Sexual Reproduction and
Meiosis
Reproduction
• Organisms can reproduce asexually mitosis, fission, & budding
• Organisms can also reproduce sexually
– Two sex cells (usually egg and sperm)
join together
– Males produce sperm, females produce
eggs
• Sex cells do not have pairs of chromosomes
• They are called haploid (single- not paired
up: sometimes represented as the letter N).
• They only have half the number of
chromosomes as body cells.
• 23 total chromosomes
• All other body cells are diploid (paired up:
2N).
– Humans have 23 pairs of chromosomes
– 46 chromosomes total
• When an egg and sperm join together, it is
called fertilization.
• The resulting cell is called a zygote
• The zygote will begin to undergo cell
division
http://www.exploratorium.edu/imagingstation/gallery.php?Asset=Zebrafish%20development&Group=&Cat
egory=Development&Section=Introduction
• The zygote formed following fertilization
has a diploid set of chromosomes (46
total).
• Half (23) from mom, half (23) from dad.
• Sexual reproduction occurs when the
genes of two parents are combined to
form an offspring.
• The offspring is a unique genetic
combination of its two parents.
So how do we get specialized sex
cells with only half the
chromosomes?
Meiosis
• Meiosis is a process that produces haploid
sex cells.
Why does meiosis happen?
• Meiosis allows for the production of sex
cells so sexual reproduction can occur.
• In meiosis I, the
nucleus divides
and produces
two new cells
with one
duplicated
chromosome
• Exactly like
mitosis.
• In meiosis II, the
division happens
again and the
sister chromatids
separate,
producing four
cells with half
(haploid) the
number of
chromosomes of
the original
nucleus.
• The end result of
meiosis:
– A cell undergoes
division twice.
– This produces four
sex cells, with half
the number
(haploid) of
chromosomes of
the original cell
Mitosis vs. Meiosis
https://www.youtube.com/watch?v=Ba9LXKH2ztU
Asexual vs. Sexual reproduction
• Asexually
produced
organisms are
genetically
identical to their
parent.
• Sexually produced organisms are
genetically different from their parent.
– This produces unique organisms who may be
more (or less) resistant to disease, more (or
less) intelligent, more (or less) physical
abilities, adaptable to environment, etc.
– There is variety among the offspring.
– https://www.youtube.com/watch?v=toWK0fIyFlY
Review
• What is it called when a sperm joins with
an egg?
• Fertilization
• After the egg is fertilized by the sperm,
what is the new cell called?
• A zygote
• How many chromosomes are in a human
sex cell?
• 23
• How many chromosomes are in a regular
human body cell (skin, cheek, liver,
muscle, etc)
• 46
• What are the names of the two sex cells
produced by humans?
• Egg and sperm
• How many times do cells divide in
meiosis?
• twice
• How many new cells are produced from a
single cell in meiosis?
• four
• How many cells are produced from a
single cell in mitosis?
• Two
• Are sex cells haploid or diploid?
• Haploid
Name each
item in the
picture:
A
B
A- Chromosome
C
B- Nucleus
D
C- Cell
D- DNA
E
E- Gene
F- Centromere
F
• Why does meiosis happen?
• To produce sex cells
• Why does mitosis happen?
• GRR
– Grow- an organism can get bigger
– Replace- replaces damaged or old cells
– Reproduce- used by some organisms to
reproduce asexually
Name each phase of mitosis