Mitosis Ch. 10

Why Divide? Growth of organism Repairs Reproduction Genetic Variation 2 Types: Mitosis: – Parent cell produces genetically identical diploid (2n) daughter cells Meiosis: – Parent cell produces genetically varied haploid (n) daughter cells

Follow the Chromosomes Chromosome  compact DNA; easier for transporting How many in humans? – 23 Homologous chromosomes  pairs of chromosomes in 2n cells; one copy from each parent How many in humans? – 46 (2n= 2x23) Word for when too many chromosomes are present; 47XXY? – Ploidy; deadly in animals but helpful in plants Sister chromatids  copies of chromosomes during mitosis

The Cell Cycle and Mitosis What is the growth and development phase of a cell called? – Interphase 3 Phases in interphase: 1)G 1 Phase  normal cell growth (Growth 1) – Various in length between cell types 2)S Phase  DNA is replicated and proteins for making chromosomes (Synthesis) – hours long 3)G 2 Phase  cell growth and preparation for division (Growth 2) – 4-6 hours long

No Growth Phase G 0 Phase  cell continues to function but no longer grows to do division – Cells can be permanently be in G 0 or pushed back into G 1 by signals What cells never leave G 0 ? – Most human nerve cells What signals start up growth again? – Growth factors, hormones, and other external signal molecules

M Phase M Phase  mitosis; completed in 5 steps 1)Prophase 2)Prometaphase 3)Metaphase 4)Anaphase 5)Telophase What is the last step to division? – Cytokinesis; splitting of the cytoplasm

Prophase 1)Chromosomes begins to form as chromatin is condensed – 2m of DNA changes to 23 pairs of chromosomes 2)Nucleolus shrinks and disappears 3)Spindle fibers formed between two centromeres 4)Centromeres move to opposite sides of the cell (spindle poles)

Late Prophase 1)Nuclear envelope disappears 2)Spindle fibers extend and attach sister chromosomes at the centromeres 3)Spindle fibers meet up with other spindle fibers across the cell

Metaphase 1)Spindle fibers lengthen and shorten to move chromosomes to the middle of the cell (metaphase plate)

Anaphase 1)Spindle fibers shorten and pull sister chromatids to opposite spindle poles 2)Sister chromatids are now called “daughter chromatids”

Telophase 1)Opposite of prophase 2)Spindle fibers disassemble 3)Chromosomes decondense 4)Nuclear envelope and nucleolus reappear

Cytokinesis In animals, protists, and many fungi: 1)Furrow forms and cuts cells in half 2)Organelles that were copied are moved to each cell In plants: 1)Cell plate forms as guide for new cell wall 2)Cell wall forms slowly and divides cell in two

The Powerful Mitotic Spindle Fibers All movement of chromosomes and cytokinesis depend on spindle fibers Spindle fiber: – complex made of microtubules and motor proteins – Grow in all directions from centrioles creating the centrosome or Microtubule Organizing Center (MTOC)

Two Spindle Types 1)Kinetochore microtubules: – Bind chromosomes to spindle poles – Chromosomes “walk” along tube – Tube is broken down as chromosome moves 2)Nonkinetochore microtubules: – Overlap cross the cell and push against each other – Lengthening tubes pushes cells apart

Cell Cycle Regulation 3 Checkpoints Each ensures the major step is completed and the cell is ready to commit to the next big step 1)G 1 /S checkpoint: cell must either commit to whole division cycle or not – May require an external factor (growth hormone) 2)G 2 /M checkpoint: commits cell to mitosis or not – Wont continue if DNA is to damaged 3)Metaphase checkpoint: cell wont finish division unless chromosomes are lined up properly

Cdks Levels in the Cycle Checkpoint regulated internally by cyclin proteins and cyclin- dependent kinase enzymes (Cdks) Cdks need cyclins to be active and they start a phosphorylation cascade leading to a target protein What does this mean for cyclin levels in the cycle? – They fluctuate through divison but Cdks levels are constant

Physical Inhibition Growth factors typically cause the levels of cyclin/Cdk activity to increase while inhibitors lower it What might inhibit cell growth after a wound has healed? – Physical contact of over cells; receptors recognize cells of the same type and stop growing Contact Inhibition  keeps cells in check inside organs and systems inside complex organism – Also limits bacterial growth in cultures – Pushes cells in G 0 phase until contact is broken

The Big “C” What happens if contact inhibition is inhibited? – Cells continue to grow and form large masses (tumors) Most cancers are caused by a shutdown of contact inhibition or an over stimulation of cyclin/Cdk complexes Tumor  mass of cells – Benign  local or no growth; may not need to be removed – Metastasis  growing and spreading through the body; must be treated Why does cancer hurt the body? – Pressure from tumors, stealing blood and nutrition, over production of signals, etc…

What Causes Cancer? Most cancers are the result of damaged DNA caused by age, chemicals (carcinogens), or naturally occurring mutations in the DNA (heredity) Oncogenes  mutated genes causing the cancer What type of oncogenes would you see in most cancers? – Cyclin/Cdk regulation – Growth hormone receptors

Cell Cycle in Prokaryotes Prokaryotes go through a similar G 1, S, and G 2 cycle, however they divide by binary fission Their single circular chromosome is replicated and then each is pulled to opposite sides of the cell The cell, once large enough, divides in half by cytokinesis Process is very short and simple, which is why bacteria can divide so quickly; most of the time is spent coping the DNA