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Binary Fission is a form of asexual
reproduction
- Production of offspring by a single parent
- some eukaryotes reproduce asexually
- They do this by mitosis
Multicellular organisms usually reproduce sexually.
-Cells from two parents join to form a NEW
individual
- offspring produced sexually are genetically
different from either parent
- 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
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
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
• Similar in size, shape, and genetic material
• 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 chromosomes 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
• 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
 When sex cells form, the chromosomes are copied once. The
nucleus then divides twice (meiosis I and meiosis II). The four cells
that result have half as many chromosomes as a normal body cell.
Meiosis I
• Before meiosis 1, each chromosome is
replicated (interphase of the cell cycle).
• Tetrads line up like the sister chromatids did 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 I
• Homologous chromosomes
pair up (SYNAPSIS) , makes
a tetrad
• As they pair up in tetrads,
chromosomes exchange
portions of their chromatids
in the process ….
CROSSING OVER
- First way genetic variation in
gametes is achieved
- Part of a chromatid on one
chromosome breaks off and
switches position with the matching
portion on a chromatid of the
homologous chromosome
- increases the # of genetic
combinations in offspring
Metaphase I
• Homologous pairs line up
along the center of the cell in
a random fashion=
independent assortment
- Another form of genetic
variation
• When the cell divides, each
daughter cell will receive a
mix of chromosomes that
differ than that of the original
cell
Anaphase I
 spindle fibers separate the paired
homologous chromosomes and pull
them toward opposite poles of the cell.
Telophase I
 Cell divides to produce two daughter
cells
 each with a complete set of
chromosomes
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 crossingover
• Each daughter cell has a set of chromosomes and alleles different from
each other and different from the parent diploid cell

moving on to Meiosis II
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
• Meiosis II doesn’t replicate the
chromosomes again
• Does not form tetrads because the
homologs were already seperated during
meiosis I
Metaphase II
• Chromosomes line up in middle
Anaphase II
• Sister chromatids separate and
move to opposite poles
Telophase II
• Meiosis II results in 4 haploid
(N) daughter cells
Gamete Formation
• Male
• Haploid gametes produced by meiosis are called spermatozoa
• Spermatogenesis begins at puberty and continues throughout one’s life
• Under hormone and environmental control
• Female
• Haploid gamete produced by meiosis is called an oocyte
• 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)
• IMPORTANT!
• Female gametes are stuck in Prophase 1 until puberty
• Complete Meiosis 1 every month and the secondary is released from
ovary
• Female gametes only reach and complete meiosis 2 if they are fertilized