MEIOSIS
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Transcript of MEIOSIS
MEIOSISMEIOSIS
Meiosis• The form of cell divisioncell division by which
gametesgametes, , with halfhalf the number of chromosomes,chromosomes, are produced.
• Diploid (2n) Diploid (2n) haploid (n)haploid (n)
• MeiosisMeiosis is sexual reproduction.sexual reproduction.
• Two divisionsTwo divisions (meiosis Imeiosis I and meiosis IImeiosis II).
MeiosisMeiosis• Sex cells Sex cells divide to produce gametesgametes
(sperm or egg)(sperm or egg).
• GametesGametes have halfhalf the # of chromosomeschromosomes.
• Occurs only in gonads (testes or Occurs only in gonads (testes or ovaries).ovaries).
Three sexual life cycles differing in the timing of meiosis and fertilization
Moss with Sporophytes
Gametophytes
MeiosisMeiosis
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II
SpermatogenesisSpermatogenesis
2n=46
Spermato-cyte
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid(n)
meiosis II
OogenesisOogenesis
2n=46
Oocyte
diploid (2n)
n=23
n=23
meiosis I
n=23
ovum
haploid(n)
meiosis II
polar polar bodiesbodies
Interphase Interphase • Similar to mitosismitosis interphase.• ChromosomesChromosomes replicate (S phase).(S phase).
• Each duplicated chromosomechromosome consists of two identical sister chromatidschromatids attached at their centromerescentromeres.
• CentrioleCentriole pairs also replicate.
Interphase Interphase
• NucleusNucleus and nucleolusnucleolus visible.
nuclear membrane
nucleolus
cell membrane
chromatin
Meiosis I (four phases)Meiosis I (four phases)• Cell division Cell division that reduces the
chromosomechromosome number by one-half.one-half.
Four PhasesFour Phases:a.a. prophase Iprophase Ib.b. metaphase Imetaphase Ic.c. anaphase Ianaphase Id.d. telophase Itelophase I
Prophase IProphase I• Longest and most complex Longest and most complex
phase (90%).phase (90%).
• ChromosomesChromosomes condense.
• SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes come together to form a tetradtetrad.
Homologous ChromosomesHomologous Chromosomes• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are
similar in shape and size.
• Homologous pairs (tetrads)(tetrads) carry genes controlling the same inherited traits.
• Each locuslocus (position of a gene)(position of a gene) is in the same position on homologues.
a. 22 pairs of autosomesautosomesb. 01 pair of sex chromosomessex chromosomes
Homologous ChromosomesHomologous Chromosomes
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
Prophase IProphase I
centriolesspindle fiber
asterfibers
Prophase IProphase I - - SynapsisSynapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
Crossing OverCrossing Over• Crossing over (variation) Crossing over (variation) may
occur between nonsister chromatidschromatids.
• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.
Crossing Over - variationCrossing Over - variation
nonsister chromatids
site of crossing over variation
Tetrad
Metaphase IMetaphase I
equator
OR
equator
Metaphase IMetaphase I• Shortest phaseShortest phase• TetradsTetrads align on the equatorequator
• INDEPENDENT ASSORTMENT OCCURSINDEPENDENT ASSORTMENT OCCURS: : genes for different traitsgenes for different traits
sort independently of one anotherinto gametes
Anaphase IAnaphase I
Anaphase IAnaphase I
• Homologous Homologous chromosomeschromosomes separate and move towards the poles.
• Sister chromatidsSister chromatids remain attached at their centromerescentromeres.
Telophase ITelophase I
Telophase ITelophase I
• Each pole now has haploidhaploid set of chromosomeschromosomes.
• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.
Meiosis IIMeiosis II
• No interphase No interphase (or very short - no more DNA DNA replicationreplication)
• Remember:Remember: Meiosis II Meiosis II is similar to mitosis but the cells are mitosis but the cells are different from each otherdifferent from each other
Prophase IIProphase II
• same as prophaseprophase in mitosismitosis
Metaphase IIMetaphase II
• same as metaphasemetaphase in mitosismitosis
equator equator
Anaphase IIAnaphase II
• same as anaphaseanaphase in mitosismitosis• sister chromatids separatesister chromatids separate
Telophase IITelophase II
Telophase IITelophase II• Same as telophasetelophase in mitosismitosis.
• Nuclei form. CytokinesisCytokinesis occurs.
• Remember:Remember: four haploid daughter cells four haploid daughter cells produced; 3 polar bodies and anproduced; 3 polar bodies and an
egg in females, 4 sperm in malesegg in females, 4 sperm in males
gametes = sperm or egggametes = sperm or egg
FertilizationFertilization• The fusion of a spermsperm and eggegg to
form a zygotezygote - a fertilized egg
n=23egg
sperm n=23
2n=46zygote
VariationVariation• Important to population as the Important to population as the
raw material for raw material for natural natural selectionselection..
• Question:Question:What are the three sexual What are the three sexual
sources of genetic variation? sources of genetic variation?
Answer:Answer:1. crossing over (prophase I)1. crossing over (prophase I)
2. independent assortment 2. independent assortment (metaphase I)(metaphase I)
3. random fertilization3. random fertilization
Remember: variation is good !
KaryotypeKaryotype• A method of organizing the chromosomes of a A method of organizing the chromosomes of a
cell in relation to number, size, and type.cell in relation to number, size, and type.
Question:Question:
• A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
Answer:Answer:
• 10 chromosomes (haploid)10 chromosomes (haploid)
Question:Question:
• A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
Answer:Answer:
• 10 chromosomes10 chromosomes
Question:Question:
• In terms of In terms of Independent Independent AssortmentAssortment -how many different -how many different combinations of sperm could a combinations of sperm could a humanhuman malemale produce produce? ?
AnswerAnswer
• Formula: 2Formula: 2nn
• Human chromosomes:Human chromosomes: 2n = 462n = 46 n = 23n = 23
• 222323 = ~8 million combinations = ~8 million combinations
1) Three major differences 1) Three major differences between meiosis and the mitosis between meiosis and the mitosis are:are:
2) Compare the number of 2) Compare the number of chromatids present in human cells chromatids present in human cells before and after DNA replication before and after DNA replication occurs in interphase.occurs in interphase.
3) Compare and contrast oogenesis3) Compare and contrast oogenesiswith spermatogenesis.with spermatogenesis.
Determine the number of chromo-Determine the number of chromo-somes in a cell whose diploid (2n)somes in a cell whose diploid (2n)number is 10 after:number is 10 after:
a. meiosis Ia. meiosis Ib. meiosis IIb. meiosis IIc. mitosisc. mitosis
What is the primary difference What is the primary difference between sexual life cycles between sexual life cycles in animals when compared to fungiin animals when compared to fungiand plants?and plants?