Mitosis

Where is mitosis happening?

I had pizza last night.  And then had heartburn.  But I bit into the pizza and instantly burned the roof of my mouthThe opening of the digestive tract where food enters and mastication begins. and my lip. In doing so, I just some of the skinThe body’s largest organ, providing protection and regulation. cellsThe basic structural and functional units of life. of my lip and some of the cells in my mouth.  Now they need repair.

Part of that repair will involve replacing the damaged cells. Those replacement cells will be provided by copying some of the nearby, undamaged cells.

Luckily this is a small wound and won’t take long to heal but it still involves producing thousands of new cells through cellular reproductionThe process of producing offspring., also called cell division. At my age the principle function of cell division is the repair and replacement of damaged cells.

Another function of cell division is growthAn increase in size and number of cells., like the growth that is happening at the tip of this onion root. You are built from approximately 60 trillion cells, and every one of those cells can trace its origin back to a single fertilized egg that formed about nine months before you were born.

Every cell in your body today was formed via cellular reproduction starting from that first cell.  Cell division ensures that the set of genetic instructions passed on to each of your cells is complete and identical.

Different kinds of cells divide at different rates depending on their role. Some human cells (like your skin cells and the lining of your digestive tract) divide every day. Others, like nerve cells, are so specialized that they never divide at all. 

This may help explain why brain injuries and nerve damage may never heal. Cells that do divide follow a sequence of steps known as the cell cycleThe sequence of events in a cell’s life, including growth, DNA replication, and division..

Cells have a life cycle of events

The cell cycles makes me think of this old punk rock song by Gang of Four.  It was called: Birth, School, Work, Death.  So, the cell cycle always begins with division from another cell and then ends with division into two new cells…birth and death.  Yes, there are two phases in between, but they are not school and work, they are interphaseThe phase of the cell cycle in which the cell grows, performs its functions, and replicates DNA. and the mitotic phase.

Interphase has three subphases

Interphase has three subphases, but two of them is when the cell is going about its usual business: Gap 1 and Gap 2. During interphase, your brain cells are computing, your muscle cells are contracting, and the cells lining your small intestine are digesting.

A cell normally stays in interphase unless it receives specific signals from its environment. Once such a “go ahead” signal is received and processed, the cell then moves on to continue the cell cycle. 

My skin cells that didn’t burn, knew of the death of the neighbors and were preparing to divide as I slept last night.  Some cells remain in interphase forever, like the neuronsThe functional cells of the nervous system that transmit signals. that make up the brain and nerves of your body.

To continue the cell cycle and begin the process of cellular reproduction, a cell must first copy (or replicate) the chromosomes within its nucleusThe control center of the cell that contains DNA and directs cellular activities.. This is accomplished during a sub-part of interphase known as the S phaseThe DNA synthesis phase of the cell cycle where chromosomes are replicated., or synthesis phase.

Copy then divide

During the S phase, every chromosome in the nucleus is copied. When a chromosome is duplicated, the two copies are called sister chromatids and they stay attached to one another at a location called the centromereThe region of a chromosome where sister chromatids attach and spindle fibers bind..

Imagine yourself standing in a room. You are a chromosome and the room is the nucleus. Now imagine that I copy you, producing an identical twin. You and your twin stay joined at the hip, attached to one another like conjoined twins (sometimes called conjoined twins).

You and your twin would make up a pair of sister chromatids with the centromere joining you at the hip. You look around and see that, within the room of the nucleus, you are one of 46 sets of identical conjoined twins, each joined at the hip. These are the 46 sets of sister chromatids found in the nucleus at the end of the S phase.

Conjoined twins and other human specimens of interest

Can I just take a moment here?  Sorry, but there is this fascinating museum of human oddities in Philadelphia Pennsylvania.  They have many preserved specimens (skeletal and otherwise) of conjoined twins. It is called the Mutter museum.  I’ve been there many times and had the pleasure of meeting the curator Gretchen Worden, who was a bizarre and fascinating woman.  It’s worth a trip if you’re in eastern US.

Mitosis

Alright then. Let’s now focus our attention on the second stage of the cell cycle: mitosis. The purpose of mitosis is to distribute those copies to the individual daughter cells.  Remember that each new cell with be a clone of the original.

The cell does not normally spend much of the cell cycle in mitosis but many biology instructors do.  Mitosis is a continuous process that is divided into four stages called prophaseThe first stage of mitosis where chromatin condenses into visible chromosomes., metaphaseA stage of mitosis where chromosomes align at the metaphase plate., anaphaseA stage in mitosis or meiosis where sister chromatids or homologous chromosomes separate., and telophaseThe final stage of mitosis where nuclear envelopes reform around separated chromosomes..  PMAT.

I like this diagram of mitosis for an overview or feel of the process.  Note that there are four chromosomes in this cell (humans would have 46) and it seems as though the chromosomes form Mom and Dad are different colors. 

Prophase is the stage when duplicated chromosomes condense and become visible. The nuclear envelopeA double membrane that surrounds the nucleus and separates it from the cytoplasm. disappears. From each end of the cell a network of microtubules (or very little tubes) forms. This network of fibers is called a spindle. Each chromosome attaches its centromere to a fiber in the spindle. The duplicated chromosomes then start to move to the center of the cell.

During metaphase, all of the duplicated chromosomes arrive and line up at the center of the cell.

Anaphase begins when the sister chromatids are separated from one another. That is, each conjoined twin is ripped apart from its partner, and the two twins move in opposite directions away from one another.

This movementA fundamental property of life involving motion of the body or its parts. is achieved by motor proteinsLarge molecules made of amino acids with various functions in the body. on each centromere that drag the chromatids along the tracks laid down by the spindle. Each chromosome is now single, separated from its duplicated partner and we can distribute these copies to the new daughter cells.

During the final stage of mitosis, telophase, the single chromosomes arrive at opposite ends of the cell. On the stage, the two sets of 46 individual dancers arrive at the opposite ends of the stage, begin to mingle, and disappear.

The chromosomes unwind, become invisible once again, and two new nuclear envelopes form, one around each set of 46 chromosomes.

We have now achieved the goal of mitosis: we’ve created two nucleiClusters of neurons in the CNS responsible for processing information., each with an identical set of chromosomes.

cytokinesis

Once mitosis is over, we still have to divvy up the rest of the cell, such as the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. and the organellestructures within a cell that perform specialized functions.. This process, known as cytokinesis, actually starts as telophase is ending. The result is two new daughter cells, each with its own nucleus and its own share of the cytoplasm.  Cytokinesis in plant cells involves this cell plate that separates the two daughter cells.  Animal cells form this cleavage furrowThe indentation in the cell membrane that forms during cytokinesis in animal cells. which divides the cells.

The cell cycle: copy, then divide

Kirby!!