Cells divide through the process of cell division in order to replicate DNA and distribute it equally between daughter cells. Cell division involves mitosis, where the nucleus divides, and cytokinesis, where the cytoplasm divides. Mitosis consists of prophase, metaphase, anaphase, and telophase where chromosomes replicate and separate. Cytokinesis then divides the cell into two identical daughter cells each with a full set of chromosomes. Meiosis produces gametes through two rounds of division resulting in four haploid cells with half the number of chromosomes. This ensures genetic variation in sexual reproduction.

1. Why cells divide
• Cells divide because:
– Demands the cell places on its DNA
– Moving enough nutrients and wastes across
the cell membrane
• Cell division – process forming two
“daughter” cell

2. Cell Division
• mitosis (division of the nucleus)
• cytokinesis – division of cytoplasm
• Chromosomes – contain DNA and
proteins
• Before cell division, each chromosome
replicates and consists of two “sister”
chromatids

3. Cont.
• Each pair of chromatids is attached at the
centromere, which will separate during cell
division and each new cell will have
identical copies of the old cell
• Cell cycle: Interphase
– G-1 phase - Cell grows
– S phase - Prepares for division (replicates
DNA)
– G-2 phase – prepares for mitosis (division)
– Cycle begins again

5. Mitosis
• 4 phases:
• Prophase – chromatin condense into
chromosomes, centriole separates and a
spindle begins to form, the nuclear
envelope breaks down
• Metaphase – chromosomes line up
across center of cell, connected to spindle
fiber (helps separate chromosomes) at
centromere

6. Cont.
• Anaphase – mitosis: centromeres that join
the sister chromatids split, chromatids
separate and become individual
chromosomes (two groups)
• Telophase – chromosomes disperse into
tangle of dense material, nuclear envelope
re-forms around chromosome clusters,
spindle breaks apart, nucleolus is visible in
daughter nucleus.

8. Cont.
• Cytokinesis – division of cytoplasm in the
M phase, cell membrane folds into cell
pinching cell into two equal parts
containing nucleus and cytoplasmic
organelles

10. Cell regulating
• Cyclin – (protein) regulates the timing of
the cell cycle in eukaryotic cells
• Regulation – internal (replication) and
external (growth) are controlled by
proteins
• Cancer – uncontrolled cell growth (disease
of the cell cycle)

11. Meiosis
• Homologous – two sets of chromosomes
(set from female, set from male), two
complete sets of chromosomes and two
complete sets of genes
• Diploid – “two sets” cell that contains
homologous chromosomes (2N=8)
• Haploid – “one set” (N=4)

12. Phases of Meiosis
• Meiosis – process of reduction division in
which the number of chromosomes per
cell is cut in half through the separation of
homologous chromosomes in a diploid cell
• Divided into two distinct phases
• Meiosis I and II

13. Meiosis I
• Prophase I – each chromosome pairs with
its corresponding homologous
chromosome to form a tetrad (4
chromatids)
• Metaphase I – spindle fibers attach to the
chromosomes
• Anaphase I – fibers pull the homologous
chromosomes toward opposite ends of the
cells

14. Cont.
• Telophase I and cytokinesis – nuclear
membrane form and the cell separates
into two haploid (N) daughter cells, each
with half the number of chromosomes as
the original cells
• Crossing-over – exchange of chromatids
which results in the exchange of alleles
between homologous chromosomes and
produces new combinations of alleles

16. Meiosis II
• Prophase II - two haploid (N) daughter
cells, each with half the number of
chromosomes as the original cells
• Metaphase II – chromosomes line up in a
similar way to the metaphase stage of
mitosis
• Anaphase – sister chromatids separate
and move toward opposite ends of the cell

17. Cont.
• Telophase and cytokinesis – Meiosis II
results in 4 haploid (N) daughter cells (2
chromosomes each)

19. Gamete Formation
• Males – haploid gamete is the sperm
• Females – only one haploid gamete, egg
• Comparing mitosis and meiosis:
– Mitosis produces 2 genetically identical diploid
(2N) daughter cells: allows the body to grow
and replace cells
– Meiosis produces four genetically different
haploid cells: used in sexual reproduction

20. Mitosis Meiosis
Asexual Reproduction Sexual Reproduction
Produces two identical cells Produces four different cells
New cell contains a full set
(diploid) of chromosomes
New cells contain half the
number of chromosomes
(haploid)
DNA replication once during
interphase
DNA replication once during
interphase before Meiosis 1
One nuclear division Two nuclear divisions
No crossing over – no genetic
variation
Crossing over during prophase
1 to ensure genetic variation
Sister chromatids pulled to
opposite poles during
anaphase resulting in 2
identical cells. The centromere
splits ensuring that identical
chromatids are pulled to the
opposite poles
Homologous chromosomes
move to opposite poles during
anaphase 1. Centromeres hold
sister chromatids together
resulting in non-identical cells
Occurs in all types of body
cells
Occurs only in sex cells
(egg & sperm)
Purpose: cell growth and
repair
Purpose: produce sex cells