MITOSIS: Making New Cells Making New DNA The Cell Cycle MITOSIS: Making New Cells Making New DNA

From one cell to many… Going from egg to organism…. and divide… the original fertilized egg has to divide… and divide…

Why do cells divide? One-celled organisms Multi-celled organisms for reproduction asexual reproduction (clones) Multi-celled organisms for growth & development from fertilized egg to adult for repair & replacement replace cells that die from normal wear & tear or from injury

The Big Challenge Before division, a cell must make copies of DNA organelles cell membrane lots of other molecules Enzymes Each new daughter cell requires a complete set of genes. Evenly divide these things between 2 new (daughter) cells

The Big Picture

Chromosomes of Human Female 23 pairs 6

Copying DNA A dividing cell duplicates its DNA creates 2 copies of all DNA separates the 2 copies to opposite ends of the cell splits into 2 daughter cells DNA starts loosely wound in the nucleus If you tried to divide it like that, it could tangle & break

Organizing & packaging DNA cell nucleus DNA has been “wound up” DNA in chromosomes in everyday “working” cell cell nucleus 4 chromosomes in this organism DNA in chromosomes in cell getting ready to divide

DNA must be duplicated… chromosomes in cell 4 single-stranded chromosomes nucleus cell DNA in chromosomes nucleus cell duplicated chromosomes duplicated chromosomes 4 double-stranded chromosomes

Some Chromosome Vocab Normal Cell Dividing Cell duplicated chromosomes Chromatin – Loosely coiled DNA. Disorganized, but accessible to enzymes and proteins. Centromere- region where chromatids are held together Chromosome – made up of 2 sister chromatids. DNA has been replicated.

Paired bases DNA structure Complementary base pairing double helix 2 sides like a ladder Complementary base pairing A pairs with T A : T C pairs with G C : G phosphate sugar N base

Copying DNA Matching bases allows DNA to be easily copied

DNA replication Copying DNA replication DNA starts as a double-stranded molecule matching bases (A:T, C:G) then it unzips… Helicase: The unzipping enzyme 13

DNA replication Enzyme DNA polymerase adds new bases DNA bases in nucleus Enzyme DNA polymerase adds new bases DNA polymerase 14

Copying DNA Build daughter DNA strand use original parent strand as “template” add new matching bases synthesis enzyme = DNA polymerase DNA Polymerase 15

Copied & Paired Up Chromosomes centromere

Copying DNA Semi-conservative Each daughter strand is ½ original DNA, ½ new.

Copying DNA DNA Polymerase can only work in one direction: 5’  3’ Can only add new nucleotides onto the sugar end of previous one. 5’ Base 3’ 5’ Base 3’ 5’ Base 3’ 5’ Base

Copying DNA Leading strand: synthesized in 5’  3’ direction (continuous synthesis) Lagging strand: synthesized in 3’  5’ direction. (Okazaki fragments.) DNA strands run in opposite directions – “anti-parallel”

Copying & packaging DNA When cell is ready to divide… copy DNA first, then… coil up doubled chromosomes like thread on a spool… now can move DNA around cell without having it tangle & break Copying DNA Coil DNA into compact chromosomes 20

The Cell Cycle: the period from the beginning of one cell division to the beginning of the next cell division. Interphase – between cell divisions. Most time of the cell cycle. Produce materials for growth Prepare for the next division Mitosis cell division

The Cell Cycle G0 phase: When cells are not dividing Mature cells. Ex: nerve cell, liver cell Can re-enter cell cycle if the correct signals are received.

Mitosis: Dividing DNA & cells Stage 1: Prophase DNA winds into chromosomes (keeps it organized) Nuclear membrane breaks down Spindle fibers form cell duplicated chromosomes nucleus

Mitosis: Dividing DNA & cells Stage 2: Metaphase chromosomes line up in middle attached to spindle fibers that will help them move duplicated chromosomes lined up in middle of cell

Mitosis: Dividing DNA & cells Stage 4: Anaphase Spindle fibers shorten, Chromosomes separate start moving to opposite ends One complete set of cell’s DNA is now at each end of the cell. chromosomes split & move to opposite ends

Mitosis: Dividing DNA & cells Stage 5: Telophase “reverse prophase” Nuclear membrane forms Chromosomes uncoil  chromatin Mitotic spindle breaks down

Mitosis: Dividing DNA & cells Stage 6: Cytokinesis “cytoplasm splitting” Membrane folds in at the center, forming a cleavage furrow cells separate now they can do their every day jobs Identical Daughter Cells.

Mitosis & Cancer: When Making New Cells Goes Terribly Wrong! 28

When is mitosis a good thing? When you have to add or replace cells growth & development repair replacement

When is mitosis a BAD thing When cells reproduce & they are not needed these cells take over organs, but don’t do the right job they just keep making copies cancer damages organs

Control of the Cell Cycle How does a healthy cell know when to divide? Cyclins – proteins that regulate progression of a cell through the cell cycle. Checkpoints – proteins detect problems at each step of the cell cycle. Initiate cell-cycle arrest if problems are detected  DNA mutations, incorrect attachment of spindle fibers to centromere Proto-oncogenes – Code for genes that initiate the cell cycle.  Have the potential to become oncogenes: cancer-causing genes. Tumor suppressor genes – code for proteins that arrest, monitor, or slow the cell cycle. 31

Control of the Cell Cycle How does a healthy cell know when to divide? Attachment requirement – cells must be able to attach to surrounding environment. Space and nutrient requirements – cells will only divide when they have enough space and nutrition Cancerous cells ignore these requirements 32

Control of the Cell Cycle If DNA gets damaged, cells stop listening to correct instructions mutations Causes of mutations: UV radiation chemical exposure radiation exposure heat cigarette smoke pollution age Genetics Viruses 33

Control of the Cell Cycle Cells usually require an accumulation of several mutations before becoming cancerous Mutations to proto-oncogenes Become over-active Mutations to tumor suppressor genes Become inactive This explains why cancer become more likely as a person gets older. Genetic Predisposition: When a few mutations are inherited, it becomes more likely that cancer will develop. Fewer mutations still needed. 34

Tumors Benign tumor abnormal cells remain at original site as a lump most do not cause serious problems & can be removed by surgery Malignant tumor cells leave original site start more tumors damage functions of organs throughout body 35

Treatments for cancers Treatments kill rapidly dividing cells chemotherapy poisonous drugs that kill rapidly dividing cells radiation high energy beam kills rapidly dividing cells localized 36