The Cell Cycle

The Cell Cycle

The cell cycleThe sequence of events in a cell’s life, including growth, DNA replication, and division. is basically an order sequence of life events for a cell.  Just as humans designate stages of life such as infant, adolescent, and geriatric, cellsThe basic structural and functional units of life. also have stages of their life.  The cell cycle also depends on another really really important factA statement based on direct observation that is repeatedly confirmed.: all cells come from pre-existing cells.  I hope that you are all asking yourselves when, then, did the first cell come from?  That’s a topic for another class I teach!  But, this also means that when a cell is “born” from another cell, it lives an ordered sequence of events. It continues this cycle until it “dies” giving birth to two other cells.  Between those two events, a cell will make proteinsLarge molecules made of amino acids with various functions in the body. and grow. It will make copies of all its DNA. Then it will grow some more. Finally, the cell will divide and distribute the copies of the DNA.

This little circular diagram over here is the cell cycle.  A cell would enter here and G1 which is known as gap or growthAn increase in size and number of cells. 1 phase.  The S phaseThe DNA synthesis phase of the cell cycle where chromosomes are replicated. is named for synthesis. During this phase, the cell is making or synthesizing DNA. Then comes G2 for some more growth. Finally, the cell enters the M phase.  Now, if you are a gamete-making cell in an ovary or testis, you will perform meiosis. This occurs during the M phase.  Any other cell in the body does mitosis.  In fact, the stem cell that leads to the creation of sperm uses mitosis to duplicate itself. Then, it pushes one of the resulting clones through meiosis. 

Chromosome Packaging

Chromosome packaging is very important. It separates multicellular complex organisms like humans from simpler bacteria.  You might think that having more chromosomes means you’re a better organismA living individual made up of cells.. However, I believe that managing all the responsibilities of life with just one chromosome is pretty fantastic. All of your little strands of DNA add up to 46 strands. Each one of these strands is called a chromosome. It’s a strand of DNA.  You should think of strands of DNA like socks. Imagine the nucleusThe control center of the cell that contains DNA and directs cellular activities. as the smallest suitcase ever. You’re going on a two-week vacation. DNA is packaged or wound around these proteins called histones. The resulting structure from the histones and the DNA is called a nucleosome. These nucleosomes then coil to make a condensed chromosome. While the cell is in the gap 1 and gap 2 phases, the DNA is dispersed. This happens because the cell uses the recipes contained in it to create proteins. During the synthesis phase, when the cell copies all its DNA, it is condensed into these nucleosomes. This also occurs in the M phase. The cell divides all of its DNA. It is condensed into the super coil. When chromosomes condense it allows us to see them with very low grade microscopes.

Gap Phases

The gap phases or growth phases are pretty simplistic in an explanation. There is one before and there is one after the S phases. Because the cell is making proteins and accessing the DNA the DNA is in its dispersed form called chromatinDNA and associated proteins in a loose, uncondensed form inside the nucleus..

S Phase

The S phase is so named for the synthesis of DNA. This is when DNA replication involving DNA polymeraseAn enzyme that synthesizes new DNA strands by adding nucleotides to a template. occurs. 1 chromosome or one strand of DNA grows a sister chromatidOne of two identical copies of a duplicated chromosome. that is attached at the centromereThe region of a chromosome where sister chromatids attach and spindle fibers bind.. Both of these structures here are chromosomes one of them is a chromosome in its duplicated state. We have to think of the duplicated state as two copies of the same chromosome attached together. Before we reach the M phase, also known as the divisional phase, the S phase is crucial. During this phase, we are copying the chromosomes. These chromosomes will be distributed to the 2 new clones.

M Phase

As I mentioned before the M phase can be either mitosis or meiosis. Is determined by the location and the purpose of the cell. There are certain cells called oocytes or spermatocytes. These are the cells in ovariesThe female gonads that produce eggs and hormones. and testes that perform meiosis. This process leads to the formation of gametes. Gametes are either sperm or ovumA mature egg cell more casually known as eggs. So that for reproductive cells though the designated for meiosis what about mitosis? Almost all of the cells in your body are capable of mitosis. However, that doesn’t mean they choose to do it.

G₀ phase

Do it off to the side of the cell cycle, sometimes you’ll see somebody note something like this. This is the G₀ phase. This is what we designate for cells that will never complete the cell cycle. There are cells that will never duplicate their chromosomes and go through gap to an M phase. Neurons for example are cells that do not like to go through mitosis. They kind of spend their time in this G₀ phase with absolutely no intention of completing the cell cycle.

Checkpoints

The cell cycle being an ordered sequence of events, has checkpoints. There are more checkpoints than the ones that you see here. These are good ones to get started with. They help to understand the function of checkpoints. The gap 1 checkpoint ensures that the cell has enough proteins. It also confirms that there is sufficient machinery to go through with DNA synthesis. If that checkpoint is passed then the cell can get out of G1 and move into the S phase. The second checkpoint at G2 does two things. It checks for the growth level of the cell. It also ensures that all of the DNA was copied correctly. If it finds incorrect copies of the DNA, other cell cycle proteins will try to fix them. If there’s no way to actually repair the DNA, the cell will undergo programmed cell death. This process is called apoptosisProgrammed cell death, an essential process for growth and development.. One significant checkpoint occurs during mitosis or meiosis. It ensures that all of the chromosomes are lined up correctly on the metaphase plateThe imaginary plane in the center of the cell where chromosomes line up during metaphase.. If the chromosomes don’t line up correctly, the resulting cells will not be clones. One of the cells will receive more chromosomes, and the other will not have enough. That’s not good if you are trying to make clones.

Hallmarks of
Cancer

You know I found this slide. The title of it was hallmarks of cancer. However, I feel this diagram shows the dangers of a cancerous cell. It also provides general guidelines that are used for staging cancers. Cancer is the cell cycle out of control. Checkpoints are being disregarded. Mutated cells are continuing to divide. Programmed cell death, or apoptosis, does not seem to be happening.  Before discussing this diagram, I want to mention that cancer doesn’t take just one mutationA change in DNA sequence that can affect gene function.. It requires many mutations collected over a lifetime for a cancerous tumor to result. The first four things listed here ABC and D accurately show how a tumor would escape cell cycle checkpoints. This escape process involves bypassing those checks. The cell is not checking its surroundings for enough room to grow. It ignores any signals to stop growing. It might even ignore signals to undergo apoptosis.

These last two pictures are really the last stages of aggressive cancers such as colon cancer. In the last stages of cancer cancerous cells start to invade tissues that are distantly located to the tumor. This is called metastasis and is basically like pieces of a tumor breaking off and circulating in the body. The other hallmark of an advanced cancerous tumor is that the tumor has started to grow its own blood vessels. So it has tricked the body. It makes the body think that it is a valid living tissue. This tissue needs to be sustained with glucoseA simple sugar that is the main source of energy for cells. and oxygen.