Meiosis
Meiosis = special type of cell division that occurs in the reproductive organs of both
plants and animals to produce gametes in animals and spores in plants
Chromosome non-disjunction = a form of abnormal meiosis where the chromosomes
fail to separate resulting in an abnormal number of chromosomes
Homologous pairs = chromosomes similar is position, shape, size and the types of
genes found on it
DNA replication is followed by 2 meiotic divisions
Meiosis 1 is a reduction division (diploid to haploid). Results in 2 cells formed each
with half the number of chromosomes (23n) of the parent cell.
Meiosis 2 is a copying division involving the 2 haploid cells formed in meiosis 1
dividing again by mitosis to form 4 daughter haploid cells
Meiosis 1
Prophase 1:
Chromosomes are condensed, becoming short and fat
The chromosomes of homologous pairs lie alongside one another via synapsis to
form homologous pairs (1 maternal + 1 paternal)
Centrioles make their way to opposite poles and spindle fibres develop
The nuclear membrane and nucleolus disappear
Homologous pairs mutually exchange segments of their inner chromatids in a
process called crossing over
Points of crossing over where the chromatids break are called chiasmata (point at
which the chromosomes are touching
Metaphase 1:
Chromatids are still crossing over!
Centromes become attached to spindle fibres
Homologous chromosomes line up on the equator using random alignment
Anaphase 1:
The homologous chromosomes are pulled apart from each other by contracting
spindle fibres and towards opposite poles
Chromosome number is halved (46 to 23)
Telophase 1:
The cytoplasm divides (cytokinesis) to form 2 haploid daughter cells
Each daughter cell has 1 of each homologous pair of chromosomes + are genetically
different from each the
Nuclear membranes reform and spindle fibres disappear
Meiosis = special type of cell division that occurs in the reproductive organs of both
plants and animals to produce gametes in animals and spores in plants
Chromosome non-disjunction = a form of abnormal meiosis where the chromosomes
fail to separate resulting in an abnormal number of chromosomes
Homologous pairs = chromosomes similar is position, shape, size and the types of
genes found on it
DNA replication is followed by 2 meiotic divisions
Meiosis 1 is a reduction division (diploid to haploid). Results in 2 cells formed each
with half the number of chromosomes (23n) of the parent cell.
Meiosis 2 is a copying division involving the 2 haploid cells formed in meiosis 1
dividing again by mitosis to form 4 daughter haploid cells
Meiosis 1
Prophase 1:
Chromosomes are condensed, becoming short and fat
The chromosomes of homologous pairs lie alongside one another via synapsis to
form homologous pairs (1 maternal + 1 paternal)
Centrioles make their way to opposite poles and spindle fibres develop
The nuclear membrane and nucleolus disappear
Homologous pairs mutually exchange segments of their inner chromatids in a
process called crossing over
Points of crossing over where the chromatids break are called chiasmata (point at
which the chromosomes are touching
Metaphase 1:
Chromatids are still crossing over!
Centromes become attached to spindle fibres
Homologous chromosomes line up on the equator using random alignment
Anaphase 1:
The homologous chromosomes are pulled apart from each other by contracting
spindle fibres and towards opposite poles
Chromosome number is halved (46 to 23)
Telophase 1:
The cytoplasm divides (cytokinesis) to form 2 haploid daughter cells
Each daughter cell has 1 of each homologous pair of chromosomes + are genetically
different from each the
Nuclear membranes reform and spindle fibres disappear
