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