Download Biology Unit 7

Biology Unit 7
Cell Division
Why would a cell want to divide?
Reasons Cells Divide
• To avoid getting too big (cells not the organism)
• To help an organism grow and organize
• To replace old cells
Cell Size Limitations
• Some of the smallest cells are no larger than 8
nanometers while some of the largest can reach up to
8cm.
• Cell size can be limited by several things:
• Diffusion
• DNA
• Surface area to volume ratio
Diffusion Limits Cell Size
• You should know that the plasma membrane allows
nutrients to enter cells while wastes leave the cell.
• Diffusion is fast over short distances but becomes inefficient
over larger distances.
• The organelles on the inside of giant cells would die waiting
for nutrients to get to them.
• If we keep cells small, we can keep the rate of diffusion fast
and allow nutrients to reach all parts of the cell quickly.
DNA Limits Cell Size
• We know that the nucleus contains blueprints for the cell’s proteins,
which are used by almost all organelles.
• There is a limit to how quickly these blueprints can be copied.
• If a single nucleus is in charge of one giant cell, proteins will not be
able to be made quickly enough for the cell to survive.
What adaptation might a giant single celled
organism have to make sure its proteins get made
efficiently?
• Have more than one nucleus!
Surface Area to Volume Limits Cell Size
• As a cell’s size increases, its volume increases much faster
than its surface area.
• So what’s the problem?
• If a cell doubles its size, it requires about 8 times more nutrients
and produces 8 times more waste.
• But, the surface area would only increase by a factor of 4.
• Since waste and nutrients are brought in through the membrane
(surface), this means that the cell would either starve or be
poisoned by excess waste.
So How Do Cells Divide?
• Because cells are replacing other cells that do a specific job, they
need to be able to divide and create exact copies of themselves.
• This is done by cells copying their chromosomes and passing them to
the new cells.
• Chromosomes are the combination of protein and DNA that contain
the genetic info in each cell.
The Cell Cycle
• The cell cycle is the sequence of growth and division
in a cell.
• Consists of 2 general parts:
• Interphase: Most of a cell’s life, normal cell function
• Mitosis: The time during which cells divide forming two
daughter cells with identical genetic information.
Interphase
• Part 1: Cells grow; protein production is high.
• Part 2: Chromosomes are copied in preparation for cell
division.
• Part 3: Mitochondria as well as other cell parts needed for
division are manufactured.
Mitosis
• Division in all types of cells except for sex cells.
• Used to help organisms grow and become organized.
• Also used for asexual reproduction.
Mitosis: 4 Stages
• “PMAT”
• Prophase
• Metaphase
• Anaphase
• Telophase
Prophase
• Longest phase of mitosis.
• Chromosomes become visible.
• Nucleus begins to disappear.
• Centrioles begin to migrate to opposite ends of the cell.
• Spindle fibers begin to form between centrioles.
Structure of a Chromosome
Prophase
Metaphase
• “Middle Phase”
• Chromosomes become attached to spindle fibers and are pulled
toward the middle of the cell.
• Each sister chromatid is attached to its own spindle fiber.
• Chromosomes line up directly down the center of the cell.
Metaphase
Anaphase
• “Apart Phase”
• Sister chromatids are separated.
• This happens when the centromere splits and chromatids are pulled
apart by shortening spindle fibers.
Anaphase
Telophase
• “Two Phase”
• Chromosomes begin to unwind.
• Spindle fibers break down.
• Nucleus reappears.
• New membrane begins to form between the two new cells.
• CELLS HAVE NOT YET SEPARATED!!!!!!
Telophase
Cytokinesis
• Not technically part of mitosis
• Cytoplasm actually separates.
• 2 identical cells are formed.
Cytokinesis
Recap Question
• Now that we have made 2 new cells, what part of the cell cycle will
they be in?
Looking into Interphase
• Interphase can be broken down into three specific phases.
• G1
•S
• G2
G1: Growth Phase 1
• Cell grows to maturity
• Produces many ribosomes/proteins
• Builds up sufficient nutrients to undergo cell division
• When this is complete, the cell passes through G1 checkpoint.
• Checks for perfect DNA, enough protein/nutrients for division.
S Phase: Synthesis Phase
• If the cell passes through the checkpoint, it enters S phase.
• DNA and the proteins to carry it are doubled during S phase
G2: Growth Phase 2
• Organelles are doubled
• Organelles specific to cell division are made
• Even more protein is made
• Cell goes through one more checkpoint before committing to mitosis
• How many chromosomes are in the cell during G2?
• 92
Cancer: Scary Facts
• Cancer is the 2nd leading cause of death in the United States after
heart disease.
• It is estimated that there will be 1,695,000 new cases in 2016.
• Approximately 40% of Americans will be diagnosed with cancer in
their lives.
• Currently, the survival rate is approximately 70%.
What is cancer?
• The malignant growth caused by uncontrolled cell division.
• May be caused by environmental changes or a change in enzyme
production.
• Enzymes that control the cell cycle are called cyclins. Scientists believe
that the one cause of cancer may be errors in cyclins.
• These errors are caused by mutated genes.
• What type of macromolecules are cyclins?
• Proteins
Environmental Causes
• There are many environmental causes of cancer.
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Air Pollution
Sun
Tobacco Smoke
Bacteria/Viruses
Diet
• Cancer can also be genetic!
• No matter the cause, the thing that leads to cancer is mutated genes.