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Transcript of Cell Growth & Reproduction - Cloud Object Storage | Store ... · 9.3 Meiosis VS Mitosis •Meiosis-...
Meiosis
Cells Divide, and Cells Die
Section 8.1
Your cells are rapidly dying and
being replaced.
Cell division produces a
continuous supply of replacement
cells.
Cell division also has a role in
reproduction, growth, and
development.
Cell Division• Cells divide to make more
cells. While all the other organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.
• Mitosis is normal cell division, which goes on throughout life in all parts of the body. Meiosis is the special cell division that creates the sperm and eggs, the gametes.
Two Types of Cell Division
Interact in the Sexual Life Cycle
Section 8.1
Each mature
individual produces
sex cells by another
form of cell division
called meiosis.
Figure 8.1Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sex cells combine at
fertilization.
Two Types of Cell Division Interact
in the Sexual Life Cycle
Section 8.1
A fertilized egg, or
zygote, divides by
mitosis. The fetus
grows and develops
into a mature adult
consisting of countless
cells with identical
DNA.
Figure 8.1Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Why Meiosis?
• In order for each generation to have the
same number of chromosomes,
gametes must have half the number of
chromosomes (N) as a body cell (2N).
• Meiosis – a type of cell division that
produces four gametes, each with half
the number of chromosomes as a body
cell.
Homologous Chromosomes
• Paired chromosomes
with genes for the
same traits are called
homologous.
• Genes are arranged in
the same order on
homologous
chromosomes.
Human Chromosomes
P. 128
KARYOTYPE
Meiosis
• Meiosis occurs in two
separate divisions,
meiosis I and meiosis
II.
• Meiosis starts with one
2N cell and ends with
four N cells.
• Meiosis is necessary
for sexual
reproduction.
Meiosis cont.
Meiosis I
• Crossing Over – non-
sister chromatids of
homologous
chromosomes exchange
genetic material.
• Homologous
chromosomes are
separated into two new
cells.
Crossing
-Over
P. 131
Meiosis II
• Sister
chromatids are
separated
• Results in four
cells, each with
one copy of
each
chromosome.
Meiosis Provides Variation
• Meiosis makes cells that are not identical.
• Meiosis results in a variety of different
gametes.
• Gametes are randomly fertilized,
producing very large numbers of possible
unique offspring.
• Crossing over in Meiosis I adds more
variation.
9.2 Phases of Meiosis• The same four stages of mitosis occur
during meiosis.
– Prophase
– Metaphase
– Anaphase
– Telophase
• The four stages occur twice, once during
meiosis I and again during meiosis II.
Meiosis I Events
• Prophase I
• The spindle appears between the
separating chromosomes.
–Synapsis (tetrad formed)
– Crossing-over may occur.
• Metaphase I, tetrads aligned at the
spindle equator.
Meiosis I (cont.)
• Anaphase I- Homologous
chromosomes separate.
–Cell divides to become two
haploid (1N) cells.
• Telophase I, nuclear envelope
reforms
Meiosis I - Summary
P. 132
Meiosis II- Second Division
• Repeats Meiosis I with Sister
Chromatids separating to form
two more cells-
• Summary: Single diploid cell
undergoes meiosis to produce four
haploid gametes.
Meiosis II Summary
P. 133
9.3 Meiosis VS Mitosis
• Meiosis- two nuclear divisions
while Mitosis- one.
• Meiosis-four daughter nuclei (1N,
haploid)
Mitosis-only two. (2N, diploid)
• Both followed by cytokinesis.
9.3 Meiosis VS Mitosis (cont.)
• Meiosis- Daughter cells
genetically dissimilar to parent
cell.
• Mitosis- Daughter cells genetically
identical to parent cell.
Meiosis
VS
Mitosis
P. 135
Mistakes in Meiosis
• Nondisjunction –
failure of
homologous
chromosomes to
separate.
• Results in gametes
that are missing
chromosomes or
that have extra
chromosomes.
Down Syndrome• Down syndrome is trisomy (3-copies) of
chromosome 21.
• Down syndrome characteristics:
–Short stature
–Eyelid folds
–Stubby fingers
–Wide gap between the first and second toes.
–Fissured tongue
–Round head
–Palm crease (the Simian line)
–Mental Retardation
Down’s
Syndrome
P. 136
Mistakes in Meiosis cont.
• Monosomy – one
chromosome
instead of a pair.
• Triploidy – three
copies of ALL
chromosomes,
resulting from
complete
nondisjunction.