1 Meiosis Meiosis Meiosis Meiosis & Sexual Reproduction Cell Division – Asexual reproduction • Mitosis – produce cells with same information • identical daughter cells – exact copies • clones – same amount of DNA • same number of chromosomes • same genetic information Asexual reproduction • Single-celled eukaryotes and simple multicellular eukaryotes reproduce asexually Binary fission in Amoeba Binary fission in Amoeba Binary fission in Amoeba Binary fission in Amoeba Budding in Yeast Budding in Yeast Budding in Yeast Budding in Yeast Budding in Budding in Budding in Budding in Hydra Hydra Hydra Hydra How about the rest of us? • What if a complex multicellular organism (like us) wants to reproduce? – joining of egg + sperm • Do we make egg & sperm by mitosis? 46 46 46 46 46 46 46 46 + + + 92 92 92 92 egg egg egg egg sperm sperm sperm sperm zygote zygote zygote zygote
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•Mitosis Meiosis •identical daughter cells •clones · Preparing for meiosis •Preceded by Interphase –Duplication of DNA –S phase of interphase of MEIOSIS (just like in
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MeiosisMeiosisMeiosisMeiosis& Sexual Reproduction
Cell Division –Asexual reproduction
• Mitosis
– produce cells with same information• identical daughter cells
– exact copies•clones
– same amount of DNA •same number of chromosomes
• same genetic information
Asexual reproduction
• Single-celled eukaryotes and simple multicellular eukaryotes reproduce asexually
Binary fission in AmoebaBinary fission in AmoebaBinary fission in AmoebaBinary fission in Amoeba
Budding in YeastBudding in YeastBudding in YeastBudding in Yeast Budding in Budding in Budding in Budding in HydraHydraHydraHydra
How about the rest of us?
• What if a complex multicellular organism (like us) wants to reproduce?
• Can you identify them?Can you identify them?Can you identify them?Can you identify them?
Four Mitotic StagesFour Mitotic StagesFour Mitotic StagesFour Mitotic Stages
•ProProProProphasephasephasephase
•MetaMetaMetaMetaphasephasephasephase
•AnaAnaAnaAnaphasephasephasephase
•TeloTeloTeloTelophasephasephasephase
MitosisMitosisMitosisMitosis
• Produces two new cellsidentical to the original cell
• Cells are diploid (2n)
Chromosomes during Chromosomes during Chromosomes during Chromosomes during Metaphase of mitosisMetaphase of mitosisMetaphase of mitosisMetaphase of mitosis
Facts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About Meiosis•• Preceded by interphase which includes Preceded by interphase which includes chromosome replicationchromosome replication
Homologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous Chromosomesseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis I
Tetrads Form in Prophase ITetrads Form in Prophase ITetrads Form in Prophase ITetrads Form in Prophase IHomologous chromosomesHomologous chromosomesHomologous chromosomesHomologous chromosomes
(each with sister chromatids) (each with sister chromatids) (each with sister chromatids) (each with sister chromatids) Join to form aJoin to form aJoin to form aJoin to form a
TETRADTETRADTETRADTETRAD
Called SynapsisCalled SynapsisCalled SynapsisCalled Synapsis
Crossing over- homologous pairs swappieces of chromosome
- Creates new combinations of traits
tetradtetradtetradtetrad
synapsissynapsissynapsissynapsisfrom mom
from dad
Sister chromatids homologs
from mom
from dad
Sister chromatids homologs
Metaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase I
Homologous pairsHomologous pairsHomologous pairsHomologous pairsof chromosomes of chromosomes of chromosomes of chromosomes align along the align along the align along the align along the
equator of the cellequator of the cellequator of the cellequator of the cell
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Independent Assortment
- chromosomes line up randomly
- independent of one another
- How many possibilities???
- 2n (n=number of chromosomes)
Independent Assortment
- in humans produces 223 (8,388,608)different combinations
Before meiosis
After meiosis
Maternal
Paternal
Anaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase I
Homologs separateHomologs separateHomologs separateHomologs separateHomologs separateHomologs separateHomologs separateHomologs separate and and and and and and and and move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.
CytokinesisCytokinesisCytokinesisCytokinesisCytokinesisCytokinesisCytokinesisCytokinesis divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.
• 2nd division of meiosis separatessister chromatids
Meiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis II
Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each chromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cell.
Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with
one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.
Sister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsare still attachedare still attachedare still attachedare still attachedare still attachedare still attachedare still attachedare still attached
Metaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase II
Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align along equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cell........
Anaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase II
Sister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsseparate and move separate and move separate and move separate and move separate and move separate and move separate and move separate and move to opposite polesto opposite polesto opposite polesto opposite polesto opposite polesto opposite polesto opposite polesto opposite poles.
Four haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cells with one with one with one with one with one with one with one with one copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome
One allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each gene
Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes along the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosome
Meiosis 1
Meiosis 2
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The value of meiosis
• Meiosis introduces genetic variation– gametes of offspring do not have same genes as gametes from parents
– This is the raw material for EVOLUTIONEVOLUTIONEVOLUTIONEVOLUTION
from Momfrom Momfrom Momfrom Mom from Dadfrom Dadfrom Dadfrom Dadnew gametesnew gametesnew gametesnew gametes
made by offspringmade by offspringmade by offspringmade by offspring
Sources of genetic variability
• Genetic variability in sexual reproduction
– independent assortment•homologous chromosomes in Meiosis 1
– crossing over•between homologous chromosomes in prophase 1
– random fertilization• random ovum fertilized by a random sperm
Any 2 parents will produce a zygote with over 70 trillion(223 x 223) diploid combinations
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both genetic similarity & differences.
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