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MITOSIS & MEIOSIS
Bio Review
MITOSIS
Part 1
THE CELL CYCLE AND MITOSIS
Due to the loss and death of cells  must replace them.
How many cells in your body?
 50-100 million trillion
 Every minute your body produces about 300 million new cells
Three stages in the cell cycle:
1. Interphase: cell carries out normal functions.
2. Mitosis: nucleus contents duplicated and divided into
two equal parts.
3. Cytokinesis: separation of two nuclei and cell contents
into two daughter cells.
PARTS OF THE CELL CYCLE
INTERPHASE
Interphase - the longest cell cycle stage (lasts 15 hrs. – months).

cell performs normal functions and grows.
In late interphase, DNA copies itself in the process of
replication.
Replication involves several steps:
1. The DNA molecule unwinds with the help of an enzyme.
2. New bases pair with the bases on the original DNA.
3. Two new identical DNA molecules are produced.
 Now, double the amount of DNA in nucleus it divides (Mitosis)
DNA UNRAVELING (VIDEO)
LATE INTERPHASE CONT’D.
Chromatin is in its loosely coiled
form so that DNA can be copied
into RNA for proteins to be made in
preparation for cell division.
At the end of interphase, the cell
continues to grow and make
proteins in preparation for mitosis
and cytokinesis.
CHROMOSOMES
As the nucleus prepares to divide,
replicated DNA in interphase joins to form
sister chromatids, joined by a centromere.
MITOSIS OVERVIEW:
Mitosis is the shortest stage of the cell cycle where the
nuclear contents divide, and two daughter nuclei are
formed.
It occurs in 4 stages:
1.
2.
3.
4.

Prophase
Metaphase
Anaphase
Telophase
Helpful saying to remember the order:
“I picked many apples today.”
MITOSIS - PROPHASE
Chromosomes start to coil and
become visible.
Pairs of centrioles start to
separate.
The nuclear membrane
disappears.
Spindle fibers start to form
between the centriole pairs.
Chromosomes move more
evenly throughout the nucleus.
MITOSIS - METAPHASE
Centriole pairs move to
opposite ends of the cell.
Spindle fibers are still
attached to the centriole
pairs.
Chromosomes line up along
the midline of the cell and
are attached to the spindle
fibers.
MITOSIS - ANAPHASE
The pair of chromatids split at
the centromere and move to
opposite ends of the spindle.
Now there are twice the number
of chromosomes within the cell
membrane.
Movement of the chromosomes
towards the opposite ends of
the cell membrane is aided by
the spindle fibers.
MITOSIS - TELOPHASE
Nuclear membranes form
around the two new sets of
chromosomes.
The spindle fiber disappears.
Chromosomes start to uncoil
(chromatin) and become less
visible.
Cell starts to make a groove
(furrow) in the middle to
eventually split into two
identical cells.
CYTOKINESIS
The division of material outside
of the nucleus.
 Occurs after telophase.
Divides the organelles and other
substances in the cytoplasm into
roughly two equal halves.
Animal cells furrow while plant
cells form a cell plate
# chromosomes in daughter cell
= the # chromosomes in parent
cell.
Daughter cells are genetically
identical to parent.
Video on Mitosis
MEIOSIS
Part 2
OBJECTIVES
By the end of the lesson you should be able to:
Describe the process of meiosis
Compare and contrast meiosis and mitosis
Explain why meiosis is needed
MEIOSIS
Sexual reproduction – 2 parents – genetic differences
The process of Meiosis results in the production of special cells called
gametes
Gametes have half the number of chromosomes as body cells
MEIOSIS
Cell division occurs twice in Meiosis
 Once at the end of Meiosis I and again at the end of Meiosis II
In Meiosis I, matching chromosomes are called homologous chromosomes
separate
In Meiosis II, sister chromatids separate
MEIOSIS
The process of meiosis shuffles genetic information and results in
variation of the gametes
Genetic diversity
MEIOSIS
AKA: Germ cell division
Purpose: to produce
gametes (i.e. sex cells:
sperm and egg)
Products: 4 haploid cells
each genetically different
 this creates genetic
variability
TERMS:
Haploid: half the total number of chromosomes found in gametes (in
humans = 23)
Diploid: total number of chromosomes found in body cells (in humans =
46)
FERTILIZATION
2 gametes come together; in humans an egg and sperm come
together
Each has 23 chromosomes, thus together there is 46 in the zygote
23
+
46
23
Gametes: why is the
sperm smaller than the
egg?
Zygote: will grow (by
mitosis) into a baby in
~9 months
MEIOSIS: REDUCING CHROMOSOME NUMBER
NUMBER OF CHROMOSOMES SEQUENCE
46
Parent Cell
92
46
46
23
23
23
All genetically different
23
Daughter
Cells
CHROMOSOME SEQUENCE
Meiosis Video
Meiosis 2 Video
MEIOSIS – PROPHASE I
Chromosomes coil and
become visible.
Nuclear membrane
disappears.
Spindle fibers form.
Homologous
chromosomes move
towards each other and
pair.
MEIOSIS – METAPHASE I
Pairs of homologous
chromosomes move to the
midline of the cell.
These pairs are aligned
on either side of the
metaphase plate.
MEIOSIS – ANAPHASE I
Homologous chromosome
pairs are pulled away
from each other towards
opposite ends of the cell.
Paired chromatids
remain attached.
MEIOSIS - TELOPHASE I
Paired chromatids may
uncoil and become less
visible.
Nuclear membranes may
form around each
chromosome cluster.
Spindle fibers disappear.
Cytokinesis occurs.
MEIOSIS II
Phase II of meiosis is the mitotic division of haploid cells.
Although there will be four new non-identical cells at the end of
meiosis II, all other processes for this division are similar to mitosis.
MEIOSIS II – PROPHASE II
Paired chromatids coil.
Spindle fibers form.
Nuclear membrane
disappears.
MEIOSIS II – METAPHASE II
Paired chromatids line
up in the middle of the
cell.
The chromatids are
attached to spindle
fibers.
MEIOSIS II – ANAPHASE II
Each pair of chromatids
split at the centromere to
form two independent
chromosomes.
Chromosomes move to
opposite ends of the cell.
MEIOSIS II – TELOPHASE II
Chromosomes uncoil and
become less visible.
Nuclear membranes from
around four new nuclei.
Spindle fibers disappear.
Cytokinesis occurs.
Meiosis Square Dance
MEIOSIS - CYTOKINESIS
Same process as in mitosis.
Four non-identical cells are
formed.
These cells are called
haploid cells (n).
# chromosomes = half the
# of chromosomes of
parent cell.
MEIOSIS EVENTS
Meiosis I
Matching chromosome pairs
(homologous chromosomes)
move to opposite poles of the cell two daughter cells result.
Meiosis II
Chromatids of each chromosome
are pulled apart - the end result is
four haploid cells, each with half
the number of chromosomes. These
develop into gametes.
CROSSING OVER
Important event that occurs between each chromosome pair in meiosis I
There is an exchange of segments of DNA and picks up new genetic
information
INDEPENDENT ASSORTMENT
This occurs when homologous pairs of chromosomes separate at the
equator and move towards opposite poles of the cell.
Video
Mitosis
vs Meiosis
FERTILIZATION
There are more than 8 million combinations possible for these 23 pairs
in any egg or sperm cell
When fertilization occurs, 70 trillion different zygotes are possible
from the combination of one sperm cell and one egg cell!
GAMETE FORMATION
Males produce 4 sperm for every round of meiosis
Females produce one egg for each round of meiosis: 3 of the 4 “eggs”
produced die off - called polar bodies
All the cytoplasm, nutrients and energy gets put into one egg