Bio 1005 Chapter 6 How Cells Divide Dr. Joseph Silver

as with many other things in biology there is with cell division (mitosis) a progression from primitive to advanced

prokaryotes primitive organisms divide by binary fission

prokaryotes have only 1 chromosome when it is time to divide the cell elongates 1 chromosome replicates they separate FtsZ proteins form a ring at midpoint FtsZ proteins form a cleavage furrow at midpoint pinches cell at midpoint forming 2 cells replicates every 20 – 60 seconds

the number of chromosomes in a cell varies from 1 to more than a 1000

in a functioning cell the genetic material is present in an unwound form looking like a bunch of loose random threads known as chromatin

the genetic material DNA has a double helix structure

1 chromosome has about 140,000,000 nucleotides which if they were words would be enough to fill about 280 books of 1000 pages that is a lot of information and if stretched out would be about 2 inches

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 of genetic material into the nucleus of a cell???????

one idea of many 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 wrap around each other to form a thread like structure 4. enzymes called condensins wrap the threads into chromatin loops

5. the loops are then attached to a scaffolding protein just as wires or bristles are attached to a wire brush 6. the chromatin loops are tightly wound into what we call a visible chromosome

we have 46 chromosomes 23 from mom 23 from dad the only time that we can actually get to see the 46 chromosomes is during cell division

when the chromosomes condense at the beginning of mitosis they are all visible scientists are able to access dividing cells stop them from dividing at the stage when all the chromosomes are visible and use biological stains so that we can see them all

each pair of chromosomes have genes for the same traits so in a perfect world we have 2 genes for every trait one from each parent this is referred to as diploid a full set of chromosomes

each pair of chromosomes are referred to as a homologue and they have genes for the same traits they do not have to be identical genes but they are genes for the same traits they are homologous

in order for cell division or mitosis to take place properly all chromosomes must duplicate and remain attached as sister chromatids 2 identical chromosomes attached to each other

sister chromatids are held together about the centromere near the center of a chromosome by a complex protein referred to as cohesins

The cell cycle consists of 1. interphase 2. prophase 3. metaphase 4 The cell cycle consists of 1. interphase 2. prophase 3. metaphase 4. anaphase 5. telophase 6. 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 dispersed 3. centrioles move to poles, chromosomes line up at cell midpoint, spindle fibers attach to centromere & centrioles 4. spindle fibers shorten, chromosomes pulled to opposite ends of cell 5. nuclear membrane reforms, chromosomes unwind, spindle fibers disappear, organelles reform 6. cell membrane pinches at center to form 2 identical cells

1. the cell cycle goes through a series of steps resulting in 2 smaller and identical cells and as the process proceeds 2. the cell is able to go through a series of checkpoints to make sure that the cell is able to proceed without errors

interphase consists of 3 parts 1. G1 2. S 3 interphase consists of 3 parts 1. G1 2. S 3. G2 which make up about 95% of the cell cycle

- during interphase the cell grows - does the work of the cell - usually the longest phase - decision is made to divide the cell - DNA is duplicated - organelles, enzymes, etc. increased - spindle begins to appear

interphase – the growth phase consists of 3 parts G1 – growth and function S – 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 in a living cell there are checks and balances to make certain that all proceeds in a correct manner

G1/S checkpoint before making the decision to duplicate the DNA the cell checks that it can proceed 1. nutritional state 2. growth factors 3. size of cell 4. is DNA OK? 5. repair/apoptosis 6. Cdk2 + cyclin e become active

S/G2 phase checkpoint 1. check for DNA breaks 2 S/G2 phase checkpoint 1. check for DNA breaks 2. check for raw materials 3. can cell proceed to G2 4. cyclin A + cdk2 become active if all is OK cell proceeds to replicate/copy DNA

G2/M checkpoint 1. check for errors in DNA 2. repair or apoptosis 3 G2/M checkpoint 1. check for errors in DNA 2. repair or apoptosis 3. condense DNA 4. cyclin B + cdk1 become active 5. M phase promoting factor (MPF) M = mitosis allows cell to continue to M phase

APC anaphase promoting complex 1. are chromosomes properly aligned 2 APC anaphase promoting complex 1. are chromosomes properly aligned 2. are spindles hooked up to chromosomes 3. is metaphase plate correct 4. are spindles hooked up to centrioles

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 control of the cell cycle cyclins – cdc and cdk (function???) cyclins – amount go up & down with cell phases MPF – mitosis promoting factor (kinase+cyclin) cyclin dependent kinases – engine that drives cell division kinases = phosphorylation or dephosphorylation APC/C = destroys cohesion complex and cyclins growth factors = 50+, specific & general,

cancer loss of control of cell cycle control and repair failure p53 = tumor suppressor gene p53 = halts mitosis if DNA damaged p53 = repair DNA or apoptosis p53 = damaged or absent involved in many cancers

proto-oncogenes – genes which mutate to oncogenes oncogenes – genes that cause cancer