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Meiosis
Making Gametes
Review
• Asexual Reproduction is also called _______________
• Mitosis starts with one parent cell and ends up with
__________________________________________
• Genetically, the daughter cells are ________________ to
the parent cell
• Making an identical copy of a cell is also called
_______________
• What type of cells in YOUR BODY divide by Mitosis?
________________
• The scientific term for Body cells is ______________
• These cells are considered 2n (diploid). What does that
mean? _______________________________________
Review Cont’d
• How many Chromosomes should a normal human have? _____
• If a human skin cell must divide to heal a wound, how many
chromosomes would each of the daughter cells have? _____
• If a human bone cell must divide because a baby is growing,
how many chromosomes would each of the daughter cells
have? _________
• What are two reasons for Mitosis? __________ &
___________
• Which organelle is in control of cell division? ___________
• What happens when the nucleus loses control? __________
Mitosis
• Asexual Cell Reproduction
• Necessary for growth & repair
• Chromosomes in Somatic Cells are in pairs
• 2N (diploid)
• Two new daughter cells are identical copies of
original parent cell
• Clones
Cell Cycle
• Sequence of phases in the
life cycle of the cell
Notice!
Mitosis is only a
SMALL part of
the cell cycle!
In what phase do
cells spend most
of their time?
_____________
Mitosis & Cell Cyle
Sex Cells are Different
• Human beings have 46 chromosomes in every
Somatic (Body) Cell
• This is a complete set
• But, if mom’s egg cell has 46…
• And, if dad’s sperm cell has 46…
• The Zygote (the fertilized egg cell) would have
92!
• Would THAT make a human?
Meiosis
•Cell division of reproductive cells
(sex cells)
• Gametes or germ cells (sperm and egg)
• Produced in Gonads
•Each new cell is not exactly like the
parent cell
• Allows for variation
• Important for the survival of our
species!
Meiosis
• In each new daughter cell, the
chromosomes are not in pairs
• NOT 2n (not diploid!)
• At the end of Meiosis, four daughter
cells contain half of the original number
of chromosomes (N)
• Each is unique!
• Not identical to the parent
Gametes (aka germ cell)
•Mature egg or sperm cell
• Produced in the Gonads
•Each gamete has half the original
number of chromosomes
•1N (haploid)
•HALF IS IMPORTANT!
Why is Half is Important?
• Meiosis makes unique sex cells!
• Half the number of chromosomes (haploid) is
important for sexually reproducing organisms!
• N = The number of chromosomes in a gamete
• To create a new organism, two parents give half of
their own chromosomes to make a baby with a
complete set of chromosomes!
• Half from mom
• Half from dad
• N + N = 2N (Zygote!)
Sexual Reproduction
• Sexual Reproduction requires two parents
• Offspring have SOME traits inherited
from each parent… but not all!
• YOUR DNA is not identical to either
parent
• It is a mix of genes from both parents
Meiosis
Sexual Reproduction
• Fusion of two (N) gametes to produce a
single zygote (2N)
• Introduces greater genetic variation,
allows genetic recombination
• A VERY GOOD THING!
Chromosome Pairing
•Homologous pair
• Homologous means similar
• Each chromosome in the pair are
Homologous to the other (carry genes
for same trait)
• Only one pair differs - sex
chromosomes X or Y
Different!
Metaphase I in Meiosis…
Chromosomes form
TETRADS (four together)
Metaphase II in Meiosis…
Chromosomes line up in the
middle and then a SECOND
DIVISION happens!
The two daughter cells are
divided into four and each
has HALF the chromosome
number!
Independent assortment of homologous chromosomes
POSSIBILITY 1
POSSIBILITY 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Figure 8.16
Combination 2
Combination 3
Combination 4
Crossing over increases genetic variability
• The exchange of genes between two homologous
chromosomes happens when in Tetrad formation
Tetrad
Chaisma
Centromere
Figure 8.18A
SPERMATOGENESIS
OOGENESIS
spermatogonium
oogonium
egg
spermatids
polar bodies
(will disintegrate)
Review Mitosis & Meiosis
•Both are forms of cell division
•Both involve replication
•Both involve disappearance of the
nucleus, and nucleolus, nuclear
membrane
•Both involve formation of spindle
fibers