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Cell Growth
C-10-1
Living things get larger because there are more
cells produced - not because the cells get bigger!
Reasons:
~ larger cells have more demand put on their
DNA
~ larger cells have more trouble moving
materials across the cell membrane
DNA overload - if a cell gets too big, there may not
be enough info on the DNA to meet the cells
needs!
ie: library in a small town - everyone can check
out a book with no problem
- if the town gets bigger, but the library
stays the same, not enough books for everyone
- people have to wait!
Exchanging materials - the rate at which food and
waste is brought across the membrane depends on
the surface area of the cell
- the rate at which the food and waste is
produced depends on the volume of the cell
ex: if a town gets bigger, but the roads stay
the same, it's harder for people to get around
town - same applies to cell materials in a large cell!
So before a cell gets too big... it divides!
Chromosomes!
Ch. 14
Chromosome - rod shaped structure that forms when a single DNA
molecule and its associated proteins coil tightly before cell division
*Made up of genes which make up DNA - segment of
DNA that carries info from the parent to the offspring
*Usually the DNA strand is uncoiled when in use.
*When the cell prepares to divide, the DNA coils up and
compacts for easier division
DNA looks like this:
Chromatid - one of a pair of strands of DNA that make up a
chromosome
*DNA replicates itself before cell division so each new cell
will have the same genes
The chromatids are joined in the center by a centromere made of
protein.
They look like this:
We have 23 chromosomes!
*body cells have 2 of each type of chromosome (diploid)
*the 2 copies of each type are homologous
chromosomes
*one copy from Dad, one from Mom
Ex:
*sex cells (gametes) only have 1 of each (haploid)
When a sperm fertilizes and egg, a zygote is formed! The new cell
is now diploid!
Chromosomes affect development:
*Most babies with less than 46 die before they become a
fetus.
* A person can live with more…like 47:
~ Down Syndrome = has extra chrom. #21
~ trisomy – extra chromosome in a 2n cell
~ can determine this with a karyotype – look at a map
of the chromosomes…
Features of Down Syndrome:
 short stature
 round face
 upper eyelids that cover the inner corner of the eyes
 some degree of mental retardation
Can happen to any race!
Usually seen born to mothers older than 35 years but can happen to
anyone.
` Due to the eggs’ DNA deteriorating over time and chrom.
not separating properly
Chrom. # 21 does not separate at time of cell replication =
nondisjunction
ie: one cell gets both chrom’s and the other gets none.
Can test through amniocentesis or chorionic villi sampling:
 are very invasive tests
 can sometimes cause premature labor
 use needles to obtain amniotic fluid or a sample of the villi
from the placenta and make a karyotype to look for genetic
abnormalities
Alterations in chromosome structure:
Mutations – changes in an organism’s genetic material
Types of mutations:
1.) Deletion – piece of chrom. breaks off and is lost when the
cell divides
2.) Duplication – piece could attach to its homologous chrom.
and then the new cell will have 2 copies
3.) Inversion – piece reattaches itself to the original chrom. in
the reverse order
4.) Translocation – piece may attach itself to a nonhomologous
chrom.
Chromosomes determine your sex
Autosomes – the 22 pairs that are NOT sex chromosomes
Sex chromosomes are the only pair the determine whether you’re
female or male!
2 forms = X and Y
The presence or absnce of the Y chromosome is what
determines the sex…this is because the hormone that makes a male
is located on the Y
 Without it, the organism is female!
With us…
Females = XX
Males = XY
Since females only have an X to donate to their offspring, sex is
always determined by the males.
Some oddities may arise like these:
XXY = Klinefelter’s syndrome = male will be strerile
 affects genital development
 may have slight breasts
 little body hair
 long limbs
XYY = male is fertile (no syndrome name)
XXX = Triplo-X syndrome = female is fertile
Mitosis and cell division!
C-10-2
The simplest form of cell division is done by bacteria – binary
fission
*This is an asexual process that produces identical offspring
Bacteria: have no nucleus and no membrane bound organelles
- DNA is circular and attached to the inner plasma
memebrane
- cell division has 2 steps:
o copy DNA
o divide the cell
It looks like this:
In eukaryoteic cells, the process is more complicated.
 undergo the Cell Cycle – sequence of growth and division
In 5 Phases:
G1 – S – G2 – M – C
1.) G1 phase – growth phase, takes place between divisions
2.) S phase – DNA is copied , at end you have a chromosome
with 2 chromatids
3.) G2 phase – growth, mitochondria and other organelles
replicate, microtubules reassemble ( form spindle apparatus)
4.) M phase – Mitosis occurs!
a. nucleus divides into 2 nuclei: each with same # chrom.
5.) C phase – cytokinesis = cytoplasm divides
* The first 3 phases are collectively called Interphase
~ this is what the cell spends most of its life in
* When interphase ends, Mitosis now begins (M phase)
Here’s what takes place now…
Prophase:
* chrom. begin to condense
* enzymes break down nuclear envelope
* cylindrical centrioles start to separate and move to opposite
ends of the cell – called “polar bodies”
* protein fibers from centriole form spindle fibers – made of
microtubules
**NOTE! Plants do NOT have centrioles!**
Metaphase:
* chrom. keep condensing
* chrom. line up in the center of the cell – “equator”
* more microtubules extend out of the centromere at the
kinetochore – hold the chroms in place
* one chromatid is attached to one centriole, one to the other
Anaphase:
* centromere divides and the 2 chromatids separate
* the new chroms move to opposite sides as the spindle fibers
shorten
Telophase:
* chroms uncoil at either end of cell
*new nuclear envelope forms
* spindle fibers break down and disappear
Cytokinesis begins – cell is cleaved in half
* cell membrane grows to close both
* done by a belt of protein threads
In plants… golgi bodies form a cell plate at the center of the
cell to build a new cell wall
- once the wall is complete, the cell separates!
Meiosis!
C-11-4
This is how our sex cells remain haploid and we get a diploid
zygote every time!
*Meiosis will cut the # chroms. in half by doing 2 successive
nuclear divisions.
~ the 1st division, Meiosis I – homologous chroms. separate
into 2 cells
~ the 2nd division – Meiosis II – the 2 chromatids of each
chrom. separate into 2 haploid cells
So… 1(2n) cell that goes through meiosis produces 4 (1n) cells!
* in animals – results in gametes
* in plants – produces spores which produce gametes
2 unique events occur in meiosis:
1.) Crossing over – Meiosis I – homologous chroms pair up
_ while next to each other, the arms of the chromatids
“cross-over” each other and swap reciprocal segments…
This is important because…
~ Genetic recombination – forms new genetic combinations
that allow for faster evolution and less defects
~ essentially, the chromatids no longer contain the same
genetic info!
2.) Skipping replication – b/c there is only 1 replication of DNA
and 2 cell divisions
* Meiosis cuts the # of chroms in ½!
Meiosis I
Stages have the same names and mitosis, but some different
things happen…
Prophase I: ( P1)
* chroms condense
* nuclear envelope breaks down
*homologous pairs cross over
Metaphase I: (M1)
*homologous chroms remain together and line up on the
equator
Anaphase I: (A1)
*homolgous chroms. separate by way of spindle fibers
* chroms do NOT separate at centromeres!!
*nondisjunction can happen here (Down syndrome)
Telophase I: (T1)
* chrom at either end of the cell
*cell divides into 2 new cells
* each new cell now has ½ of the original # of chroms as
parent cell
Meiosis II:
EXACTLY the same as mitosis only the chromosomes DO NOT
replicate
Phases now called: P2, M2, A2, T2
All steps go: I, P1, M1, A1, T1, C, P2, M2, A2, T2, C
Let’s see it all together…
Regulating the Cell Cycle
C-10-3
Some cells divide often while others stop dividing
once they are grown.
Controls on Cell Division:
- place cells in a nutrient broth in a petri dish
- cells will grow until they cover the bottom
- when they touch, they stop growing
- if remove cells from center, cells will grow until
the gap is closed!
- shows that cell division can be turned on and
off
* same happens in the body when a cut or injury
happens!
~ cells divide to until they close the gap - then
slows down and returns to normal
Cell Cycle Regulators
~ cyclin - protein that regulates the timing of the
cell cycle
- essentially tells the cell when it's time to
divide
~ discovered in early 1980's
Also 2 types of regulatory proteins:
1.) Internal regulators:
* respond to events inside the cell
* allow cycle to proceed only when certain
processes have happened
ex: - all chrom. have been replicated
- all chrom. attached to spindle fibers
2.) External regulators:
* respond to events outside the cell
* direct cells to speed up or slow down the
cycle
* ex: growth factors - important during
embryonic development and wound healing
* others stop growth when neighboring cells
bump into each other preventing over
growth
Why is this important?
Uncontrolled growth = CANCER!!
Cancers do not know when to stop growing!
~ form tumors which can damage surrounding
tissues
~ can break free and cause the tumor to
spread
~ all cancers have lost the regulatory ability
which is why they grow uncontrollably
Healthy cells have a gene called p53 which stops
the cell cycle right before mitosis to make sure all
chrom. are correctly replicated.
~ if p53 gets damaged, the cycle is not
properly regulated!
Ex: ~ sun damages your DNA
~ p53 stops mitosis until the damaged DNA is
repaired and replicated
~ this ensures no damaged DNA gets
replicated
~ if p53 is not working = damaged DNA gets
replicated during cell division = skin
cancer!
~ if cell is simply beyond repair, the cell will
be killed rather than divide a damaged DNA
strand - called apoptosis
* why your skin peels after a sunburn!
Stem Cells
~ are unspecialized cells that have potential to
differentiate
~ found in embryos before day 10 - then start
to specialize
~ important to cells that cannot regenerate
once damaged - nerve and brain cells
~ important for potential new organs or organ
parts to lessen rejection for the recipient
Kinds of stem cells
1.) totipotent -early embryonic cell - can develop
into whole organisms
2.) pluripotent - can form almost all tissues but not
whole organism - further specialize into
specific cells
3.) multipotent - specific cells that can give rise to
more specific cells
ex: blood cells  both red and white
skin stem cells  diff. types of skin cells
Sources of stem cells
* human embryos are main source but are now
developing techniques from skin cells!!
Discussion:
1.) Hypothesize why adults don't have stem cells
that can give rise to every kind of cell in the body.
2.) How might this science be used to treat some
of these diseases or conditions:
~ Heart disease
~ Parkinson's
~ Alzheimer's
~ Diabetes
~ Burn victims
~ Spinal cord injuries
3.) What moral and ethical issues do you see with
harvesting and using stem cells from embryonic or
fetal tissue?
~ Would you be less opposed if they came
from somewhere else?