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
• 1st stage – mitosis (division of the nucleus)
• 2nd stage – 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:
– Cell grows
– Prepares for division (replicates DNA)
– Divides into two daughter cells
– 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 New cells contain half the
(diploid) of chromosomes number of chromosomes
(haploid)
DNA replication once during DNA replication once during
interphase interphase before Meiosis 1
One nuclear division Two nuclear divisions
No crossing over – no genetic Crossing over during prophase
variation 1 to ensure genetic variation
Sister chromatids pulled to Homologous chromosomes
opposite poles during move to opposite poles during
anaphase resulting in 2 anaphase 1. Centromeres hold
identical cells. The centromere sister chromatids together
splits ensuring that identical resulting in non-identical cells
chromatids are pulled to the
opposite poles
Occurs in all types of body Occurs only in sex cells
cells (egg & sperm)
Purpose: cell growth and Purpose: produce sex cells
repair