Background information for
Sexual Reproduction
Mr. Shrout
Mitosis
• Mitosis produces genetically identical cells
and is involved in growth and asexual
reproduction.
• This form of cell division can quickly produce
new cells (organisms), but does not produce
any variations.
• Variations are produced by the mechanisms
involved in Sexual Reproduction
Chromosome Numbers
• All organisms have a specific number of
chromosomes
– Homologous pairs: In organisms that reproduce
sexually, those chromosomes occur in pairs, one
member of each pair originating with each parent
– Examples:
• Humans have 46 chromosomes, 23 homologous pairs
• Fruit flies have 8 chromosomes, 4 homologous pairs
Homologous chromosomes
•Each pair of homologous
chromosomes has genes for
the same traits.
eye color
(brown?)
Homologous Chromosomes
eye color
(blue?)
homologous
chromosomes
double stranded
homologous chromosomes
Sister chromatids
karyotype
• A karyotype is a photographic inventory of an
individual’s chromosomes
• Human female karyotype- 46 total, 23 pairs
• Notice the chromosomes in this picture aren’t
duplicated
Human female karyotype
46 chromosomes
23 pairs
XX
Human male karyotype
46 chromosomes
23 pairs
XY
Cell Types
• Organisms have 2 main cell types:
– Somatic cells: cells found in normal body tissues that
aren’t associated with reproduction
• These cells have the full number of chromosomes
– Germ Cells: cells that produce cells that function in
sexual reproduction
• These cells produce gametes: sperm (or pollen) and egg
• Gametes don’t have the full number of chromosomes of an
organism
• Why?
How about the rest of us?
• What if a complex multicellular organism (like
us) wants to reproduce?
– joining of egg + sperm
• Do we make egg & sperm by mitosis?
What if we did, then….
46
egg
+
46
92
sperm
zygote
Doesn’t work!
Some cells have different numbers of
chromosomes
• Normal body cells are Diploid- they have the full
amount of chromosomes abbreviated 2n
• But all gametes (sperm and egg cells) are
Haploid- they have half the normal amount of
genetic information abbreviated 1n
– One copy of each chromosome
– This is so that when the gametes combine the
offspring has the correct amount of genetic
information
How do we make sperm & eggs?
• Must reduce 46 chromosomes  23
– Sperm and eggs must be haploid: having half the
number of chromosomes of the organism
23
46
meiosis
zygote
23
egg
46
23
46
23
sperm
gametes
fertilization
Stages of Meiosis
The Phases of Meiosis
Interphase
•the cell replicates its
chromosomes.
•After replication, each
chromosome consists of
two identical sister
chromatids, held
together by a
centromere.
Interphase
The Phases of Meiosis
Prophase I
•The chromosomes coil
up and a spindle forms.
•As the chromosomes
coil, homologous
chromosomes line up
with each other gene by
gene along their length,
to form a four-part
structure called a tetrad.
Prophase I
The Phases of Meiosis
Prophase I
•The chromatids in a tetrad
pair tightly
•In fact, they pair so tightly
that non-sister chromatids
from homologous
chromosomes can actually
break and exchange genetic
material in a process known
as crossing over.
Prophase I
The Phases of Meiosis
Metaphase I
•During metaphase I, the
centromere of each
chromosome becomes
attached to a spindle
fiber.
•The spindle fibers pull
the tetrads (homologous
pairs) into the middle
Metaphase I
The Phases of Meiosis
Prophase I + Metaphase 1
•Crossing over can occur at any location on a chromosome,
and it can occur at several locations at the same time.
•Genetic recombination results from crossing
over during prophase I and metaphase 1 of meiosis
–This increases variation further
The Phases of Meiosis
Anaphase I
Anaphase I
•begins as homologous
chromosomes, each with its
two chromatids, separate and
move to opposite ends of the
cell.
•This critical step ensures
that each new cell will
receive only one
chromosome from each
homologous pair.
*Random assortment*
The Phases of Meiosis
Telophase I
Telophase I
•Nucleus reforms,
chromosomes may or may
not unwind
•The spindle is broken
down and the cytoplasm
divides to yield two new
cells
•Cells are now HAPLOIDhalf the amount of genetic
information
Meiosis 1 overview
• 1st division of meiosis
Copy DNA during
interphase
 4 chromosomes
 diploid
 2n
prophase 1
Divide 1
Line Up in
tetrads
metaphase 1
telophase 1
 2 chromosomes
 haploid
 1n
The Phases of Meiosis
Prophase II
Meiosis II
•The second division in
meiosis is simply a mitotic
division of the products of
meiosis I.
•During prophase II, a
spindle forms in each of the
two new cells and the
spindle fibers attach to the
chromosomes.
•Nucleus dissolves
The Phases of Meiosis
Metaphase II
Metaphase II
•The chromosomes, still
made up of sister
chromatids, are pulled to
the center of each cell
and line up randomly at
the equator.
The Phases of Meiosis
•Anaphase II
• the centromere of each
chromosome splits,
allowing the sister
chromatids to separate
and move to opposite
poles of each cell.
Anaphase II
The Phases of Meiosis
Telophase II
Telophase II
• nuclei reform, the spindles
break down, and the
cytoplasm divides.
•At the end of meiosis II,
four haploid cells have been
formed from one diploid
cell.
•These haploid cells will
become gametes,
transmitting the genes they
contain to offspring.
Meiosis 2 overview
telophase 2
telophase 1
Line Up 2
• 2nd division of meiosis
– looks like mitosis
 2 chromosomes
 haploid
 1n
metaphase 2
gametes
The Phases of Meiosis
•At the end of meiosis II, four haploid cells have been
formed from one diploid cell.
•These haploid cells will become gametes,
transmitting the genes they contain to offspring.
Gamete Formation
• Formation of Gametes (egg and sperm- sexual
reproductive cells)
– Meiosis in males occurs in the testes and produces 4
sperm cells. The process is called spermatogenesis.
– Meiosis in females occurs in the ovaries and produces 1
egg and 3 polar bodies, which disintegrate. The process is
called oogenesis.
• Variation: Meiosis allows for crossover and genetic
recombination (gene mixing), which creates the
differences between parents and offspring.
Meiosis & mitosis
• Meiosis to make
gametes
– sperm & egg
• Mitosis to make
copies of cells
– growth
– repair
– development
Meiosis 1 (diploid -> haploid)
Meiosis 2
(haploid 2 cells -> haploid 4 cells)
Comparison of Mitosis & Meiosis
• MITOSIS
MEIOSIS
Key Features of Meiosis
• Genetic Recombination- a set of processes
that produce genetic variation during sexual
reproduction (producing variation is the whole
point of sexual reproduction!!)
– Random Assortment- during anaphase 1,
homologous chromosomes move randomly to
opposite ends
• not all of the chromosomes from one parent go in one
direction
Key Features cont..
– Crossing Over- During prophase 1 and metaphase
1, sections from homologous chromosomes can
be exchanged
• genes from one parent can be exchanged with genes
from the other parent
– Fertilization- combination of gametes from the
two parents leading to the formation of a zygote
(fertilized egg)
• This combines genetic information from two individuals
Crossing Over
Random Assortment
Fertilization