Meiosis
A process in which the number of chromosomes per cell
is cut in half through the separation of homologous
chromosomes to form gametes, or sex cells
• Two distinct divisions, called meiosis I and meiosis II
In Humans…
•All human body cells have 46 chromosomes (coiled up DNA).
What does that mean?
• Every cell MUST have the same amount of chromosomes.
WHY does every cell have the same amount of chromosomes?
All human body cells (somatic cells) that make up your body came from a single cell.
Somatic cells:
• The 46 chromosomes come in pairs
• These pairs are called homologous chromosomes
Homologous Chromosomes
• Homologous Chromosome: Chromosomes that are the SAME (length and traits).
• Key Point: They are similar, but NOT IDENTICAL
Mitosis Review
• The daughter cells are IDENTICAL to the parent cell
HOW??
• They have the same number of chromosomes
Chromosome Number
• How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis?
• If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain?
• In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
46
92
23
Diploid Cells
•A diploid – when the cell has a full set of chromosomes ( ½ from mom and ½ from dad)
•Somatic cells are DIPLOID cells
•For Humans: DIPLOD CELL = 46 Chromosomes
Haploid Cells
•A haploid – when the cell has half the set ( ½ ) of chromosomes
•Sex cells are HAPLOID cells
•For humans a diploid cell has 46 chromosomes.
•How many chromosomes would a sex cell (haploid) have?
23 chromosomes
In Sexual Reproduction…
• Two haploid sex cells come together to form a diploid cell.
• These sex cells are also called Gametes
Haploid sex cell23 Chromosomes
Haploid sex cell23 Chromosomes
Diploid cell 46 Chromosomes
Female sex cell =
EggMale sex cell =
SpermEgg + Sperm =
Zygote
Fruit Fly Chromosomes
• Not all organisms have the same number
of chromosomes
• Fruit Fly Example: Somatic Cell has 8
Chromosomes
• Four chromosomes come from its male
parent; Four come from its female parent
• These two sets of chromosomes are
called:
• Homologous Chromosomes
Diploid Cells
• Diploid: Full set of homologous
chromosomes
• Represented by the symbol 2N
• Fruit Fly: the diploid number is 8. (2N = 8)
• Humans: 2N = 46
Haploid Cells
• Haploid: Half set of chromosomes
• Gametes (sex cells sperm, egg) are
haploid
• N = single set of chromosomes in a sperm
or egg cell
• Fruit Fly: Haploid Number is 4 (N = 4)
• Humans: N=23
Mitosis: Makes Somatic Cells
Diploid
46 Chromosomes
Diploid
46 Chromosomes
Diploid
46 Chromosomes
Creates ____________________________. 2 identical diploid cells
Meiosis: Makes Sex Cells
Diploid
46 Chromosomes
Haploid23
Chromosomes
Haploid23
Chromosomes
Haploid23
Chromosomes
Haploid23
Chromosomes
Creates ____________________________. 4 different haploid cells
Gametes to Zygotes •Haploid cells produced by meiosis are gametes:
•male animals – gametes sperm
•plants – pollen contain haploid sperm cell
•female animals – gametes eggs
•plants - egg cell contain haploid cells
•Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote
•Zygote undergoes cell division by mitosisand eventually forms a new organism
• Somatic cells : ______________
• Examples: _________________________________
• Number of chromosomes: _______________
• Gametes: ____________
• Examples: _____________________
• Number of chromosomes: ________________
Body Cells
Sex Cells
Brain cell, skin cell, etc.
Egg and sperm
46 (diploid)
23 (haploid)
RELATIONSHIP BETWEEN THESE TYPES OF CELLS:Somatic cells will always have ___________ the number of chromosomes as gametes
TWICE
Key differences between types of cells
Check For Understanding:
1. Unlike mitosis, meiosis results in
a) 4 identical, diploid cells
b) 2 genetically different haploid cells
c) 2 genetically different diploid cells
d) 4 genetically different haploid cells
CHECK FOR UNDERSTANDING
1. If a horse has 12 chromosomes in its gametes, how many chromosomes are in it’s skin cells?
a) 12b) 24c) 48d) 6
2. If there are 10 chromosomes in the parent cell at the start of meiosis, how many chromosomes will be in the daughter cells at the end of meiosis?
a) 10b) 20c) 5d) 40
Check for Understanding
Question: What is the relationship between the number of chromosomes in the zygote and the number of chromosomes in a gamete?
A ZYGOTE (diploid) HAS TWICE THE
NUMBER OF CHROMOSOMES AS THE
GAMETES (haploid)…
(Because two haploids come together
to form the diploid zygote)
Mitosis or Meiosis?
•Makes gametes
•Makes somatic cells
•Creates haploid cells
•Creates diploid cells
•Asexual reproduction
Mitosis
Meiosis
Mitosis
Mitosis
Meiosis
Mitosis or Meiosis?
• Creates cells with the same number of chromosomes as the original
• Involved in sexual reproduction
• Creates cells with ½ the number of chromosomes as the original
• Creates 4 daughter cells
• Creates genetically identical offspring
Mitosis
Mitosis
Meiosis
Meiosis
Meiosis
Check for Understanding
1. If human were to have 46 chromosomes in its skin cells, how many chromosomes are in a diploid cell?
2. What about a haploid cell?
3. In Humans, N= ___?
46
23
23
Check for Understanding
• In spinach, 2N = 12.
• What is the haploid number?
• How many chromosomes are in a spinach leaf?
• The Red King Crab has 208 chromosomes.
• What is the diploid number?
• How many chromosomes are in the sperm cells?
• The Great White Shark has 82 chromosomes.
• What is N?
• How many chromosomes would be in a brain cell?
612
208104
41
82
Check for Understanding
1. Name an example of a cell that is a GAMETE.
2. An organism has 15 chromosomes in its sperm cell. How many chromosomes will be in that organism’s skin cell (somatic cell)?
3. Mitosis ends with a ________ (Diploid or Haploid) cell.
4. Meiosis ends with a ________ (Diploid or Haploid) cell.
Egg or
Sperm
30
Diploid
Haploid
Meiosis Overview
• Goal: Sexual reproduction (one cell from mom + one cell from dad)
• 2 Phases:
• Meiosis I
• Meiosis II
• End Result: 4 cells with HALF the chromosomes of the original cell (4 HAPLOID CELLS)
Stages of Meiosis
Interphase 1
Before meiosis I begins, cell
undergoes interphase
Chromosomes
replicate
Meiosis 1 Begins: Prophase I
Chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.
Prophase I
• Homologous Chromosomes pair up and form tetrads,
they undergo a process called crossing-over
(chromatids of the homologous chromosomes cross
over one another)
• crossed sections of the chromatids are exchanged
(new combinations of genes)
Chromosomes Cross-Over
• During Prophase 1, sister chromatids cross-over.
• That means that the DNA from your mom and the DNA from your dad mix.
• Each chromosome at the end will have DNA from both parentsand is DIFFERENT
Metaphase I• Spindle fibers attach to each homologous
chromosome
• Paired homologous chromosomes line up
across the center of the cell.
•
Anaphase I
•Spindle fibers pull each homologous
chromosome pair toward opposite ends
of the cell
Telophase I and Cytokinesis
•Nuclear membranes reform
•Cytokinesis follows Telophase I, forming two
new cells.
Meiosis I Ends
•Results in two cells (daughter cells) each of which has four chromatids
Meiosis II Begins • Unlike the first division, neither cell goes through
a round of chromosome replication before
entering meiosis II NO INTERPHASE
Prophase II
• Chromosomes—each consisting of two
chromatids—become visible
Metaphase II
• Chromosomes line up in the center of each cell
Anaphase II
• Paired chromatids separate
Telophase II and Cytokinesis
- Each of the four daughter cells produced in meiosis
II receives two chromatids, they are HAPLOID
Why is crossing over important?
• Crossing over leads to genetic diversity(which is the difference in a species).
• Genetic Diversity is the major advantage of sexual reproduction because it provides genetic variation.
Summary/ OverviewM
eio
sis
M
ito
sis
Meiosis v. Mitosis
Meiosis
• Two divisions• Homologous chromosomes line
up and move to separate daughter cells.
• Reduces chromosome number by half
• Produces 4 genetically different haploid cells.
• Early step in sexual reproduction.
Mitosis
• One Division• Two sets of genetic material
separate, each daughter cell receives one complete set of chromosomes
• Doesn’t change chromosome number
• Makes 2 identical diploid cells• Form of asexual reproduction
Chromosomal Mutations
• Chromosomal mutations are
unexpected changes in a
particular chromosome
• This includes the addition of
an extra chromosome or the
deletion of a chromosome.
