The Cell (Part 3)
Cells need to divide. Why? well because cells die and need to be replaced. Also, in order for our bodies to grow our cells must divide. Some cells, such as cells of the deep epidermis and epithelial cells lining the intestinal tract divide quite rapidly, while cells like neurons and muscle fibers do not divide at all once mature. Let’s examine the M phase of the cell cycle, which is the phase in which cell division occurs.
M Phase: M phase, or mitotic phase, is broken down into two distinct sub phases. Mitosis, which is further divided into more subphases, and cytokinesis.
- Mitosis: the division of the genetic material and the nucleus
- Cytokinesis: The division of the cytoplasm and plasma membrane
Another type of cell division called meiosis results in four daughter cells, each with half of our genetic code. This process of cell divison is used to produce our sex cells. We will discuss this type of division when we learn about reproduction. For now, let’s talk about the phases and events that occur during Mitosis
- Early Prophase: During early prophase our chromatin condenses and forms the x-shaped chromosomes. This prevents our genetic material from being damaged during cellular division. Each chromosome consists of two copies of our DNA, each copy is a bar shaped structure called a sister chromatid. These chromatids are held together centrally by a structure called the centromere. Later, in anaphase when the chromatids separate, they will each be called chromosomes. Confusing huh? Make sure to look at the pictures and leave comments if you need help. The nucleoli also dissapear in this phase, and the centrosomes that were duplicated during interphase now begin to push eachother towards the opposite poles of the cell. It is these centrosomes that will be responsible for pushing the cell apart, and pulling the sister chromatids apart later in mitosis.
- Late Prophase: During the second half of prophase, the nuclear envelope will dissolve. This allows the newly formed mitotic spindle, which consists of the two centrosomes and their associated microtubules, to interact with the chromosomes. The microtubules within the mitotic spindle can be categorized into two groups. Some of the microtubues growing from each centrosome will be used to push the poles of the cell apart (polar microtubules), other specialized microtubules will attach to the centromere of each chromosome to special proteins called kinetochores (kinetochore tubules). The polar microtubules continue to grow, pushing the cell apart, while the Kinetochore microtubules pull on the chromosome towards their respective centrosome. This results in a sort of tug of war that results in the chromosomes slow migration towards the cell’s center.
- Metaphase: The centrosomes are now at completely opposite poles of the cell thanks to their polar microtubules forcing them apart. Meanwhile the kinetochore microtubules have aligned all of the chromosomes along the cell’s quator, otherwise referred to as the metaphase plate. Once at the equator, special enzymes begin to degrade the centromere which binds the two sister chromatids together.
- Anaphase: This phase begins the second the sister chromatids split to form two new chromosomes each. It is the shortest phase of mitosis, and consists of the kinetochore microtubules “reeling in” the chromosomes towards the opposite poles of the cell. This causes the chromosomes to appear v-shaped as they move through the thick cytoplasm. While the chromosomes migrate to the poles of the cell, the polar microtubules continue to force the poles of the cell away from eachother.
- Telophase: As soon as the chromosomes are done being pulled towards their respective centrosome, and chromosomal movement stops, telophase begins. Telophase is the final phase of mitosis and it ressembles the first phase, prophase, only in reverse. The first event of telophase is the unpacking of the chromosomes. The chromosomes uncoil and revert back to chromatin. Remember, the chromosomes were pulled apart, so when they uncoil it results in the formation of two masses of chromatin at either pole of the cell. A nuclear envelope then forms around each chromatin mass and the nucleoli reappear. The mitotic spindle also dissambles during telophase. Once these events are complete mitosis is over. At this point the cell is binucleate. Meaning that there is a nucleus at either pole of the cell.
Cytokinesis: Remember that the mitotic phase actually has two sub-phases, mitosis and cytokinesis. When mitosis ends we have a cell with a nucleus at either side. During cytokinesis the plasma membrane constricts and actually pinches the cell into two identical daughter cells. This is accomplished by motor proteins within the actin microfilament terminal web just deep to the plasma membrane.
Okay guys, protein synthesis can get a bit intense. Take your time, and use all of the resources available to you on this site to learn this important cellular process.
Protein synthesis, put plainly is the production of proteins. Cells are essentially little protein factories. The blueprints of the many types of protein are contained within our DNA otherwise known as our genetic “code”.
Remember that DNA is composed of long sequences of 4 nucleotides. These nucleotides are abbreviated as A, T, C, G. and these bases repeat in special combinations that code for specific amino acids. Long sequences of these nucleotides code for long chains of amino acids linked together by peptide bonds (a protein molecule) These sequences of nucleotides that code for proteins are called genes. You can think of the nucleotides A, T, C, G as the letters of the genetic alphabet. much like computer code that contains sequences of 0’s and 1’s, the genetic code contains code comprised of the nucleotides. Sequences of 3 nucleotides that code for a specific amino acid are called a triplet. By arranging triplets in a specific order, a gene that codes for a specific protein is formed. Below we will talk about the two phases of protein synthesis, transcription and translation.
Simple right? here’s the problem… DNA cannot leave the nucleus, and if you recall from earlier, ribosomes, the organelles that translate our genetic code into proteins, are located in the cytoplasm. So what happens? Enter mRNA. mRNA is similar in structure to DNA, except that it is single stranded and contains the nucleotide uracil instead of thymine. Using one strand of the uncoiled DNA referred to as the template strand, RNA polymerase assembles an mRNA strand that is complementary to the DNA template. This process is called transcription, and results in an mRNA strand that is complementary to the template strand (A’s intead of U’s/C’s instead of G’s etc) and identical to the other strand of DNA which is called the coding strand. Once assembled, this mRNA molecule leaves the nucleus in order to start the next phase of protein synthesis, translation.
Translation is the conversion or “translation” of a mRNA molecule into a sequence of amino acids called a protein.
Once in the cytoplasm, the large and small ribosomal subunits find a special sequence of 3 nucleotides called the start codon (AUG). Remember a codon is a triplet of nucleotides found on mRNA that codes for a specific amino acid. Let’s take a look at the steps involved in translation.
- Initiation: Once the start codon is located, 3 things combine to initiate translation. The large and small ribosomal subunits bind to the mRNA like a vice binding a 2 x 4. When they bind to the mRNA they also bind an initiator tRNA molecule with an attached methionine amino acid in the P site of the ribosome (remember there are 3 sites within the ribosome that bind tRNA… the E site, the P site, and the A site).
- Codon Recognition: Now that translation has begun, the tRNA molecule with the anticodon that matches the codon exposed in the A site will bind to that codon and occupy the A site. At this point we have a tRNA in the P site with an attached methionine amino acid, and a tRNA with some other amino acid located in the A site.
- Peptide Bond Formation: The Methionine amino acid bound to the tRNA in the P site is now transferred to the amino acid bound to the tRNA molecule in the A site and a peptide bond between the amino acids is formed. The result is the early stages of an amino acid chain also known as a peptide chain (the early stages of a protein).
- Translocation/Elongation: During this stage the entire ribosome shifts one codon forward. The result is that the tRNA molecule that was previously in the P site, is now being located in the E site, and the tRNA that was in the A site, is now in the p site. The tRNA in the E site is now released into the cytoplasm. At the end of this step there is a ribosome with a tRNA linked to a peptide chain located in the P site… and an empty A and E site.
- Termination: This process continues until a stop codon is reached, at which point the fully formed amino acid chain, otherwise known as a polypeptide, is released into the cytoplasm.
That’s how the cell makes protein!!! mRNA is transcribed in the nucleus from a DNA template, that mRNA migrates out into the cytoplasm where it is then translated into a polypeptide chain by a ribosome.
Alright guys… Question time! Comment below for extra help or clarification.