Nuclear Division (MEIOSIS)

Meiosis

• the process by which a cell nucleus divides to produce daughter nuclei each containing half the number of chromosomes of the original nucleus 

meiosis is sometimes referred to as…

reduction division bcs it reduces the number of chromosomes

Prophase I

• genetic material condenses into chromosomes

• nuclear membrane disintegrates

• centrioles move to opposite poles of the cell 

• spindle fibres form from asters 

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• homologous chromosomes pair up to form bivalents - process called SYNAPSIS

• homologous chromosomes join at several points called chiasmata

• crossing over occurs at each chiasma where genetic material is exchanged between non-sister chromatids

Metaphase I

• centromere of chromosomes is attached to spindle fibres

• bivalents line up at the equator through spindle fibres

• one homolgous chromosomes faces 1 pole and the other faces the opposite one

  • THIS ALIGNMENT IS COMPLETELY RANDOM

Anaphase I

• homologous chromosomes are pulled apart by spindle fibres

• separates chromosomes into two haploid sets

• independent assortment occurs - what chromosomes go to which side is random

Telophase I and Cytokinesis

• nuclear membrane reforms + nucleolus

• homologous chromosomes reached the opposite poles

• spindle fibers disappear

• cleavage of cytoplasm may occur

Prophase II

• chromosomes condense

• centrioles duplicate and move to opposite poles of cell

• nuclear membrane disintegrates

• spindle fibres appear

• NO PAIRING OF CHROMOSOMES AND CHIASMATA DONT DEVELOP

Metaphase II 

• spindle fibres at each centriole attach to each sister chromatid 

• spindle fibres drag the sister chromatids and line them up at the center

• chromosomes are oriented at random with respect to one another 

Anaphase II

• sister chromatids are pulled apart by spindle fibres

• sister chromatids go to each side randomly

Telophase II and Cytokinesis

• Sister chromatids reach opposite poles

• spindle fibres disintegrate

• nuclear membrane reformes

• cleavage of cytolasm/cell walls in plants

• 4 NEW HALPOID DAUGHTER CELLS R PRODUCED ALL GENETICALLY DIFFERENT TO EACH OTHER

How does meiosis generate genetic variation 

• crossing over and synapsis 

• random alignment in M1 AND M2

• independent assortment in A1 and A2

Crossing over and Synapsis

alleles are exchanged at chiasmata

produces new combinations of genes on chromosomes

each time meiosis occurs chiasmata form at any point on the chromosomes - such that an infinite number of diff combination of genes can be produced

always equal amounts crossed over - chromosome stay the same length and have the same genes

Random Alignment in M1/M2

• which homologous chromosome is facing which side is purely up to chance

  • maternal or paternal could face either pole

• random orientation of sister chromatids

Independent assortment

• it is pure chance as to which chromosome or sister chromatid get’s into which gamete

• eg not all maternal chromosomes will go into 1 gamete, you could have 40% mom 60% dad - literally any combo 

Significance of Meiosis in Sexual Reproduction

• keeps the number of chromosomes constant which each generation

• if it was 2n - then the new gen would be 4n and the no of chromosomes would double w every generation

  • bcs mitosis keeps the number of chromosomes constant

Other sources of generation genetic variation 

• random fertilization

  • it is pure chance as to which egg fertilizes which sperm

  • since each gamete is unique a unique zygote is ALWAYS created

• mutations 

  • occur during dna replication/cell division 

  • responsible for changes in gene pool

  • driving force of evolution