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Meiosis and Sexual Reproduction
No More Clones!
Isn’t Love Grand?
Objectives
• Understand the meaning of “SEX”
• Understand the process of meiosis
• Understand the need to undergo meiosis as
sexual organisms – to produce change or
variation
Need For Sex = Need for Change
• Sex: results in the new
genetic combinations.
• Babies look different
than parents
• New babies = new gene
combinations
• Meiosis is key to
enhancing this variation
Sex Terms to Know
• Diploid: having 2 sets of each chromosome type
– You are a Diploid or 2N critter!
• Haploid: having 1 set of chromosome type
– Egg / Sperm are Haploid
• Homologous chromosomes:
– Daddy chromosome and Mummy chromosome
– Each is a copy. You send either mummy or daddy
chromosome into your sex cell (egg or sperm)
How they Match Up:
• Mitosis
– produces 2 daughter cells
– daughters are clones of
the parent
– daughter cells are diploid
– growth, replacement,
repair
• Meiosis
– produces up to 4 daughter
cells
– daughters are distinct from
parents
– daughter cells are haploid
– generation of gametes (sex
cells): egg & sperm
Meiosis (the process)
•
•
•
Meiosis: nuclear division with a
reduction in chromosome
number by half
Goal: make 4 sex cells
Process:
1. Double all DNA
Chromosomes doubled
2. First Division
•
2N  1N
3. Second Division
•
1N 1N
Meiosis Overview
Essentially, Double,
then Divide Twice
then
Package 1N (set) of
Chromosomes
into Egg or Sperm
Cell
Genetic Variation due to Eggs and
Sperm Happening to Meet and Fall
in Love 
• Variation in the DNA of a
zygote
• Random fertilization:
8+ million possible
combinations of sperm
and egg =
70,368,744,000,000
possible combinations
Variation #2 -- Choosing Mommy or Daddy
Chromosome for Egg or Sperm Cell
Variation #3 Cross Over
-- Mummy and Daddy
Chromosomes Hold Hands
and Swap Genes
• New chromosomes end up
in the egg or sperm cells
(4,951,760,200,000,000,000,000,000,000
different zygotes with a single crossover)