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Cell Reproduction
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!
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
Checkpoints:
Cell inspects itself prior to cell division to
make sure everything is good to go!
Several checkpoints...
1.) G1 checkpoint - checks surroundings before
copying DNA
if good - go to S phase
if bad - rests and waits
2.) G2 checkpoint - checks for mistakes in DNA
copying and makes sure cell is big enough to divide
3.) Mitosis checkpoint - checks at metaphase to
see if chrom. are lined up correctly and attached
to spindle fibers
~ ensures DNA is evenly split between
daughter cells
Cancer
~ uncontrolled division of cells
~ can cause tumors which invade and damage
healthy tissues
* Caused by loss of control of DNA
~ if division isn't regulated, damaged DNA
could be copied and therefore cause more
defective cells to be produced - causes tumors
* Tumors - 2 kinds
~ benign - does not spread
~ malignant - can spread - called metastasis
- invades and destroys healthy nearby
tissues
* Treatment
~ drugs kill cancerous cells - chemotherapy
~ radiation therapy - high energy rays focus on
the cells and kill them
* Prevention
~ avoid cancer causing agents!
- UV rays
- cigarettes / smoke