Download IB BIO I Meiosis Van Roekel Meiosis – type of cell division, known as

IB BIO I
Meiosis
Van Roekel
Meiosis
Meiosis – type of cell division, known as a reduction division, which cuts the number of chromosomes
from a parent cell in half in order to produce sex cells, also known as gametes. Necessary because
during fertilization, each parent will provide half of the chromosomes to the new offspring, so the
offspring will have genes from both parents
Meiosis Vocabulary
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Diploid Cells – cells that has two copies of each chromosome (2n, where n = # of chromosomes.
In humans 2n=46)
Haploid Cells – cells with a single copy of each chromosome (n, where n = # chromosomes. In
humans n=23)
Somatic Cells – normal body cells that have two copies of each chromosome (diploid cells)
Gamete Cells – sex cells with one copy of each chromosome (haploid cells)
Homologous Chromosomes - pairs of chromosomes that are similar in size and shape and carry
the same genes. One comes from the mother, the other comes from the father. (Can also be
seen as pairs of sister chromatids)
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)
o 1 Diploid cell  4 Haploid Cell (cuts the number of chromosomes from the parent cell in
half)
Creates genetic diversity through independent assortment, random orientation, and crossing
over,
o This results in each haploid cell have different forms of each gene, or in other words,
four unique haploid cells
o Genetic diversity is important for the evolution of populations and species.
Stages of Meiosis (occurs in two separate divisions)
Interphase - Cell will still go through interphase (G1, S, and G2) before meiosis, which results in two
identical
for every chromosome
Meiosis I:
Stages:
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Prophase I - Each set of
match up with their
homologous pair and fasten together in a group of four chromosomes called a
.
o Crossing over occurs, which is the
of chromosomal
between
, occurs during this stage.
 Results in new
of genes on each chromosomes, leading to
genetic
Metaphase I - homologous chromosomes (tetrad) line up at the
of the cell and
are attached to microtubules from centrosomes
IB BIO I
Meiosis
Van Roekel
o
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Homologous chromosomes line up on the equator of each cell in
and
, known as
Anaphase I – Homologous chromosomes move towards
of cell.
Sister chromatids
and move as a unit towards the same
side of the cell.
Telophase I – nuclear envelope
around separated homologous chromosomes.
Each daughter cell has
the number of chromosomes, but contains sister chromatids,
which must be separated next.
Cytokinesis –cytoplasm
, forming
new
cells.
At end of meiosis I, each cell is now considered
and has
the number of
chromosomes as the parent cell (
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
, microtubules form and begin
to attach to sister chromatids while centrosomes move towards opposite sides of the cell
Metaphase II –
line up at
in random
order (referred to as
). Microtubules are attached to
each of the sister chromatids at centromeres
Anaphase II – Sister Chromatids
and pulled towards
of the cells. Now considered
.
Telophase II/Cytokinesis –
at opposite sides around separated
chromosomes and
creating four daughter cells.
Each of the four daughter cells are
haploid cells (half
number of chromosomes)
Genetic Variety
Meiosis produces
sex cells that when used in fertilization can produce offspring that show
. Gametes are unique because of: crossing over, random
orientation, and the law of independent assortment.
Crossing Over:
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Occurs during
and results in the
of genetic
information between
and
chromosomes.
Resulting chromosomes contain sections of genetic material that originated in two different
people, so sister chromatids are no longer
.
Occurs between
, when chromatids intertwine
and break at same position.
Place where chromatids connect to each other is called the
(pluralchiasmata)
IB BIO I
Meiosis
Van Roekel
Random Orientation:


Occurs during
I and II
Random
and
of
line up at equator of cell, resulting in different forms of the genes being
as they
to each gamete
Law of Independent Assortment:
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The
of one pair of
(form of a gene) is independent
of the
of another pair of
.
Traits that determine
(flower color & flower height)
are passed down
of each other as a result of random orientation
Genes/alleles that pass to daughter cells depend on
of
during metaphase I
Results in
, where n is the number of chromosomes and 2
represents the number in each homologous pairs
In humans:
Sex is determined by the presence of the Y chromosome. XX =
. XY =
.
Division Error – occurs when chromosomes do not separate as expected, resulting in
of chromosomes
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Non-disjunction – occurs when two or more
stick together instead of separating
Trisomy – when offspring receives
chromosomes instead of
o Errors have different results depending on what chromosomes is affected
o Trisomy 21 – occurs when
chromosomes receives 3 chromosomes, resulting in
Down’s Syndrome
o Results when
occurs on the 21st chromosome. (21st
homologous chromosomes do not separate during Anaphase I)
o Risk increase with age of the mother, especially over the age of 35
o Typically associated with physical growth delays, characteristic facial features, and mild
to moderate intellectual disability
Karyotype –
of
found in cell, arranged based on
. Photo is taken during metaphase of mitosis. Obtained one of two ways:
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Amniocentesis: use a hypodermic needle to extract some
around the developing baby
Chorionic Villus Sampling: obtain
sample from the
uterus wall
Must be able to read and interpret Karyotypes
and
in the
IB BIO I
Meiosis
Mitosis
DNA replication occurs during interphase
before cell division
cell division (prophase, metaphase,
anaphase, telophase)
No
Produces
Produces
of homologous
chromosomes
genetically
daughter cells
cells for
and
repair
Van Roekel
Meiosis
DNA replication occurs during interphase
before cell division
cell divisions (prophase I & II,
metaphase I & II, anaphase I & II, telophase I &
II)
of homologous
chromosomes
Produces
genetically
daughter cells
Produces
(sex cells) by reducing
number of chromosomes in
and
introduces genetic
.