The Cell Cycle, Cloning, and Plant Genetics

In this class, we started by playing the homework computer game together.  It was a game from NobelPrize.org (an excellent site for games about Nobel prize discoveries) about the cell cycle.

Most importantly,
1.  Cells use the cell cycle to reproduce
2.  The cell cycle is very well controlled.  Cells are not allowed to reproduce if they can't pass through the checkpoints.
3.  When something goes wrong and cells can reproduce out of control, that is called cancer.
4.  There are several steps of the cycle:  growth, synthesis, growth, separation, division

Here is the link to the game we played.

We read through everything together, and learned these facts (and more):

• Cells in our bodies are going through the cell cycle, reproducing themselves, every second
• Skin cells and blood cells are dividing all the time, but other cell types, like liver cells and brain cells, do not divide very often.  
• In order to divide, a cell needs to receive a signal that it's ok to proceed.  Sometimes this signal is the death of a nearby cell.
• Once the cell has received the signal, it grows by 20%, which is called the Gap 1 phase.
• Then it reaches Checkpoint 1, where the cell is checked to confirm that it has grown enough and that its DNA (the blueprint for creating the entire organism) is not damaged.
• If it passes the checkpoint, it goes into Synthesis phase, where it duplicates its DNA
• Note:  each cell needs to have exactly the right amount of DNA, which in humans and most other organisms is two complete copies of every chromosome.  Any more or less could be disaster for the organism.
• Plants, however, often have 4 or even 8 copies of each chromosome, and end up being larger.
• Humans who have one extra chromosome 21 (a small chromosome without many genes on it) have Down syndrome.  Humans who have an extra X chromosome are ok because every cell inactivates one copy of the chromosome, and humans who have an extra Y chromosome are ok because there are not many genes on it.  But an extra copy of any other chromosome usually leads to death before birth and is a very severe problem.  
• So it is essential for cells to make sure they have exactly the right amount of DNA before they start duplicating it, and they need to make sure they copy it perfectly.
• We have enzymes whose job is to proofread the DNA.
• Once the DNA has been duplicated, the cell needs to grow again, so it enters Gap 2 phase.
  
• Then is Checkpoint 2, when the cell is checked to confirm that it has made enough DNA, that there are no DNA errors, and that the cell has grown large enough.
• If damaged DNA is detected, the cell can either repair the damage, or the cell can commit suicide.
• If it passes this checkpoint, then the chromosomes all line up in the center of the cell, attach to spindles, and separate apart so that each half of the cell has one complete set of chromosomes.
• Checkpoint 3 is to make sure that each chromosome is attached to spindles.  
• If it passes, then the cell can divide and there are now 2 cells, identical to the first.

The next game we played illustrated exactly how to clone a mouse using a procedure called Somatic Cell Nuclear Transfer.

• The idea is to create a genetic clone of Mimi, a brown mouse.
• First, we discussed the difference between clones, siblings, and identical twins.
  
• To clone Mimi, we will take a body cell (somatic cell) from Mimi, probably a skin cell, an egg cell from Megdo (a brown mouse), and have Momi be the surrogate mother (a white mouse who will let the embryo grow into a baby mouse in her womb.
• We will know if it worked by the color of the baby mouse.
• The steps are:

1. Take the somatic cell from Mimi and a fertilized egg from Megdo.
2. Remove the nucleus from the fertilized egg. (this is called enucleation)
3. Transfer the nucleus from the somatic cell into the enucleated egg.
4. Wait a little while for the nucleus to adjust to its new home, then add a chemical to tell the cell to begin dividing so it can create an embryo.
5. Put the embryo into the surrogate mouse (Momi).
6. After 19 days, the baby mouse will be born -- what color will it be (if the procedure worked)?

I mis-spoke to the class by telling them that there is currently an international ban on human cloning and research focused on cloning humans.  This is controversial, and only a few countries actually have a ban.  The United States does not have a ban on human cloning.

Lastly, we began a game focused on plant genetics.  It started with an introduction to what Gregor Mendel did.  We saw that with his pea plants he was able to observe a pattern in how characteristics showed up in baby plants when he chose who the parents would be.

The traits (characteristics) we worked with were:

• pea color -- green or yellow
• flower color -- pink or white
• stem length -- long or short

There were many other traits he looked at, and most of them had two possible versions (like green or yellow peas, as described above).  By observing what happened, Mendel figured out that each plant has two copies of a gene, which is the part of the DNA that determines that particular trait.  They get one copy from the mom and one copy from the dad.  One version always seems to "win" over the other, so that if a plant has one copy of the green pea gene and one copy of the yellow pea gene, their peas are not greenish yellow, but instead are yellow.  The green pea gene is "hiding" within the plant.

Our job in the game was to try to create plants with a specific set of target characteristics, as quickly as possible.  You can see both what the plants look like and what their genes are.  Try it out, and see how far you get!!!