Upgrade to remove ads
Chapter 11: The Cell Cycle and Cell Division
Terms in this set (93)
If the DNA content of a human skin cell at the beginning of mitosis is 12 picograms, what is the DNA content of a cell in G1?
Apoptosis is present in most organisms, with a common molecular pathway. This indicates that apoptosis is important in terms of evolution. What might be a selective advantage for apoptosis?
Apoptosis may be a way to eliminate cells that might develop mutations causing the cell to be harmful to the organism, such as a normal cell turning into a cancer cell. In addition, as an organism develops, organs must be a defined size and shape. Apoptosis may eliminate excess cells that would make an organ too big or misshapen.
How do cell cycle treatments differ from treatments based on specific targets (see Figure 11.24)? Would the side effects of the two types of treatments be different?
The cell cycle treatments affect the cell cycles of all dividing cells in the body, not just those of the tumor cells. By contrast, targeted drugs affect altered proteins present only in tumor cells. The side effects of the general cell cycle drugs would be on organs and systems that rely on dividing cells. For example, blood cells undergo apoptosis after a period in the bloodstream and must be replaced by dividing cells; if division in these cells is blocked by an anticancer drug, the patient may develop side effects such as poor immunity (too few white blood cells) and anemia (too few red blood cells).
What are the four events of cell division in both prokaryotes and eukaryotes?
1. A reproductive signal. An intracellular or extracellular signal initiates cell division.
2. Replication of DNA. The cell's genetic material must be duplicated so that each of the two new cells will have a complete, identical set of genes.
3. Segregation of replicated DNA. The replicated DNA must be equally distributed to each of the new cells.
4. Cytokinesis. Enzymes and organelles for the new cells must be synthesized, and new material must be added to the cell membrane (and the cell wall, in organisms that have one), in order to separate the two new cells.
What is binary fission?
In prokaryotes, the cell grows in size, replicates its DNA, and then separates the cytoplasm and DNA into two new cells in a process called binary fission.
What are common signals for the initiation of cell division in prokaryotes?
External factors such as environmental conditions and nutrient concentrations are common signals for the initiation of cell division in prokaryotes.
What is a chromosome?
A chromosome consists of a long, thin DNA molecule with proteins attached to it.
How many chromosomes do most prokaryotes have?
Most prokaryotes have just one main chromosome.
How is DNA compacted to fit in a cell?
To fit in the cell, bacterial DNA must be compacted. The DNA folds in on itself, and positively charged (basic) proteins bound to the negatively charged (acidic) DNA contribute to this folding.
What two regions of prokaryotic chromosomes are involved in cell reproduction?
1. Ori: the site where replication of the circular chromosome starts (the origin of replication)
2. Ter: the site where replication ends (the terminus of replication).
How do prokaryotic cells divide?
Chromosome replication takes place as the DNA is threaded through a replication complex of proteins near the center of the cell. Replication move bidirectionally. While DNA replicates, anabolic metabolism is active, resulting in cell growth. In rapidly dividing prokaryotes, DNA replication occupies the entire time between cell divisions.
How does the DNA segregate during cell division in prokaryotes?
Segregation of DNA molecules: as replication proceeds, the ori regions move toward opposite ends of the cell. DNA sequences adjacent to the ori region bind proteins that are essential for this segregation. This is an energy-requiring process, since the binding proteins hydrolyze ATP.
How does cytokinesis function in prokaryotes?
The cell membrane pinches in to form a ring of fibers similar to a purse string. The major component of these fibers is a protein called FtsZ that is related to eukaryotic tubulin.
As the membrane pinches in, new cell wall materials are deposited, which finally separate the two cells.
What functions as the reproductive signal in eukaryotic cell division?
Reproductive signal: the signals for cell division are usually not related to the environment of a single cell, but to the function of the entire organism.
How do eukaryotic cells replicate (basic)?
Replication: eukaryotes have more than one chromosome (humans have 46). DNA replication in eukaryotes is usually limited to part of the period between cell divisions.
Segregation in eukaryotes?
Segregation: newly replicated chromosomes are closely associated with each other (sister chromatids), and a mechanism called mitosis segregates them into two new nuclei.
Cytokinesis is eukaryotes?
Cytokinesis: cytokinesis proceeds differently in plant cells than in animal cells.
What commonly limits the reproduction of a prokaryotic cell by binary fission?
A common signal for the initiation of binary fission is adequate nutrients.
Why must DNA be replicated and segregated before the cell can divide?
If the cell divided before DNA was fully replicated, each new cell would not receive a full complement of the genetic material, DNA.
Why are DNA replication and cell division more complex in eukaryotes than in prokaryotes?
In eukaryotes there is more DNA, the DNA is in numerous molecules, the DNA molecules are much larger, and the DNA is located in a separate cell compartment, the nucleus.
What are the basic stages of the eukaryotic cell cycle?
The cell cycle can be divided into mitosis/cytokinesis and interphase.
What are the subphases of interphase?
G1, S, G2
What is G1 phase?
G1 phase typically lasts for 11 hours. Each chromosome is a single, unreplicated DNA molecule with associated proteins. Variations in the duration of G1 account for most of the variability in the length of the cell cycle in different cell types.
What is S phase?
S phase typically lasts for 8 hours. DNA replication occurs - each chromosome is duplicated and thereafter consists of two sister chromatids. During S phase, the centrosome doubles, and at the beginning of prophase the two centrosomes separate from one another, moving to opposite ends of the nuclear envelope.
What is G2 phase?
G2 phase typically lasts 4 hours. The cell makes preparations for mitosis.
What is G0 phase? Give examples of cells in G0.
Some cells in G1 enter an inactive resting phase of the cell cycle called G0. Examples: cardiac muscle and neurons.
What is the role of Cdk's in the cell cycle?
Progress through the cell cycle depends on the activities of cyclin-dependent kinases (Cdks). By catalyzing the phosphorylation of certain target proteins, Cdks play important roles at various points in the cell cycle.
What is the restriction (R) point?
The control point at the G1-to-S transition is called the restriction (R) point. The G1-to-S transition: the commitment is made to DNA replication and subsequent cell division.
How do you regulate Cdks?
An effective way to regulate Cdks is to regulate the presence or absence of cyclins. If a cyclin is not present, its partner Cdk is not active. Cyclins are present cyclically: they are made only at certain times in the cell cycle.
What is checked at each checkpoint of mitosis?
Different cyclin-Cdks act at cell cycle checkpoints, signaling pathways that regulate the cell cycle's progress. G1 - DNA damage S - incomplete replication or DNA damage G2 - DNA damage M - chromosome unattached to spindle
What specific protein is produced if DNA is damaged by radiation? And what happens to the cell?
If DNA is damaged by radiation (before R checkpoint), a signaling pathway results in the production of p21 protein.
This protein binds to the G1-S cdk, preventing cyclin binding and keeping the cdk in its inactive state while repairs are being made to the DNA.
When the DNA damage pathway is no longer operating, p21 breaks down, allowing the cyclin-Cdks to function.
Cell will undergo apoptosis is the DNA is damaged beyond repair.
What are growth factors?
If cells are to divide, they can be stimulated by external chemical signals called growth factors. Growth factors bind to specific receptors on target cells and activate signal transduction pathways that end with cyclin synthesis.
What are two examples of growth factors?
1. Platelets gather at a bleeding wound to initiate blood clotting. Platelets produce and release a protein called platelet-derived growth factor which diffuses to the adjacent cells in the skin and stimulates them to divide and heal the wound.
2. Two types of growth factors, interleukins and erythropoietin, stimulate the division and specialization, respectively, of precursor cells of white blood cells and red blood cells.
The protein p16 acts on the cell cycle in a fashion similar to that of p21. Recently it was found that there is more p16 protein in the cells of older people than in younger people. What are the implications of this for aging?
P16 blocks the interaction of cyclin and its Cdk that act at the G1-S boundary. If there is more p16 in older people, it may mean that their cell cycle is blocked and they cannot repair damaged tissues by cell replacement.
What is chromatin?
The complex of DNA and proteins is referred to as chromatin.
How are sister chromatids held together?
Throughout G2, the sister chromatids are held together along most of their length by a protein complex called cohesin. At mitosis, most of the cohesin is removed, except in a region called the centromere.
What are condensins?
At the end of G2 and the beginning of mitosis, a second group of proteins called condensins coats the DNA molecules and makes them more compact.
What are histones?
Histones are positively charged proteins around which DNA is wrapped about 1.8 times. The charged R groups on these amino acids bind to the negatively charged phosphate groups on DNA by ionic attractions.
What are nucleosomes?
DNA-histone and histone-histone interactions result in the formation of beadlike units called nucleosomes.
Can a DNA in its compacted form be replicated?
No. During interphase, DNA is accessible to proteins involved in replication and transcription. Once a mitotic chromosome is formed, its compact nature makes it inaccessible to replication and transcription factors.
What is the spindle apparatus?
The spindle apparatus is a dynamic microtubule structure that moves sister chromatids apart during mitosis.
What is the centrosome? What are centrioles?
Before the spindle can form, its orientation is determined by the centrosome, an organelle in the cytoplasm near the nucleus. In many organisms, the centrosome consists of a pair of centrioles, each one a hollow tube formed by nine microtubule triplets.
Do plants and fungi have centrosomes?
The cells of plants and fungi lack centrosomes, but distinct microtubule organizing centers at each end of the cell play the same role.
When does the spindle form? How does it form?
The spindle begins to form during prophase.
Each half of the spindle develops as tubulin dimers aggregate from around the centrioles and form long fibers that extend into the middle region of the cell.
The microtubules are initially unstable, constantly forming and falling apart, until they contact kinetochores or microtubules from the other half-spindle and become more stable.
What are kinetochores and when do they form?
Late in prophase, specialized structures called kinetochores develop in the centromere region, one on each chromatid.
What forms the mitotic center/pole?
Each of the two centrosomes, now on opposite sides of the nucleus, serves as a mitotic center, or pole, toward which the chromosomes will move.
What are the two groups of microtubules in the spindle?
Polar microtubules form the framework of the spindle and run from one pole to the other.
Kinetochore microtubules attach to the kinetochores on the chromosomes.
How is the separation of eukaryotic chromatids regulated?
The separation of chromatids occurs at the beginning of anaphase and is controlled by an M phase cyclin-Cdk, which activates another protein complex called the anaphase-promoting complex (APC).
How does anaphase occur?
Separase is a protease that hydrolyzes the cohesin protein holding the sister chromatids together.
A cell cycle checkpoint, often called the spindle assembly checkpoint, occurs at the end of metaphase to inhibit the APC if one of the chromosomes is not properly attached to the spindle.
After separation, the chromatids are called daughter chromosomes.
How do the two sets of daughter chromosomes move to the poles in anaphase?
The kinetochores contain molecular motor proteins, including kinesin and cytoplasmic dynein, which use energy from ATP hydrolysis to do the work of moving the chromosomes along the microtubules.
The kinetochore microtubules shorten, drawing the chromosomes toward them.
The centrosomes move apart, aiding in separation.
Explain cytokinesis in animal cells.
In animal cells, cytokinesis begins with a furrowing of the cell membrane. This contractile ring is composed of microfilaments of actin and associated myosin, which form a ring on the cytoplasmic surface of the cell membrane. These two proteins interact to produce a contraction, pinching the cell in two.
Explain cytokinesis in plant cells.
In plant cells, as the spindle breaks down after mitosis, membranous vesicles derived from the Golgi apparatus appear along the plane of cell division, roughly midway between the two daughter nuclei. The vesicles are propelled along microtubules by the motor protein kinesin, and fuse to form a new cell membrane. At the same time, they contribute their contents to a cell plate, which is the beginning of a new cell wall.
What are the various levels of "packing" by which the genetic information contained in linear DNA is condensed during prophase?
During interphase, DNA is somewhat condensed by histone proteins into nucleosomes, and these fold over one another to form chromatin fibers. During prophase, the fibers attach as loops to proteins, and these in turn loop extensively to form the chromosome.
Taxol is an anticancer drug that binds to microtubules, preventing them from adding or removing subunits and causing them to form bundles. How do you think this drug works to stop cancer? Is Taxol specific to cancer cells?
Chromosomes are attached to spindle microtubules, and molecular motors on the microtubules move the chromosomes along. In addition, spindle microtubules shorten from the poles, and this causes the attached chromosomes to move to the poles. Taxol prevents these processes, and so inhibits cell division. The drug also inhibits the division of normal cells that enter M phase.
How would the cell cycle be affected if there were a non-functional cohesin protein?
A nonfunctional cohesion would not allow close attachment of chromatids during cell division, and there would not be a centromere. The two chromatids would be separate and there would be no organization for kinetochore attachment. There would be ineffective segregation of one chromatid of a pair to each daughter cell.
What is asexual reproduction?
Asexual reproduction, sometimes called vegetative reproduction, is based on the mitotic division of the nucleus.
What is sexual reproduction?
Sexual reproduction in an organism that is not identical to its parents. It requires gametes created by meiosis; two parents each contribute one gamete to each of their offspring.
Does asexual or sexual reproduction result in more genetic variability?
While asexually reproducing organisms can have genetic diversity, meiosis generates much more diversity, which is available for natural selection and evolution. Independent assortment, random fertilization, and crossing over.
What are somatic cells?
In most multicellular organisms, the body cells that are not specialized for reproduction, called somatic cells, each contain two sets of chromosomes, which are found in pairs.
What are homologous chromosomes?
The members of such a homologous pair are similar in size and appearance.
The two chromosomes in a homologous pair (called homologs) bear corresponding, though not identical, genetic information. (They carry the same genes but not the same alleles).
What are gametes?
In contrast to somatic cells, gametes contain only a single set of chromosomes - one homolog from each pair. The number of chromosomes in a gamete is denoted by n, and the cell is said to be haploid.
How is a zygote produced?
During reproduction, two haploid gametes fuse to form a zygote in a process called fertilization. The zygote has two sets of chromosomes, just as the somatic cells do in a diploid organism. Its chromosome number is denoted by 2n, and the zygote is said to be diploid.
What are the three overall goals of meiosis?
1. It reduces the chromosome number from diploid (2n) to haploid (n).
2. It ensures that each of the haploid products has a complete set of chromosomes.
3. It generates genetic diversity among the products.
What are the major features of meiosis I?
Homologous chromosomes come together to pair along their entire lengths. No such pairing occurs in mitosis.
The homologous chromosomes pairs separate, but the individual chromosomes, each consisting of two sister chromatids, remain intact. (The chromatids will separate during meiosis II).
What happens at the end of meiosis I?
At the end of meiosis I, two nuclei form, each with half of the original chromosomes (one member of each homologous pair). Since the centromeres did not separate, these chromosomes are still composed of two sister chromatids.
What is the end result of meiosis II?
The sister chromatids are separated during meiosis II, which is not proceeded by DNA replication. The end product is four cells, containing haploid number of chromosomes and are not genetically identical.
Explain meiosis I.
Homologous chromosomes pair by adhering along their lengths in a process called synapsis. This lasts from prophase I to the end of metaphase I.
The four chromatids of each pair of homologous chromosomes form a tetrad, or bivalent.
During this time, chromatin continues to coil and compact.
Homologous chromosomes are held together by physical attachments mediated by cohesins; regions having these attachments are called chiasmata.
A chiasma reflects an exchange of genetic material between nonsister chromatids on homologous chromosomes - crossing over.
Crossing over results in recombinant chromatids, and it increases genetic variation among the products of meiosis by shuffling genetic information among the homologous pairs.
Breakage and reunion of DNA molecules to form recombinant chromatids involves breakage of phosphodiester bonds that hold adjacent nucleotides on DNA together and formation of new bonds linking DNA molecules of two homologous chromatids.
What is nondisjunction? Give some examples on how this might happen.
Nondisjunction results in the production of aneuploid cells. In meiosis I, a pair of homologous chromosomes may fail to separate. In mitosis or meiosis II, sister chromatids may fail to separate. Homologous chromosomes may fail to remain together during metaphase I and then migrate to the same pole in anaphase I.
What is aneuploidy?
Aneuploidy is a condition in which one or more chromosomes are either lacking or present in excess. This could occur because of a breakdown in the cohesins that keep sister chromatids and tetrads together during prophase.
What is translocation?
In a process called translocation, a piece of chromosome may break away and become attached to another chromosome.
What is a karyotype?
A rearranged photomicrograph reveals the number, shapes, and sizes of a chromosome in a cell, which together constitute its karyotype.
What is a polyploid?
Under some circumstances, triploid (3n), tetraploid (4n), or higher-order polypoid nuclei may form. Triploid individuals are usually sterile because the nucleus cannot undergo normal meiosis since 1/3 of the chromosomes would lack partners.
What are the conditions for allopolyploid?
Allopolyploid conditions: Haploid gametes from two species (A and B) mated to form a diploid zygote (chromosomes AB). Nondisjunction of all chromosomes occurred during mitosis in the fertilized egg, resulting in a tetraploid (AABB), which grew up to be a fertile adult.
How do crossing over and independent assortment result in unique daughter nuclei?
In crossing over, there is an exchange of some genetic material between non-identical chromosomes of a pair. So the resulting chromosomes carry new combinations of genes, which can be passed on to offspring in a gamete and fertilization.
In independent assortment, it is random which chromosome of a homologous pair ends up in a particular gamete. So different gametes will usually have a different set of chromosomes; that is, chromosome 1 from the father, chromosome 2 from the mother, and so on. Fertilization therefore results in diploids, each of which has a different set of chromosomes.
Compare prophase I of meiosis with prophase of mitosis. Compare anaphase I of meiosis with anaphase of mitosis.
Prophase I meiosis: Chromosomes have chromatid pairs attached, and the two homologs are lined up gene for gene beside one another.
Prophase mitosis: Chromosomes have attached chromatid pairs but are not lined up beside one another.
Anaphase I meiosis: Homologous chromosomes, each with two attached chromatids, separate and move to the poles.
Anaphase of mitosis: Chromatids separate and become single daughter chromosomes and move to the poles.
Rarely, a person is born with an extra X chromosome (XXY). How can this arise by aneuploidy?
In the formation of male gametes, the X and Y chromosomes fail to separate in meiosis I anaphase. At the end of meiosis II, there will be two types of gametes: half without an X or Y, and half with X and Y. If the latter fertilizes a normal egg with a single X chromosome, the offspring will be XXY. However, if there is a similar nondisjunction in the formation of female gametes, there will be eggs with two X chromosomes. If the XX egg is fertilized by a normal, Y-containing sperm, the offspring will be XXY.
What is necrosis?
The first type of cell death, necrosis, occurs when cells and tissues are damaged by mechanical means or toxins, or are starved of oxygen or nutrients. Necrotic cells often swell up and burst, releasing their contents into the extracellular environment and possibly causing inflammation.
What is apoptosis?
Apoptosis, or programmed cell death, is a genetically determined series of events that occurs during normal developmental processes and in adult tissues as well.
Why might an animal cell undergo apoptosis? Examples?
1. The cell is no longer needed by the organism. Example: unneeded tissue between a fetus' fingers disappears as the cells undergo apoptosis in response to specific signals.
2. The longer cells live, the more prone they are to genetic damage that could lead to cancer. Example: epithelial cells on the surface of an organism may be exposed to radiation or toxic substances and die after only days or weeks.
What are the events of apoptosis?
The events of apoptosis are similar in many organisms. The cell becomes detached from its neighbors and its chromatin is digested by enzymes. The cell forms membranous lobes, or "blebs," that break up into cell fragments. The surrounding, living cells usually ingest the remains of the dead cell by phagocytosis.
What is the hypersensitive response in plants?
Apoptosis is also used by plant cells, in an important defense mechanism called the hypersensitive response. Plants can protect themselves from disease by undergoing apoptosis at the site of infection by a fungus or bacterium. They don't form blebs (because cell wall), but they digest their own cell contents in the vacuole and then release the digested components into the vascular system.
What are some signals that can lead to apoptosis?
Signals such as hormones, growth factors, viral infection, certain toxins, or extensive DNA damage can lead to apoptosis.
Some apoptotic pathways target the mitochondria; the cell dies quickly if its mitochondria can't carry out cellular respiration.
What are caspases?
Caspases are enzymes that are activated during apoptosis. They are proteases that hydrolyze target molecules in a cascade of events. The cell dies as the caspases hydrolyze proteins of the nuclear envelope, nucleosomes, cytoskeleton, and cell membrane.
How do cancer cells differ from normal cells?
Cancer cells differ from the normal cells from which they originate in two ways:
1. Cancer cells lose control over cell division.
2. Cancer cells can migrate to other locations in the body.
What are tumors?
Tumors are large masses of cells which do not respond to growth factor controls and instead divide continuously.
What is the difference between benign and malignant tumors?
Benign tumors resemble the tissue they came from, grow slowly, and remain localized.
Malignant tumors do not look like their parent tissue; they have irregular structures, such as variable nucleus sizes and shapes.
What is metastasis?
The spreading of cancer, called metastasis, results in organ failures and makes the cancer very hard to treat.
What are oncogenes?
Oncogene proteins are positive regulators in cancer cells. They are derived from normal positive regulators that have become mutated to be overly active or that are present in excess, and they stimulate the cancer cells to divide more often. Oncogene products could be growth factors, their receptors, or other components in the signal transduction pathways that stimulate cell division.
What is an equation that describes the growth rate of a cell population?
Growth rate of cell population = rate of cell division - rate of apoptosis
What are tumor suppressors?
Tumor suppressors are negative regulators in both cancer and normal cells, but in cancer cells they are inactive. An example is the RB protein.
What are some cancer treatments?
Cancer treatments target the cell cycle The most successful and widely used treatment for cancer is surgery. Other approaches are taken to treat or cure cancer, and these generally target the cell cycle. An example is 5-fluorouracil, which blocks the synthesis of thymine.
What types of events initiate necrosis, and how do these events differ from those that initiate apoptosis?
Signals for necrosis include great damage to a cell or starvation.
Cancer-treating drugs are usually given in combination to target several stages of the cell cycle. Why might this be a better approach than using a single drug?
Cancer cells are not synchronous in the cell cycle. At a given point in time, some are in G1, some in S, and so on. So targeting all the phases might be better than targeting just one.
YOU MIGHT ALSO LIKE...
Bio Ch. 12
Biology Chapter 8
Bio. Chapter 10 (Cell division and Mitosis)
OTHER SETS BY THIS CREATOR
Chapter 13: DNA and its Role in Heredity
Chapter 12: Inheritance, Genes, and Chro…
OTHER QUIZLET SETS
W4 Psychological approaches: Individual therapies…
Daily Geo 1-20 to 36-40
Chapter 14 ~ Other Health Impairments