chapter 10 how cells divide dr. joseph silver. as with many other things in biology there is with...
TRANSCRIPT
Chapter 10
How Cells Divide
Dr. Joseph Silver
as with many other thingsin biology
there is with cell division (mitosis)a progression
from primitive to advanced
prokaryotes divideby
binary fission
prokaryoteshave only 1 chromosomewhen it is time to divide
the 1 chromosome replicates
1. origin of replication2. enzyme complex moves along in both
directions3. until 1 chromosome becomes 24. enzymes contact terminus site
5. cell elongates6. pulling chromosomes apart7. septum forms at mid point
8. cell pinches into 29. we now have 2 cells each with 1
chromosome
fig 10.3shows how mitosis has
changed during progressionform simple to complexin cells having a nucleus
from this point on we will studymodern
eukaryotic cell divisionusually referred to as
mitosis
the termmitosis
was first useda little more than
100 years ago
it means “thread”
the number of chromosomesin a cell varies
from1 to more than a 1000
in a functioning cell the genetic material
is present in anunwound form
looking like a bunch ofloose random threads
known as chromatin
the genetic materialDNA
has a double helix structure
one eukaryotic chromosomeif laid out in a straight line
would be about 2 inches long
your text states the following1 chromosome has
about 140,000,000 nucleotides which if they were words
would be enoughto
fill about 280 books of 1000 pagesthat is a lot of information
SO
one of our cells has 46 chromosomes
46 x 2 = 92 inches (about 8 feet)
so the question is how
do you get 8 feet ofgenetic materialinto the nucleusof a cell???????
the answer is shownin fig. 10.5
1. every 200 nucleotides are wrapped around an 8 protein positively
charged complex called a histone
2. this complex is called a nucleosome
3. many nucleosomes wraparound each other
to form a thread like structure
4. enzymes called condensinswrap the threads into
chromatin loops
5. the loops are then attachedto a scaffolding protein just as wires or
bristlesare attached to a wire brush
6. the rosettes or chromatin loopsare tightly wound into
what we call a visible chromosome
like so many other things is science
if you put 3 scientists in the roomyou will hear 4 different ways that something may happen
there are many things that we do not fully understand
we have 46 chromosomes23 from mom23 from dad
the only time that we can actuallyget to see the 46 chromosomes
is during cell division
when the chromosomes condenseat the beginning
of mitosis they are all visible
scientists are able to accessdividing cells
stop them from dividingat the stage when all the chromosomes are
visibleand use biological stains
so that we can see them all
look at fig 10.6
each pair of chromosomeshave genes for the same traits
soin a perfect world
we have 2 genes one fromeach parent for every trait
this is referred to as diploida full set of chromosomes
each pair of chromosomesare referred to as a
homologueand
they have genes for the same traitsthey do not have to be identical genesbut they are genes for the same traits
they are homologous
in order for cell divisionor mitosis
to take place properlyall chromosomesmust duplicate
and remain attachedas sister chromatids
2 identical chromosomes attached to each other
sister chromatids are held together about the
centromere near thecenter of a chromosome
by a complex protein referred to as
cohesins
The cell cycle consists of1. interphase 2. prophase
3. metaphase4. anaphase5. telophase6. cytokinesis
1. cell grows, DNA duplicated, cell organelles increase, centrioles duplicate, spindle fibers appear
2. nuclear membrane disappears, spindle fibers increase, chromatin condenses to chromosomes,
organelles dispersed3. centrioles move to poles, chromosomes line up at
cell midpoint, spindle fibers attach to centromere4. spindle fibers shorten, chromosomes pulled to
opposite ends of cell 5. nuclear membrane reforms, chromosomes
unwind, spindle fibers disappear6. cell membrane pinches at center to form 2
identical cells
1. the cell cycle goes througha series of steps
resulting in 2 smaller and identical cellsand
as the process proceeds
2. the cell is able to go through a seriesof checkpoints to make sure that thecell is able to proceed without errors
interphase consists of3 parts
1. G12. S3. G2
which make up about 95% of the cell cycle
during interphase the cell growsdoes the work of the cellusually the longest phase
decision is made to divide the cellDNA is duplicated
organelles, enzymes, etc. increasedspindle begins to appear
interphase – the growth phaseconsists of 3 parts
G1 – growth and functionS – DNA duplicated
G2 – everything that the cell needs in order to become 2 smaller cells
is made ready and increased if necessary
G0 – growth & division arrested, cell is dormant
as with so many things ina living cell there are
checks and balancesto make certain that all proceeds
in a correct manner
G1/S checkpointbefore making the decision to
duplicate the DNA the cell checks that it can proceed
1. nutritional state2. growth factors
3. size of cell4. is DNA OK?
5. repair/apoptosis6. Cdk2 + cyclin e become active
S phase checkpoint
1. check for DNA breaks2. check for raw materials3. can cell proceed to G2
4. cyclin A + cdk2 become active
if all is OK cell proceedsto replicate/copy DNA
G2/M checkpoint
1. check for errors in DNA2. repair/apoptosis3. condense DNA
4. cyclin B + cdk1 become active5. M phase promoting factor (MPF)
M = mitosis
allows cell to continue to M phase
APCanaphase promoting complex
1. are chromosomes properly aligned2. are spindles hooked up to chromosomes
3. is metaphase plate correct4. Cdc20 and Cdh1 enzymes activate
process5. separases destroys cohesions
interphase-growth-DNA copied-centrioles copied-spindle fibers appear-organelles copied-checkpoints OK or not
prophase-chromatin condenses-chromosomes visible- sister chromatids appear- nuclear envelope breaks down- organelles dispersed-cytoskeleton disassembled-centrioles move to poles-spindles expand
metaphase-chromosomes at mid plate-spindles attached to chromosomes-spindles attached to centrioles
telophase-chromosomes at opposite ends-nuclear envelope reforming-Golgi & ER reform-spindles being reabsorbed-chromosomes unwinding- cytoplasm pinched off
control of the cell cycle
cyclins – cdc and cdk (function???)cyclins – amount go up & down with cell phasesMPF – mitosis promoting factor (kinase+cyclin)
cyclin dependent kinases – engine that drives cell divisionkinases = phosphorylation or dephosphorylationAPC/C = destroys cohesion complex and cyclins
growth factors = 50+, specific & general,
cancerloss of control of cell cycle
p53 = tumor suppressor genep53 = halts mitosis if DNA damaged
p53 = repair DNA or apoptosisp53 = damaged or absent in many cancers
proto-oncogenes – genes which mutate to oncogenes
oncogenes – genes that cause cancertumor suppressor genes – when mutate = cancer
see fig 10.24