Download Ch11 Meiosis

Meiosis
Modes of Reproduction

Asexual reproduction
– Process by which a single parent reproduces
itself

Sexual Reproduction
– Process by which cells from two different
parents unite to form the first cell of a new
organism
Sexual Reproduction
With sexual reproduction, there is the
passing of alleles.
 Alleles: one of a number of different forms
of a gene
Where do we find alleles?


Gametes: specialized cell involved in sexual
reproduction
(i.e. Male = sperm
Female = egg)
Meiosis
A
process of reduction division:
the number of chromosomes per
cell is cut in half through
seperation of homologous
chromosomes in a diploid cell
Chromosomes

Homologous chromosome: 2 sets of
chromosomes; 1 from male & 1 from female
– chromosomes that each have a corresponding
chromosome from the opposite sex parent

Diploid =

Haploid =
2 sets of homologous chromosomes (2n)
single set of chromosomes (n)
Meiosis I & II

There are two cycles of Meiosis:
1.Meiosis I
1.Meiosis II
Meiosis I

Meiosis I
– each chromosome is replicated and connects
to it’s homologous chromosome forming a
tetrad. Chromosomes exchange portions of
chromatid in crossing over.
– Forms 2 cells of different genetics.

Crossing Over
– Process where homologous chromosomes
exchange portions of their chromatids during
meiosis.
Tetrads & Crossing Over
Prophase I
Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad.
 There are 4 chromosomes in a tetrad.
 Crossing-over may occur here
 Crossing-over is when chromosomes overlap and
exchange portions of their chromatids.

Prophase I
Metaphase I
Spindle fibers attach to the chromosomes
 Chromosomes line up in middle or equator

Anaphase I

The fibers pull the homologous
chromosomes apart toward opposite ends of
the cell.
Telophase I & Cytokinesis
Nuclear membranes form.
 The cell separates into 2 cells.

Meiosis II

Meiosis II
– The 2 diploid cells (2n) from meiosis I enter
this, but does not start with DNA replication.
– Each chromosome has 2 chromatids that get
split in anaphase.
– Forms 4 haploid (n) cells.
Prophase II
No DNA replication occurs.
 Each cell has half the number of
chromosomes as the original cell.

Metaphase II

The chromosomes line up similar to
metaphase in mitosis.
Anaphase II

Sister chromatids separate and move to
opposite ends of the cell.
Telophase II

Meiosis II results in 4 haploid cells.
Gametes

Sperm: male gamete
– In humans holds 23 chromosomes

Egg: female gamete
– In humans holds 23 chromosomes
Fertilization

Process in sexual reproduction where male and
female reproductive cells (gametes) join to form
a new cell
Mitosis vs Meiosis
Results in
Cells are
Occurs in
Mitosis
2 Diploid Cells
(2N)
Genetically
Identical
Somatic (Body)
Cells
Meiosis
4 Haploid Cells
(N)
Genetically
Different
Sex Cells