Meiosis and Cell Regulation

Introduction to Meiosis Meiosis - cell division where the numbers of chromosomes is reduced. 2 sets of cell division occur Meiosis I and Meiosis II 4 Cells are made Each cell has ½ the number of chromosomes as the parent cell

Diploid (2n) Any cell that contains 2 complete sets of chromosomes Examples: body cells (skin, hair, heart, liver, lung, etc.) Most cells that make up humans are diploid In humans these cells have 46 chromosomes

Haploid (n) Any cell that contains 1 complete set of chromosomes Example: gametes (sex cells) (sperm in males and eggs in females) In humans these contain 23 chromosomes

Homologous Chromosomes A pair of chromosomes that has the same genetic material. One chromosome comes from the mother, one from the father. Humans have 46 chromosomes, 23 homologous pairs

Meiosis

Meiosis I Prophase I Homologous chromosomes pair up to form a tetrad Crossing over occurs and allows for genetic information to be exchanged Nuclear envelope starts to disappear Spindles form

Meiosis I Chromosomes line up in the middle of the cell Metaphase I Chromosomes line up in the middle of the cell Spindle fibers attach to the chromosomes at the centromere

Meiosis I Homologous pairs are separated and move to opposite ends Anaphase I Homologous pairs are separated and move to opposite ends

Meiosis I Chromosomes uncoil and form 2 nuclei Telophase I Chromosomes uncoil and form 2 nuclei Nuclear membrane begins to form The spindles break down A cleavage furrow begins to form The cell prepares to divide

Meiosis II Chromosomes condense Nuclear envelope starts to disappear Prophase II Chromosomes condense Nuclear envelope starts to disappear Spindles form in each new cell

Meiosis II Chromosomes line up randomly at the center of the cell Metaphase II Chromosomes line up randomly at the center of the cell

Meiosis II Centromeres split Anaphase II Centromeres split Sister chromatids move apart to opposite ends

Meiosis II 4 nuclei form around chromosomes Spindles break down Telophase II 4 nuclei form around chromosomes Spindles break down The cells prepare to divide

After Meiosis II and Cytokinesis 4 Haploid cells are formed So in humans, how many chromosomes in each?

What are we separating?

Benefits of Meiosis Formation of Gametes Gamete = Sex Cell (egg and sperm) Contain one set of chromosomes Haploid In humans Typical cell = 46 chromosomes Gamete = 23 chromosomes Keeps chromosome number constant after fertilization

Benefits of Meiosis During prophase, crossing over occurs During metaphase, chromosomes line up randomly at the equator These 2 reasons are why we don’t look exactly like one parent or another

Crossing Over Exchange of chromosomal segments between a pair of homologous chromosomes Takes place in Prophase I of meiosis

Independent Assortment Independent Segregation of genes during the formation of gametes

Genetic Variation in Meiosis Variation - the genetic (and physical) differences between members of a population Organisms that reproduce sexually have greater variation, therefore a greater chance of survival

Importance of Variation Diversity = Stability As variation in a population increases, so does the likelihood that individuals will survive despite changes in the environment If there are significant environmental changes, a trait that was once beneficial may become a disadvantage where a trait that was not helpful may become an advantage

Regulation In some types of cells the cell cycle occurs very quickly. EX) Skin cells, blood cells, hair follicle cells, and cells of the digestive track.

Regulation Other cells move through the cycle more slowly, and others stop. Neurons stop dividing while you are a toddler. Since no new neurons are made, the body doesn’t repair brain damage and spinal cord injuries

Regulation The cell cycle is closely controlled. Although we do not know all of the controls, we know some. There are a group of proteins called cyclins that regulate the timing of the cell cycle in eukaryotes

Two Types of Cyclins Internal Regulators Respond to internal events EX) They respond to whether or not a full copy of DNA has been made

Two Types of Cyclins External Regulators Respond to the surroundings EX) They respond to whether or not there is space around the cell to allow for new cells.

Cancer In some cells the regulators on the cell cycle malfunction and result in cancer. There are many types of cancer. Cancer cells grow uncontrollably using up all the nutrients leaving the healthy cells to die around them

Cancer We will most likely need to completely understand the regulations of the cell cycle to cure cancer.

Cancer One treatment for cancer is chemotherapy Chemotherapy is the range of toxins that can be given to cancer patients that in some way harms cells that are going through the cell cycle quickly. (Also affect cells that normally go through the cycle quickly) Side effect—Nausea, hair loss, anemia (lack of oxygen)