arranged in the arranged in the same order same order

• Meiosis distributes Meiosis distributes members of each members of each pair to separate pair to separate nucleinuclei

Humans have 1 pair of sex chromosomes and 22 pairs of autosomes

Cell Cycle With Meiosis Cell Cycle With Meiosis

G1cytoplasm doubles, including organelles

S

chromosomesduplicate

G2synthesis of proteins and assembly of components for division

cytokinesis

MeiosisInterphase includes Interphase includes

G1 = growth phase 1G1 = growth phase 1 S = synthesis phaseS = synthesis phaseG2 = growth phase 2G2 = growth phase 2

Interphase

Meiosis IIcy

toki

nesis

Mei

osis

I

Meiosis includes two Meiosis includes two chromosomal divisionschromosomal divisions

Meiosis IMeiosis IMeiosis IIMeiosis II

Cytokinesis = division ofCytokinesis = division of cytoplasmic contents cytoplasmic contents

Applying Your KnowledgeApplying Your Knowledge

A.A. When does chromosomal division occur? When does chromosomal division occur?

B.B. When do the cytoplasmic contents double?When do the cytoplasmic contents double?

C.C. When does chromosome duplication occur?When does chromosome duplication occur?

1.1. G1 phaseG1 phase

2.2. S phase S phase

3.3. G2 phaseG2 phase

4.4. Meiosis I and IIMeiosis I and II

Meiosis Involves Two Chromosomal DivisionsMeiosis Involves Two Chromosomal Divisions

Reduction Division: Separation of homologous

chromosomes, reducing the chromosome number to

haploid

Equational Division: Separation of sister

chromatids, no change in chromosome number

Duplicated Duplicated ChromosomesChromosomes

in Early Meiosis in Early Meiosis

Homologous Homologous Chromosome Pair Chromosome Pair

Duplicated chromosomes joined at their centromeres

Centromere = constricted region

Nonsister Chromatids are joined to differentcentromeres

Prophase I of Meiosis IProphase I of Meiosis I

Chromosomes thickenChromosomes thickenGenetic exchange occurs between nonsister Genetic exchange occurs between nonsister chromatids through crossing-over chromatids through crossing-over

Metaphase I of Meiosis IMetaphase I of Meiosis I

Homologous pairs line up at the cell equatorHomologous pairs line up at the cell equatorSpindle fibers attached to centromeres cause Spindle fibers attached to centromeres cause chromosomal movement chromosomal movement

Spindle fiber

Anaphase I and Telophase I Anaphase I and Telophase I of Meiosis Iof Meiosis I

Anaphase IAnaphase IMembers of homologous pair Members of homologous pair separate and are pulled to separate and are pulled to opposite poles of the cell opposite poles of the cell

Telophase ITelophase IChromosomes reach oppositeChromosomes reach opposite poles poles

+ cytokinesis

CytokinesisCytokinesisCytoplasm divides into two Cytoplasm divides into two separate cells separate cells

Daughter cells are haploid with duplicated chromosomes

Prophase II and Metaphase II of Prophase II and Metaphase II of Meiosis IIMeiosis II

Prophase IIProphase IIChromosomes thickenChromosomes thicken

Metaphase IIMetaphase IIChromosomes line up Chromosomes line up at cell equator at cell equator

Anaphase II and Telophase II of Anaphase II and Telophase II of Meiosis IIMeiosis II

Anaphase IIAnaphase IISister Chromatids Sister Chromatids separate and move to separate and move to opposite poles opposite poles

Telophase IITelophase IIChromosomes have Chromosomes have reached opposite poles reached opposite poles + cytokinesis

CytokinesisCytokinesisCytoplasm divides into Cytoplasm divides into

separate cells separate cells Daughter cells are haploid

and have genetic differences

Applying Your KnowledgeApplying Your Knowledge

A.A. When do sister chromatids separate?When do sister chromatids separate?

B.B. When does crossing-over occur?When does crossing-over occur?

C.C. When do homologous pairs line up at When do homologous pairs line up at the cell equator? the cell equator?

1.1. Prophase IProphase I

2.2. Metaphase IMetaphase I

3.3. Anaphase IAnaphase I

4.4. Metaphase IIMetaphase II

5.5. Anaphase IIAnaphase II

Sperm formationSperm formation•Equal divisions of cytoplasmEqual divisions of cytoplasm

•Four functional products per meiosisFour functional products per meiosis

Egg formationEgg formation•Unequal divisions of cytoplasmUnequal divisions of cytoplasm

•Small polar bodies formedSmall polar bodies formed•One functional product per meiosisOne functional product per meiosis

Fertilization occurs after Meiosis I and

stimulates Meiosis II

Comparing Mitosis and MeiosisComparing Mitosis and Meiosis

MitosisMitosis MeiosisMeiosis

Cell type at start Cell type at start

Cell type at endCell type at end

Are products identical? (Y/N)Are products identical? (Y/N)

Number of cells producedNumber of cells produced

Number of chromosome Number of chromosome duplicationsduplications

Pairing of homologues (Y/N)Pairing of homologues (Y/N)

Crossing-over (Y/N)Crossing-over (Y/N)

Number of chromosome Number of chromosome divisionsdivisions

Comparing Mitosis and MeiosisComparing Mitosis and Meiosis

MitosisMitosis MeiosisMeiosis

Cell type at start Cell type at start

Cell type at endCell type at end

Are products identical? (Y/N)Are products identical? (Y/N)

Number of cells producedNumber of cells produced

Number of chromosome Number of chromosome duplicationsduplications

Pairing of homologues (Y/N)Pairing of homologues (Y/N)

Crossing-over (Y/N)Crossing-over (Y/N)

Number of chromosome Number of chromosome divisionsdivisions

DiploidDiploid

DiploidDiploidDiploidDiploid

HaploidHaploid

YesYes NoNo

22 44

11 11

NoNo YesYesNoNo YesYes

11 22