MEIOSIS and Sexual
Reproduction
Vocabulary
Meoisis
Diploid
Gamete
fertilization
Somatic cell
Crossing Over
Autosomal chromosome Independent
Sex chromosome
assortment
Homologous chromosomes
Sperm
Meiosis I
Egg
Meiosis II
Haploid
tetrad
Mitosis Vs. Meiosis
• What can you tell me about both?
Organisms that reproduce Sexually are
made up of two different types of cells.
1. Somatic Cells are “body” cells and contain the
normal number of chromosomes ….called the
“Diploid” number (the symbol is 2n). Examples
would be … skin cells, brain cells, etc.
2. Gametes are the “sex” cells and contain only ½
the normal number of chromosomes…. called the
“Haploid” number (the symbol is n)….. Sperm
cells and ova are gametes.
Meiosis Forms Gametes!!!!!
• The Male Gamete is the Sperm and is
produced in the male gonad the Testes.
• The Female Gamete is the Ovum (ova = pl.)
and is produced in the female gonad the
Ovaries.
Meiosis in males is called
spermatogenesis and produces sperm.
Meiosis in females is called oogenesis
and produces ova.
Sperm are small because they
contain no organelles. Must be
able to move.
One egg produced that holds all of
the organelles.
fertilization
Spermatogenesis –
Formation of sperm
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
One egg
produced.
Oogenesis
Large holds all
organelles
*** The polar bodies die… only one ovum
(egg) is produced from each primary oocyte.
Egg Cell
• Sexual Reproduction: Fusion of
two gametes that results in the
combo of the genetic material
from the two parents.
Fertilization
• Sperm + Ovum (egg) ----
Zygote
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Sex Chromosomes OR Sex cells?
• There are sex cells and then there are
sex chromosomes!! Don’t get them
confused!
• Sex cells – sperm and egg
• Sex chromosomes XX and XY
Autosomal Chromosomes 1 – 22
(
In Humans
the
“Autosome
s” are sets
1 - 22
Homologous Chromosomes
(because
a homologous pair consists of 4 chromatids it is called a “Tetrad”)
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Sex Chromosomes
“Sex Chromosomes”
…….the 23rd set
This person has 2
“X” chromosomes…
and is a female.
23
Types of Chromosomes
• Homologous Chromosomes: Pair of chromosomes
(maternal and paternal) that are similar in shape and
size. They carry the genes from parents to offspring.
• Each locus (position of a gene) is in the same
position on homologues.
• Humans have 23 pairs of homologous chromosomes.
1 - 22 pairs of autosomes – not related
to the sex of the organism.
1 pair of sex chromosomes – control
the development of sexual characteristics.
Types of chromosomes
Autosomal chromosomes– chromosome
1 -22 set
Sex chromosome – set 23
• Egg – sex chromosome X
• Sperm – sex chromosome X or Y
• Male – XY
Female - XX
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
All Organisms have a
characteristic number of
chromosomes!
• The number has nothing to do with
complexity!
• Fruit fly - 8
• Horse - 64
• Fern - 1200
Chromosome numbers
All are even numbers –
diploid (2n) sets of
homologous
chromosomes!
Ploidy = number of
copies of each
chromosome.
Diploidy
Meiosis
Cell division – ‘reduction
division’
Production of sex cells –
gametes
Meiosis errors
Mitosis vs. Meiosis
• Most cells in the body produced by
mitosis.
• Only gametes are produced by
meiosis.
Why do we need meiosis?
• Meiosis is necessary to halve the number
of chromosomes going into the sex cells
• Why halve the chromosomes in gametes?
At fertilisation the male and female sex cells
will provide ½ of the chromosomes each –
so the offspring has genes from both
parents
Meiosis
Meiosis is Two cell divisions
(called meiosis I and meiosis II)
with only one duplication of chromosomes.
Every gamete is different because of the
mixing of genes during metaphase I.
Crossing over of genes!
Germ Cells - type of cells that for gametes.
Females form all eggs before birth
Males form sperm whole life
Mitosis
Meiosis
Produce identical
cells
Unique cells
Diploid cells
Haploid cells
Takes place whole
life of organism
Takes place at only
certain times
Asexual
reproduction
Sexual reproduction
Body cells
Germ cells
•
•
•
•
Meiosis – key differences from
mitosis
Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.
Meiosis I involves:
– homologous chromosomes pair up form a tetrad.
– crossing over of non-sister chromatids.In Metaphase I,
homologous pairs line up at metaphase plate. This
mixes up genes
– In Anaphase I, sister chromatids do NOT separate.
– End Product, separation of homologous pairs of
chromosomes, rather than sister chromatids of
individual chromosome.
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling
the same inherited traits.
• Each locus (position of a gene) is in the same
position on homologues.
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes
How Does Sexual Reproduction
creates genetic variation???
1. Independent Assortment :Meiosis results in
genetic variation by shuffling the genes that are
placed in the gamete.
No daughter cells formed during meiosis are
genetically identical to either mother or father
2. Fertilization: During sexual reproduction, fusion
of the unique haploid gametes produces truly unique
offspring.
3. Crossing over - During the prophase 1 of
meiosis, chromosome segments will switch with
homologous chromosomes.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Another Way Meiosis Makes Lots of
Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of
different gamete types produced by independent
Interphase I - Like mitosis
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
• Centriole pairs also replicate.
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Meiosis I (four phases)
• Cell division that reduces the chromosome
number by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase 1: DNA has been replicated.
Spindle fibers and centrioles form – Nuclear
membrane and nucleolus disappears.
Chromosomes are tetrads!
Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).
Prophase I
spindle fiber
aster
fibers
centrioles
Metaphase 1: Homologous chromosomes
align at the equatorial plate.
Spindle fibers attach.
• Crossing over occurs with the
homologous chromosomes
Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
• Chiasmata (chiasma) are the sites of crossing
over.
Metaphase I
OR
metaphase plate
metaphase plate
Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
(mom and dad’s chromosomes randomly align on
each side.)
2. Variation
3. Crossing over occurs
Anaphase 1: Homologous pairs
separate with sister chromatids
remaining together.
Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromeres.
Anaphase I
• Telophase 1: Two .daughter cells are
formed with each daughter containing
only one chromosome of the
homologous pair.
Telophase I
• Each pole now has haploid set of
chromosomes.
• Cytokinesis occurs and two haploid daughter
cells are formed.
Telophase I
Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis
Meiosis 2
Second division of meiosis: Gamete formation
• Prophase 2: DNA does not replicate.
Same as 1
• Metaphase 2: Chromosomes align at
the equatorial plate.
• Anaphase 2: Centromeres divide and
sister chromatids migrate separately to
each pole.
• Telophase 2: Cell division is complete.
Four haploid daughter cells are
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis. NO
CROSSING OVER or HOMOLOGOUS
CHROMOSOMES!
metaphase plate
metaphase plate
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember:
four haploid daughter cells
produced.
gametes = sperm or egg
Telophase II
Independent Assortment
Process occurring in Metaphase I where it
is random which one of the pair is
placed in the germ cell.
Mitosis
Meiosis
Produce identical
cells
Unique cells
Diploid cells
Haploid cells
Takes place whole
life of organism
Takes place at only
certain times
Asexual
reproduction
Sexual reproduction
Body cells
Germ cells
Variation
• Important to population as the raw
material for natural selection.
• Question:
What are the three sexual sources of
genetic variation?