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Chapter 9 Meiosis
Chapter 9Meiosis
asexual: one parent sexual: two parents
What is MITOSIS?
ASEXUALASEXUAL or SEXUAL?
Asexual vs. sexual reproduction asexual: one parent creates an identical copy sexual: two parents create a new, different cell
What is MITOSIS?
ASEXUAL or SEXUAL?
Somatic (body) vs. Reproductive (sex)
somatic (body) cells: all the cells in your body EXCEPT the sex cells (ex: liver, skin, brain, etc)
reproductive (sex) cells: haploid (half the DNA) cells used for reproduction ONLY sperm (male) and egg (female)
Chromosome Number Remember the DNA
buddies? Homologous
chromosome: chromosomes in diploid cells that work in pairs (one from dad, one from mom);
Homologs have the same genes in the same order along the chromosome, but they may not be EXACTLY the same. Ex- blue eyes vs brown eyes; blonde vs black hair
Chromosome Number diploid: “two sets”; all homologous pairs are
present
- human body cells have 46 chromosomes - human body cells have 46 chromosomes (diploid number = 46 or 23 pairs)(diploid number = 46 or 23 pairs)
- fruit fly has 8 chromosomes (diploid number = - fruit fly has 8 chromosomes (diploid number = 8 or 4 pair)8 or 4 pair)
haploid: “one set”; no homologous pairs are present; HALF the normal amount of DNA- human sex cells have 23 chromosomes (haploid = 23)- fruit fly sex cells have 4 chromosomes (haploid = 4)
Karyotype
Karyotype: a picture of all the homologous pairs; used to make sure all DNA is present without extra or missing chromosomes; last pair = sex chromosomes (XX female, XY male)
To read: make sure no extra or missing and identify sex
Is this a male or female?
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
Purpose of Meiosis
To produce HAPLOID (half DNA/no pairs) eggs or sperm, so fertilization can produce a fertilized egg (zygote) with new chromosome pairs (diploid).
“Crossing Over” In meiosis, variety is BEST! The
offspring should be DIFFERENT. To mix up the DNA, sometimes crossing over occurs between homologous pairs.
Homologous pairs break pieces off and trade to create brand new combinations. “Mom” and “Dad” chromosomes become “Mad” and “Dom”
Genetic Diversity Independent
assortment (where the chromosomes distribute randomly) and crossing over create new combinations of genes for DIVERSITY
Stages of Meiosis (See DIAGRAM) Meiosis starts the same as mitosis with a duplication of the DNA. In
order to create half cells, there are TWO divisions. One to split the pairs and one to split the duplicated chromosomes.
Interphase Meiosis I (Division of the homologous pairs)
– Prophase I– Metaphase I– Anaphase I– Telophase I– Cytokinesis
Meiosis II (Division of the chromosomes)– Prophase II– Metaphase II– Anaphase II– Telophase II– Cytokinesis
Prophase I Pause Prophase I is longest phase of meiosis; crossing
over occurs here Human egg production in a baby girl actually
starts before birth and pauses in prophase I until puberty, then 1 egg/month continues until the eggs are gone
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.
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
In Between Meiosis I and Meiosis 2…
There is NO Interphase II There is NO Interphase II There is no DNA replication
Separation of
Chromatids
Meiosis I – Splits the
homologous pairs
Meiosis II – splits the
chromosome
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:
all four 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 the the diploid diploid
number ofnumber ofchromosome
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 too many or missing chromosomes
Nondisjunction Occurs when chromosomes do not separate in
meiosis; 1 extra or 1 missing chromosome in gamete
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.
