The Cell (part 2)

The Centrosome:

Most of the cell’s microtubules originate somewhere near the cell’s nucleus at an area called the centrosome. The centrosome is where microtubules are organized, and can also be constructed/torn down. It is from here that the polar and kinetochore microtubeles (mitotic spindle, which we will discuss at length in the mitosis section) originate. Within the Centrosome’s grainy looking matrix is a pair of centrioles. Centrioles are non membranous barrel shaped organelles composed of nine triplets of microtubules each (see image). These centrioles are organized at right angle to eachother. Besides being found in the centrosome, centrioles are found at the bases of cillia and flagella

Cilia, Flagella, and Microvilli

Cilia are thin, hairlike cellular extension that can move. Using motor proteins, cilia can propel substances across the free surface of epithelial tissue. One example is the cilia that line our respiratory tract and propel mucous up and out of our respiratory system. Cilia are formed when a centriole divides and the new centriole migrates towards the cell membrane. Once at the membrane it generates microtubules which push upwards into the cell membrane. As the membrane protrudes and expands, cilia are formed. A special type of cilia called a primary cilium is found in many types of cells. This cilium does not propel substances but is used rather to probe the external environment using the sensors scattered throughout its membrane.

Flagella are very similar to cilia, but much larger. Flagellum are used to propel cell’s rather than move substances, as  is the case with cilia. The only example in humans of flagella are sperm cells.

Microvilli

Microvilli are small but numerous extensions from a cell’s apical or free surface. This type of cellular extension is common in tissues that absorb things, such as the simple epithelium lining the intestinal tract. The presence of microvilli greatly increases the surface area that cells and tissue can use to absorb substance. Unlike cilia and flagella which are composed internally of microtubules, microvilli get their form and shape from actin, specifically from actin protruding out of the terminal web.

The Nucleus

How many times have we heard that the nucleus is the control center of the cell? Well, it is absolutely true, but what in the world does that mean? I believe that describing the nucleus as the control center; as if it has a mind of its own, can be a bit misleading. Essentially, all that the nucleus really does is contain our genetic material (DNA). When you think about the nucleus in this way it sort of makes the nucleus sound like microscopic Tupperware. What is truly amazing is what is found inside of that Tupperware. Who knew leftovers could be so fascinating!

The Nuclear Envelope

Before we dive into the inner workings of the nucleus lets examine its membrane, the nuclear envelope. This envelope has two layers of phospholipids, much like the mitochondrial membrane. The outer layer connects to the rough endoplasmic reticulum, while the inner layer is packed with lamin proteins that give the nucleus strength and shape while helping to organize DNA in the nucleus. Scattered throughout this double membrane envelope are nuclear pores that allow certain substances to pass in and out of the nucleus. Large proteins and mRNA molecules pass through the pores using an energy dependent process, while smaller solutes diffuse in and out of the nucleus passively. Contained within the Nuclear envelope is the nucleoplasm. Let’s check that out next.

The Nucleoplasm

Similar the cytoplasm, the nucleoplasm contains solutes dissolved in fluid that suspend the other elements of the nucleoplasm (nucleoli, chromatin), much like how the cytosol suspends the organelles in the cytoplasm. The elements contained within the nucleoplasm are described below:

• Nucleoli:  Nucleoli look like little nuclei inside of the nucleus. Most nuclei contain 1-2 nucleoli depending on the type of cell. Nucleoli are responsible for assembling ribosomal sub-units, and like ribosomes they are non membranous. You can expect that growing cells, or other cells that must produce a lot of protein will have many nucleoli. Why? well because nucleoli produce ribosomal sub-units, which combine to form ribosomes, that translate mRNA into protein. Keep in mind that the ribosomes aren’t necessarily produced by nucleoli, but rather the ribosomal sub units are produced here. Many of these sub units must leave the nucleus through nuclear pores to be assembled in the cytoplasm.

• Chromatin: Chromatin consists of our genetic material (DNA) and associated proteins. In a cell that is not dividing, it appears as a gel like substance floating in the nucleoplasm. If we look much more closely at chromatin, we see that it is a mass of DNA strands wrapped around proteins and packed in unique units called nucleosomes. Chromatin is approximately 30% DNA, 60% disc shaped histone proteins, and 10% RNA chains. If we take a closer look at chromatin, we see that it consists of groups of 8 histone proteins with DNA wrapped around each group (see picture). This allows the DNA to be more tightly condensed and organized. Each group of histone proteins and the associated DNA is called a nucleosome. The Nucleosome is the fundamental unit of chromatin, and allows for the large amount of genetic material (about 2 meters worth of DNA) to be packed inside of the nucleus. Although nucleosomes provide an organized way of storing DNa, it is not nearly compact enough to allow for orderly ans safe cellular division. Before a cell can divide, the chromatin must further condense and coil to form small X-shaped chromosomes. Chromosomes are very compact and dense, which prevents DNA from tearing or breaking during cellular division. Let’s examine the events of the cell cycle, and cellular division next.

The Cell Cycle and Cellular Division

The Cell Cycle: The cell cycle can be compared to a human beings lifetime. Cells are “born” they grow, and then they produce. Some cells grow very quickly and reproduce many many times, while others grow slowly, or perhaps seldom/never reproduce. At a very basic level we can divide the cell cycle into two very distinct phases: Interphase, and the mitotic phase (cellular division phase)

• Interphase: Very simply put, interphase is the phase in which the cell is not dividing. It starts at cell formation and ends at cellular division.  During this phase the cell is carrying out its routine functions which may include cell growth, exporting proteins, filtration, excretion, and many other cellular activities. This phase can be broken down into 3 subphases which are listed below:

1.  G1 Subphase: During G1, cells carry out their routine functions. Additionally the centrosome begins to duplicate during this phase. During cellular division it is necessary to have two centrosomes. This phase can very greatly in length based on cell type. In cells such as neurons, this phase may be indefinite, meaning that cells never leave this phase. In cases such as these, the cells are said to be in the G0 phase. Other cells, like special types of cells in the epidermis, may be in this phase for only hours.
2. S phase: During S Phase, our genetic material is copied. All of the DNA is duplicated exactly, ensuring that the two cells produced during cellular division are genetically identical. All of the components of chromatin are duplicated during this process, including the histone proteins that form the nucleosomes.
3. G2 Subphase: During this very short phase, all of the necessary proteins and enzymes required for cellular division are synthesized. Also, centrosome duplication finishes during this phase. By the end of this phase, the cell is ready to divide.

Before we dive in to mitosis (cellular division) lets take a look at the main event of S phase, DNA replication. The process of DNA replication occurs over the course of four distinct steps. Before we examine these steps, let’s lay some groundwork. Throughout our genetic material are sites called origins of replications. These origins of replication have a specific series of nucleotides (remember nucleotides are the units of DNA that code for amino acids). This specific nucleotide code tells an enzyme called DNA polymerase to begin the synthesis of DNA. Let’s take a look at the 4 steps of DNA replication

1. Enzymes attach to the previously mentioned zones of replication and separate the DNA strands forming replication bubbles. at one end of the replication bubble is a replication fork where the DNA is being unwound.
2. Once the replication bubbles are formed and the DNA is unwound, the DNA strands are ready to be duplicated. A short RNA primer which is roughly ten bases long attaches to the origins of replication on either strand.
3. From the RNA primer, an enzyme called DNA Polymerase adds nucleotide to the primer, thus creating a complementary strand of DNA. One interesting feature of DNA polymerase is that it moves in only one direction. Therefore one DNA polymerase follows the unwinding of DNA towards the replication fork (this strand is called the leading strand) The other DNA polymerase must synthesise new DNA in sections, requiring a new RNA primer to initiate the creation of each new DNA segment. This strand is called the lagging strand.
4. Lastly, ligase enzymes splice and attach the short segments of the lagging strand together, and the RNA primers are replaced by DNA.

Thats it guys, stay tuned for part 3 where we will talk about the mitotic phase and protein synthesis.

Related articles

• The Cytoplasm (aandpsource.wordpress.com)
• Quiz #1: The Cell (aandpsource.wordpress.com)
• Practice Quiz #1: The Cell (aandptutor.wordpress.com)
• Cell Biology 101: Defining Centriole (biology.answers.com)
• Learning the Stages of Mitosis: Prophase (biology.answers.com)
• The machinery of mitosis: Kinetechores, centrioles and chromosome pumps (phys.org)