Meiosis:
reduction division that results
in half the number of
chromosomes to make
sex cells (gametes)
What’s a gamete?
 Male = sperm
 consists of the nucleus (in the head), a tail, and
mitochondria for energy
 Female = eggs
 Nucleus, mitochondria, ribosomes and a reserve
supply of food for developing organism
What is ploidy
when referring to cells?
 The number of chromosome sets within a cell
 Each set is designated
n so
 one set is n is haploid,
 two sets is 2n is diploid,
 three sets is 3n is triploid,
 four sets is 4n is tetraploid,
How are gametes different from somatic
cells?
Somatic cells
Gamete cells
 Body cells
 Sex cells
 46 c’somes
 23 c’somes
 Diploid – 2 sets of
 Haploid- only 1 set of
chromosomes
c’somes
Meiosis consists of ONE
replication of c’somes followed by
TWO divisions
 MEIOSIS I
 Homologous chromosomes separate
 This never happens in mitosis
 Results in two daughter cells with the same # of c’somes
as parent
 2n  n
 Crossing over occurs
 MEIOSIS II
 Each of the two daughter cells again separate its
chromosomes (without copying like between 2 events of mitosis)
 Results in 4 daughter cells with ½ the # of c’somes as the
parent cell


In males: all 4 cells become sperm  spermatogenesis
In females: one cell becomes an egg, the other 3 die 
oogenesis
Comparison of Mitosis & Meiosis
Mitosis
Results in 2
daughter cells
Meiosis
Results in 4
daughter cells
Daughter cells are
diploid
Daughter cells are
haploid
Daughter cells are
identical to each
other and to parent
Daughter cells are
different from
each other and
from parent
Lets talk about CROSSING OVER
Happens in Prophase of Meiosis I
 Paired Homologous chromosomes exchange
genes while next to each other

You don’t inherit entire chromosomes from your parents—
only pieces of them
 Produces new combinations of genes
creating more variation and diversity
within a species

Important for natural selection and evolution
Crossovers are completely random!
Determination of Sex
Females: two X chromosomes (XX)
Males: XY
All offspring receive
 an X chromosome from mom
 and either an X or a Y from dad
X chromosome:
 Essential for LIFE
Y chromosome:
 Not essential for
life
 large and
contains
thousands of
genes
 Small and contains
only 23 genes
CHECK YOUR UNDERSTANDING
 What are the male and female gametes?
 How many chromosomes does a gamete have?
 What is the ploidy of a gamete?
 When does crossing over occur? Why is it
important?
 How many sperm cells are formed by meiosis of
one cell? How many egg cells?
 Summarize the differences between mitosis
and meiosis
 What sex chromosome is always passed down
by a mother?
 What 2 sex chromosomes may be passed down
by a father?