Chapter 7 Section 1
 Mitosis:
 occurs because the cell
gets too big and for
healing.
 produces genetically
identical diploid daughter
cells.
 Meiosis:
 occurs in the production of
gametes.
 produces haploid
daughter cells.
 Somatic Cells –
 They are all the cells in the
body except for sperm and
eggs
 Always diploid!
 Produced by mitosis.
 Gametes –
 In males – sperm/pollen In
females – eggs
 Always haploid!
 Produced by meiosis.
 Each sperm and egg cell
contains half the normal
number of chromosomes –
essential to sexually
reproducing organisms.
 When a egg is fertilized by
the sperm, the offspring
inherits one chromosome for
each homologous pair from
each parent.
 It gains the normal number
of chromosomes.
G1 Phase
 Meiosis produces gametes
through two stages of cell
division.
 Meiosis begins with a cell
that has the normal number
of chromosomes.
 Due to the cell having
completed the S (synthesis)
phase of the cell cycle, the
chromosome has replicated
DNA.
After S Phase
G2 Phase
 Meiosis I: separation of
homologous chromosomes –
with each of the 2 new
daughter cells get one
chromosome from each pair.
 Meiosis II: each
chromosome is separated
into two chromosomes, so
that each of the four
daughter cells get half the
normal number of
chromosomes
Second reduction
First reduction
Unreplicated chromosomes
 During meiosis, two cell divisions – divided into 2
stages – stages I and II.
 Each stage, the cell undergoes the same phases as
mitosis: prophase, metaphase, anaphase and
telophase.
 Prophase I:
 Nuclear envelope breaks
down.
 Homologous
chromosomes pair up
 Crossing over occurs.
 Crossing-over occurs in
Prophase I after the
homologous chromosomes
pair up.
 DNA strands from one
chromosome join with the
other, breaking off and
piecing themselves back
together.
 END RESULT: each
chromosome contains pieces
of DNA from its homolog
 Each homolog has a
slightly different set of
alleles.
 Crossing-over shuffles the
genetic material so that
each sister chromatid has a
different combination of
alleles.
 Instead of the two original
allele combinations, there
are now four
 Crossing over makes it
possible for offspring to
inherit unique
combinations of alleles.
 This increases the
genetic variation in the
gene pool of a species.
 Metaphase I:
 Spindles attach to
the centromeres
of the
homologues.
 Homologous
chromosomes
align at the
equator.
 Anaphase I:
 Spindles
shorten
 Homologous
chromosomes
separate
 Telophase I:
 Cytokinesis occurs.
 Two cells have
formed.
At the end of telophase I, one parent cell has been
divided into two new daughter cells with half the
number of chromosomes – haploid cell.
Prophase I
 Prophase II:
 Chromosomes
condense
 Spindle fibers form
Prophase II
Metaphase I
 Metaphase II:
 Spindles attach to the
centromeres of the
sister chromatids.
 Chromosomes align
at the equator.
Metaphase II
Anaphase I
 Anaphase II:
 Spindle fibers shorten.
 Sister chromatids
separate.
Anaphase II
Telophase I
 Telophase II:
 Chromosomes
begin to unwind
and stretch out.
 Nuclear envelope
reforms.
 Cytokinesis
occurs.
Telophase II
At the end of telophase II, two daughter cells have
been divided into four new daughter that are
haploid.
Mitosis
Number of daughter cells
Chromosome number
Crossing-over occurs
(yes/no)
Sister chromatids separate
Daughter cells genetically
identical to parent cell
Daughter cells genetically
identical to each other
Meiosis I
Meiosis II