Chapter+12+Cell+Division+&+Chapter+13+Meiosis

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**Chapter 12 Cell Cycle and Regulation**


 * The Key Roles of Cell Division **

1. Explain how cell division functions in reproduction, growth, and repair. [] Cell division is essentially the reproduction of cells. Once these new cells are produced, the cells grow and repair the damaged cells or replace the cells that have gone through the process of apoptosis.

Andrew Chang and Jason Weiss

2. Describe the structural organization of a prokaryotic and eukaryotic genome.

Prokaryotes and Eukaryotes genomes differ in a few ways. Prokaryotic genomes are found in a single sided strand of DNA, and are in a circular shape.

Eukaryotic genomes are packaged into chromosomes. It forms a double helix.

--Jason Weiss and Andrew Chang

3. Describe how the chromosome number changes throughout the human life cycle.



All human cells, except gametic cells, are diploid.

Mitosis replicates and replenishes somatic cells throughout the body. Meiosis creates variation in haploid (23 chromosome) egg and sperm cells. -Philip

4. List the phases of the cell cycle and describe the sequence of events that occurs during each phase.

G1 (Gap 1) is the first stage and it is where cells increase in size and start to synthesize RNA and proteins. The next stage, S, which stands for synthesis, is where DNA replication occurs and where two daughter cells are produced. The third stage is G2 (Gap 2) is where cells continue to grow. There is also a G2 checkpoint that makes sure that the cell is able to continue on into Mitosis.The fourth stage, M for mitosis, is where cells start to divide because the chromosomes separate along with cytoplasm. The separation of chromosomes is also known as nuclear division and the separation of the cytoplasm is known as cytokinesis.

http://www.cellsalive.com/cell_cycle.htm (it's a pretty good video) **Nice Video MJ**

Elena

5. List the phases of mitosis and describe the events characteristic of each phase. Prophase - Chromatin condenses into chromosomes, the nucleolus disappears, and centrioles begin moving to opposite ends of the cell. Prometaphase - the nuclear membrane dissolves, proteins attach to the centromeres, kinetochores are created. Metaphase - spindle fibers align the chromosomes along the middle of the nucleus Anaphase - paired chromosomes separate at the kinetochores and move to opposite sides of the cell Telophase - new membranes form around the daughter nuclei, chromosomes and spindle fibers disperse Cytokinesis - a fiber ring composed of actin around the center of the cell contracts, pinching the cell into two daughter

http://www.youtube.com/watch?v=VGV3fv-uZYI - JH

6. Describe the spindle apparatus, including centrosomes, microtubules, asters, and centrioles (in animal cells).

Most of the events that occur in mitosis depend on the mitotic spindle (spindle apparatus), which forms originally during prophase. This apparatus contains fibers that are made of microtubules as well as proteins and soon elongate. In animal cells, the spindle apparatus begins at the centrosome, which functions in the cell to organize all of the cell’s microtubules. Specifically, a pair of centrioles is located at the center of the centrosome but is not required for cell division and is not even present in plant cells. In the spindle apparatus during interphase, the centrosome will duplicate to form two centrosomes, staying nearby at the nucleus. In prophase and prometaphase, these centrosomes will move apart where the spindle apparatus will grow from. As prometaphase concludes, the two centrosomes, with one of each located at different poles of the spindle, will be at different ends of the cell. At this point, an aster, which is a radial array of short microtubules, will extend from each centrosome. Now, the spindle apparatus will be composed of centrosomes, spindle, microtubules and the asters- Big Red (DK)

What a guy!

7. Compare cytokinesis in animals and plants.
 * Cytokinesis is the final step in both mitosis and meiosis (I and II). In a plant cell, cytokinesis will occur by means of a cell plate forming between the two nuclei of the telophase cell. This cell plate grows and grows until it spans the width of the cell. The cell plate later becomes the cell wall for the daughter cells, seen in the diagram below. In animal cells, cytokinesis happens as the cleavage of the telophase cell. Cleavage is the pinching of the cytoplasm of the cell to form two new daughter cells with two separate nuclei. the area where cleavage occurs is called the cleavage furrow, seen in the pictures below. Despite these differences between the plant and animal cell, the main goal of cytokinesis is the formation of daughter cells. -Danny G. a.k.a. DG3 **

[]



8. Describe the process of binary fission in bacteria and explain how eukaryotic mitosis may have evolved from binary fission. **BINARY FISSION:**

1. DNA replicates 2. attaches each copy to a different part of the cell membrane 3. as the cell begins to pull apart, the replicate and original chromosomes split 4. cytokinesis leads to two identical daughter cells.


 * Emily H. **

9. Describe the roles of checkpoints, cyclin, Cdk, and MPF in the cell cycle control system.
 * Regulation of the Cell Cycle **

Cell cycle **checkpoints** are control mechanisms that help ensure that cell division in eukaryotic cells goes correctly. These checkpoints verify whether the processes at each phase of the cell cycle have been accurately completed before progressing to the next stage. The first checkpoint is located at the end of the G1 phase, just before entry into S phase. The restriction point is controlled mainly by a Cdk inhibitor that ensures that the **cyclin-dependent kinase** **(Cdk)** can no longer act along with **cyclin** to cause the cell cycle progression. The second checkpoint is located at the end of G2 phase, triggering the start of the M phase (mitosis.) In order for this checkpoint to be passed, the cell has to check several factors to ensure the cell is ready for mitosis. If this checkpoint is passed, the cell initiates the many molecular processes that signal the beginning of mitosis.The CDKs associated with this checkpoint are activated by phosphorylation of the CDK by the action of a **"Maturation promoting factor" (Mitosis Promoting Factor, MPF**). Finally the mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes have/should aligned at the mitotic plate and be under bipolar tension.---Sabrina Zionts



10. Describe the internal and external factors that influence the cell cycle control system. The direct regulation of the cell cycle system involves an internal control system consisting of cyclins and cyclin-dependent kinases, and an external control factors like growth factor. For an example of internal controls, Anaphase, the separation of sister chromatids, does not begin until all of the chromosomes are properly attached to the spindle at the metaphase plate. to ensure that daughter cells do not end up with missing or extra chromosomes. An example of an external factor can be seen with the growth factor PDGF. Fibroblast, a type of connective tissue cell, have PDGF receptors triggers a signal that allows the cell to pass the G1 checkpoint and divide. -- Joanne H.

11. Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls.

When cells fail to undergo apoptosis, they divide and multiply uncontrollably. The goal of mitosis is to form two identical daughter cells from one parent cell, but cancer cells override the normal steps of this process. Chemical signals regulate cell division, yet cancer cells are able to undergo cell division without the boost of external signals. This is often caused by a DNA mutation in one of the several genes normally used to control growth, such as p53, and once the genes begin to behave abnormally, cells undergo rapid and uncontrollable mitosis without dying. Cancer cells also may have an abnormal cell surface, so they break off and metastasize instead of forming normal connections with normal cells.- JLM



12. Distinguish between benign, malignant, and metastatic tumors.

A tumor is defined to be a mass of abnormal cells within otherwise normal tissue and can generally be categorized into two categories: benign and malignant. A benign tumor is stagnant at its original site and typically does not cause serious health hazards. The lack of migration present in benign tumors is due to insufficient genetic and cellular alteration to survive elsewhere in the body. On the contrary, malignant tumors are cancerous and present serious health related risks. Genetic and cellular mutations present in malignant tumors allows for survival in areas of the body other than its native region. Angiogenesis, the systematic incorporation of blood vessels, is often used by metastatic tumors for nourishment and growth. Metastatic tumors fall under the category of malignant tumor because it simply describes a tumor undergoing metastasis (the proliferation of cancer cells from an original site throughout distant regions of the body). -AHC

13. Describe the function of proto-oncogenes, oncogenes, tumor suppressor genes. Discuss ras and P53. A proto-oncogene regulates cellular division and apoptosis in a cell. When a proto-oncogene is defective, it can become an oncogene. Oncogenes cause cells to divide without any supervision or regulation. An example of a proto-oncogene is RAS. When RAS is mutated, it usually leads to cancer in humans because the RAS proteins are always turned on. This causes a high rate of cellular division, ultimately causing cancer. Tumor suppressor genes are also called anti-oncogenes. Both alleles of the gene must be damaged in order for it to not function correctly. These genes either promote apoptosis or regulate the cell cycle. Apoptosis only occurs if the damage done to a cell cannot be repaired, so to keep the mutated cell from dividing, tumor-suppressor genes induce programmed cell death. P53 is a tumor suppressor gene that aids in regulating the cell cycle. It is found in the majority of cancers, including leukemias and lymphomas.

[] This might be the winner fo rthe best video....this guy is nuts MJ Ella **Chapter 12 Meiosis and Sexual Cycles** 14. Explain in general terms how traits are transmitted from parents to offspring. An organism’s traits and appearance, otherwise known as phenotype, are determined by various proteins. However, these prtoeins are in turn controlled by our genetic material. This material is the nucleic acid DNA. DNA remains in the nucleus of cells and is contained in a form otheriwse known as a chromosome. When gametes are formed, half of the genetic material is passed along. Fertilization combines the gametes, and thusly the genetic material of each parent into the new offspring. The new DNA codes for proteins in the offspring.
 * The basis of heredity**

This site is very useful for discovering more about how chromosomes pair up with eachother []

-Tom Haile

15. Distinguish between asexual and sexual reproduction. While both processes produce offspring, they do so with different mechanisms of reproduction. Asexual reproduction occurs with a single parent, male or female, while sexual reproduction necessitates two parents, a male and a female. The offspring of the asexual mechanism is genetically identical to the parent, as offspring are produced by mitotic divisions. With the sexual mechanism, however, the makeup of the offspring is not genetically identical to the parents. As previously stated, asexual reproduction occurs through typical cell division (mitosis) following nuclear division. Sexual reproduction occurs through different cell division (meiosis) that produces haploid gametes following nuclear division. These gametes fuse to produce a diploid zygote. Asexual reproduction is advantageous in that it is a relatively quick process, so an asexual organism can quickly colonize an area (without even spending time to look for a mate), yet its disadvantage is that there is no variation. For instance, if an asexual organism has a disease, all of the offspring will be affected. Sexual reproduction's major advantage is that it results in the vast variation we see today, however, the slow speed of the process can be considered a disadvantage. -Supritha



16. Explain how haploid and diploid cells differ from each other. State which cells in the human body are diploid and which are haploid.
 * The role of meiosis in sexual cycles **

Haploid and diploid cells differ primarily in the number of sets of chromosomes; diploid cells have two homologous copies of each chromosome while haploid cells have only one copy of each chromosome. Within humans, somatic cells are diploid (described as 2n) while sex cells, or gametes, are haploid (1n). Somatic cells refer to any cell within a multicellular organism that is not a gamete, germ cell, gametocyte, or undifferentiated stem cell. In humans, diploid cells/somatic cells make up the bones, blood, internal organs, skin, and connective tissue. In contrast, haploid cells/sex cells make up eggs and sperm. Because haploid cells only have one copy of each chromosome, haploid cells in humans only have twenty-three chromosomes while diploid cells in humans have forty-six chromosomes. Haploid cells are formed from diploid cells by meiosis, while diploid cells reproduce and form other diploid cells in mitosis. Enjoy the following German film on haploid/diploid cells: [] For those of you who may not speak German: [] -Kellen Svetov

17. Distinguish among the three life-cycle patterns characteristic of eukaryotes, and name one organism that displays each pattern. One life cycle typical of eukaryotes is the haploid life cycle. It this particular life cycle, the organism spends the majority if its life with only half the chromosomes it can have. In fact, the only haploid life cycle to be diploid is the zygote stage. One example of an organism with this life cycle is the spirogyra. Another life cycle is the diploid life cycle. Humans are a prime example of this life cycle because we have 46 chromosomes, while at the same time our gametes are haploid with only 23 chromosomes. Finally there is the haplo-diloid life cycle. Typically organisms with this life cycle grow are diploid organisms, but when they reach maturity, they release haploid spores. These haploid spores fall to the ground and form haploid structures that will eventually develop into diploid organisms. This cycle is most typical of plants and fungi.--Tom Haile

18. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase. Meiosis consists of Prophase I Prometaphase I Metaphase I Anaphase I Telophase I and Cytokinesis. Meiosis II consists of all the same phases yet the end product is four haploid cells consisting of 23 chromosomes. In Prophase I crossing over occurs with the formation of a chiasmata. The nuclear envelope degrades and the centrioles polarized as they move to the ends of cell. In Prometaphase the kinetochore forms from the centriole for one chromosome rather one chromatid like in mitosis. In Metaphase the chromosomes line up at the metaphase plate. Also there is a 50 /50 chance whether the daughter cells receives chromosomes from the mother or father. In anaphase I Chromosomes are pulled towards the centrioles. Two daughter cells are now present each with 23 are haploid and have chromosomes with 2 chromatids. In Telophase I the nuclear envelope develops around the new daughter cells getting ready for Meiosis 2. Then in cytokinesis the cleavage furrow occurs in order to split the cells. In meiosis II the same steps occur yet there are slight differences. Prophase II is virtually the same. Now in Meiosis II there are 2 daughter cells undergoing meiosis II. In Metaphase II is rotated 90 degrees facing the other way as before. Then in Anaphase II the sister chromatids are pulled apart to separate parts of the cell. THe same occurs in Telophase II except there are now a final result of 4 Haploid cells. - Elijah Kroloff

[|Meiosis 1 Animation] [|Meiosis 2 Animation]

19. Descr ibe the process of synapsis during prophase I and explain how genetic recombination occurs. Synapsis is when replicated homologous chromosomes pair up. During synapsis, sections of the chromosomes are exchanged. Each of these sections carries unique genetic information. By replacing certain sections with others, a whole new genome is created. This process only occurs during prophase I. This means that genetic recombination only occurs once in the two rounds of meiosis. http://www.youtube.com/watch?v=op7Z1Px8oO4&feature=related Hannah Dresdner

20. Describe three events that occur during meiosis I but not during mitosis.

The first major difference between between mitosis and meiosis occurs in prophase I when homologous chromosomes come together and synapse. During synapsis a process unique to meiosis occurs: crossing over. During crossing over portions of chromosomes overlap creating genetic variation from the parental cell. The second major difference occurs in anaphase I of meiosis. In anaphase of mitosis sister chromatids are separated and pulled to opposite poles of the cell. Instead of sister chromatids being separated in anaphase I of meiosis, homologous chromosomes are the ones separated. This is because a in meiosis I a tetrad is being pulled apart instead of a single pair of sister chromatids. The third major difference occurs in Interphase II of meiosis. In mitosis, the cell cycle effectively ends after telophase and then goes starts over again in interphase where G1, S, and G2 occur and then mitosis again. However, in Interphase II of meiosis no DNA replication occurs. This allows for the two diploid daughter cells to then go through meiosis II where they then divide into four unique haploid daughter cells.

This dinosaur's somatic cells probably divided by mitosis, and he/she created gametes though meiosis. Gordie Kelch

21. Explain how independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms. Independant Assorment: This is the random reassortment of the homologus chromosomes that line up along the Metaphase plate during Meiosis. This video is funny and also does a good job of explaining how the precess occurs. When the pairs randomly cross over and independently assort themselves a new combination of genetic information occurs which is genetic diversity. Crossing over does the same thing it ensures that the gene combination on the newly created pair is radically different from the two original choromosomes. The daughter cells will have a combination of the genetic traits of the two parent cells. Diversity is achieved.
 * Origins of Genetic variation**

Independant Assorment:[] Crossing Over: [] Random Fertilization: [] (This video is great at explaining all three of these concepts.) -Jared Danger Molk

22. Explain why heritable variation is crucial to Darwin‘s theory of evolution by natural selection. Heritable variation is the variation in characteristics caused by genetic factors. These variations may be caused by genetic and environmental factors, for example. This is crucial to Darwin’s theory of natural selection because his theory states that favorable heritable traits become more common in successive generations of populations, while the unfavorable traits become less common. In this way, natural selection ensures that the favorable phenotype is more likely to survive. So the genetic and environmental variations work in the favor of natural selection: the favorable trait gets passed on and the less favorable is lost. -Monika K.