Mitosis

• Why do cells need to divide?

Recap…

• Cell theory…– Cells are the basic structural unit of life– Cells are the functional units of life– Cells come from pre-existing cells

Overview• Why do cells need to divide?– Repair, growth, development

• Types of reproduction– Sexual

• Genetically different• 2 parents• Takes time to develop, better chance of survival

– asexual• Genetically identical• One parent• Many offspring very quickly

DNA• Blueprint of life, nucleic acid• Chromatin– Granular genetic material, spread out in nucleus of

non-dividing cells• Chromosomes– Condensed genetic material, in dividing cells

• Sister chromatids– Identical copies of Chromosomes joined by a

centromere (“centro-” middle)

Humans• 46 chromosomes – 46 sister chromatids– One from your mom, one from your dad

Cell Cycle: Life of a Cell

Cell Cycle• Interphase– 90 % of cell’s life, non dividing– G1 phase

• Grows, makes organelles

– S phase• DNA Synthesis…DNA replicates

– G2 phase• Cell prepares to divide, makes sure it has all important organelles for

division

• M phase– When the cytoplasm and nucleus of the cell divides

Cell Cycle

• There are check points in G1, S, and G2– Make sure cell is ready to move onto the next

phase (has all necessary organelles, copied DNA, etc.)

• Once the cell has past the G1 checkpoint, it will complete the cell cycle

• Some cells stay in the G1 phase all their life (muscle cell, brain cells)

Regulators of Cell Cycle• Cyclins– Protein that regulates the timing of the cell cycle

in eukaryotic cells– Levels of cyclins rise and fall throughout the cell

cycle• Cyclin-dependent Kinases (cdks)– Enzymes that are activated when they bind with

cyclin and they make the cell cycle continue

Regulators• Internal– Factors within the cell that control cell cycle– Cyclin and CDKs• Allow cell cycle to proceed only when certain processes have

occurred– Replication of chromosomes– Chromosome Attachment to spindle before anaphase

• External– Factors Outside the cell• Growth factorsmolecules that bind to cell surface that signal

cell to divide• Similar cells have molecules that have opposite effect so that

when it becomes to crowded, cells stop dividing

M-phase• Consists of mitosis and

cytokinesis• Mitosis– Process by which the nucleus

of a cell divides– One parent cell makes two

identical daughter cells– This is how organisms repair

tissue and grow and develop• Cytokinesis-division of the

cytoplasm

Depending on cell type…

• Mitosis can take a few minutes or a few days• Muscle cells (non-dividing)• Nerve cells (non-dividing)• Skin cells (divide all the time)• Digestive Tract cells (divide all the time)

Life Span of Some Human Cells

Cell type Life span Cell division

Lining of esophagus 2-3 days Can divide

Lining of small intestine 1-2 days Can divide

Lining of the large intestine

6 days Can divide

Red blood cells Less than 120 days Cannot divide

White blood cells 10 hours to decades Many do not divide

Smooth muscle Long-lived Can divide

Cardiac (heart) muscle Long-lived Cannot divide

Skeletal muscle Long-lived Cannot divide

Neurons (nerve) cells Long-lived Most do not divide

Easy way to remember M-Phase

• PMAT • Pilates Make Annie Toned• Practice Makes Athletes Train

– Prophase– Metaphase– Anaphase– Telophase

Prophase• 50-60% of time• Chromosomes become visible• Centrioles develop in cytoplasm near nuclear envelope• Centrioles separate and migrate to opposite ends of

nuc. Env.• Centrosome– Region where Centrioles are found– Organize the “spindle”

• Fan like microtubule structure that helps separate chromosomes

• Plants do NOT have Centrioles

End of prophase

• Chromosomes coil together tightly• Nucleolus disappears• Nuclear envelope breaks down

Metaphase

• Few minutes• Chromosomes line up in middle (M in

metaphase MIDDLE)• Microtubules connect centromere of each

chromosome to the 2 poles of spindle

Anaphase

• Centromeres joining sister chromatids separate and become individual chromosomes

• They are dragged by fibers to opposite poles• Ends when chromosomes stop moving

Telophase

• Opposite of prophase• Condensed chromosomes disperse into tangle

of material• Nuclear envelope reforms• Spindle breaks apart• Nucleolus becomes visible• At the end 2 identical nuclei in one cell

Cytokinesis

• Happens at the same time as Telophase• Division of cytoplasm• Animal Cells– Cell membrane drawn inward until it pinches off

and forms 2 id daughter cells• Plant Cells– Cell plate forms between nuclei– Cell Plate develops into separate membrane – Cell wall appears

Regulators of Cell Cycle• Cyclins– Protein that regulates the timing of the cell cycle

in eukaryotic cells– Levels of cyclins rise and fall throughout the cell

cycle• Cyclin-dependent Kinases (cdks)– Enzymes that are activated when they bind with

cyclin and they make the cell cycle continue

Regulators• Internal– Factors within the cell that control cell cycle– Cyclin and CDKs• Allow cell cycle to proceed only when certain processes have

occurred– Replication of chromosomes– Chromosome Attachment to spindle before anaphase

• External– Factors Outside the cell• Growth factorsmolecules that bind to cell surface that signal

cell to divide• Similar cells have molecules that have opposite effect so that

when it becomes to crowded, cells stop dividing