Meiosis
The Birds and the Bees Humans and most other animals reproduce sexually. This means that two cells fuse to make a new baby organism. These specialized cells are called gametes or sex cells Sperm and egg cells are human gametes
Meiosis What is it? Who does it? All sexually reproducing organisms Making sex cells (sperm & eggs) Where does it happen? Why does it happen? In males- in the testes In females- in the ovaries To allow organisms to reproduce Why is a different process needed? 1)Chromosome # needs to be cut in half 2) Creates genetic diversity (can help evolution of species)
Meiosis allows sex cells to join to form a complete set of chromosomes/instructions to make a new organism.
Remember, humans have 23 pairs of chromosomes Pairs 1-22 are said to be Homologous chromosomes (they go together because they have same traits but are not identical)
Meiosis is a special kind of cell division that halves the number of chromosomes in gametes. This insures that the correct number of chromosomes are passed on.
The goal is to make haploid sex cells
What would happen if two cells with 46 chromosomes fused? 46 chromosomes +46 chromosomes 92 chromosomes... Oh no!
Nondisjunction =chromosomes fail to dis-join or separate: Trisomy=gamete with 3 of one type Monosomy=gam ete with 1 of one type Tripoidy= zygote with three of one type
Down Syndrome= 3 of #21
Down s Syndrome
Klinefelter s = XXY
Some Symptoms Delayed puberty Loss of libido - reduced interest in sexual activity Impotence Reduced facial hair - reduction in need to shave Tiredness Reduced muscle power and stamina Change in body shape - increased fat and breast development Infertlity Osteoporosis - brittle bones resulting in fracture Depression and sometimes dyssocial behaviour Early heart disease
Turner s Syndrome XO sex chromosome Instead of XX female Or XY for male:
Meiosis has two stages: Meiosis 1 and Meiosis 2
Meiosis 1 This is the stage where the chromosome number is halved Includes 4 stages: Prophase 1 Metaphase 1 Anaphase 1 Telophase 1
Meiosis 1 Prophase 1 is similar in some ways to prophase in mitosis. Chromosomes condense Spindle fibers appear Nucleus and nucleolus disappear
UNLIKE in mitosis, homologous chromosomes line up next to each other during prophase This process is called synapsing Lined up homologues are called tetrads
When homologous chromosomes hang out so close to each other, they sometimes swap parts! This is called crossing over
Synapsis and Crossing Over in Action
More Crossing Over
Check Out the Tetrads!
Crossing over increases genetic diversity! Why is this important for a species longterm survival?
During metaphase 1, tetrads line up on a metaphase plate.
Then spindle fibers pull the homologous chromosomes apart in anaphase 1
In telophase 1, two daughter cells are formed. They are NOT identical! (Why?)
After Meiosis 1 Homologous chromosomes have been separated. Two non-identical daughter cells have been formed. The chromosome number has been cut in half.
The Big Picture
Chromosomes are NOT duplicated again between Meiosis 1 and Meiosis 2 Why not?
Meiosis 2 is similar to mitosis Two identical daughter cells are formed from each of the cells created in Meiosis 1 Includes 4 Stages: Prophase 2 Metaphase 2 Anaphase 2 Telophase 2
Meiosis II
Meiosis 2 Prophase 2: spindle reforms and chromosomes move toward the metaphase plate Metaphase2:sisterchromatidslineduponthe metaphase plate Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell Telophase2andCytokinesis:nuclei form at either pole and each cell is finally divided into two identical daughter cells
All Together Now
Let s compare mitosis with meiosis..