Stages of Meiosis, Gametogenesis, Sex Determination, & Abnormalities.

MEIOSIS

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