Chapter 9
CELLULAR REPRODUCTIONMEIOSIS
Key Terms
Sexual
⦿
Dyad
reproduction
⦿
Synapsis
⦿
Meiosis
⦿
haploid
⦿
Gametes
⦿
Fertilization
⦿
Tetrad
⦿
Just the Basics
⦿
Sexual reproduction- two parents pass
chromosomes to offspring
⦿
Meiosis- nuclear division, daughter cells
receive haploid (n= 23 chromosomes) number
of chromosomes in varied combinations
Human Life Cycle
⦿
All reproductive events occurring from
one generation to the next
⦿
Two types of nuclear division included:
● Mitosis
● Meiosis
Meiosis
⦿
Two major functions
● Reducing chromosome number
● Shuffling chromosomes and genes
○ Gametes- sperm (males), egg (female)
Our Beginnings….
⦿
Fertilization- joining of a single egg and
sperm
● Random
● Allows for more shuffling
Meiosis
⦿
Important because it reduces the number of
chromosomes
● Gametes must contain half as many chromosomes as
the body cells
○ Sperm- 23
○ Egg- 23
Overview of Meiosis
⦿
Two divisions
● Meiosis I
● Meiosis II
⦿
Results in 4 daughter cells
Before Meiosis Begins
⦿
Each chromosome has duplicated
⦿
Composed of two sister chromatids
During Meiosis I
⦿
Homologous (same) chromosomes of each
pair come together
⦿
Synapsis- line up side by side
● Results in a tetrad- association of four chromatids
● Chromosomes of tetrad stay close together
Later in Meiosis I
⦿
Homologous chromosomes of each pair
separate
⦿
One chromosome from each goes to each
daughter cell
● No rule restrictions
Between Meiosis I and II
⦿
NO duplication of chromosomes needed
⦿
Daughter nuclei have half the chromosome
number
● Still duplicated
● Called dyads- each chromosome composed of 2 sister
chromatids
During Meiosis II
⦿
Dyad separates
● Call them daughter chromosomes
⦿
Results in 4 new daughter cells with haploid
number
Phases of Meiosis
⦿
Meiosis I
⦿
Meiosis II
● Prophase I
● Prophase II
● Metaphase I
● Metaphase II
● Anaphase I
● Anaphase II
● Telophase I
● Telophase II
Importance of Meiosis
⦿
Keeps chromosome number constant
● Haploid gametes
⦿
Introduces genetic variation
● Multiple combinations possible in daughter cell
● For example:
○ Humans: (223)2 or 70,368,744,000,000 zygote possibilities
Prophase I
•Tetrads
form
•Nuclear
envelope fragments
Metaphase I
•Tetrads
align at equator
Anaphase I
•Homologous
chromosomes
separate
•Dyads
move to poles
Telophase I
•Two
haploid daughter
cells formed
•Cytokinesis
Prophase II
•Chromosomes
•Nuclear
condense
envelope fragments
Metaphase II
•Dyads
equator
align at spindle
Anaphase II
•Sister
chromatids
separate
•Daughter
chromosomes
move to pole
Telophase II
•Four
haploid daughter
cells formed
•Cytokinesis