Oh! MEIOSIS• Cell division in which one DIPLOID CELL

produces four HAPLOID CELLS called sex cells or gametes

• Meiosis is reduction division; the cells produced contain half the number of chromosomes as before meiosis

• Meiosis occurs in two steps. During meiosis I, the chromosome pairs are divided between the two cells produced. During meiosis II, the two sister chromatids are segregated into each daughter cell

Stages Of Meiosis!

Genes and Chromosomes• Organisms have tens

of thousands of genes!

• A thousand or more are arranged on each chromosome

• Chromosome number of a species is not related to the complexity or the organism

Chromosomes in different Organisms:

0

200

400

600

800

1000

1200

1400

1st Qtr

HumanGarden PeaDogChimpanzeeAppleIndian Fern

Diploid Time:

• A cell with 2 of each kind of chromosome

• 2n

• Organisms have 2 factors (or alleles) for each trait

• One allele is located on each of the paired chromosomes

Happy Haploids:

• A cell with one of each kind of chromosome• n• Organisms produce gametes that contain

one of each kind of chromosome• Parent organisms give one factor (or allele)

for each trait to each of their offspring

Homologous Chromosomes

• Together, the 2 chromosomes of each pair in a diploid cell help determine what the individual looks like

• These paired chromosomes are called H.C.

• Have genes for the same traits arranged in the same order

• However, because there are different possible alleles for the same gene, the 2 chromosomes in a homologous pair are not identical to each other

Genes on a chromosome

Interphase• Remember that in

Interphase the cell carries out its usual metabolic activities and replicates its chromosomes

• Just hanging out doing its cell thing

Prophase 1

• Chromosomes coil up and a spindle forms• Each pair of homologous chromosomes comes

together to form a four-part structure called a tetrad

Tetrads Unite!

• Consists of 2 homologous chromosomes, each made up of 2 sister chromatids

• The chromatids in a tetrad pair tightly• In fact, they pair so tightly that non-sister

chromatids from homologous chromosomes sometimes actually exchange genetic material in a process known as Crossing Over

Crossing Over

• Results in new combinations of alleles on a chromosome

• This is why there are so many unique folks on this planet!

Metaphase 1

• Tetrads line up in the midline of the spindle• Unique to meiosis• Homologous line up together in pairs

Anaphase• Homologous chromosomes separate and move to

opposite ends of the cell• Centromeres don’t split like in mitosis• Ensures that each new cell will receive only one

chromosome from each homologous pair

Telophase 1

• Spindle is broken down and the chromosomes uncoil

• Cytoplasm divides to form 2 new cells (each cell only has half the genetic information of the original cell because it has only one chromosome from each homologous pair

• However, another cell division is needed because each chromosome is still doubled, containing 2 identical sister chromatids

OH Meiosis II!

• Meiosis 2 is similar to mitosis. However, there is no "S" phase (Chromosomes don’t replicate

• The chromatids of each chromosome are no longer identical because of recombination

• Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid

Prophase II

• Spindle forms in each of the 2 new cells• Chromosomes are still made up of sister

chromatids

                                      

Metaphase II

• Chromosomes line up at the equator or middle

                                      

Anaphase II

• Centromeres of each chromosome splits

• This allows the sister chromatids to separate and move to opposite poles

                                      

Telophase II

• Nuclei re-form• Spindles break down• Cytoplasm divides • (These are the same

events as in mitosis)                                       

Meiosis is Finally Over!

• Four haploid cells have been formed from the original diploid cell

• Each haploid cell contains one chromosome from each homologous pair

• These haploid cells will become gametes, transmitting the genes they contain to offspring!

Difference Between

Mitosis and Meiosis!