Chapter 12 Notes PDF

**Role of centriole: -** [Its' role includes the formation of mitotic spindle\ ]Mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis. In animal cells, the assembly of spindle microtubules begins in the centrosome and the microtubules begin organizing...

**Role of centriole: -** [Its' role includes the formation of mitotic spindle\ ]Mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis. In animal cells, the assembly of spindle microtubules begins in the centrosome and the microtubules begin organizing center. And aster which is the radial array of short microtubules extends from each chromosome.\ The root word for aster is star which you might recognize from astrology, astronomy and the aster looks like light emanating as from a star. The spindle include the chromosomes, the spindle microtubules and the asters. The purpose of the spindle is to grab onto the condensed chromosomes. Two centrosomes, (one on each end of the cell) so, the spindle is being shout out from both ends of the cell. Once each chromosome is grabbed onto by the spindle from both centrosomes, the sister chromosomes can be pulled apart. [Centrosomes are not pulling the chromosomes to them, instead the cell has mechanism called a kinetochore to move a chromosomes apart.] **The Kinetochore** *The kinetochore is a complex of proteins that is an associated with the centromere, that the mitotic spindle can reach out and attach to.* \*Kinetic energy is the motion, and the kinetochore is from kinetic. Kinetochore is the structure that facilitates movement of the chromosomes. [The microtubules from mitotic spindle are not pulled again, instead the kinetochore is able to move itself up along the tubules. And in doing so the microtubules become shortened by the depolymerization. ] The monomer of microtubules which is tubulin gets chopped off as the kinetochore ends move up along the spindle. In prometaphase some of the mitotic spindles reaches out from the centrioles to attach to these kinetochores while other spindle microtubules attach to opposite strands of spindle microtubules. The spindle microtubules that are attached to the chromosomes begin to move them apart them with in a cell. But the other spindle microtubules which aren't attached to the chromosomes are called non-kinetochore microtubules. And they will overlap and push against each other to help to elongate the cell. **Metaphase: -** At metaphase, the chromosomes have been assembled at the center of the cell which we call the metaphase plate due to movement of kinetochore. Important aspects of metaphase: - It's the sister chromatids which are lined up at the metaphase plate. sister chromatids are completely identical. The centrosomes are at the opposite end of the cells with their asters of spindle microtubules emanating and attaching to the chromosomes. [Spindles from both sides have attached to each chromosome centromere. It's important because this is how the identical sister chromatids will be separate from each other. (This is the checkpoint for the cell). If the mitotic spindle is not attached with both sides, the cell will abort mitosis]. **Anaphase: -** The replicated chromosomes are split, and the identical sister chromatids can move to opposite sides of the cell. Because they are able to move by the mechanism of the kinetochore. The kinetochore functions by moving along the microtubules towards the opposite end of the cell. As it moves along the mitotic spindles which is made of microtubules. The microtubules shortened by the depolarization; it means it's the deconstruction of the polymer. IN this case, the microtubules chain by removing monomers of tubulin protein as it moves. Those non kinetochore microtubules from opposite poles are overlapping and they're pushing against each other to help elongate the cell. **Telophase** Genetically identical daughter nuclei begin to form at opposite end of the cell so here we see the reformation of the nuclear envelope to surround and protect our DNA. Each cell also contains one centrosome with its pair of centrioles. - Telophase and even parts of anaphase can happen in unison with cytokinesis. Which is the division of the cytoplasm, mitosis is the division of the genetic material. Cytokinesis is the division of the cytoplasm. You will also notice that these cells are beginning to pinched in its center. In animal cells, cytokinesis occurs by a process called cleavage. The pinched in part of the cell is called the cleavage furrow. This is accomplished by a contractile ring of microfilaments that is tightened and squeeze until the cell is pinched into two. Plant cells have a different process for cytokinesis, remember plant cells have both a plasma membrane and rigid cell wall which is made of cellulose. Cytokinesis for plant cells a cell plant forms to separate the two identical daughter nuclei into separate cells. Recap of the role of mitotic spindle at each of the phases so, For prometaphase: - Some of the spindle microtubules have attached to the kinetochores of this, the chromosomes and they begin to move those chromosomes toward the center of the cell.\ At metaphase: - the chromosomes are all lined up at the metaphase plate which is as imaginary line right at the midpoint between the spindle two poles. At anaphase, the sister chromatids separate and move along the kinetochore microtubules toward opposite end of the cell. In Telophase: - genetically identical daughter nuclei forms at opposite end if the cell. And cytokinesis begins during anaphase and the telophase and the spindle eventually disassembles. Remember\< Mitosis is the division of the genetic material, The DNA and all things that involved in this process. Prokaryotes like bacteria and archaea have a unique form of cell division called binary fission. In this, the chromosomes replicate beginning at the origin of replication and the origin of replication is just what is sounds like it would be a special part of the genome that Initiate DNA replication.\ Remember, Prokaryotes like bacteria have a circular genome called plasmid unlike eukaryotes, we have multiple strands of linear chromosomes. Once the replication of the genome completes, the daughter chromosomes actively move apart and the plasma membrane pinches inward dividing the cell into two. **Binary Fission in bacteria: -** This is an image of an agar plate with some plated bacteria, and we can see that bacteria takes place in different colonies and each of those dots is a colony of genetically identical bacteria, and they are called clones withing each dot. **The cell cycle control system: -** The eukaryotic cell cycle is regulated by a molecular control system. The cycle control system. The frequency of cell division in varies with the type of the cell. For example, the cells in the skin and those that line your intestinal epithelium are the fastest dividing cells in the human body. These differences result from regulation at the molecular level. The sequential events of the cell cycle are directed by a distinct control system which is very similar to a clock. As you can see in the image, the clock has specific checkpoint where the cell cycle stops until a go-ahead signal Is received. We have three distinct checkpoints in red, so lets check them, Note: - The cell cycle control system is regulated by both internal and external signal at these checkpoints. An example if an internal signal was given while describing the metaphase. If you remember, if the kinetochore does not attach to the spindle microtubules properly, a molecular signal send out that delays anaphase. Some external signals are growth factors which are protein released by certain cells that stimulate other cells to divide. A clear example of an external signal is density depended on inhibition in which crowded cells stop dividing. Most animal cells exhibit something called anchorage dependance in which they must be attached to a substratum which cuts as a foundation in order for the to divide. Cancer cells exhibit neither density dependance exhibition nor anchorage dependance. For many cells from G1 checkpoint seems to be the most important, if a cell receives a go ahead signal at the G1 checkpoint it will usually complete S, G2 and M phase and then divide \\. If the cell doesn't receive the go ahead signal, it will exit a cell cycle and switch into a non-dividing stage which is called G0 phase. **The cell cycle \_ Go checkpoint.** Cells at the G0 phase are not actively preparing to divide. The cell is in quiescent or inactive stage that occurs once cell exit the cell cycle. Some enter G0 temporarily due to environmental conditions such as the lack of availability of nutrients and the stimulation from growth factor. The cell will remain in this stage until the conditions are approved or until an external signal trigger the cell to enter to enter the G1 phase. Other cells that never or rarely divide such as mature cardiac muscle cell and your nervous system cells remain in G0 permanently. **The cell cycle \_ G2 checkpoint** The G2 checkpoint bars entry into the mitotic phase if certain conditions are not met. Ad with the G1 checkpoint, the cell size and protein reserves are assessed and the most important role of the G@ checkpoint is to ensure that all of the chromosomes have been replicated and that replicated DNA is not damaged. If the Checkpoint detects a problem with the DNA, the cell cycle is halted and the cell attempts to either completes its DNA replication or repairs the damaged DNA. **The cell cycle \_M checkpoint** It occurs near the end of the metaphase stage of mitosis. The M checkpoint phase is also known as the spindle checkpoint because it determines whether all of the sister chromatids is correctly attached to the spindle microtubules. The cycle will not proceed until the kinetochore of each pair of sister chromatid are firmly anchored to at least two spindle fibers arising from the opposite poles of the cell. This way the anaphase condenses the two sister chromatids will be pulled apart and separate it proper in different directions. **Cyclins & Cdk's Regulator molecules** These two groups of proteins called cyclins and cyclin-depend kinases which is abbreviated as Cdk's are called positive regulates and this is because they are responsible for the progress of the cell through the varies checkpoint. Increases of in the concentration of cyclin proteins can be triggered by the both external and the internal signal. Cyclins regulate the cell cycle only when they are tightly bound to cdk's to be fully active. The cdk cycling complex results must also be phosphorylated. Cdk's are the enzymes. \*Remember kinesis are enzymes that phosphorylate meaning the add a phosphate group onto other proteins and phosphorylation activates the proteins by changing its shape. The proteins phosphorylated by cdks cyclin dependent kinesis are involved to advancing the cells in the next phase. Review- Interphase which is comprises of G1 and G2 and mitotic M phase which is comprises of prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. Almost as important as understanding the phase of cell cycle is the understanding how the cell cycle is regulated in eukaryotic cells. One irregularity leads to cancer cells, and they are many ty\[es of cancers and notable cause of cancer is dysregulation of certain genes which can lead to loss of cell cycle control. **Cancer cells: -** Proto-oncogene are the genes that code for the positive cell cycle regulators, meaning they will make the cell cycle go. This gens are normal genes that when mutated in certain ways become oncogenes which are genes to become the cause of cancerous,. You probably recognize the part of the word oncogene, onco and oncologiost are the doctor who specialize in treating cancer patients. Tumor suppressor gene Are the segments of DNA that code for negative regulator proteins. So these genes can prevent the cells from undergoing uncontrolled division. We see an example here is the gene for P53. If we have normal P53 this protein will be able to cause the cell to a go through apoptosis which is programmed cell death so, if there is anything wrong with cell this protein will help end the cell and the cell won't continue to divide and to create more bad cells. However, if there is something wrong with this gene and it's mutated and we create a P53 protein that doesn't work properly. It can't stop the cell from dividing and so the cell cycle is going to continue, and we can have cells that are cancerous. When a normal cell converts to cancer cell, this process is called transformation.