meiosis - sharpschool
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MEIOSIS : CELL TYPES
You have Body cells and Gametes
Body cells are also called somatic cells.
Germ cells develop into gametes. Germ cells are located in the ovaries and testes.
Gametes are sex cells: egg and sperm.
Gametes have DNA that can be passed to offspring.
MEIOSIS : ALL ABOUT CHROMOSOMES
• Your body cells have 23 pairs of
chromosomes.
– Homologous pairs of
chromosomes have the same
structure and genetic
material.
– For each homologous pair,
one chromosome comes from
each parent.
• Chromosome pairs 1-22 are
autosomes.
• Sex chromosomes, X and Y,
determine gender in mammals.
Your cells have autosomes & sex chromosomes
MEIOSIS : DIPLOID AND HAPLOID
Diploid (2n) cells have two copies of every chromosome.
Body cells are diploid.
Half the chromosomes come from each parent.
In humans this means we have 2 sets of 23
chromosomes (total of 46)
MEIOSIS : DIPLOID AND HAPLOID
– Gametes are haploid.
– In humans gametes have 22 autosomes and 1 sex
chromosome. (23 total)
Haploid (n) cells have one copy of every chromosome.
MITOSIS REVIEW
• Mitosis and meiosis are types of nuclear
division that make different types of cells with
different number of chromosomes.
• Mitosis makes more diploid cells.
• Number of Chromosomes in parent is Equal to
the number of chromosomes in offspring.
Goal of Mitosis is Chromosome number must be
maintained.
INTRODUCTION TO MEIOSIS
Meiosis makes haploid cells from diploid cells.
This happens because cells go through TWO
rounds of division during meiosis.
Meiosis reduces chromosome number and creates genetic
diversity.
Meiosis Occurs in Sex cells
Meiosis Produces gametes.
MEIOSIS DETAIL KEY CONCEPT
During meiosis, diploid cells undergo two cell
divisions that result in haploid cells.
Meiosis I and Meiosis II
Meiosis I and meiosis II each have four phases, similar to
those in mitosis.
Biggest difference is the how the DNA organizes for each
division.
In Meiosis I Pairs of homologous chromosomes form tetrads.
homologous chromosomes
sister
chromatids
sister
chromatids
MEIOSIS DETAIL KEY CONCEPT
During meiosis, diploid cells undergo two cell
divisions that result in haploid cells.
Meiosis I pairs of homologous chromosomes (tetrads)
separate.
Homologous chromosomes are similar but not identical
genetically
In Meiosis II SISTER CHROMATIDS separate.
MEIOSIS I DETAILS
• Meiosis I occurs after DNA has been replicated.
• Meiosis I divides homologous chromosomes in four phases.
Prophase I
Metaphase I
Anaphase I
Telophase I
PROPHASE I
Each chromosome pairs with its corresponding homologous
chromosome to form a tetrad.
There are 4 chromosomes in a tetrad.
The pairing of homologous chromosomes is the key to
understanding meiosis.
Crossing-over may occur here
Crossing-over is when chromosomes overlap and exchange
portions of their chromatids.
METAPHASE I
Tetrads line up along the equator
Spindle fibers attach to the
centromeres of each chromosome in
the tetrad.
ANAPHASE I
The cell begins lengthening.
The spindle fibers pull the homologous
chromosomes apart and toward opposite
ends of the cell.
Sister chromatids attached at
centromeres move together
Disjunction Occurs – process of
separation
There are ½ as many chromosomes as
in the original cell but the chromosome is
double stranded.
TELOPHASE I
Movement of homologous chromosomes continues until there is
a haploid set at each pole
Each chromosome = linked sister chromatids
At each pole, now, there is a complete haploid set of
chromosomes (but each chromosome still has two sister
chromatids).
Nuclear membranes reforms.
A cleavage furrow appears.
The cell separates into 2 daughter cells.
PROPHASE II
• Meiosis I results in two
haploid (N) cells.
• Each cell has half the
number of
chromosomes as the
original cell.
• Spindle Reforms
• Sister Chromatids are
present
METAPHASE II
The Sister Chromatids
line up .along the
equator (middle) of the
cell similar to
metaphase in mitosis
TELOPHASE II
• Nuclear membranes form
around each set of
chromosomes at opposite ends
of the cell
• Spindle fibers break apart
• Cell undergoes cytokinesis
• End result is four HAPLOID cells
with genetically unique
information.
COMPARE AND CONTRAST
–Meiosis has two cell divisions while
mitosis has one.
– In mitosis, homologous chromosomes
never pair up.
–Meiosis results in haploid cells; mitosis
results in diploid cells.
• Meiosis differs from mitosis in significant
ways.
GAMETOGENESIS
In Males:
SPERMATOGENESIS
– Sperm become streamlined and motile.
– Sperm primarily contribute half of the DNA to an
embryo.
Haploid cells develop into mature gametes.
•Gametogenesis is the production of gametes.
•Gametogenesis differs between females and
males.
GAMETOGENESIS
In Females:
Oogenesis
– Eggs contribute half of DNA, all of the cytoplasm,
and all organelles to an embryo.
– During meiosis, the egg gets most of the
contents; the other cells form polar bodies.
– So only one functional gamete per cycle of
meiosis