Chapter 10Cell 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 DNA2. Diffusion and osmosis are slow

• Rate of exchange depend on surface area3. 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 cellsCell division is the process by a cell

divides into two new daughter cellsThe cell begins by copying its DNA,

each daughter cell gets one copy

Cell DivisionCell division in prokaryotes is simpler

and occurs by binary fission◦After DNA replication, cell contents

are separated into two partsIn 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 DNADNA 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 Cycle1. 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

Cytokinesis3. 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 ControlCell 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 ControlCyclins – proteins that regulate the

timing of the cell cycleMany other proteins involved in

regulating the cell cycle including internal and external regulatorsCancer – uncontrolled cell division. Cancer cells do not respond to the signals that regulate the growth of most cells.

CancerCauses 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 TreatmentsThere is no cure for cancer, and there

probably never will beHowever, 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 11Introduction to Genetics

DNA and Sexual ReproductionWhy 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 NumberHumans 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)

MeiosisMeiosis 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 IPrior 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 IIAfter 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