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
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