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MEIOSISMEIOSIS
Meiosis
• The form of cell divisioncell division by which gametes, gametes, 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 I meiosis I and meiosis IImeiosis II).
FertilizationFertilization
• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg
n=23egg
sperm n=23
2n=46zygote
MeiosisMeiosis
• Sex cells Sex cells divide to produce gametesgametes (sperm (sperm or egg)or egg).
• GametesGametes have halfhalf the # of chromosomeschromosomes.
• Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).
Male: spermatogenesisMale: spermatogenesis
Female: oogenesisFemale: oogenesis
• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.
SpermatogenesisSpermatogenesis
2n=46
humansex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
Meiosis – mouse testes Meiosis – mouse testes Parent cell
4 gametes
1st division
2nd division
Interphase IInterphase I
• Similar to mitosismitosis interphase.
• ChromosomesChromosomes replicate (S phase).(S phase).
• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.
• CentrioleCentriole pairs also replicate.
Interphase IInterphase I
• 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 I
b.b. metaphase Imetaphase I
c.c. anaphase Ianaphase I
d.d. telophase Itelophase I
Prophase IProphase I
• Longest and most complex phase (90%).Longest and most complex phase (90%).
• ChromosomesChromosomes condense.
• SynapsisSynapsis occurs: homologous chromosomes homologous chromosomes come together to form a tetradtetrad.
• TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).
Prophase IProphase I - - SynapsisSynapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
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.
• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.
a. 22 pairs of autosomesautosomes
b. 01 pair of sex chromosomessex chromosomes
KaryotypeKaryotype• A method of organizing the A method of organizing the chromosomes chromosomes of a of a
cell in relation to number, size, and type.cell in relation to number, size, and type.
Homologous ChromosomesHomologous Chromosomes
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.
Homologue
Homologue
AutosomesAutosomes(The Autosomes code for most of the offspring’s traits)(The Autosomes code for most of the offspring’s traits)
In Humans the “Autosomes” are sets 1 - 22
21 trisomy – Downs 21 trisomy – Downs SyndromeSyndrome
Can you see the extra 21st chromosome?
Is this person male or female?
Sex ChromosomesSex ChromosomesThe Sex Chromosomes code for the sex of the offspring.The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a ** If the offspring has two “X” chromosomes it will be a femalefemale.. ** If the offspring has one “X” chromosome and one “Y” ** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a chromosome it will be a malemale..
XX chromosome - female XY chromosome - male
In Humans the “Sex
Chromosomes” are the 23rd set
Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”
X chromosomeX chromosomeY chromosomeY chromosome
Crossing OverCrossing Over
• Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata.
• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.
• Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing crossing overover.
Crossing Over - variation Crossing Over - variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
Another Way Meiosis Makes Lots of Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-OverDifferent Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of Crossing-over multiplies the already huge number of different gamete types produced by independent different gamete types produced by independent
assortment.assortment.
Sex ChromosomesSex Chromosomes
XX chromosome - female XY chromosome - male
Prophase IProphase I
centriolesspindle fiber
asterfibers
Metaphase IMetaphase I• Shortest phaseShortest phase
• TetradsTetrads align on the metaphase platemetaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n
Example:Example: 2n = 42n = 4then then n = 2 n = 2
thusthus 2 22 2 = 4 combinations= 4 combinations
Metaphase IMetaphase I
metaphase plate
OR
metaphase plate
Question:Question:
• In terms of In terms of Independent AssortmentIndependent Assortment - -how many different combinations of how many different combinations of sperm could a 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
Anaphase IAnaphase I
• Homologous chromosomes Homologous chromosomes separate and move towards the poles.
• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.
Anaphase IAnaphase I
Telophase ITelophase I
• Each pole now has haploidhaploid set of chromosomeschromosomes.
• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.
Telophase ITelophase I
Meiosis IIMeiosis II
• No interphase II No interphase II
(or very short - no more DNA replicationDNA replication)
• Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis
Prophase IIProphase II
• same as prophaseprophase in mitosismitosis
Metaphase IIMetaphase II
• same as metaphasemetaphase in mitosismitosis
metaphase platemetaphase plate
Anaphase IIAnaphase II
• same as anaphaseanaphase in mitosismitosis
• sister chromatids separatesister chromatids separate
Telophase IITelophase II
• Same as telophasetelophase in mitosismitosis.
• Nuclei form.
• CytokinesisCytokinesis occurs.
• Remember:Remember: four haploid daughter cells four haploid daughter cells produced.produced.
gametes = sperm or egggametes = sperm or egg
Telophase IITelophase II
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
VariationVariation
• Important to population as the raw Important to population as the raw material for material for natural selectionnatural selection..
• Question:Question:
What are the three sexual sources ofWhat are the three sexual sources of
genetic variation? genetic variation?
Answer:Answer:
1. crossing over (prophase I)1. crossing over (prophase I)
2. independent assortment (metaphase I)2. independent assortment (metaphase I)
3. random fertilization3. random fertilization
Remember: variation is good!
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)
FertilizationFertilization
• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg
n=23egg
sperm n=23
2n=46zygote
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
Sources
• www.ursulinehs.org/powerpoint/meiosis.ppt
• www.biology4teachers.com/Cell%20Division/MEIOSIS.ppt
• http://www.iteachbio.com/Life%20Science/LifeFunctionsandTheCell/Meiosis.ppt