MEIOSIS
Stages of Meiosis, Gametogenesis, Sex Determination, & Abnormalities.
Stages and Steps of Meiosis
Interphase
•  Same as in Mitosis
•  3 stages – G1, S, G2
•  DNA replication and cell growth
Meiosis I
Prophase I
•  Nuclear membrane begins to dissolve
•  Homologous chromosomes (made up of sister chromatids) match up and
intertwine in a process called Synapsis
•  Crossing over occurs which is the exchange/mixing of genetic information
between two non-sister chromatids
•  Crossing over results in chromosome recombination
Metaphase I
•  The pair of homologous chromosomes, each containing 2 sister
chromatids, is called a tetrad
•  The tetrads move toward the centre of the cell and align along the
metaphase plate.
•  Independent assortment occurs, which is the RANDOM arrangement of
the homologous chromosome along the metaphase plate
Anaphase I
•  The homologous chromosomes separate towards opposite poles of the
cell
•  Only 1 chromosome from each homologous pair will be found in the new
daughter cells
Telophase I
•  Division of cytoplasm and organelles
•  Nuclear membrane reappears and reforms as the cell begins to divide into
two cells *Cytokinesis
•  Ends in two haploid cells
•  Unlike mitosis, the chromosomes are NOT IDENTICAL (due to crossing
over)
Meiosis II
*** the cell does not undergo interphase, it jumps directly into Meiosis II
Prophase II
•  DNA replication did not occur as interphase is skipped
•  One set of chromosomes still contain sister chromatids, HOWEVER they
are not identical due to crossing over that occurred in Prophase I
•  Nuclear membrane dissolves
Metaphase II
•  Chromosomes line up in a single file along the cell’s metaphase plate
•  Sister chromatids remain attached by their centromere
Anaphase II
•  Chromatids are pulled apart and pulled to opposite poles of the cell
•  The nuclear membranes begins to reform around the chromatids
Telophase II
•  Division of cytoplasm and organelles
•  Chromosomes unwind, nuclear membrane reforms and the cell begins to
divide into two cells *Cytokinesis
•  End with four haploid cells called gametes
Gametogenesis
•  Is the formation of sex cells in meiosis.
•  In animals it takes place in the testes and ovaries.
•  Produced by meiosis, sex cells contain the haploid number of
chromosomes.
•  The production of sperm is called spermatogenesis
•  The production of egg cells (ova) is called oogenesis.
The Differences
Oogenesis
•  Cytoplasm is not equally divided between cells during meiosis.
•  One daughter gets cell most of the cytoplasm, which becomes a single
ovum (eggs cell)
•  The other 3 remaining cells are called polar bodies, which die.
Spermatogenesis
•  Cytoplasm is equally divided during both stages of meiosis
•  Results in 4 equally sized sperm cells
•  Streamlined for maximum motility
Karyotypes & Sex Chromosomes
•  Karyotype is a picture of chromosomes that are arranged by number, size, or
some other characteristic
Sex Chromosomes
•  Found in most eukaryotic organisms
•  In most mammals - matching pair of
homologous chromosomes in females and
partially matching in males
•  In males one chromosome is smaller than
the other
•  The larger of the two is a homologous
chromosomes to that in the female
Sex Determination
•  The larger type of sex chromosome is the X chromosome
•  The smaller type is the Y chromosome
•  X & Y chromosomes contain matching regions
•  Allows for synapsis and act as homologous pair = meiosis
•  Non sex chromosomes are called autosomes
•  XX = Female
XY = Male
•  XX/XY is know as the sex-determination system
•  You always get an X from the Mom, therefore dad determines the sex
•  If you get an X from dad = female. Get a Y from dad = male
•  Not all animals use this sex-determination cycle
Abnormalities Meiosis
•  Can contribute to genetic disorders and infertility.
•  Random meiotic errors, disease, and environmental factors
Entire chromosomes can be lost or gained in a rare error during meiosis
•  Usually a result of non-disjunction
Non-disjunction
•  The failure of homologous chromosomes to separate during meiosis
•  Results in one daughter cell with an extra chromosome and one daughter
cell missing a chromosome
•  Not able to function normally
•  Can be found via preparation of a karyotype
Outcomes of Non-disjunction Errors
•  Trisomy – Means 3 homologous chromosomes instead of a homologous
pair
•  Monosomy – Means 1 chromosome instead of a homologous pair