Meiosis Notes
Review Mitosis
•
What was the purpose of Mitosis?
•
What were the phases of the cell cycle?
Meiosis - Yes it is different from Mitosis!!
•
MITOSIS is division that occurs in any cell in the body called somatic
cells (body cells)
•
MEIOSIS is cell division by which gamete cells (reproductive cells--sperm and egg) are produced
DNA review to REMEMBER!!
•
Chromosomes = tightly coiled DNA, called chromatid or sister chromatid
– Contain genes which determine hereditary traits
– We have a total of 46 chromosomes in EVERY cell of our body
– We get…
• 23 from Mom
• 23 from Dad
*During the S phase of Interphase, Chromatids are duplicated to
make sister chromatids
SO……
•
We call the pair of matching sister chromatids Homologous
Chromosomes
– Genes are your traits
• Eye color, hair color, height, foot length, etc…
– Alleles are alternate forms your traits can take
• Usually one is dominant (brown eyes) vs. recessive (blue
eyes)
Somatic and gamete cells have different #’s of chromosomes, we refer to them
as…..
• DIPLOID (2N) = Somatic cells (Body Cells)
– describes a cell that has two copies of each chromosome: 1 from
mom and 1 from dad
– Cell starts with 46 chromosomes and each of the 2 new cells has
46 chromosomes.
•
HAPLOID (N) = Gametes (Sex Cells)
– describes a cell that has half the number of chromosomes.
– Cell starts with 46 chromosomes and each of the 4 new cells has
23 chromosomes.
Sexual Reproduction
• During sexual reproduction, an egg is fertilized by a sperm to create a
diploid zygote.
• Sperm + Egg = 46
• (23) + (23)
•
What if gametes (sex cells) went through Mitosis?
– Mitosis produces 2 identical daughter cells so…
– Diploid produces diploid
– If sperm and egg were diploid they would create a polyploid
(“many sets”) zygote when they came together
– Obviously we do not have “many sets” of chromosomes do we?
– Sperm + Egg = 96
– (46) + (46)
SO WHAT’S THE SOLUTION?
• MEIOSIS! - Where a Diploid cell produces 4 Haploid daughter cells
• Meiosis is a process of reduction division in which the number of
chromosomes per cell is cut in half through the separation of homologous
chromosomes in a diploid cell.
Genetic Recombination
• So, when a haploid sperm fertilizes a haploid egg a diploid zygote is the
result
• Meiosis helps create Genetic recombination, where 2 different sets of
chromosomes come together, keeps you from being an exact copy of one
of your parents!
• It’s what creates genetic diversity in our world!!
Meiosis includes TWO stages – MEIOSIS I and MEIOSIS II
 Interphase will only happen BEFORE MEIOSIS I stage
Interphase
Same as in Mitosis
– G1: cell growth, duplication of organelles, normal cell functions
– S: DNA synthesis/replication of DNA
– G2: job of cell carried out, further growth, prep for cell division
MEIOSIS I
Prophase I
• Nuclear membrane and nucleolus disappear
• Centrioles begin to separate and spindles begins to form
• The homologous chromosomes pair up creating a tetrad = 2 sets of sister
chromatids = 4 individual chromatids
• The homologous chromosomes pair up, gene for gene, down their entire
length.
During Prophase I
• Crossing-over can occur
– Involves the exchange of genetic material between homologous
chromosomes
– Breaks arms of each chromosome and switches the material
Metaphase I
• Spindle fibers attach to the tetrads
• The tetrads line up along the cell equator
• Each side of the equator has chromosomes from both parents (mixed up)
Anaphase I
• The paired homologous chromosomes are pulled away and move toward
opposite ends of the cell
• Sister chromatids remain attached!!!!
Telophase I and Cytokinesis
• The nuclear membrane forms again in some species, the spindle fibers
dissemble and the cell undergoes cytokinesis.
• 2 daughter cells are produced that are not identical
– A “reduction division” has occurred because each daughter cell has
half of the number of chromosomes the original parent cell had
Important Note: Meiosis II will occur in both of the daughter cells created in
Meiosis I
Review
•
Meiosis I – Homologous Chromosomes separate
• Begins w/ 1 diploid cell
• 46 chromosomes (in homologous pairs)
• Prophase I, Metaphase I, Anaphase I, Telophase I
• Ends with 2 Haploid cells
• 23 sister chromatids in each new cell.
Meiosis II
Prophase II
• The DNA DOES NOT duplicate
• The nuclear membrane disappears
• centrosomes and centrioles move to opposite sides of the
cell
• Spindle fibers start to assemble
Metaphase II
• Spindle fibers attach to the sister chromatids at the centromere
• The 23 sister chromatids (in each cell) line up along the align at the cell
equator
Anaphase II
• Centromeres break, spindle fibers shorten, and the sister chromatids are
pulled apart from each other and move to opposite ends of the cell
• So 23 individual chromatids go to each side!!!
Telophase II and Cytokinesis
• Spindle fibers break down
• Nuclear envelopes and nucleolus reform
• Cell membrane pinches in and cytoplasm is divided
• 4 haploid daughter cells are produced with 23 individual chromatids
• A “reduction division” has occurred
Review
• Meiosis II – Sister chromatids separate
• NO DNA replication
• Prophase II, Metaphase II, Anaphase II, Telophase II
• Ends with 4 haploid cells
• 23 individual chromatids in each cell
Results of Meiosis
• The 4 haploid daughter cells…
• Have only 1 set of genes
• Are not considered to be in any phase because they will not grow
and divide
• Now referred to as gametes (sex cells)
Review Questions
•
What type of cells are produced from meiosis?
•
What happens to the chromosome number in the cells that are produced
from meiosis?
•
What do genes code for?
•
This is called a karotype
• it is a picture of your 23 pairs of chromosomes
• Autosomes – pairs 1-22
• Sex chromosome - pair 23
Spermatogenesis
• Meiosis in males to produce sperm
• Thousands of reproductive cells undergo meiosis each day to produce
large numbers of sperm
Oogenesis
• Meiosis in females to produce ova (eggs)
• Only occurs in one cell once a month
• The cytokinesis at end of meiosis I and meiosis II is uneven and cytoplasm
is divided unequally
• One cell gets the majority of the cytoplasm and becomes the egg
• The other cells are called polar bodies and do not participate in
reproduction