This is a DBQ (Data Based Questions Exercise).  It is required by the school and your results will be analyzed and compared with others from the school.  You will be graded on your ability to come up with conclusions from the information given.  Here is the grading criteria.  

Mitosis, Mutations and Cell Maturation  

Think of all the things inside your body that need to function to keep you healthy and fit. Your red blood cells need to bring oxygen to every other cell, your small intestine cells need to absorb nutrients from the food you eat, your muscles need to contract to make you move, your heart needs to constantly keep the oxygen and nutrients flowing, your kidneys need to regulate the amount of water and waste in your body, and your brain needs to control it all – and that’s just to name a few. 

There is no such thing as a typical cell – all have a specific job. Your body has millions of cells (over 200 specific kinds), each with its own structure and function.  These cells must work together to keep you healthy and fit.  All of your cells go through a lifecycle with four main phases:

1. G1: in which a parent cell grows and completes its’s normal job,
2. S phase: in which the cell copies its DNA,
3. G2: in which a cell grows and prepares for division
4. Mitosis: in which the cell divides into two identical daughter cells.

These daughter cells then start the same process, continuing the cycle.    

Normally, a cell must pass checkpoints before it can begin the next of the four phases. A cell will use external cues (is there is enough room in the organ for more cells?) and internal cues (is there DNA damage?) to decide if it is ok to move on to the next phase.  If errors or damage are detected, the cell cycle is halted, and the cell attempts to either complete DNA replication or repair the damaged DNA.  If the damage is irreparable, the cell may undergo apoptosis, or programmed cell death. This self-destruction mechanism ensures that damaged DNA is not passed on to daughter cells and is important in preventing cancer.

Age of cells

We lose about 96 million cells per minute.  Pretty crazy right!? Luckily, thanks to mitosis, we also gain about 96 million cells per minute.  The lifespan of a cell can vary widely, depending on what type of cell it is, and how hard it has to work.   For example, the cells lining the inside of your small intestine are replaced every 5 days, as they are constantly exposed to extremely harsh bacteria and acidic conditions.  White blood cells can last for about 13 days while constantly searching for and destroying foreign bacteria and viruses.  Your skin cells last for about 30 days (unless you scrape or itch them more!), your red blood cells last about 120 days, and your liver cells last for about 18 months.   

Your heart muscle cells, however have an extremely slow replacement rate.   This is particularly bad news for anyone at risk of a heart attack, which kills heart cells by blocking their blood supply.  One study found that when you’re 25, about 1% of your heart cells are replaced every year.  By the time you are 75, however, this number drops to 0.5%.  Calculations suggest that about half of your heart’s muscle cells are replaced over a lifetime (Bergman and Bhardwaj 2009).

There are some cells in your body that don’t ever undergo mitosis, namely neurons.  For example, cells in your cerebellum, the brain's outside layer that governs memory, thought, language, attention and consciousness, are the same when cells throughout your life.   Because these cells do not replace themselves, if they become damaged, we are susceptible to diseases like dementia.  Spinal cord injuries act much in the same way.  Spinal cord neurons are like long wires, and any scar tissue inhibits connections.  If your brain is not able to send signals to your legs and muscles, you lose your ability to control them or even move. 

Stem Cell Therapy

Scientists are now looking to stem cells to help replace damaged heart and nerve cells.  Stem cells, unlike the specialized cells mentioned above, have no specific job - yet.  They are usually found in developing embryos, but there are a few reserves in adults (bone marrow, liver, brain).  These adult stem cells remain dormant in an individual until activated by tissue damage or injury.

Although stem cells do not have any one function, they are able to specialize into different types of cells.  Every cell in your body is derived from the stem cells that made up your embryo, meaning that these stem cells can be induced to become any cell type.

This makes stem cells particularly interesting to researchers working on tissue and organ regeneration.  Traditionally, an organ transplant was the only way to replace ones that were beyond repair, but scientists are looking into inducing stem cells to regenerate damaged tissue under the right conditions.  Stem cells from just below a burn victim’s skin, for example, have been induced to develop skin tissue and eventually grafted on the victim.

Studies are being conducted using stem cells to regenerate cells that very rarely divide – namely cardiac muscle and nervous tissue.  By injecting your own stem cells into the damaged area, the hope is they can be stimulated to generate new and healthy cardiac muscle cells. Additionally, replacement cells and tissues may be used to treat brain disease such as Parkinson's and Alzheimer's by replenishing damaged tissue, bringing back the specialized brain cells that keep unneeded muscles from moving.

Cancer

We’ve read about the cell cycle and how often cells should divide, but what happens when things go wrong?  When DNA becomes damaged via mutagens like UV radiation, or even just age, they normally undergo apoptosis (basically they terminate their own life).  However, if there is repeated damage to cells or DNA, they can become cancerous. 

Cancer is essentially mitosis gone crazy. Cancer cells pass those checkpoints during the cell cycle and keep mutating.  Not only do they lose the ability to control division, but they also stop doing their “normal” cell activities.  For example, a cancerous growth on the surface of your lungs will reduce the area of healthy lung cells to exchange oxygen and carbon dioxide.  Although they are on the surface of the lungs, they do not perform the function of lung cells.

T-cells in your body are constantly moving around, looking for any bacteria, viruses or foreign invaders to battle.  Cancer cells are invisible to your T-Cells because they ARE your cells.  This is why it takes so long for your body to realize there actually is a problem.  It also makes cancer tricky to battle for researchers – how do you kill cancer cells without killing the rest of your cells?

Chemotherapy drugs are specifically designed to attack cells that are flying through these checkpoints and constantly undergoing mitosis.  One unfortunate side effect is hair loss and thin brittle nails.  If you think about it, hair and nails are the only cells on our body that we can regularly “see” growing.  Since these cells must constantly divide to grow, chemotherapy also attacks these cells as well.