BellringerBellringer

If normal cells have 46 chromosomes, If normal cells have 46 chromosomes, how many chromosomes do you think how many chromosomes do you think gametes (sex cells) have and why?gametes (sex cells) have and why?

MeiosisMeiosis

Division of Sex CellsDivision of Sex Cells

MeiosisMeiosis

A process of reduction division in which the A process of reduction division in which the number of chromosomes per cell is cut in half number of chromosomes per cell is cut in half through the separation of homologous through the separation of homologous chromosomes in a diploid cell.chromosomes in a diploid cell.

DiploidDiploid – 2 sets of chromosomes – 2 sets of chromosomes HaploidHaploid – 1 set of chromosomes – 1 set of chromosomes HomologousHomologous – chromosomes that each have a – chromosomes that each have a

corresponding chromosome from the opposite corresponding chromosome from the opposite sex parent sex parent

MeiosisMeiosis

MeiosisMeiosis

Principle of genetics requires 2 thingsPrinciple of genetics requires 2 things 1. Each organisms must inherit a single copy 1. Each organisms must inherit a single copy

of every gene from each of it’s “parents.”of every gene from each of it’s “parents.” 2. When an organisms produces gametes, 2. When an organisms produces gametes,

those two sets of genes must be separated those two sets of genes must be separated from each other so that each gamete from each other so that each gamete contains just one set of genes. contains just one set of genes.

Meiosis StagesMeiosis Stages

Meiosis usually involves 2 distinct stagesMeiosis usually involves 2 distinct stages Meiosis IMeiosis I Meiosis IIMeiosis II

Prophase IProphase I

Each chromosome pairs with its corresponding Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.homologous chromosome to form a tetrad.

There are 4 chromosomes in a tetrad.There are 4 chromosomes in a tetrad. The pairing of homologous chromosomes is The pairing of homologous chromosomes is

the key to understanding meiosis.the key to understanding meiosis. Crossing-over may occur hereCrossing-over may occur here Crossing-over is when chromosomes overlap Crossing-over is when chromosomes overlap

and exchange portions of their chromatids.and exchange portions of their chromatids.

Prophase IProphase I

Metaphase IMetaphase I

Spindle fibers attach to the chromosomesSpindle fibers attach to the chromosomes Chromosomes line up in the middle in Chromosomes line up in the middle in

pairspairs

Metaphase IMetaphase I

Anaphase IAnaphase I

The fibers pull the homologous The fibers pull the homologous chromosomes toward opposite ends of chromosomes toward opposite ends of the cell.the cell.

Anaphase IAnaphase I

Telophase I & CytokinesisTelophase I & Cytokinesis

Nuclear membranes form.Nuclear membranes form. The cell separates into 2 cells.The cell separates into 2 cells.

Telophase ITelophase I

Prophase IIProphase II

Meiosis I results in two haploid (N) cells.Meiosis I results in two haploid (N) cells. Each cell has half the number of Each cell has half the number of

chromosomes as the original cell.chromosomes as the original cell.

Prophase IIProphase II

Metaphase IIMetaphase II

The chromosomes line up similar to The chromosomes line up similar to metaphase in mitosis.metaphase in mitosis.

Metaphase IIMetaphase II

Anaphase IIAnaphase II

Sister chromatids separate and move to Sister chromatids separate and move to opposite ends of the cell.opposite ends of the cell.

Anaphase IIAnaphase II

Telophase IITelophase II

Meiosis II results in 4 haploid cells.Meiosis II results in 4 haploid cells.

Telophase IITelophase II

Gamete FormationGamete Formation

In males, meiosis results in 4 sperm cellsIn males, meiosis results in 4 sperm cells In females, meiosis results in 1 egg cell In females, meiosis results in 1 egg cell

and three polar bodies, which are not and three polar bodies, which are not used in reproduction.used in reproduction.

Mitosis vs MeiosisMitosis vs Meiosis

MitosisMitosis MeiosisMeiosis

Results inResults in 2 Diploid Cells 2 Diploid Cells (2N)(2N)

4 Haploid Cells 4 Haploid Cells (N)(N)

Cells areCells are Genetically Genetically IdenticalIdentical

Genetically Genetically DifferentDifferent

Occurs inOccurs in Somatic (Body) Somatic (Body) CellsCells

Sex CellsSex Cells

Cell Cycle RegulationCell Cycle Regulation•Cyclin regulate the timing of the cell cycle in eukaryotic cells.•Internal regulators

•Proteins that respond to events inside the cell.•External regulators

•Proteins that respond to events outside the cell•Directs cells to speed up or slow down the cell cycle.•Growth factors stimilate the growth and division of cells.

•Uncontrolled cell growth can lead to cancer•Cancer: a disorder in which some of the body’s own cells lose the ability to control growth. They do not respond to the signals that regulate the growth of most cells.

•Most cancer cells have a defect in p53 gene, which normally stops the cell cycle until all chromosomes have been properly replicated.