MITOSIS AND MEIOSIS
8.2, 8.3, & 10.2
LEVELS OF BIOLOGICAL ORGANIZATION
• Cells  Tissues
• Tissues  Organs
• Organs  Organ Systems
• Organ Systems  Organisms
FROM ZYGOTE TO TRILLIONS OF CELLS
• Zygote=Cell that forms when sperm and egg join
• Sperm gives you DNA from dad, egg gives you DNA from mom
• Shortly after sperm fertilizes egg, the zygote splits into a 2nd cell
• These 2 cells split into 4, then 8, then 16, etc.
• Each of these cells contains IDENTICAL DNA (Recall DNA replication)
WHY MUST CELLS DIVIDE
• Diffusion limits cell size
• Cells get materials from surroundings through their membrane. This is also how they
expel wastes
• If the cell gets too large, it would take a long time for these processes to occur
• DNA also limits cell size
• DNA  RNA  Proteins
• There is a limit on how fast this can happen.
• If the cell is too large, it takes too long to make all of the structures from the proteins.
SURFACE AREA-TO-VOLUME
CELL CYCLE
I-PMAT
• The cell cycle can be divided into 2 (or 3) stages
• Interphase, Mitosis, and Cytokinesis
• Interphase and Mitosis are further divided by certain events that happen
• Interphase=G1, S, G2
• Mitosis=Prophase, Metaphase, Anaphase, Telophase
• NEITHER INTERPHASE NOR CYTOKINESIS ARE PARTS OF MITOSIS
INTERPHASE (G1, S, G2)
• Think “Intermission”
• Most of the cell’s life is spent in Interphase
• Period of growth and development
• G1 (Growth Phase 1)
• Cell grows in size and synthesizes mRNA and proteins
• S Phase (Synthesis phase)
• DNA replicates
• G2 (Growth Phase 2)
• Cell grows again, organelles are duplicated, and proteins are synthesized to prepare for
cell division
MITOSIS
• Period of division
• 4 parts (PMAT)
• Prophase (“Prologue”)
• Metaphase (“Middle”)
• Anaphase (“Apart”)
• Telophase (“Two”)
• Happens only in somatic (body) cells
PROPHASE (“PROLOGUE”)
• First and longest stage of mitosis
• Prophase begins as strands of chromatin coil around proteins called histones and become
a chromosome (tightly wound strand of DNA and proteins)
• Each copy of DNA (from Interphase) coils and forms 2 identical strands
• Called sister chromatids
• These sister chromatids join together at the center (called a centromere)
• Each chromosome has a “partner” (remember, you get one from your mom and one from your dad)
•
These are called homologous chromosomes
• The nucleus disappears
• Centrioles move to opposite ends of the cell
• A structure called a spindle forms
• Made up of rod-like structures called microtubules
DNA TO CHROMOSOMES
CHROMOSOME STRUCTURE
METAPHASE (“MIDDLE”)
• Spindle fibers attach to centromeres of chromosomes
• Spindles extend from centriole on one side and attach to sister chromatid on that same
side
• Fibers help chromosomes line up in the middle
• Very important because it ensures that ½ of the chromosomes end up in each cell
ANAPHASE (“APART”)
• Spindle fibers shorten and pull sister chromatids apart at the centromere
• Each identical sister chromatid moves to one of the sides of the cell
TELOPHASE (“TWO”)
• Chromatids reach opposite ends
• Chromatids begin to unwind into chromatin
• Nuclei form around chromatin
• Spindle breaks down
• Cells begin to split
• This ends Mitosis
CYTOKINESIS
• Division of cytoplasm
• Cell divides and 2 new daughter cells are formed
• Process differs in plants and animals
• In plant cells, a structure called a cell plate forms and separates the 2 daughter cells
• In animals cells, the cells “pinch in” until the cells are separated
CONTROL OF THE CELL CYCLE
• Cancer=uncontrolled cell division
• Proteins (called cyclins) created during protein synthesis help to regulate the cell cycle
• When these proteins are damaged by mutations of DNA, the cell’s normal cycle is disrupted and cells
divide out of control
• Forms a tumor
• Cancer cells take nutrients from healthy cells, and can occasionally move around in the body.
• When cancer cells move around, it is called metastasis.
• This forms new tumors throughout the body
MEIOSIS
• Occurs only in sex cells (males-sperm and female-egg cells)
• The goal is to cut the original number of chromosomes in half
• Offspring need to inherit from both parents
• If the number was not cut in half, the offspring would have 2x the chromosomes as the parents
• Terms
• A diploid cell is a cell that contains 2 of each chromosome (di=2)
• These are typical body cells
• A haploid cell is a cell that contains 1 of each chromosome (hap~half)
• These have ½ the number of chromosomes as typical cells
PHASES OF MEIOSIS
• Very similar to mitosis
• IPMATPMAT
• Starts with Interphase
• Only occurs once before meiosis begins
• Basically does mitosis twice
• Meiosis I
• Prophase I, Metaphase I, Anaphase I, Telophase I
• Meiosis II
• Prophase II, Metaphase II, Anaphase II, Telophase II
• Each ends with cytokinesis
MEIOSIS-INTERPHASE
• Exactly the same as in Mitosis
• Occurs only before Meiosis I, NOT Meiosis II
MEIOSIS-PROPHASE I
• Very similar to Prophase in Mitosis
• One huge difference
• Homologous chromosomes line up and form a structure called a tetrad
• They tangle up and can actually break portions off and exchange DNA
• Process is called crossing over
• Increases genetic diversity by potentially allowing you to pass on traits from both grandparents to
offspring
MEIOSIS-METAPHASE I
• Again, similar to Metaphase in Meiosis, but with a key difference
• Spindle fibers attach to tetrads, and line them up in the middle
• This means that there are 2 chromosomes next to one another in the center, instead of one as was
the case in Mitosis
Mitosis
Meiosis
MEIOSIS-ANAPHASE I
• Probably the most different from Mitosis
• Whereas in Mitosis, sister chromatids separate at the centromere and move towards
opposite ends of the cell, in Meiosis, tetrads are pulled apart and chromosomes (the pair
of sister chromatids) move to each side of the cell
• This ensures that there will be a full chromosome in the daughter cells
MEIOSIS-TELOPHASE I
• Occurs exactly as it did in Mitosis, except this time there is an entire chromosome
(pair of sister chromatids) in each of the 2 new cells
• Another division is necessary to split the genetic material in half
• Cytokinesis occurs, same as Mitosis
• 2 diploid cells are formed
MEIOSIS II
• NO INTERPHASE
• This process occurs EXACTLY THE SAME AS MITOSIS
• Only it is occurring in 2 cells at once
RESULTS OF MITOSIS AND MEIOSIS
• In Mitosis, 1 cell splits into 2 daughter cells
• The daughter cells are identical
• In Meiosis I, a single cell splits into 2 daughter cells
• Each of these cells contains the same amount of chromosomes as the parent cell
• Still diploid, and not identical
• In Meiosis II, the 2 daughter cells from Meiosis I undergo another cell division, and
split into 2 more daughter cells
• Each of these cells contains ½ of the original genetic material
• Now haploid, and each unique