This document summarizes the process of meiosis. It describes meiosis as producing haploid gametes through one round of DNA replication followed by two cell divisions, resulting in four genetically unique cells. It explains that meiosis increases genetic variation through the processes of crossing over and independent assortment during meiosis I. Finally, it notes that meiosis occurs in both males and females but results in four sperm cells in males and one egg plus three polar bodies in females.

1. 6a: Meiosis
● Describe cells during the cell cycle as either
haploid or diploid
● Describe the stages of meiosis
● Identify male and female gametes
● Describe the process of crossing over and
independent assortment as means of
creating genetic diversity

2. Some background...
● You are a multi cellular organism that
started as a single cell... how?
● How many chromosomes did your first cell
have?
● How was that first cell made?
● How many cells must those cells have had?
● What process made those cells?

3. Some background...
● You are a multi cellular organism that started as a single
cell... how?
– Mitosis!
● How many chromosomes did your first cell have?
– 46
● How was that first cell made?
– Sperm fertilized an egg
● How many cells must those cells have had?
– Half of 46 = 23
● What process made those cells?
– Meiosis!

4. Vocabulary
● Gamete: A sex cell (sperm for male & egg in
females)
● Fertilization: The process in which an egg
cell fuses with a sperm cell and male and
female genetic material combines
● Zygote: A fertilized egg. Contains two sets of
chromosomes and will divide by mitosis to
develop into an embryo

5. Chromosomes
● A somatic (body) cell contains two sets of
chromosomes. One from mom and one from dad.
● These chromosomes exist in pairs called
homologous chromosomes.
● Homologous Chromosomes: Contain genes for
the same traits in the same locations, although
they may contain different variations of those
genes.
– (eg. Both contain eye colour gene, one for blue and the
other for brown.)

6. Homologous Chromosomes

7. Chromosome Number
● Diploid:
– Cell containing two sets of chromosomes
– Represented as 2n (in humans 2n = 46)
– All body cells are diploid
● Haploid:
– Cell has only one set of chromosomes
– Contains only 1 chromosome from each homologous pair.
● eg. Sex chromosomes (1 X or 1 Y)
– Represented as n (in humans n = 23)
– Only gametes are haploid

8. Meiosis
● Process that produces gametes (egg or
sperm) with half the number of
chromosomes (haploid)
● In meiosis there is one round of DNA
replication (during interphase) and two cell
divisions
● Produces 4 genetically unique haploid cells

9. Life Cycle
(mitosis and meiosis)

10. Meiosis
A: DNA replication
B: Meiosis I
C: Meiosis II

11. Stages of Meiosis
● Meiosis is divided into two parts each with
prophase, metaphase, anaphase, telophase
● Meiosis I:
– Pairs of homologous chromosomes line up at
center and separate (different from mitosis where
sister chromatids separate )
– Produces two daughter cells that are haploid
● Meiosis II:
– Sister chromatids are separated (like mitosis)
– Produces four daughter cells that are haploid

12. Meiosis I

13. Meiosis II

14. Result of Meiosis
● Meiosis produces four genetically unique
daughter cells
● Each cell contains one chromatid (daughter
chromosome) from each homologous pair
● Cells are haploid (humans = 23
chromosomes)

15. Genetic Variation in Meiosis
● Every gamete is unique from all other
gametes produced because of two
processes that occur during meiosis I
1. Crossing Over
2. Independent Assortment

16. Crossing Over
● Occurs between each homologous pair
● Pairs of non-sister homologous chromatids
“cross over” and exchange DNA
● Multiple cross overs can occur leading to
infinite possibilities

17. Independent Assortment
● Homologous chromosomes line up
randomly at metaphase I, so the mixture of
chromosomes that end up in each daughter
cell after separation varies

18. Gamete Formation
● Meiosis is the same in male and females, after
meiosis further development occur to specialize
a cell as either egg or sperm
● Male: Meiosis occurs in testes and produces 4
sperm cells
– Produced from puberty on (millions every day)
● Female: Meiosis occurs in ovaries and produces
1 egg and 3 smaller polar body cells
– Produced before birth (about 1-2 million eggs
present at birth)

19. Gamete Formation