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MEIOSIS
Reduction Division
A Source of Distinction
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Meiosis
Terms to Know
• n – the number of chromosomes in a cell
• Diploid - a cell with a double set of
chromosomes (2n) [somatic cells]
• Haploid - a cell with a single set of
chromosomes (n) [gametes]
• Homologous - similar chromosomes that
carry genes for the same trait (eye color,
height, blood type, etc. ) But are not
identical ( red/brown, tall/short, a/b, etc.)
DIPLOID & HAPLOID CELLS
• ALL BODY CELLS
(SOMATIC CELLS)
HAVE
CHROMOSOMES
IN PAIRS
• BODY CELLS ARE
CALLED DIPLOID
CELLS (2n)
• HUMANS HAVE
THE 2n # OF
CHROMOSOMES
HOMOLOGOUS CHROMOSOMES
• HOMOLOGOUS CHROMOSOMES ARE THE
PAIRED CHROMOSOMES THAT CONTAIN THE
SAME TYPE OF GENTIC INFORMATION, SAME
BANDING PATTERNS, SAME CENTROMERE
LOCATION, ETC.
• THEY MAY HAVE DIFFERENT ALLELES, SO
NOT PERFECTLY IDENTICAL
• WHY DO THEY HAVE DIFFERENT ALLELES?
CAME FROM DIFFERENT
PARENTS
WHY MEIOSIS?
• MITOSIS – RESULTS IN GENETICALLY
IDENTICAL OFFSPRING – INCLUDING
THE # CHROMOSOMES
• WHAT WOULD HAPPEN IF THE EGG AND
SPERM HAD THE SAME # OF
CHROMOSOMES AS THE BODY CELLS?
EGG = 46 CHROMOSOMES
SPERM = 46 CHROM.
ZYGOTE = 46 + 46 = 92 CHROMOSOMES =
NOT HUMAN
DIPLOID AND HAPLOID CELLS
CONT’D
• HAPLOID CELLS
– ONLY HAVE 1 OF EACH TYPE OF
CHROMOSOME (DIPLOID CELLS HAVE 2 OF
EACH TYPE)
– SYMBOL IS (n)
– SEX CELLS HAVE THE n # OF
CHROMOSOMES
MEIOSIS
• A TYPE OF CELL DIVISION WHICH
PRODUCES GAMETES CONTAING HALF
THE NUMBER OF CHROMOSOMES AS
THE BODY CELLS (Reduction Division)
• 2 STAGES – MEIOSIS I & MEIOSIS II
• START W/ 1 DIPLOID CELL, END UP W/ 4
HAPLOID CELLS (GAMETES)
• 4 DAUGHTER CELLS ARE GENETICALLY
DIFFERENT FROM EACH OTHER AND
MOTHER CELL
MEIOSIS
•
•
•
•
SPERM – MALE GAMETE (n)
EGG – FEMALE GAMETE (n)
FERTILIZATION PRODUCES A ZYGOTE (2n)
THIS TYPE OF REPRODUCTION IS CALLED
SEXUAL REPRODUCTION
STAGES OF MEIOSIS
• MEIOSIS I
– PROPHASE I, METAPHASE I, ANAPHASE I,
TELOPHASE I (PMAT)
• MEIOSIS II
– PROPHASE II, METAPHASE II, ANAPHASE II,
TELOPHASE II (PMAT)
MEIOSIS
Terms to Know
• Synapsis - when homologous
chromosomes join to form a tetrad
• Tetrad - a structure consisting of two
homologous chromosomes (four pieces of
chromatin)
• Independent assortment - the process by
which the chromosomes from one parent
arrange themselves and separate
independently of each other
MEIOSIS
Terms to Know
• Crossing over - The exchange of portions of
chromatin between homologous chromosomes
during synapsis (prophase I)
(can be lead to mutations i.e. inversion; deletion;
insertion)
• Gametes - specialized haploid cells involved in
sexual reproduction
Meiosis I: Overview
Replication
Phrophase 1
Metaphase 1
Anaphase 1
Telophase 1
Meiosis I
Interphase
• DNA replication occurs
Prophase I
• Homologous chromosomes
pair up and form a tetrad
(synapsis)
• Homologous chromosomes
may exchange portions of
their chromatin. This
creates new chromosomes.
This process is called
crossing over
Crossing-Over
One Way Meiosis Makes Lots of Different Sex Cells
This occurs during Prophase I
Crossing-over multiplies the already huge number of different gamete
types produced by independent assortment.
Meiosis I
Metaphase I
• Tetrads line up along the center
of the cell
• Each tetrad lines up
independently of the others
(Independent Assortment).
• Some tetrads have maternal
chromosomes on one side and
paternal on the other, while
other tetrads may have the
opposite arrangement.
Independent Assortment
One Way Meiosis Makes Lots of
Different Sex Cells (Gametes) –
 The example on the right starts with 3 pairs of
homologous chromosomes.
How many distinct gametes are possible?
2n = 23 = 2 x 2 x 2 = 8
Independent assortment produces 2n
distinct gametes, where n = the number of
homologous chromosomes.
In humans, n = 23 and 223 ≈ 8,000,000.
That’s a lot of diversity by this mechanism alone.
Meiosis I
ANAPHASE I
→
• Homologous
• chromosomes
separate
***chromosomes are
not the same as the
original cell***
TELOPHASE I
→
Nucleus forms around
chromosomes and two
new cells are formed
(diploid)
Meiosis II- Overview
Meiosis II
NO DNA REPLICATION
Prophase II
• Chromosomes condense
Metaphase II
• Chromosomes line up
along the center of the
cell.
• Chromosomes again
assort themselves
independently.
Meiosis II
Anaphase II
• Chromosomes separate
and move to opposite
sides of the cell
Telophase II
• Nuclear membrane
forms around
chromosomes and cells
separate
• New cells are called
gametes
IMPORTANT THINGS TO KNOW
• TETRAD – WHAT THE HOMOLOGOUS
CHROMOSOMES ARE CALLED WHEN THEY
PAIR UP DURING PROPHASE I
• CROSSING OVER – OCCURS DURING
PROPHASE I
– CREATES GENETIC VARIABILITY
(RECOMBINATION OF GENES: RESULTING FROM
CROSSING OVER)
• IN MEIOSIS I, HOMOLOGOUS CHROMOSOMES
SEPARATE (ANAPHASE I)
• IN MEIOSIS II, SISTER CHROMATIDS
SEPARATE
RESULTS OF MEIOSIS
• Reduction in amount of genetic material:
individual cells start in a diploid state
and result in 4 haploid cells
• Combination and composition of
chromosomes has been sorted randomly
• EACH GAMETE IS UNIQUE
Meiosis – A Source of Distinction
Meiosis does two things 1) Meiosis takes a cell with two copies of every
chromosome (diploid) and makes cells with a single copy of
every chromosome (haploid).
This is a good idea if you’re going to combine two cells to
make a new organism. This trick is accomplished by
halving chromosome number.
In meiosis, one diploid cells produces four haploid cells.
Meiosis – A Source of Distinction
The second accomplishment of meiosis:
2) Meiosis scrambles the specific forms of each gene that
each sex cell (egg or sperm) receives.
This makes for a lot of genetic diversity. This trick is
accomplished through independent assortment and crossingover.
Genetic diversity is important for the evolution of
populations and species.
Keys to Understanding Meiosis
Chromosomes are paired.
Chromosomes carry genes.
The gene forms on a pair of chromosomes may be identical .. or different.
Brown eyes
Brown eyes
Tall
Tall
Brown eyes
Blue eyes
Tall
Short
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
Boy or Girl? The Y Chromosome “Decides”
Oogenesis
A Path of Meiosis in Humans
Woman are less busy in
meiosis than men –
meiosis produces only a
few hundred mature
eggs over a lifetime.
Note: Meiosis in the human female
results in: one fertile egg and three
infertile “polar bodies”
Spermatogenesis –
A Path of Meiosis in Humans
Men are busy – meiosis produces
roughly 250,000,000 sperm per day.
“Putting It All Together” - Fertilization
The Key Difference Between Mitosis and Meiosis is the Way
Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
The first (and
distinguishing)
division of meiosis