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Biology notes, Lectures 9 and 10 (1) (1).doc

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Western University
Biology 1001A
Tom Haffie

Lecture 2 1. The general mechanisms by which vaccines protect against diseases. • Vaccines protect against disease by priming the immune system. They introduce a harmless version of the disease into the body that allows the immune system to gain “memory” of the invader for the next time it encounters the disease • Later, due to priming by the vaccine, the immune system is able to respond faster and with more intensity with antibodies to the real, damaging form of the disease 2. Why developing a vaccine against HIV is relatively challenging, compared to other diseases. • HIV has a high mutation rate so it changes the markers on its surface too often to develop an effective vaccine. Each time the markers change, the vaccine is useless and the immune system can’t recognize the virus 3. Why people are encouraged to get a flu vaccine each year (as opposed to one time only). • Every year, the flu virus mutates so the previous year’s vaccine is rendered useless by the mutation of the virus Lecture 2 lecture 1. General global distribution of HIV infection • HIV is felt globally (a pandemic) • Every inhabited continent has been stricken with HIV. • In Canada, in the US, and in Europe, HIV is less common • In Sub-Saharan Africa, Aids and HIV are an incredibly prevalent disease • Sub-Saharan Africa accounts for about 60% of all people infected with the disease and 90% of children living with the disease 2. General temporal trends in HIV infection rates • Over the last 20 years, HIV infection rates have increased but may be reaching a plateau • Deaths due to AIDS have been on the decline 3. Factors that explain why no cure or universal vaccine has been developed for HIV/AIDS • The virus mutates and changes over time • Our immune defenses are crippled by HIV 4. Reasons why viruses are not considered “alive”. • Viruses can’t self reproduce, they need a host (obligate parasites) • No metabolic processes, not made up of cells • Viruses are not cellular 5. Reasons why anti-viral drug therapies often have serious side effects. • Viruses need a host cell to reproduce and use host cell mechanisms for that purpose • Severe side effects because viruses are using the host cell almost every step of the way so there is no way to target the virus without targeting the host cell • Anti-viral drugs can kill the virus, but there will be a lot of collateral damage as well 6. Major steps in life cycle of HIV. • The glycoproteins on the surface of HIV mediates attachment to the protein receptors on the host cell and fuses with it • The viruses injects RNA and reverse transcriptase into the host cell which splices its genetic code into the host cell’s DNA with the help of integrase • Transcription of the viral DNA results in the production of RNA which can serve as the genome for new viruses and can be translated to produce viral proteins • Complete HIV particles are assembled and in macrophages, HIV buds out without rupturing the cell while in T cells, HIV exits by rupturing and killing the cell 7. Specific role of integrase and reverse transcriptase in retroviral life cycle. • Integrase splices viral DNA into host DNA • Reverse transcriptase reverse transcribes the viral RNA into double stranded viral DNA. It is highly error prone (no proofreading enzymes) so it causes a lot of mutations 8. Mechanism of action of AZT. • Anti-retroviral therapies inhibit virus-specific enzymes0 • AZT mimics thymidine and inhibits reverse transcriptase 9. Reasons why effectiveness of AZT decreases over time. • AZT resistance began to emerge as the virus evolved • In a population of viruses, some will be more resistant to AZT than others • The presence of AZT selects for individuals with resistance so over time, the population of the viruses becomes primarily populated by resistant individuals as they reproduce successfully in the presence of AZT while the other do not • Evolution and natural selection in action 10.Rationale for multi-drug (drug cocktail) approach to treating viral infections. • If multiple points of HIV action are targeted, it is much harder for resistance to evolve 11.Principles of evolution of HIV: variation, heritability, differential reproduction, change in genotype of population. • Variation: in a large population of HIV particles, some virions will have different mutations than others, with mutations occurring at a high frequency. This creates a large variability in the genotypes of the HIV particles • Heritability: mother virions produce daughter virions that are similar so the mutations in a population are passed on from one generation to the next (heritability) • Differential reproduction: selection pressure favours reproduction of some virions over other • Change in genotype of population: eventually, the susceptible virions die off. Only the resistant virions are able to reproduce. The population thus changes over time 12.Role of CCR5-Δ32 mutation in human resistance to HIV infection. • Some people will not catch HIV regardless of repeated exposure to HIV or, if they do, the disease progresses very slowly. These people have a deletion in one of the alleles that code for proteins on an immune cell receptor 13.Global distribution of CCR5-Δ32 allele. • The allele is not spread evenly over the population • It is uncommon in African countries and more common in Scandinavia and North Eastern Europe as well as among Ashkenazi Jews 14.Likely explanations of modern distribution of CCR5-Δ32 allele. • Perhaps this allele was selectively favoured in Northern Europe during historical epidemics like the plague and smallpox • Perhaps it’s just chance and history Lecture 3 1. Characteristics shared by all life. • All life displays order, harnesses and utilizes energy, reproduces, responds to stimuli, exhibits homeostasis, grows and develops, and evolves 2. In what way properties of life are "emergent". • The characteristics of life come about, or emerge, from many simpler interactions that in themselves do not have the properties found at higher levels • Not only is the functional or structural complexity of living systems more than the sum of the parts, it is fundamentally different 3. Characteristics of the "habitable zone" of a solar system. • The region of space around a star where the heat from it allows for surface temperatures to be within a range that allows water to exist in a liquid state 4. Conditions of a primitive Earth. • The atmosphere contained an abundance of water vapour as well as large quantities of hydrogen (H )2 carbon dioxide (CO ), 2mmonia (NH 3, and methane (CH ). 4 • There was almost a complete absence of oxygen (O ) 2 • According to Oparin and Haldane, the atmosphere was a reducing atmosphere because of the presence of such molecules (eg. ammonia, hydrogen, methane) • These molecules contain an abundance of electrons and hydrogen and would have entered into reactions with one another that would have yielded larger and more complex organic molecules • Today, in our oxidizing atmosphere, high levels of oxygen prevent such molecules from being formed because oxygen would itself accept the electrons from organic molecules and be reduced to water • The lack of oxygen also meant there was no ozone layer • Without the ozone layer, energetic ultraviolet light was able to reach the lower atmosphere to provide energy needed to drive the formation of key biological molecules 5. Types of molecules that were, and were not, synthesized by the Millar-Urey experiment. • Urea, amino acids, and lactic, formic, and acetic acids were formed • When cyanide and formaldehyde were added to the simulated primitive atmosphere, amino acids, fatty acids, the purine and pyrimidine components of nucleic acids, sugars such as glyceraldehyde, ribose, glucose, and fructose, and phospholipids were formed • Debate has developed over whether or not Earth’s primitive atmosphere contained enough methane and ammonia to be considered reducing. • Polymers were not produced but clay catalysts may have helped 6. Importance of liposomes in evolution of first cells. • A critical step along the path of life is the formation of a membrane- defined compartment • Such a compartment would allow metabolic reactions to take place in an environment distinctly different than the external surroundings, the concentration of key molecules could be higher, and greater complexity could be maintained in a closed space • A probiont is the term given to a group of abiotically produced organic molecules that are surrounded by a membranelike structure • An early type of probiont could have been similar to a liposome (lipid bilayer cell) • Liposomes are selectively permeable and can swell and contract depending on the osmotic conditions of their environment 7. Characteristics of mimivirus that suggest it should be considered to be alive. • It can get sick, it has a large number of coding genes, it is really big 8. Characteristics of virophage. • A virophage infects a virus that infects another cell and hijacks its machinery to replicate itself. It is much smaller than its host virus • The satellite virus can perform horizontal gene transfer between viruses Lecture 3 lecture 1. Age of the Earth. • 4.6 billion years ago, Earth formed 2. Age of start of life on Earth. • Based on geological evidence, 3.6 billion years ago cyanobacteria were around • But they weren’t the first form of life. Maybe 4 billion years ago life formed 3. Domains of life. • Bacteria, archaea, eukaryotes. LUCA is common to all forms of life 4. Characteristics of LUCA • Last Universal Common Ancestor: all life is derived from this single form of life • Embodied many key characteristics now seen in all forms of life (see below) 5. Characteristics shared by all domains of life. • All use DNA, RNA, and protein (Central Dogma), DNA for genetic system, DNA to RNA to protein transfer of information, cells made of lipids, common system of protein assembly, ATP and glucose and glycolysis for energy 6. Reason why the term “prokaryote” is inappropriate. • Prokaryotes don’t exist. Archaea and bacteria used to be lumped together and called prokaryotes but archaea are far more similar to eukaryotes than bacteria. They share no common ancestor except LUCA. Nothing in common • No single group of organisms can be lumped into “prokaryotes” • “Pro” means anteriority, but there is no evidence prokaryotes came before eukaryote 7. Reductive evolution explanation for rise of bacteria and archaea. • Evolutionary does not always move from simple to complex • LUCA may have been a proto-eukaryote but by reductive evolution, bacteria and archaea got rid of the eukaryote attributes • Streamlined themselves 8. Advantages of evolutionary simplification (streamlining). • To survive in harsh environments, to reproduce faster, to conserve energy 9. Relationship between homochirality and life • Chirality means handedness • You cannot superimpose one chiral partner on the other • Two enantomers (optical isomers) have the same groups but differ in the orders they are attached. Have identical chemical and physical properties but have vastly different biological properties (thalidomide can be an antiemetic or a teratogen through chirality) • Life is homochiral. We only use one form of the chiral pair • This is essential to the evolution of life (L amino acids, D sugars) • But Miller-Urey could not explain this because the products were racemic (50/50) • The cost to the cell to use both chiral partners would be huge (cell would have to make two of everything) • We don’t know where this homochirality came from 10.Reasons why we think RNA was the first of the three molecules of the Central Dogma to evolve. • Central Dogma: DNA to RNA to protein. Enzymes are needed to drive this synthesis and enzymes are proteins. We need the product to catalyze the intermediary steps • But RNA could function as an enzyme catalyst (ribozymes) and carry information so it was likely the first form of information transfer in ancient cells • After RNA was formed, protein probably came along, followed last by DNA 11.Force that drives RNA folding. • RNA is three dimensional and can fold into a very specific shape • Watson-Cricks base pair interactions hold the molecule in its specific shape 12.Characteristics of a ribozyme. • A ribozyme is an RNA molecule that can catalyze a reaction • It plays a role in precursor tRNA processing, introns excision, mRNA processing, etc. 13.Mechanism whereby a ribozyme cleaves RNA. • A ribozyme catalyzes a reaction by forcing the substrate into a conformation so it strains that substrate into a shape that favours catalysis • The ribozyme cleaves RNA by removing a proton from oxygen which becomes very reactive and forms a bond with the phosphodiester group which cleaves it from the RNA 14.Characteristics of amino transferase activity. • Ribosomes catalyze peptidyl transferase activity (formations of peptide bonds in the translation process of protein biosynthesis). Ribosomes are 2/3 RNA, 1/3 protein (ribonucleoprotein) 15.Reasons why a ribosome is considered a ribozyme. • A ribosome catalyses the formation of protein chains 16.Advantage that protein has over RNA as a catalyst. • More variability in sequence (20 amino acids vs. 4 base pairs) • Enzymes have higher rates of catalysis and are more diverse in structure and function 17.Advantage that DNA has over RNA as a repository for genetic information. • Thymine replaces uracil because a cytosine to uracil mutation is common. Now DNA polymerase can see that the uracil is a mutation (increases the fidelity of the process) 18.Chemical basis for the advantage that DNA has over RNA as a repository for genetic information. • DNA is far more stable than RNA because of the difference on the 2prime group on the 5 carbon. RNA has a hydroxyl group, DNA doesn’t. RNA degrades very quickly because of the hydroxyl group but we can isolate DNA from millions of years ago • As well, this change also alters the shape of the molecule just enough to make it much more likely that two DNA strands will interact to form the classic double helix • Complementary strands allow you to rely on the other copy as a back- up Lecture 4 1. The approximate times by which the first cells, and the first eukaryotic cells, had appeared. • The first organisms may have been single-celled methane producing bacteria that existed more than 3.5 billion years ago • The first eukaryotic cells appeared 2.5 to 2.8 Bya 2. The two-kingdom, five-kingdom and three-kingdom (three domain) systems for classifying living things. • Until about 50 years ago, all organisms were classified into plants or animals • Life was then classified into prokaryotes and eukaryotes using cellular characters • Then, the five kingdom scheme came into being. Organisms were divided into five groups: one for prokaryotes, four for eukaryotes (protists, fungi, plants, and animals) • About 30 years ago, life was reclassified using molecular evidence into three kingdoms: Eubacteria, Archaea, and Eukarya 3. Main characteristics distinguishing members of the Eubacteria, Archaea, Eukarya domains of life. • Eubacteria includes the major form of bacteria and cyanobacteria • Archaea are unicells with cell walls made of different molecules than those found in Eubacteria. Archaea tend to live under harsher conditions • Eukarya includes some unicellular organisms (slime moulds, ciliates, etc.) and the three groups of multicellular organisms (fungi, plants, and animals). • The Archaea have histones, and their genome is far different from Eubacteria. They are more closely related to Eukarya than Eubacteria 4. Meaning of horizontal gene transfer and why this makes it challenging to recreate the universal tree of life. • Horizontal gene transfer is the transfer and incorporation of one organism’s or species’ DNA into the DNA of a different organism or species • Horizontal gene transfer has occurred between prokaryotes and from prokaryotes to eukaryotes • HGT between individuals contrasts dramatically with the vertical transmission of genes from generation to generation • HGT obscures phylogenetic relationships when otherwise distantly related organisms share a gene or sequence obtained through HGT rather than through common ancestry • As much as one third of the genome of some prokaryotes was acquired through HGT 5. Monophyletic vs. polyphyletic groupings of organisms. • Monophyletic means one origin. It is used for lineages that share a single common ancestor • Polyphyletic means many origins. It is used for groups that have more than one (multiple) independent origins Lecture 4 lecture 1. Most recent common ancestor (MRCA) for a given group(s), given a phylogenetic tree. • MRCA of all humans: 3,000 years ago • MRCAs are the place where the phylogenetic tree branches off the main branch 2. Why the idea that “humans are descended from chimps” is inaccurate. • Humans did not evolve directly from chimpanzees, we simply share a common ancestor with chimps. The lineages diverged and one formed hominids and the other formed chimps and bonobos • Our MRCA was not a chimp, it was intermediate between a chimp and a human 3. Order of main branching events in tree of life (dates not testable). • Rendezvous 1: Chimps and bonobos, 5-6 million years ago • Rendezvous 2: Gorillas, 7 millions years ago • Rendezvous 3: Orangutans, MRCA of all great apes • Rendezvous 4: Gibbons, MRCA of all apes • Rendezvous 5, 6: Old and New World monkeys • Rendezvous 7: Tarsiers, 58 million years ago • Rendezvous 8: Lemurs and lorises, MRCA of all primates • Rendezvous 9: mass extinction event, end of dinosaurs • Rendezvous 10: Rodents and rabbits • Rendezvous 11: Laurasiatheres (evolved in the north in Laurasia) • Rendezvous 14 and 15: Marsupials and monotremes (MRCA of all mammals) • Rendezvous 16: Reptiles and Rendezvous 17: Amphibians, MRCA of all tetrapods • Rendezvous 18 to 22: Lungfish, Coelacanths, Ray-finned fish, Sharks, Jawless fish. MRCA of all vertebrates, 530 mya • Rendezvous 26: Protostomes and Rendezvous 34, 35, 36: Fungi, Amoebozoans, Plants • Rendezvous 38: Archaea, MRCA of all eukaryotes and Rendezvous 39: Bacteria 4. Cause of global catastrophe associated with mass extinction 65 mya. • End-Cretaceous mass extinction thought to be caused by a meteor hitting the Earth and kicking up a giant dust cloud. Changed the temperature and the availability of light • Half the species on Earth went extinct. Global catastrophe. End of the age of the dinosaurs, beginning of the age of the mammals 5. Relative proportion of protostome vs. deuterostome species. • Deuterostomes are dwarfed in terms of species diversity compared to protostomes • So many more protostomes, because they include insects, mollusks, etc. 6. Information provided by genetic relatedness vs. traditional groupings of organisms (“reptiles”, “fish”) • When we look at their phylogeny and their patterns of descent and relatedness, birds are actually just a type of reptile. Things like snakes and lizards are more closely related to birds with a more recent common ancestor than they do with turtles • “Fish” can be no more closely related to each other than to us (ex. Lungfish and Sharks) 7. Distribution of multi-cellularity in tree of life. • Multi-cellularity has evolved in only three groups: plants, animals, and fungi • Plants can be unicellular or multicellular. Amoebozoans like slime moulds can live as a unicellular organism or group to form multicellular groups 8. Why estimating numbers of species is uncertain. • There are more species undiscovered than there are discovered • Most of the species on Earth have not been described • Many species live in environments that are hard to reach (ocean depths, lizard lungs) 9. Role of similarities due to common descent (DNA genome) vs. convergence (eyes) in constructing a phylogenetic tree. • There are some things that are found in every living creature (DNA, glycolysis, ATP, etc.) • We have these traits in common because we descended form a single common ancestor • But some similarities reflect shared ancestry while other traits are similar due to convergence (eyes have evolved multiple times, like sonar) • If evolution were to rerun, would the outcomes be the same? Lecture 5 1. Meaning of "C-value". • The amount of DNA contained within a haploid nucleus (1n) 2. "Paradox" or "enigma" associated with C values • The puzzle surrounding the extensive nuclear genome size among eukaryotic species • Genome size does not correlate with organismal complexity (C-value paradox) • The C-value enigma refers to the issue of variation in the amount of non-coding DNA found within the genomes of different eukaryotes 3. Meaning of haploid (n) and diploid (2n) • Haploid means the organism has only one of each chromosome, diploid means the organism has two copies of each chromosome as a pair of homologues • The number of chromosome sets is called the ploidy of an organism 4. Relationship between C and n as measures of genome size • In bacteria and archaea, larger genomes tend to reflect increased gene number • Members of the Eukarya domain vary widely in both C, gene number, and n • There are no rules relating organism complexity and genome size or chromosome number • When DNA is being replicated in mitosis, C doubles, but n does not change 5. Proportion of the human genome that codes for protein. • Most is non-coding, only 2% of the human genome codes for protein • Introns occupy about 24% of the genome • The rest of the DNA occupies the space between the genes with more than 50% having no known function Lecture 5 lecture 1. Non-nuclear genomes in typical plant and animal cells. • Animals have mitochondrial DNA, plants have mitochondrial and chloroplast DNA 2. Trend in C value from prokaryotic vs. eukaryotic cells. • The amount of DNA is one genome is called C. Genome size is quite variable • In general, genome size goes up among eukaryotes compared to prokaryotes but within a taxonomic group, C-value has a very wide range 3. Relationship between C value and organismal complexity • There seems to be no relationship between C-value and organismal complexity • This is the C-value enigma 4. Relationship between C value and ploidy • One C value is distributed over one set of chromosomes • Two sets of chromosomes (2n) is equal to 2C. The organism is diploid. Ploidy refers to the number of sets of chromosomes • In trying to understand genomes, the number of chromosomes is not very useful 5. Distribution of linear vs. circular chromosomes in the various domains of life. • Linear chromosomes are characteristics of Eukarya • Archaea and bacteria (prokaryotes) have circular chromosomes • Mitochondria and chloroplasts also have circular genomes 6. Role of nucleosomes in DNA packaging in chromosomes • A chromosome is packaged chromatin: DNA plus protein • The DNA is wound around nucleosomes (made up of histone proteins) to condense it 7. General trends in costs of DNA sequencing • The cost of DNA sequencing is dropping quickly • From three million dollars to 10 thousand now 8. Relative distribution of various component of genome sequence ("junk" vs. essential DNA) • 10% is essential DNA, with about 2% coding for proteins. 10% is intron sequences • 25% is unknown but more than 50% of DNA is thought to be “junk” Lecture 6 1. Purine and pyrimidine base-pairing in DNA/RNA • DNA contains four different nucleotides with each containing the five- carbon sugar deoxyribose, a phosphate group, and either adenine, guanine, thymine and cytosine • Two of the bases (adenine and guanine) are purines and the other two (thymine and cytosine) are pyrimidines • Purines bond with pyrimidines (A and T, C and G). These relationships are Chargaff’s rules • In RNA, thymine is replaced with uracil. The base pairing remains the same (A with U) • This is complementary base pairing. The purine and pyrimidine pair is stabilized by hydrogen bonds and is just the right size to fit between the DNA backbone 2. Outcome of the classic Meselson and Stahl experiment • Meselson and Stahl tagged the DNA with non-radioactive, heavy nitrogen 15 isotopes (which has one more neutron than normal nitrogen 14) by growing E. coli in a culture medium containing nitrogen 15. The DNA incorporates the denser nitrogen into its structure • Then they transferred the bacteria to a culture containing the nitrogen 14 so all the newly replicated DNA would incorporate it and be tagged with the lighter nitrogen 14 • Just before the transfer to the nitrogen 14 medium and after each round of replication following the transfer, they extracted DNA from cell samples • DNA formed one density band when centrifuged after one replication, two after two replication. The first density band contained equal amounts of N 14 and N ; the second two had an N 14and N 14 band and an 15 14 N N band. This matched the predicted results using the semi- conservative replication model, supporting the hypothesis 3. Direction of movement of DNA polymerase on the template strand • The deoxyribose sugars are linked by the breaking of phosphate groups. This means DNA polymerase can only add molecules at the 3 I end (on the hydroxyl group) • Each phosphate group is a bridge between the 3 carbon of one sugar I and the 5 carbon of the other sugar • Because of the anti-parallel nature of DNA, the template strand is read I I in the 3 to 5 end. In one replication fork, DNA polymerase moves in two different directions, always from the 3 end to the 5 end of the template strand 4. Meaning of semi-conservative, semi-discontinuous, leading and lagging strand • Semi-conservative replication: both original DNA molecules serve as template strands and pair with the new strands to form an exact copy of the original DNA molecules composed of one new and one old strand • Semi-discontinuous replication: because the two strands of DNA are I anti-parallel, only one strand can be synthesized continuously in the 3 to 5 direction towards the replication fork. The other must be replicated in short strands called Okazaki fragments away from the replication fork • Leading strand: the new strand of DNA synthesized in the direction of unwinding • Lagging strand: the new strand of DNA synthesized in the direction opposite unwinding 5. General action of proteins in Fig. 12.15. • DNA helicase unwinds the DNA • Primase synthesizes RNA primers in the 5 to 3 direction (in the direction of unwinding in the leading strand and in the opposite direction in the lagging strand) • Topoisomerase prevents twisting ahead of the replication fork as DNA unwinds • DNA polymerase III adds DNA nucleotides to the RNA primer (3 to 5) I I • DNA polymerase I removes the RNA primers and replaces then with DNA bases • A break in the backbone remains after DNA polymerase is done, DNA ligase seals it Lecture 6 lecture 1. Basic structure of double-stranded DNA • DNA is a double helix composed of two DNA molecules running antiparallel (conferring polarity) and stabilized by hydrogen bonds between the base pairs • This antiparallel arrangement is highly significant for the process of replication I I • 3 has hydroxyl group, 5 has phosphate 2. Components necessary for DNA synthesis • DNA synthesis requires a phosphate group to break and supply energy to the replication process (at the 5 end)I 3. Direction of elongation of a given DNA strand I I I • Bases always added on the 3 end to the free 3’OH, 5 to 3 4. Structure of a replication bubble • Two replication forks stuck together is one replication bubble • Two forks at one origin, so at one origin, there are two leading strands, and two lagging strands basically diagonal to each other • There are multiple origins all across your genome 5. Relationship between replicated DNA and metaphase chromosomes • Chromatids are identical DNA molecules attached at the centromere • You have the same number of chromosomes whether they are replicated or not, replication increases the amount of DNA (C), not the number of actual chromosomes (n) • DNA replicates in S-phase, in G2, your genome is composed of two DNA helixes, and in mitosis, the DNA condenses into chromatids and separates into daughter cells • Note: most cells are in G1 or G0, not in mitosis. Even if they are cancerous, even if they are repairing a wound, even actively cycling cells are mostly in interphase 6. Why chromosomes shorten at each replication • An RNA primer is removed at the 3 end of the template and cannot be I replacedbecause DNA polymerase can only add new bases on the 3 end and at the 3 end of the template strand, the 5 end of the new strand is exposed and so cannot be extended 7. Mechanism by which telomerase adds telomeres to chromosomes • Telomerase extends the ends of the DNA by extending the 3 end through I an RNA template. • The DNA added is a telomere (it is a repeated stretch of DNA at the ends of chromosomes) • RNA primer is then added to the new telomere as usual and the 5 end I I I extended as usual, from 3 to 5 end. Afterwards the primer is removed once more. Lecture 7 1. Stages and main characteristics of the stages of mitosis. • Prophase: DNA condenses into sister chromatids, translation and transcription stop • Prometaphase: nuclear envelope disappears, microtubules attach to replicated chromosomes • Metaphase: chromatids line up at the spindle midpoint/metaphase plate • Anaphase: cohesion dissolves and sister chromatids (now called chromosomes) are pulled by motor proteins in kinetochore along spindles to opposite poles of the cell • Telophase: The spindle disassembles, the nuclear membrane reappears and the chromosomes decondense. Cleavage furrow forms. End stage of mitosis 2. Stage of cell division, given a micrograph of a dividing cell. 3. Role and mechanism of the mitotic spindle. • The mitotic spindle does not actually pull the chromatids. Motor proteins as part of the kinetochore walk their way along the microtubules attached to the centromere • The region where sister chromatids are closest to each other is called the centromere • A microtubule attaches at the kinetochore protein complex which is located at the centromere due to protein attraction. Spindle from opposite poles have to attach to chromosomal kinetochores for anaphase to proceed 4. Role of cell cycle check points. • Ensures one stage has completed properly before the cell proceeds to
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