Meiosis

MEIOSIS

• Gregor Mendel• 1822• Austrian monk• University of

Vienna• In charge of the

Garden

What Gregor Mendel Knew…

• Each organism must inherit a single copy of every gene from each of its “parents”

• Each of the organisms gametes must contain just one set genes– When gametes are formed, there must be a

process that separates the 2 sets of genes so each gamete gets one set

Karyotype• A photograph of a person's chromosomes,

arranged according to size

Chromosome Number

• Homologous chromosomes– Chromosome that has a corresponding

chromosome from the opposite-sex parent• Fruit fly has 8 chromosomes– 4 from mom– 4 from dad

Diploid

• Di= two sets• Cell that contains both sets of homologus

chromosomes– Cell contains

• 2 complete sets of chromosome• 2 complete sets of genes

• Number of chrms in diploid cell represented by 2N

• For Drosophilia (fruit fly) 2N=8• Mendel said:– Each adult cell contains two copies of each gene

Haploid

• Means “one set”• Refers to cells that contain only one set of

chromosomes• Gametes (sex cells)• Represented by N• Drosophilia fruit fly– N=4

How are haploid (N) gametes made from

diploid (2N) cells?

Meiosis

• 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

Meiosis

• 2 distinct stages• Meiosis I – A diploid cell enters here

• Meiosis II– At the end of this, the diploid cell that entered

meiosis has become 4 haploid cells

Meiosis I• Before meiosis 1, each chromosome

is replicate• Then they divide like in mitosis• What happened in mitosis?– PMAT

• Tetrad– STRUCTURE MADE WHEN EACH

CHROMOSOME PAIRS UP WITH ITS HOMOLOGOUS CHROMOSOME

– 4 CHROMATIDS IN A TETRAD

Prophase 1

• Each chromosome pairs with its homologous chromosome making a tetrad

• As they pair up in tetrads, chromosomes exchange portions of their chromatids in the process …. CROSSING OVER

Crossing Over

Metaphase1• Spindle fibers attach to chromosomes

Anaphase 1• The spindles pull homologous

chromosomes apart to opposite poles/ends

Telophase 1• Nuclear membranes form and cell

separates into two new cells

Now what do we have?• 2 new daughter cells• Are they identical to the parents?– No– Let’s say the parent started with 8 chromosomes– Each daughter cell has 8 chromosomes but they are

different because of crossing-over

• Each daughter cell has a set of chromosomes and alleles different from each other and different from the parent diploid cell

Meiosis II

• Unlike Mitosis, Neither cell goes through a round of chromosome replication

Prophase II

• Meiosis I resulted in 2 “seemingly” diploid cells– Remember they are genetically different b/c of

crossing over in prophase I• We still need to cut this number in half to

reach our goal of 4 haploid cells

Metaphase 2• Chromosomes line up in middleAnaphase 2

• Sister chromatids separate and move to opposite poles

Telophase 2• Meiosis II results in 4 haploid (N) daughter

cells

Gamete Formation

• Male– Haploid gametes produced by meiosis are called sperm

• Female– Haploid gamete produced by meiosis is called an egg– Cell divisions at the end of meiosis one and two are

uneven so one cell gets most of the cytoplasm (the EGG) and the other three are called polar bodies (don’t participate in reproduction)

Mitosis vs. Meiosis• Mitosis– Results in the production of two genetically identical

DIPLOID cells– Daughter cells have sets of chromosomes identical to

each other and to parent cell– MITOSIS allows body to grow and replace other cells– Asexual reproduction

• Meiosis– Results in four genetically different HAPLOID cells– MEIOSIS is how sexually reproducing organisms make

gametes

Genes