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• Chromosomes that are alike and not involved in determining sex are autosomes.
• The system of sex determination described for Protenor is called the X-O system. 
• Mitosis is the distribution of chromosomes between two daughter cells
• cytokinesis is the partitioning of the cytoplasm between the two daughter cells. 
• Interphase typically occupies about 90% of the total cell cycle
• The first portion of interphase is gap phase 1 (G1).

  It is usually the longest interval of interphase and is a period of cell growth and the metabolic activities characteristic of the particular cell type.
•  During the S (DNA synthesis) phase, growth continues, but this phase also involves DNA replication.
• an exact copy of the DNA is made during the S (synthesis) phase. 

  This process is called replication
• A chromatid is a copy of a chromosome produced by replication. 

  Each chromatid attaches to its other copy, or sister, at a point of constriction called a centromere. 
• Prophase: During prophase, the nuclear envelope breaks down, spindle fibers form, and chromatin condenses into visible chromosomes.
• Mitosis is the division of the nucleus into two genetically identical daughter cells. It consists of four stages: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.
• The second gap phase is shorter than G1 and is devoted mainly to protein synthesis.
• MEIOSIS: THE BASIS OF SEXUAL REPRODUCTION
• Sexual reproduction requires a genetic contribution from two different sex cells. 

  Egg and sperm cells are specialized sex cells called gametes.
• In animals, a male gamete (sperm) unites with a female gamete (egg) during fertilization to form a single cell called a zygote.
• During most of the life of a cell, chromosomes are in a highly dispersed state called chromatin. 
• inactive portions of chromosomes produce dark banding patterns with certain staining procedures and thus are called heterochromatic regions, whereas active portions of chromosomes are called euchromatic regions.
• Bound to each centromere is a disk of protein called a kinetochore, which eventually is an attachment site for the microtubules of the mitotic spindle.
•  FIRST STAGE OR THE PROPHASE: begins when chromosomes become visible with the light microscope as threadlike structures. The nucleoli and nuclear envelope begin to break up, and the two centriole pairs move apart.
• an array of microtubules called asters, which brace each centriole against the plasma membrane.
• The formation of asters, spindles, centrioles, and microtubules is collectively called the mitotic spindle (or mitotic apparatus). 
• metaphase (replicated chromosomes) begin to align in the center of the cell, along the spindle equator. 
• . After the centromeres divide, the sister chromatids are considered full-fledged chromosomes (called daughter chromosomes). 
• Anaphase, the shortening of the microtubules in the mitotic spindle, and perhaps the activity of motor proteins of the kinetochore, pulls each daughter chromosome apart from its copy and moves it toward its respective pole. 
• Telophase begins once the daughter chromosomes arrive at the opposite poles of the cell. 

  During telophase, the mitotic spindle disassembles. A nuclear envelope re-forms around each set of chromosomes, which begin to uncoil for gene expression, and the nucleolus is resynthesized. The cell also begins to pinch in the middle. Mitosis is over, but cell division is not.
• The final phase of cell division is cytokinesis, in which the cytoplasm divides. 

  Cytokinesis usually starts sometime during late anaphase or early telophase. A contracting belt of microfilaments called the contractile ring pinches the plasma membrane to form the cleavage furrow. 

  The furrow deepens, and two new, genetically identical, daughter cells form.
• In animals, a male gamete (sperm) unites with a female gamete (egg) during fertilization to form a single cell called a zygote.

  The zygote is the first cell of the new animal.
• All of the cells in the bodies of most animals, except for the egg and sperm cells, have the diploid (2N) number of chromosomes. 
• Gametes are produced by cells set aside for that purpose early in development. These cells are called germ-line cells and eventually undergo a type of cell division called meiosis
• The nuclei of the two gametes combine during fertilization and restore the diploid number
• Spermatogenesis produces mature sperm cells and follows the sequence previously described. 

  All four products of meiosis often acquire a flagellum for locomotion and a cap-like structure that aids in the penetration of the egg. 
• Oogenesis producesa mature ovum or egg. 

  It differs from spermatogenesis in that only one of the four meiotic products develops into the functional gamete. uses
• formation of oocytes begins with oogonia which is a diploid cell that will further divide to form matured egg cells That take up before the birth of the female organisms
• formation of oogonia is not considered as part of oogenesis proper
• first true step of oogenesis: oocytogenesis where oocytes develop after females are born