Meiosis • The form of cell division cell division by which , , with half half the number of chromosomes, chromosomes, are produced. • Diploid (2n) Diploid (2n) haploid (n) haploid (n) • Meiosis Meiosis is sexual reproduction. sexual reproduction. • Two divisions Two divisions ( meiosis I meiosis I and meiosis meiosis II II).
Meiosis. The form of cell division by which , with half the number of chromosomes, are produced. Diploid (2n) haploid (n) Meiosis is sexual reproduction. Two divisions ( meiosis I and meiosis II ). Meiosis. Sex cells divide to produce gametes (sperm or egg) . - PowerPoint PPT Presentation
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Meiosis
• The form of cell divisioncell division by which, , with halfhalf the number of chromosomes,chromosomes, are produced.
• 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 chromosomesHomologous 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.
Telophase ITelophase I
Meiosis IIMeiosis II
• No interphase II No interphase II
(or very short - no more DNA replicationDNA replication)
• Remember: Meiosis IIRemember: 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 material Important to population as the raw material for 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)
• A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?