Chapter 9 Meiosis
Chapter 9Meiosis
asexual: one parent sexual: two parents
What is MITOSIS?
ASEXUALASEXUAL or SEXUAL?
Asexual vs. sexual reproduction asexual: one parent sexual: two parents
What is MITOSIS?
ASEXUAL or SEXUAL?
Somatic (body) vs. Reproductive (sex)
somatic (body) cells: all the cells in your body (muscle, liver, heart, tissue, etc.) EXCEPT for sex cells
reproductive (sex) cells: sperm (male) and egg (female) cells
Chromosome Number Pairs of matching
chromosomes are homologous chromosomes
homologous: a chromosome that has a corresponding chromosome from the opposite sex parent (one from the dad, one from the mom)
Homologs have the same genes, in the same order along the chromosome
Karyotype
Homologous chromosomes can be arranged into a karyotype
Humans have 23 pairs of chromosomes
Is this a male or female?
Chromosome Number diploid: “two sets”; both sets of homologous
chromosomes
- human body cells have 46 chromosomes - human body cells have 46 chromosomes (diploid = 46)(diploid = 46)
- fruit fly has 8 chromosomes (diploid = 8)- fruit fly has 8 chromosomes (diploid = 8) haploid: “one set”; a single set of
chromosomes (or half the needed DNA)- human sex cells have 23 chromosomes (haploid = 23)- fruit fly sex cells have 4 chromosomes (haploid = 4)
Human Chromosomes
What if? What would happen if two diploid
cells (with 2 complete sets of 23 chromosomes or 46 total) came together during reproduction?
How many chromosomes would that new cell have?
92 chromosomes! 92 chromosomes! Would it be human?
Mitosis You have already learned how a cell makes a
complete copy of its chromosomes in
But … How do sex cells end up with half the
chromosomes that body cells have????
MeiosisMeiosis
Life Cycle Germ (or sex) cells fuse during fertilization to make a
zygote
Purpose of Meiosis
To produce eggs and sperm with only 23 chromosomes, so fertilization can produce a fertilized egg (zygote) with 46 chromosomes.
Meiosis
Specialized type of cell division that produces haploid cells from diploid parent cell
Stages of Meiosis 2 cell divisions after replication:
– Meiosis I– Meiosis II
Each division has 4 stages:– Prophase– Metaphase– Anaphase– Telophase
Meiosis I: Interphase I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Meiosis I: Prophase I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over may occur.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
“Crossing Over” In meiosis, chromosomes get
together in homologous (similar) pairs & crossing over occurs.
Pieces from one chromosome break off and rejoin the other!
Genetic Diversity Independent
assortment and crossing over create new combinations of genes in gametes
Prophase I Pause Prophase I is longest phase of meiosis Production of proteins and structures in egg In humans, meiosis starts before birth, pauses
in prophase I until puberty, then 1 egg/month continues for up to 40 years
Meiosis I: Metaphase I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Meiosis I: Anaphase I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
At the end of Meiosis I … 2 new cells are formed; although each new
cell now has 4 chromatids (as it would after mitosis), something is different
neither of the daughter cells has the two complete sets of chromosomes that it would have in a diploid cell
the two cells produced by Meiosis I have sets of chromosomes and alleles that are different from each other and different from the diploid cell that entered Meiosis I
Now on to Meiosis II …
There is NO Interphase II There is NO Interphase II so there is no DNA replication
Separation of
Chromatids
Meiosis II: Prophase II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Meiosis II: Metaphase II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Meiosis II: Anaphase II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Meiosis II: Telophase II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
At the end of Meiosis II … You have FOUR (4) daughter cells with the
haploid number (N) of chromosomes!
If this took place in …… a male:
the new haploid cells will grow flagella and become sperm
… a female:only 1 of the 4 haploid cells will survive and become an egg
Male + female =
A zygote with
4646 chromosome
s!
Problems in Mitosis and Meiosis What happens if something goes wrong during
cell division?
– In Mitosis, incorrect cell division or uncontrolled cell division leads to cancer. Cells divide incorrectly or too fast. This can lead to the
growth of tumors or abnormal cells that do not do their job correctly.
– In Meiosis, incorrect cell division leads to birth defects from non-disjunction.
Nondisjunction Occurs when chromosomes do not separate in
either anaphase I or anaphase II Gamete is missing 1 chromosome or has 1
extra chromosome Fertilization will produce a zygote with 45 or 47
chromosomes
Down’s Syndrome Trisomy 21 results from
nondisjunction of chromosome 21
These kids have 3 chromosomes at pair 21.