Mitosis

Inside the nucleus of each of your cells is an exact copy of the DNA that you received from your parents. As you know, every eukaryotic cell consists of molecules — water, proteins, lipids, carbohydrates nucleic acids and some other things — all organized inside a membrane. The membrane is referred to as “semi-permeable.” This means some things can cross the membrane and other things can not. In a normal environment, the cell controls what is inside and what is outside of itself. The organization inside the eukaryotic cell is complex, and includes many organelles. Organelles are lipid-bound structures that contain molecules organized to do specific functions, for example photosynthesis in plants, cellular respiration in nearly all cells, and my favorite, pigment granules inside pigment cells. Prokaryotic cells (like bacteria) are equally well designed, but they don’t have membrane-bound organelles inside themselves.

The nucleus of eukaryotic cells is the central core of the cell. It has two membranes around it, and it contains the DNA. DNA is the genetic material that is passed from generation to generation as a coded molecule made of nucleotides (we discussed in the last few posts). DNA is a physical molecule that is kept safe in the nucleus of your cells and copied exactly every time a new cell is made. It does not shift around or change its code (unless there is a mistake, which is very rare). It also does not leave the nucleus of the cell.

DNA has nothing to do with our political lives and is not found in our behaviors or our social structures — Bill Moyers notwithstanding, and if anyone knows Bill Moyers I hope he will read this little book so he can do a more accurate job of representing biology. Our understanding of the limiting parameters of biology (and our response to those limits) will determine whether or not humans on earth can continue the lifestyle to which we have become accustomed. So to Bill Moyers I say:

“Democracy does not contain DNA, and biology is not a whim of human language. Biology is a fact of  nature that will be here whether or not humans continue on this earth. It is far more important to our survival than either our democracy or our ability to make cute metaphors.”

Oh, oops, I got off on a rant, but you get the point. DNA is not a social reality. DNA in nature is designed to maintain the code of life and to regulate the biology of the cell. The DNA molecule carries the coded instructions for operating your body, cell by cell, and that’s all that it does. And that is one reason DNA is so carefully controlled in the cell, so it should not make mistakes in the code or wander out of the nucleus of the cell.

In the past few posts we have given an overview of DNA replication. DNA replication happens inside the nucleus of each cell when it comes time for the cell to replicate (that is when one cell divides to make two cells). To divide, the cell gets bigger, stretches out longer, and then pinches itself in half in the middle. Before it divides, the cell must make another copy of the genetic code, and then it must have a way to make sure that each new cell gets one of each of the chromosomes (the genetic code), so the two new cells are genetically identical. So that is when DNA replication occurs. We explained DNA replication in the last couple of posts.

After the DNA replicates, then the cell has two exact copies of each chromosome. Each chromosome is one enormously long DNA molecules, plus some proteins and other things that cluster around the DNA. All the DNA in all the chromosomes are your genetic code.

Because it is so long, the chromosome has to wind itself up into a shorter space before it replicates. These shorter, wound up (condensed) chromosomes are what you may have seen in pictures. The original chromosome and the copy chromosome (of each) remain attached to each other, and they all are still in the nucleus of the cell. Then, just before the cell begins to pinch itself into two cells, the nucleus dissolves. The condensed chromosomes line up in a space like a flat circle that is referred to sometimes as a “plate” between the two ends, preparing for mitosis.

Mitosis is the process of cell division that gives rise to two identical daughter cells.

The double chromosomes line up across the middle of the elongated cell, so that one of the duplicated chromosomes faces each end. Structures (microtubules, they are made of molecules) form in each end of the cell that look like little strings. The copy chromosomes are still attached to the original. The microtubules from one end attach to one of each duplicated chromosome, and those from the other end of the elongated cell attach to the other of each duplicated chromosome. Then one of each different chromosome is pulled to the left and the other is pulled to the right. The cell pinches itself in the middle until there are two cells that each have one of every different kind of chromosome, a nucleus forms around the chromosomes, and they stretch out long again so they are no longer condensed. The two daughter cells are genetically identical to each other and to the original cell.

This is the process of mitotic cell division (mitosis) and I’m sure you can find it on the web and in many books, in great detail, with lots of names for all the different stages of division. The detail is not as important to us as the bottom line, which is:

1. Every chromosome must replicate its DNA so each new cell will have the exact genetic code as the parent cell;

2. The replicate chromosome stays attached to the original chromosome while they line up in the center of the elongated replicating cell.

3. One copy of each chromosome is pulled to the left and another copy is pulled to the right in the elongated cell.

4. If all these processes are done properly, then each new cell has an exact copy of the DNA code of the original cell.