Transcript Mitosis and Meiosis

Mitosis
Forming Diploid Cells
All Cells Except Gamete Cells
(in ovaries and testes)
Interphase
• 46 chromosomes in humans
• Chromatin
• Replicates DNA
– Sister chromatids
– Centromere
• Prepares for nuclear division
Prophase
• Centrioles split and move to opposite ends
– Attachment for spindle fibres (“guide wires”)
– No centrioles in plant cells
• Chromosomes condense
• Centromeres attach to spindle fibres
• Nuclear membrane dissolves
– Separation of chromosomes and cell organelles
Metaphase
• Chromosomes line up at equatorial plate
• Nuclear membrane completely dissolved
Anaphase
• Centromeres divide
• Sister chromatids become singular
chromosomes
• Chromosomes move to opposite poles
• Identical set of chromosomes at each pole
Telophase
• Chromosomes lengthen
• Spindle fibres dissolve
• Nuclear membrane forms
– 46 chromosomes
Cytokinesis
• Cytoplasm divides
• Furrow develops
• Pinch off cell into two parts
• Plant cells – cell plate forms between chromatin
masses
– Cell plate becomes new cell wall
Meiosis
Forming Haploid Cells
Gametes
Specific to Meiosis
• Two stages of cell division
• Chromosome number in daughter cells is half
the number in the parent cells
• Haploid = n  Diploid = 2n
n=23
2n=46
Meiosis I
• DNA Replication takes place before
• Prophase I:
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Nuclear membrane dissolves
Centrioles split
Chromosomes come together as homologous pairs
Spindle fibres form
Synapse and crossing over
*Tetrad (four chromatids)
• Metaphase I:
– Homologous chromosomes attach to spindle
fibres
– Line up along equatorial plate
• Anaphase I:
– Each chromosome separates from homologue
– Move to opposite poles (segregation)
• Telophase I:
– Nuclear membrane forms
– Not identical chromosomes (sister chromatids)
– Cytokinesis
Meiosis II
• DNA does NOT replicate
• Prophase II:
– Nuclear membrane dissolves
– Spindle fibres form
– Centrioles move to opposite poles
• Metaphase II:
– Chromosomes line up along equatorial plate
– Attached at centromere (and to spindle fibres)
• Anaphase II:
– Chromatids break apart
– Move to opposite poles
– Nuclear membrane forms
• Telophase II:
– Cytokinesis (cytoplasm seperates)
– Four haploid daughter cells
• Chromosome number has been reduced to half
• These cells may become gametes