Transcript Meiosis Power Point

Meiosis
Review Mitosis
• What was the purpose of Mitosis?
– cell division in multi-cellular organisms
– Reproduction in uni-cellular organisms
– What were the phases of the cell cycle?
• Interphase – G1, S, G2
• Mitosis Phase - PMAT!
• Cytokinesis
Meiosis
Yes it is different from Mitosis!!
• Mitosis is division that occurs in any cell in the body
called somatic cells (body cells)
• MEIOSIS is cell division by which gamete cells
(reproductive cells---sperm and egg) are produced
Remember!!!
• Chromosomes = tightly coiled
DNA (chromatid, sister chromatid)
– Contain genes which determine
hereditary traits
• We have a total of 46 chromosomes in EVERY cell of our
body
– We get…
•23 from Mom
•23 from Dad
*During the S phase of Interphase,
Chromatids are duplicated to make
sister chromatids
Dad
Mom
SO……
• We call the pair of matching sister chromatids Homologous
Chromosomes
• Homologous
chromosomes
contain the same
genes for the same
traits, but not
necessarily the
same Alleles
•******This is
different than
mitosis because the
sister chromatids
DO NOT pair up.****
– Genes are your traits
• Eye color, hair color, height, foot length, ect…
– Alleles are alternate forms your traits can take
• Usually one is dominant (brown eyes) vs. recessive (blue eyes)
*Somatic cells and Gametes have
different numbers of chromosomes
• DIPLOID (2N) = Somatic cells (Body Cells)
– describes a cell that has two copies of each
chromosome: 1 from mom and 1 from dad
– Cell starts with 46 chromosomes and each of the 2
new cells has 46 chromosomes.
• HAPLOID (N) = Gametes (Sex Cells)
– describes a cell that has half the number of
chromosomes.
– Cell starts with 46 chromosomes and each of the 4
new cells has 23 chromosomes.
Sexual Reproduction
• During sexual reproduction, an egg is fertilized
by a sperm to create a zygote
• The zygote should be diploid, that is, it should
have the same number of chromosomes as all
normal human body cells
• Sperm + Egg = 46
• (23) + (23)
Sexual Reproduction
• What if gametes (sex cells) went through
Mitosis?
– Mitosis produces 2 identical daughter cells
so…
– Diploid produces diploid
– If sperm and egg were diploid they would
create a polyploid (“many sets”) zygote when
they came together
– Obviously we do not have “many sets” of
chromosomes do we?
SO WHAT’S THE SOLUTION?
MEIOSIS!
• Where a Diploid cell produces 4 Haploid
daughter cells
• Meiosis is a 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.
Genetic Recombination
• So, when a haploid sperm fertilizes a
haploid egg a diploid zygote is the result
• Meiosis helps create Genetic
recombination, where 2 different sets of
chromosomes come together, keeps you
from being an exact copy of one of your
parents!
• It’s what creates genetic diversity in our
world!!
Stages of Meiosis
• Meiosis I
• Meiosis II
Meiosis I
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Cytokinesis
Interphase
Same as in Mitosis
– G1: cell growth, duplication of
organelles, job of cell carried out
– S: DNA synthesis/replication of
DNA
– G2: job of cell carried out, further
growth
Prophase I
• Nuclear membrane and nucleolus disappear
• Centrioles begin to separate and spindles begins to
form
• The homologous chromosomes pair up creating a
tetrad = 2 sets of sister chromatids = 4 individual
chromatids
• The homologous
chromosomes pair up,
gene for gene, down their
entire length.
During Prophase I
• Crossing-over can occur
– Involves the exchange of genetic
material between homologous
chromosomes
– Breaks arms of each chromosome and
switches the material
Prophase I
CROSSING OVER!!
Metaphase I
• Spindle fibers attach to the tetrads
• The tetrads line up along the cell equator
• Each side of the equator has
chromosomes from
both parents (mixed up)
Anaphase I
• The paired homologous chromosomes are pulled
away and move toward opposite ends of the
cell
• Sister chromatids remain
attached!!!!
Telophase I and Cytokinesis
• The nuclear membrane forms again in
some species, the spindle fibers
dissemble and the cell undergoes
cytokinesis.
• 2 daughter cells are produced that
are not identical
– A “reduction division” has occurred
because each daughter cell has half of
the number of chromosomes the original
parent cell had
Important Note:
Meiosis II will occur in both of
the daughter cells created in
Meiosis I
Review
• Meiosis I – Homologous Chromosomes separate
– Begins w/ 1 diploid cell
– 46 chromosomes (in homologous pairs)
– Prophase I, Metaphase I, Anaphase I,
Telophase I
– Ends with 2 Haploid cells
– 23 sister chromatids in each new cell.
Meiosis I
Interphase
Prophase I
Metaphase I
Cells undergo a round of
DNA replication, forming
duplicate Chromosomes.
Each chromosome pairs with its Spindle fibers attach to the
corresponding homologous
chromosomes.
chromosome to form a tetrad.
Anaphase I
The fibers pull the homologous
chromosomes toward the
opposite ends of the cell.
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
• Cytokinesis
Prophase II
• The DNA HAS NOT been
duplicated
– The nuclear membrane
disappears
– centrosomes and centrioles
move to opposite sides of the
cell
– Spindle fibers start to assemble
Metaphase II
• Spindle fibers attach to
the sister chromatids at
the centromere
• The 23 sister chromatids
(in each cell) line up along
the align at the cell
equator
Anaphase II
• Centromeres break, spindle
fibers shorten, and the
sister chromatids are
pulled apart from each
other and move to opposite
ends of the cell
• So 23 individual chromatids
go to each side!!!
Telophase II and Cytokinesis
• Spindle fibers break down
• Nuclear envelopes and nucleolus
reform
• Cell membrane pinches in and
cytoplasm is divided
• 4 haploid daughter cells are
produced with 23 individual
chromatids
– A “reduction division” has
occurred
Results of Meiosis
• The 4 haploid daughter
cells…
– Have only 1 set of genes
– Are not considered to be
in any phase because
they will not grow and
divide
– Now referred to as
gametes (sex cells)
Review
• Meiosis II – Sister chromatids separate
– NO DNA replication
– Prophase II, Metaphase II, Anaphase II, Telophase II
– Ends with 4 haploid cells
– 23 individual chromatids in each cell
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two haploid (N)
daughter cells, each with half the
number of chromosomes as the
original.
The chromosomes line up in a similar The sister chromatids separate and
way to the metaphase stage of
move toward opposite ends of the
mitosis.
cell.
Telophase II
Review Questions
• What type of cells are produced from meiosis?
– Gametes (sex cells)
• What happens to the chromosome number in the
cells that are produced from meiosis?
– Cell chromosome number is cut in half - Start with 2n
and end with 1n (end with 4 haploid gametes each
genetically different from the parents)
• What do genes code for?
– traits
Meiosis Mistakes
Mistakes in Meiosis
• Nondisjunction: failure of homologous chromosomes to
separate properly during meiosis I
– both chromosomes of a homologous pair move to the same pole
of the cell rather than separating
Mistakes of Meiosis
Effects of Nondisjunction:
1. Trisomy: zygote will have an extra chromosome
–
The gamete with an extra chromosome is fertilized by a normal
gamete resulting in a zygote with an extra chromosome
–
Organisms usually survive
–
Example: Down syndrome – flat face, short neck, some degree
of mental retardation
2. Monosomy: zygote will be missing a chromosome
–
a gamete with a missing chromosome fuses with a normal
gamete during fertilization resulting in a zygote that lacks a
chromosome
–
Organisms usually do not survive
–
Example: Turner syndrome – human females only have one xchromosome rather than two
• This is called a karotype
– it is a picture of your 23 pairs of chromosomes
• Autosomes – pairs 1-22
• Sex chromosome - pair 23
Meiosis Visuals
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
Spermatogenesis
• Meiosis in males to
produce sperm
• Thousands of
reproductive cells
undergo meiosis each
day to produce large
numbers of sperm
Oogenesis
• Meiosis in females to
produce ova (eggs)
• Only occurs in one cell
once a month
• The cytokinesis at end of
meiosis I and meiosis II is
uneven and cytoplasm is
divided unequally
• One cell gets the majority
of the cytoplasm and
becomes the egg
• The other cells are called
polar bodies and do not
participate in
reproduction
Comparing Mitosis
and Meiosis
Mitosis vs. Meiosis
Mitosis
Meiosis
Starts as one cell, ends as two
cells
Starts as one cell, ends as four
cells
Starts 2N (diploid), ends 2N
Starts 2N, ends 1N (haploid)
New cells are identical to each
other AND to parent
New cells NOT identical to
each other OR parent
Used to produce body (somatic)
cells
Used to produce gametes (sex
cells)
Mitosis results in the
production of two genetically
identical diploid cells, whereas
meiosis produces four
genetically different haploid
cells