Cell division occurs through mitosis and meiosis. Mitosis produces two identical daughter cells from one parent cell during growth and repair. Meiosis reduces the chromosome number by half and produces genetic variation through independent assortment and crossing over during gamete formation for sexual reproduction. The cell cycle is tightly regulated and consists of interphase, mitosis, and cytokinesis. Errors in meiosis can result in genetic disorders.

1. Chapter 11
Cell Division

2. Terminology
• Nucleic Acid
• Gene
• Chromatin
– Histones
– Nucleosomes
– Linker DNA
• Chromosome

3. Terminology
• Plasmid
• Condensed
Chromosome
• Centromere
• Telomere
• Sister Chromatids

4. Cell Division
The life cycle of an organism is linked to
cell division.
Unicellular organisms use cell division
primarily for reproduction.
In multicellular organisms, cell division
is also important in growth and repair
of tissues.

5. Cell Division
Reproductive signal :To initiate cell
division
Replication
Segregation :Distribution of the DNA into
the two new cells
Cytokinesis: Separation of the two new
cells

6. Prokaryotic Cell Division
Binary Fission
External factors such as nutrient
concentration and environmental
conditions are the reproductive signals
that initiate cell division.
For many bacteria, abundant food
supplies speed up the division cycle.

7. Binary Fission
Plasmid

8. Eukaryotic Cell Division
Highly Controlled/ Regulated
Two Types
Mitosis separates them into two new nuclei, identical
to the parent cell.
Meiosis is nuclear division in cells involved in sexual
reproduction.
The cells resulting from meiosis are not identical
to the parent cells.

9. Cell Cycle
The period between
cell divisions,
divided into
mitosis/cytokinesis
and interphase.

10. Interphase
The cell nucleus is visible and cell
functions including replication occur.
Begins after cytokinesis and ends when
mitosis starts.
Has three subphases: G1, S, and G2

11. Interphase Subphases
G1: Gap 1
Between end of cytokinesis and onset of S phase;
chromosomes are single, unreplicated structures
(Normal Cell Function)
S Phase
DNA replicates; one chromosome becomes two sister
chromatids
G2: Gap 2
End of S phase, cell prepares for mitosis
M phase
Mitosis and cytokinesis occur during this phase

12. Centrosomes
• Determines the plane of cell division
• It doubles during S phase and will determine the
spindle orientation
• Each centrosome can consist of two centrioles
• The centrosomes move to opposite ends of the
nuclear envelope during G2-to-M transition.
• Orientation determines the plane at which the cell
will divide and the spatial relationship of the two
new cells.
• Plants don’t have centrosomes (but have something
similar)

13. Mitosis
Can be divided into phases:
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

14. Prophase
Cohesin disappears except at
the centromere; chromatids
become visible.
Kinetochores, for movement,
develop in the centromere
regions.
Centrosomes serve as mitotic
centers or poles;
Microtubules form between the
poles to make the spindle.

15. Prometaphase
The nuclear envelope breaks down.
Chromosomes consisting of two chromatids attach to
the kinetochore mictotubules.

16. Metaphase
• Chromosomes line
up at the midline of
the cell.

17. Anaphase
• The separation of
sister chromatids
• Cohesin is
hydrolyzed by
separase.
• Critical check point

18. Telophase
Occurs after
chromosomes have
separated:
•Spindle breaks down
•Chromosomes uncoil
•Nuclear envelope and
nucleoli appear
•Two daughter nuclei are
formed with identical
genetic information

19. Cytokinesis
Division of the
cytoplasm differs in
plant and animals.
Animals:
Contractile Ring

20. Cytokinesis
Plants:
Cell Plate

21. Types of Reproduction
Asexual
- Mitosis
The offspring are
clones (genetically
identical to the
parent).

22. Types of Reproduction
Sexual
The offspring are not identical to the parents.
It requires gametes created by meiosis; two parents
each contribute one gamete to an offspring.
Gametes (and offspring) differ genetically from each
other and from the parents.

23. More Terminology
Somatic cells: body cells not specialized for
reproduction.
Homologous pairs of chromosomes: Each parent
contributes one homolog.
Genome: all the genetic information in a cell
Diploid (2n): Contains two sets of each chromosome
Haploid (n): Contains one set of each chromosome
Fertilization: Two haploid gametes (female egg and
male sperm) fuse to form a diploid zygote

24. Karyotype
The number, shapes, and
sizes of the metaphase
chromosomes in a cell.
Individual chromosomes
can be recognized by
length, position of
centromere, and
banding patterns.
Cytogenetics uses
karyotypes to aid in the
diagnosis of certain
diseases.

25. Meiosis
Consists of two nuclear divisions (1 and
2) but DNA is replicated only once.
The function of meiosis is to:
• Reduce the chromosome number from
diploid to haploid
• Ensure that each haploid has a
complete set of chromosomes
• Generate diversity among the products

26. Interphase
Prior to Meiosis the cell must commit to
sell division moving from the G1 phase
to the S phase
During S phase the DNA is Replicated
During G2 phase the cell prepares for
Meiosis

27. Prophase 1
Very Long Phase
The homologous
chromosomes pair:
synapsis.
The four chromatids of
each homologous pair
form a tetrad.
Chiasmata form between
non-sister chromatids.
Crossing Over can occur

28. Crossing Over
Source of Genetic
Diversity
Form Recombinant
Chromatids

29. Prophase/Prometaphase 1
• Nuclear Envelope Breaks Down
• Spindle Forms
• Centrosomes move to the poles
• Cohesin is limited to the centromeres

30. Metaphase 1
Homologous Paris line
up down the
Metaphase Plate
Independent assortment
(how the homologous
pairs line up) during
Metaphase/Anaphase I
also allows for chance
combinations.
“More Genetic Diversity”

31. Anaphase 1
Homologous
chromosomes
separate

32. Telophase 1
Occurs in some
organisms
Nuclear envelope
reaggregates,
followed by an
interphase called
interkinesis.
In other organisms,
meiosis II begins
immediately.

33. Meiosis II
Similar to Mitosis
Differences:
1) DNA does not replicate
before meiosis II
2) In meiosis II the sister
chromatids may not be
identical because of
crossing over
3) The number of
chromosomes at the
equatorial plate in meiosis II
is half the number of those
in mitosis

34. Mitosis vs. Meiosis

35. Meiotic Errors
Nondisjunction:
Homologous pairs fail to
separate at anaphase I;
sister chromatids fail to
separate, or
homologous
chromosomes may not
remain together

36. Cell Death
Necrosis Apoptosis
Cell is damaged or is genetically
starved for oxygen programmed cell
or nutrients. The cell death.
swells and bursts