Sexual
Reproduction
MEIOSIS
MEIOSIS
•The production of sex cells (gametes)
•Male gamete (sperm), Female gamete
(egg/ovum)
•2 Stage cell division in which the
chromosome number is reduced by one
half
MEIOSIS
•A human cell normally contains 46
chromosomes. This number is referred to as the
diploid chromosome number (2n)
•When the same human cell
undergoes meiosis, it produces
sex cells (gametes) that have
23 chromosomes
•This number is referred to as
haploid (half) and is given the
symbol “n”
23
chromosomes
male gamete
(sperm)
23
chromosomes
female gamete
(egg)
46 chromosome zygote
To summarize:
Meiosis
Mitosis
2n
2n
46
2n
46
n
46
n
23
46
23
23
23
23
23
MEIOSIS
Involves 2 cell divisions that
produce 4 haploid cells
Meiosis I = “reduction division” and
“genetic recombination”
Meiosis II = separation of 2
chromatids (maintains haploid
number)
Stages of Meiosis
Meiosis I
Prophase I
tetrad
•Nuclear membrane dissolves
•Centrioles move to opposite poles
•Spindle fibres appear
•Chromosomes come together as
homologous chromosomes (4 chromatids
called a tetrad)
•Aligning of homologous chromosomes is
called synapsis
•Chromatids often intertwine and exchange
segments (crossing over a.k.a. genetic
recombination)  Causes diversity
Stages of Meiosis
Meiosis I
Metaphase I
•Homologous pairs
attach to the spindle
fibres and line up at the
equatorial plate (middle
of cell)
Stages of Meiosis
Meiosis I
Anaphase I
•Homologous
chromosomes move to
opposite poles
•Each side gets only 23
chromosomes
Stages of Meiosis
Meiosis I
Telophase I
•Cytoplasm divides forming two
cells
•Each new cell is now haploid
•Nuclear membrane reappears
Stages of Meiosis
Meiosis II - similar to Mitosis..only without DNA
replication
Prophase II
•Nuclear membrane
dissolves
•Spindle fibres begin to
form
Stages of Meiosis
Meiosis II - similar to Mitosis..only without DNA
replication
Metaphase II
•Chromosomes line up along
the equatorial plate
Stages of Meiosis
Meiosis II - similar to Mitosis..only without DNA
replication
Anaphase II
•Chromatids move to opposite
poles
Stages of Meiosis
Meiosis II - similar to Mitosis..only without DNA
replication
Telophase II
•Cytoplasm divides and nuclear
membrane appears
•Meiotic division is complete
with four daughter cells each
with 23 chromosomes
Meiosis is important because...
It allows for sexual reproduction
It allows for greater range of
characteristics
It ensures (via sexual reproduction)
the recombination of genes
Summary: Genetics
Videos
Genetics 101 Part 1, Part 3
Homework
Complete Mitosis/Meiosis worksheet
Read pages 169-172
#7, 8, 10-12 (page 172)