1. MEIOSIS

2. Mendel: Forgotten?
Gregor Mendel published his findings on

genetics in 1865 but his work didn’t get much
attention
In the early 1900’s scientists did experiments

similar to what Mendel had done
They searched for papers detailing results

similar to theirs and realized that their
discoveries were not new.
Genes were still a mystery though. Scientists

didn’t know where they were located or how
they passed information from once cell to

3. One Makes Two
Asexual reproduction: only one parent cell is

needed for reproduction
Parent cell goes through mitosis and divides

into two daughter cells
Most single-celled organisms reproduce in this

way as do most of the cells in your body

4. Two Make One
Sexual reproduction: two parent cells join

together to form a new individual
Parent cells are known as sex cells, different

from ordinary body cells
Human body cells have 46 chromosomes or

23 pairs of homologous chromosomes

5. Human sex cells have only 23 chromosomes

Male sex cells are called sperm

Female sex cells are called eggs, or ova

Each sperm and each egg has only one of the

chromosomes from each homologous pair

6. Less is More
When an egg and sperm join to form a new

individual, each parent donates ½ of a
homologous pair of chromosomes
This ensures the offspring will have a normal

number of chromosomes in each body cell

7. Meiosis to the Rescue
Sex cells are made during meiosis

Meiosis produces new cells with half the usual

number of chromosomes
When sex cells are made, the chromosomes

are copied once, and then the nucleus divides
twice.
Resulting sperm and eggs have ½ the number


8. Back at the Lab
Walter Sutton read Mendel’s studies which

showed that the egg and sperm must
contribute the same amount of genetic
information to the offspring
Using some of his own observations, he came

up with the idea that:
Genes are located on chromosomes

9. What was that again?
Mitosis:

P

M

A

T/C


10. Meiosis: First Time Through
Prophase I

Metaphase I

Anaphase I: Chromosomes separate from their

homologous partners and move to opposite
sides of cell
Telophase/Cytokinesis I: Two cells are

formed, each with the same amount of
chromosomes as the parent cell

11. Chromatids vs. Homologous
Chromosomes

12. Meiosis: Going Again
Prophase II: Nuclear membranes dissolve

again, meiotic spindle fibers form
Metaphase II: Spindle fibers attach and line

paired chromatids up at equator
Anaphase II: Chromatids pull apart and move

to opposite poles
Telophase/Cytokinesis II: Four new cells have

formed from original cell, each with half the
number of chromosomes present

13. Differences Between Mitosis and
Meiosis
Mitosis Meiosis
One cell divisision Two cell
divisions
Two daughter cells Four daughter
cells
Daughter cells have Daughter cells have
same number of half the number of
chromosomes as parent chromosomes as parent
cells cells

15. Male or Female?
Sex chromosomes carry genes that determine

whether the offspring is male or female.
In humans, females have two X chromosomes

while males have one X chromosome and one
Y chromosome.
Each egg cell produced by a female contains

one X chromosome. A sperm cell produced by
a male can contain either an X or Y
chromosome.

17. Male or Female?

18. Male or Female?

19. Think/Pair/Share
Explain the difference between sex cells and

sex chromosomes.
If there are 14 chromosomes in pea plant

cells, how many chromosomes are present in
a sex cell of a pea?
How many times do the chromosomes make

copies of themselves during meiosis? How
many times do cells divide during meiosis?