Chapter 10
Cell Growth and Division
Why are cells not larger?
Cells are small because:
1. DNA “overload”
• The larger a cell become, the greater the
demand on its DNA; greater need of the
resource of DNA
2. Diffusion and osmosis are slow
• Rate of exchange depend on surface area
3. Surface area can’t keep up with volume
• As cell increases in size, volume increases faster
than the surface area
•
Surface area/Volume ratio
How do we overcome this limit to
cell size?
If we can’t grow by having larger cells, then we
must have more cells
 Cell division is the process by a cell divides into
two new daughter cells
 The cell begins by copying its DNA, each
daughter cell gets one copy

Cell Division
Cell division in prokaryotes is simpler and
occurs by binary fission
◦ After DNA replication, cell contents are
separated into two parts
 In eukaryotes cell division occurs by:
Mitosis – division of the nucleus
Cytokinesis – division of the cytoplasm


Some organisms (especially unicellular)
reproduce by mitosis and cytokinesis (asexual)
Cell division
The Forms of DNA
DNA is passed down from one generation to
the next in the form of Chromosomes
(during anaphase and telophase) - precisely
compacted chromatin
 Loose chromatin (during interphase) threads of DNA & some protein
 Chromatids (during prophase and metaphase)
– ½ of a duplicated chromosome
◦ Sister chromatids separate from each other
when the cell divides

Chromosome
Cell Cycle


1. Interphase – when the cell is not dividing, but
doing its duty, and perhaps, preparing for division.
2. Mitosis:
·
Prophase – centrioles separate, spindle
forms, paired chromatids, nuclear envelope is
breaking apart.
·
Metaphase – Sister chromatids align on the
equator.
·
Anaphase – Sister chromatids separate and
chromosomes move apart.
·
Telophase – Chromosomes concentrate at
ends and nuclear envelopes reform.
Cell Cycle
Cytokinesis
3. Cytokinesis is the division of the cytoplasm
◦ Typically occurs at the same time as telophase
◦ Animal cells: cell membrane pinches off
◦ Plant cells: cell plate forms midway, gradually
developing into a separating membrane
Cytokinesis
Cell Cycle
Cell Division Control

Cell growth and cell division are carefully
controlled
◦ Cells will grow into open space, but when
cells contact other cells they respond by not
growing
◦ Controls for cell growth (cell division) can be
turned on and off
◦ Similar affect occurs in you injure yourself
Cell Division Control
Cyclins – proteins that regulate the timing of
the cell cycle
 Many other proteins involved in regulating the
cell cycle including internal and external
regulators
Cancer – uncontrolled cell division. Cancer
cells do not respond to the signals that regulate
the growth of most cells.

Cancer

Causes of cancer:
1. Environmental (smoking, UV radiation,
viruses…)
2. Genetic (many have a defect in gene p53
which stops the cell cycle until all chromosomes
have been properly replicated).
Cancer Treatments
There is no cure for cancer, and there probably
never will be
 However, there are a number of current and
future cancer treatments
◦ Radiation therapy- ionizing radiation designed
to kill cancer cells; damages cancer and
healthy cells
◦ Chemotherapy- drugs that destroy cancer
cells; traditionally affect all rapidly dividing
cells

Cancer Treatments
◦ Surgery- cut out the cancer cells; is not
possible for all cancers
◦ Other treatments- angiogenesis inhibitors,
targeted therapies, immunotherapy,
electroporation, nanoparticles
◦ Irreversible electroporation
Chapter 11
Introduction to Genetics
DNA and Sexual Reproduction
Why do many organisms combine DNA from
two parents to make offspring?
 How does a sperm and egg (gametes) combine
to form 1 cell with appropriate amount of
DNA?
◦ If two normal human cells combined to form
the new embryo, how much DNA would the
embryo have?
◦ How would you overcome this problem?

Chromosome Number
Humans have 2 pairs of 23 chromosomes (46
total)
◦ 1 set comes from the female, the other from
the male parent
◦ The sets are called homologous
chromosomes which code for the same trait
but are different
 A cell that has both sets of homologous
chromosomes is called diploid (2 sets)
 A cell (gametes) that only contain 1 set of
chromosomes is called haploid (1 set)

Meiosis

Meiosis is a process of reduction division in
which the number of chromosomes per cell is
cut in half through the separation of
homologous in a diploid cell
◦ Has two distinct sections: Meiosis I and
Meiosis II
◦ At the end of Meiosis II, 1 diploid cell has
become 4 haploid cells
Meiosis I

Prior to Meiosis I each chromosome is
replicated (like mitosis)
◦ Meiosis I is similar to mitosis
◦ Difference is that in prophase I each
chromosome pairs with its corresponding
homologous chromosome to form a tetrad
◦ As a result, crossing over occurs which results
in exchanging portions of their chromatids
Crossing Over
Meiosis II
After Meiosis I, the two cells enter a second
meiotic division (no replication beforehand)
◦ Prophase II, Metaphase II, Anaphase II,
Telophase II/ Cytokinesis
◦ Cell now only have half the standard DNA
(chromosomes) – haploid
◦ Each of the 4 cells created are unique
(genetically different)
 For males the cell created become sperm, in
females an egg (both are gametes)

Meiosis