IB BIO I
Meiosis
Van Roekel
Meiosis – type of cell division, known as reduction division that halves the number of chromosomes in a
cell in order to produce sex cells, also known as gametes.
Meiosis does two things:
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Takes one cell with two copies of every chromosome (diploid cell) and creates four cells with a
single copy of every chromosome (haploid cell)
Creates genetic diversity through independent assortment, random orientation, and crossing
over, resulting in each haploid cell have different forms of each gene (four unique haploid cells)
Stages of Meiosis
Meiosis I –
Homologous Chromosomes:
Cell will still go through interphase and synthesis, which results in two identical sister chromatids for
every chromosome
Stages:
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Prophase I – each set of chromatids match up with homologous pair and fasten together
(synapsis) in a group of four chromatids called a tetrad. Crossing over, or the exchange of
segments between homologous chromosomes, occurs during this stage.
Metaphase I - homologous chromosomes line up at the equator of the cell and are attached to
spindle fibers at centromeres
Anaphase I – Spindles separate homologous chromosomes and move them towards opposite
poles of cell. Sister chromatids remain attached.
Telophase I – nuclear envelope forms around separated chromosomes
Cytokinesis – separates cells when cytoplasm divides, forming two new daughter cells.
At end of meiosis I, each cell is now considered haploid and has half the number of chromosomes as the
parent cell (single copy of each chromosome). However, each chromosome still consists of a pair of
sister chromatids that must be separated.
Meiosis II –
Stages:
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Prophase II – nuclear envelope breaks down, spindles form and begin to attach to sister
chromatids
Metaphase II – sister chromatids line up at equator in random order (referred to as random
orientation). Spindle fibers attach to sister chromatids at centromeres
Anaphase II – centromeres on sister chromatids separate, and sister chromatids are pulled in
opposite directions towards opposite poles.
Telophase II/Cytokinesis – nuclei form at opposite poles and cytoplasm pinches forming four
unique haploid daughter cells.
IB BIO I
Meiosis
Van Roekel
Genetic Variety
Meiosis results in unique sex cells that when used in fertilization can produce offspring that show
genetic diversity. Gametes are unique because of crossing over, random orientation, and the law of
independent assortment.
Crossing Over:
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Occurs during prophase I and results in the exchange of genetic information between paternal
and maternal chromosomes.
Resulting chromosomes contain sections of genetic material that originated in two different
people, so sister chromatids are no longer identical.
Must occur between homologous chromosomes, and occurs when chromatids intertwine and
break at same position. Location of connection is referred to as chiasma/chiasmata
Random Orientation:
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Occurs during:
Random arrangement of chromosomes as they line up at equator of cell, resulting in different
alleles being passed to each gamete
Law of Independent Assortment:
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Separation of one pair of alleles is independent of the separation of another pair of alleles.
Traits that determine different factors (flower color & flower height) are passed down
separately of each other
Genes/alleles that pass to daughter cells depend on orientation of chromosomes during
metaphase I
Results in 2n distinct gametes, where n is the number of chromosomes and 2 represents the
number in each homologous pairs
Sex is determined by Y chromosome
Division Error – occurs when chromosomes do not separate as expected, resulting in unequal
distribution of chromosomes
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Non-disjunction –
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Trisomy –
Errors have different results depending on what chromosomes is affected
Trisomy 21 – occurs when 21st chromosomes receives 3 chromosomes, resulting in Down’s
Syndrome
Karyotypes – photographs of chromosomes found in cell, arranged based on size and shape. Photo is
taken during metaphase of mitosis. Obtained one of two ways:
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Amniocentesis:
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Chorionic Villus Sampling:
IB BIO I
Meiosis
Mitosis
DNA replication occurs during interphase
before cell division
One cell division (prophase, metaphase,
anaphase, telophase)
No synapsis of homologous chromosomes
Produces two genetically identical diploid
daughter cells
Enables multicellular adult to form from zygote,
produces cells for growth and tissue repair
Van Roekel
Meiosis
DNA replication occurs during interphase
before cell division
Two cell divisions (prophase I & II, metaphase I
& II, anaphase I & II, telophase I & II)
Synapsis of homologous chromosomes
Produces 4 genetically different haploid
daughter cells
Produces gametes (sex cells) by reducing
number of chromosomes in half and introduces
genetic variation.