exam 1 study guide
exam 1 study guide BIOL 11100 - 001
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BIOL 11100 - 001
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This 10 page Study Guide was uploaded by Brianna Nemeth on Saturday September 19, 2015. The Study Guide belongs to BIOL 11100 - 001 at Purdue University taught by Denise Lore Zielinski, Mark Edward Browning in Fall 2015. Since its upload, it has received 86 views. For similar materials see Fundamentals Of Biology II in Science at Purdue University.
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Date Created: 09/19/15
MitosisMeiosis 1 What does the duplication of cells require In order to make a new cell it is necessary to do 3 main things 1 Replicate genetic material In eukaryotic cells this means the duplication of all chromosomes each consisting of a single DNA molecule 2 Accurate segregation of each duplicated chromosome to each daughter cell This ensures that the two new cells each have the same number and kind of chromosomes 3 Division of the cytoplasm such that each daughter cell receives all the subcellular organelles needed to maintain cellular life 2 How is the genetic material of prolltayotic cells organized and how do the cells divide Most prolltaryotes have a single circular chromosome Prolltaryotic cells divide by binary fission This process is much simpler than the elaborate process seen in eukaryotes It does not require a complex cell cycle and the segregation of chromosomes is much simpler Contrary to what was thought for many years the process of chromosome segregation is an active process that requires the participation of a number of proteins This is not accomplished by the simple growth of the plasma membrane with passive segregation of the attached chromosome However replication and segregation of the chromosome are concerted processes during binary fission That is they occur together with the newly replicated chromosome being segregated to opposite poles by a mechanism that yet unknown This requirement for replication immediately followed by segregation may limit how complex a chromosomal organization this type of cell can have 3 What are centromeres and lltinetochores and what are their roles in the function of chromosomes What is the function of cohesion Centromeres are the constrictions seen in chromosomes often in the center of the chromosome socalled metacentric chromosomes Kinetochores are the proteins found at the centromere that function as microtubule attachment sites during mitosis The attachment of polar microtubules to the lltinetochores of sister chromatids is necessary for accurate segregation of chromosomes Centromeres can be thought of as genetic entities they are DNA sequences and their behavior can be followed genetically by following the behavior of traits that are tightly linllted see next section on genetics for information on linlltage Kinetochores are cell biological entities that are found at the physical location of centromeres Both are necessary for the proper functioning of this structure during mitosis Cohesin is a complex of proteins that hold two sister chromatids together near the kinetochore Cleavage of cohesion irreversibly separates the two chromosomes Because replication of chromosomes and segregation of chromosomes occurs at different times cohesion is very important 4 How are chromosomes organized in eukaryotic cells How are they compacted to fit into cells and how are they compacted further to allow them to be separated during cell division What is the structure of the nucleosome What is its role in the structure of chromosomes Chromosomes are highly compacted in cells as they can be literally mm long they must be compacted to fit into um long cells This compaction is hierarchical and can be thought of as occurring in stages The degree of compaction changes with the cell cycle as they must be maximally compacted for segregation during cell division At all times chromosomes are organized into structures we call nucleosomes These consist of a protein core called the histone octamer About 200 bp of DNA is wrapped 15 times around this histone octamer lillte string around a ball leaving a short linker stretch between adjacent nucleosomes This can be then coiled into a still higher order structure often called a solenoid where the string of nucleosomes form a higher order coil This structure composed of nucleosomes pacllted together in a coil is about 30 nm in diameter and is called rather originally the quot30 nm fiber also sometimes called the solenoid for its shape This is the state of interphase chromatin To achieve accurate segregation of chromosomes during cell division it is necessary to compact them even further 5 What are the phases of the cell cycle and what occurs during each phase What does it mean to say that cell division is an oscillation between interphase and mitosis The cell cycle can be divided somewhat arbitrarily into 5 phases G1 gap 1 S synthesis G2 gap 2 mitosis and cytokinesis The two gap phases G1 and G2 plus the DNA synthesis phase S phase are together called interphase Interphase takes up the majority of the cell cycle and the cycle itself can be seen as oscillating between interphase or preparing to divide and mitosis or actively dividing The G1 phase is important because it is when the cell decides to actually divide at all It is also important to prepare the cell for S phase During S phase the cell replicates all of its DNA and the centrosomes replicate as well The cell must accomplish the accurate replication of all of its DNA to be able to proceed to mitosis During G2 the cell prepares to divide by mitosis Mitosis involves the actual segregation of genetic material Cytolltinesis is the division of the cytoplasm 6 What are cyclins and what is their role in the cell cycle What are Cdllt enzymes and what is their function during the cell cycle Cyclins are proteins that are produced and destroyed in concert with the cell cycle They were first recognized based on their periodic synthesis and destruction which correlates with the progression of the cell cycle Mitotic cyclins are made at increasing levels as the cell progresses through interphase and then are rapidly degraded during mitosis The role of cyclins in the cell cycle is to form a complex with enzymes that are protein kinases thus Cyclin Dependent Kinases or Cdk s The Cdllt enzymes are not active alone but require binding to cyclin for activity The Cdllt enzymes control the progress through the cell cycle by phosphorylating a variety of cellular proteins needed for division This includes the proteins that turn on the expression of genes encoding new protein needed during the cycle as well as activating existing protein The Cdk s are also controlled by phosphorylation of the kinase itself and by the presence absence of cyclins The case of regulation of the cell cycle during anaphase is a little more subtle and relates to a key difference between prokaryotes and eulltaryotes The signal to proceed through anaphase goes through the anaphase promoting complex APC Once the APC is activated that molecule in turn activates separase an enzyme that hydrolyses the cohesion proteins holding the two chromatids together The relationship between the replication of chromosomes and their segregation is thus during the eukaryotic cell cycle the replication and segregation of chromosomes are not consecutive events the way they are in prokaryotes This means that after chromosomes are replicated they must remain attached until the segregation event for maximal accuracy Thus the two chromatids of a newly replicated chromosomes remain attached at their centromere until anaphase In fact the centromere appears to be a single centromere holding together the two chromatids in microscope images This is an oversimplification but not a bad way to think about it 7 Are any steps in the cell cycle irreversible What are the implication of this for the control of the cell cycle and the mechanics of the cell cycle There are two steps that may be thought of as irreversible the decision to divide at all called START and the decision to segregate chromosomes during anaphase of mitosis The irreversible nature of START is obvious once you have committed to replicating chromosomes you had better divide the cell as well or you will double the number of chromosomes 8 What are the phases of mitosis and what is happening during each phase Why is anaphase so important What would happen if you had mitosis without cytolltinesis The phases of mitosis are Prophase Prometaphase Metaphase Anaphase and Telophase Brie y during prophase the chromosomes condense and become visible The centrioles move to opposite ends of the cell The spindle apparatus appears and the nuclear envelope dissolves Prometaphase is characterized by the attachment of the chromosomes to the spindle apparatus The attachment occurs at the kinetochore and the microtubules that make up the spindle apparatus start to move the chromosomes toward the center of the cell Metaphase is characterized by the arrangement of all chromosomes at the equator or metaphase plate of the cell with polar microtubule fibers from opposite poles attached to the centromere of each chromatid This involves some pulling of chromosomes towards one pole then the other until they are all arranged in the middle and are also under tension with sister chromatids of each chromosome oriented towards opposite poles During anaphase chromosomes separate their connection at the centromere and move towards opposite poles This movement is usually called anaphase A and is accompanied or followed by movement of the two poles away from each other called anaphase B The actual mechanics of anaphase A appear to involve the degradation of proteins holding the chromosomes together controlled by the anaphase promoting complex APC These proteins that hold chromosomes together are called cohesins Thus the cohesins are the glue that holds chromatids together after replication and the APC triggers the removal of this glue at the appropriate time During telophase the spindle apparatus disassembles the chromomosomes start to uncoil and the nuclear envelope is reassembled Mitosis without cytolltinesis will produce a cell with twice the amount of DNA as its precursor cell 9 What is the difference between a homolog and a sister chromatid Do all chromosomes have sister chromatids In a diploid cell for each chromosome there are two homologues a paternal and maternal homolog After S phase each homolog is composed of two sister chromatids held together at the centromere as we have just seen in mitosis Thus only cells in G2 through metaphase have sister chromatids Notice that we call an unreplicated chromosome and a replicated chromosome the same thing one chromosome even though they look different and have different amounts of DNA 10 What is the significance of meiosis I and how does this differ from mitosis How does meiosis II differ from mitosis Meiosis I is the defining division of meiosis often called the reductive division because at the end the resulting cells will have half the number of chromosomes To understand this you must understand that we count chromosomes by counting centromeres not chromatids see 10 above Meiosis I is characterized by the homologues of each chromosome finding their quotpartnerquot and pairing along the entire length of the chromosome This involves the construction of an elaborate structure the synaptonemal complex between them While they are complexed with one another crossing over can occur This event recombines parts of one chromosome with its homologue The result is that the chromosomes an individual inherits are not exact copies of the ones their parents had Instead each chromosome is a mosaic of parts of that chromosome from each parent This dramatically increases the possible genetic diversity that arises through sexual reproduction Homologues will remain associated through metaphase with homologues becoming attached to microtubules from opposite poles This leads eventually to homologues moving to opposite poles during anaphase instead of sister chromatids as in mitosis We say that during meiosis I homologues disjoin and segregate and during mitosis sister chromatids disjoin and segregate Meiosis II is much like a mitotic division except that it is not preceded by DNA replication and the cells will have half the DNA of the original cell after the division The behavior of the chromosome during metaphase and anaphase is the same as in mitosis and distinct from the pairing seen in meiosis I 11 Compare and contrast meiosis and mitosis Mitosis results in cells that are essentially identical or at least with identical genetic material while meiosis produces cells that have half the genetic material half the number of chromosomes and are not genetically identical Mitosis produces two cells while meiosis generally produces 4 cells Mitosis involves a single round of replication and a single nuclear division while meiosis involves a single round of replication but two rounds of nuclear division Homologues pair during meiosis I and remain associated until anaphase when they segregate During this pairing process chromosomes will also exchange material by crossing over a cytologically visible process which correlates with genetic recombination as we will see later Homologues behave independently during mitosis and sister chromatids segregate during anaphase 12 Anueploidy is a condition in which gametes lacllt or have duplicate copies of a particular chromosome Fertilization of such a gamete by a normal gamete can result in zygotes that have abnormal numbers of chromosomes For example Down syndrome is most frequently caused by the presence of three copies of chromosome 21 trisomy 21 Given your knowledge of gamete formation hypothesize a mechanism by which aneuploidy can occur Be explicit about the stages of meiosis that are relevant Anueploidy may occur if homologous chromosomes fail to separate during anaphase I or if sister chromatids fail to separate in anaphase II Genetics Concepts 1 What was the view of genetics prior to Mendel How did he change this The view of inheritance before Mendel was that traits from each parent mix in the offspring or quotblending inheritance This was not a quantitative concept so one of the biggest differences in methodology between Mendel and the majority of his predecessors was his quantitative approach His quantitative approach simple by today s standards but was way ahead of its time and led to the use of probability to predict the outcome of crosses One of the most important conceptual difference was the idea of particulate inheritance This is the idea that traits are carried by discrete physical units that do not blend We now call these genes 2 What is a locus How is this related to a gene and an allele A locus is the physical location of a gene on a chromosome Genes are the physical units of inheritance Traditionally genes are defined as a segment of DNA that codes for a protein but we will define genes in different ways throughout the semester Alleles are the different forms of a gene that can exist The copies of genes found on different chromosomes can be different alleles of that gene 3 When we cross round by wrinlltled we get only round progeny Why is this What do we call the round and wrinkled phenotypes Only the round progeny appear in an F1 generation because the round is dominant to wrinkled The physical basis of this dominance will be something that we approach from different views of the gene The round phenotype specified by the round allele would be the dominant phenotype or allele and the wrinkled phenotype specified by the wrinkled allele would be the recessive phenotype or allele 4 What is the principle of segregation What is the physical basis behind this principle The principle of segregation says that during the process of gamete formation different forms of a gene alleles are segregated into gametes This means that they are physically separated into different gametes During fertilization individual single copy alleles from different parents are united restoring diploidy so that each embryo has two copies of each gene A more modern statement would be that different alleles of a gene segregate during meiosis Note that this is only visible in a heterozygote as homozygotes make only one type of gamete because all copies of a gene are the same allele The physical basis for segregation is the behavior of chromosomes during meiosis 5 What is the relationship between genotype and phenotype for homozygotes and heterozygotes The relationship between genotype and phenotype for a homozygote is simple as there are two copies of the same allele In Mendelian genetics a heterozygote takes the phenotype of the dominant allele This means that when you see the dominant phenotype you do not know the genotype either homozygous dominant or heterozygous but if you see the recessive phenotype you do know the genotype homozygous recessive In either case genotype leads to phenotype keeping in mind rules of dominance 6 What is the principle of independent assortment What is the physical basis for this principle How does the process of meiosis allow us to predict the Mendelian ratio seen for a dihybrid cross The principle of independent assortment says that different genes will behave independently in the production of gametes and thus in genetic crosses To put it in more modern terms alleles of different genes segregate independent of each other So although alleles happen to be found together in one individual they do not have to stay together if they are on different chromosomes they assort and segregate independently of one another The physical basis for this is that different chromosomes behave independently during the process of meiosis the chromosomes from one parent don t have to stay together during segregation That is when homologues pair and move to the metaphase plate during meiosis I the homologues of different chromosomes will align on the metaphase plate independently In a single gamete the chromosomes are a mix of some paternal and some maternal chromosomes 7 What do we mean by the term quotprogeny testing How does this relate to the concepts of genotype and phenotype When we look at an individual who displays the dominant phenotype we cannot tell their genotype that is if they are a homozygous for the dominant allele or heterozygous We can determine this experimentally if we do a test crosscross this individual to the homozygous recessive genotype and observe the progeny If the recessive allele segregates then the original parent was a heterozygote 8 Is dominance absolute How can dominance be altered How does this affect your view of the concepts of genotype and phenotype Dominance is not absolute there are many variations on strict dominance In incomplete dominance the phenotype of the heterozygote appears intermediate between the phenotypes of the two homozygotes In codominance the phenotype of the heterozygote shows the phenotypes due to both alleles In either case the heterozygote has a phenotype that is distinct from both homozygotes This all indicates that dominance can be a continuum or not exist at all This means that the simple view from Mendel does not always hold In these cases of altered dominance the nature of the relationship between genotype and phenotype is even more clear genotype leads to phenotype 9 How do alterations of dominance affect the results from simple crosses The phenotypic ratio is reduced to the genotypic ratio as now all of the genotypes have unique phenotypes Thus we see a 121 ratio instead of the 31 monohybrid ratio The usual monohybrid ratio is really a condensed form of the genotypic ratio 10 What is pleiotropy What do we mean when we talk about quotgene interactions Give an example from class of such a genetic interaction How does this affect your view of genotype and phenotype Pleiotropy is when a single gene product influences multiple traits This occurs because of branching biochemical pathways and multifunctional proteins In many cases the protein product of a pleiotropic allele may be dominant when acting in one capacity and recessive when acting in another function Gene interactions are not contrary to independent assortment although it may appear to be on the surface The usual ratio expected of independent assortment will be altered but it is still there much like with incomplete dominance and codominance The interaction is at the level of the function of the genes one gene obscures the action of another gene The example we used in class was coat color in Labrador retrievers In this case when an individual is homozygous recessive for one gene the phenotype for another gene is not seen This is called recessive epistasis This is a common phenomenon in coat colors in mammals The gene with the epistatic effect determines the presence or absence of color the gene whose phenotype is obscured determines the nature of the color If there is no color at all how can black or brown be significant In terms of genotype and phenotype it leads to even greater complexity as now we must consider the effects of multiple genes together when we think about phenotype The effect of one gene will only be seen in the presence of certain alleles for another gene or to use genetic terminology in a particular genetic background We cannot just think about the effects of different genes individually then sum these effects Considering the overall complexity of metabolism it was naive to think that we could only consider one gene at a time 11 What is sexlinlltage and how can you recognize it What is the physical basis for sexlinlltage Sexlinkage is the alteration in patterns of inheritance based on the sex of the parents This is seen for loci that are on the socalled sex chromosomes These are chromosomes that differ between the sexes and these chromosomal differences lead to altered patterns of inheritance The heterogametic sex the one with different sex chromosomes will always show recessive traits In Drosophila if we cross whiteeyed males with redeyed females all progeny are red eyed If we do the reciprocal cross with redeyed males and whiteeyed females we would normally expect no difference What we actually observe is redeyed females and whiteeyed males This difference in reciprocal crosses is the hallmark of sexlinlltage In humans it leads to males showing sexlinllted traits more often they are the heterogametic sex and to inheriting these traits from their maternal grandfathers through their mothers 12 What is linkage and how do we define linkage How does this relate to the behavior of chromosomes during meiosis Linlltage is an alteration of independent assortment such that an excess of parental gametes are produced This alters Mendelian ratios significantly and in ways that cannot be viewed as a modified ratio the way some genetic interactions can We usually analyze this using test crosses crossing to the homozygous recessive so we can concentrate on a single parent In this case we can easily observe the excess of parental genotypes characteristic of linkage This has a simple basis two loci that are close together on the same chromosome would not be expected to segregate independently If they are very close together we should only see the parental combinations 13 How can two genes on the same chromosome ever lead to recombinant gametes How can this quotpartial linlltage be related to genetic map distances What is the unit of genetic maps The production of recombinant gametes from linllted genes is due to the phenomenon of crossing over where chromosomes physically exchange material This is also called genetic recombination as it does lead to recombinant genotypes This process is infrequent so these recombinant genotypes will always be fewer than the nonrecombinant ones parentalgtrecombinant thus linlltage The farther apart two loci are the more frequently recombination will occur between them Thus recombination frequency is actually a measure of quotgenetic distance Although this is a somewhat abstract notion the idea has a concrete reality the farther apart two loci are on a chromosome the higher the probability of recombination occurring between them The percent recombination is the unit used in genetic mapping and is also called the centimorgan 1 recombination1 map unit1 cM 14 Can genes on the same chromosome ever assort independently How can this occur and what does it say about events during meiosis If genes are far enough apart on the same chromosome recombination will be more frequent between them In this case something interesting happens when you have two events you have restored the parental combinations If there are enough events then odd numbers are recombinant and even numbers are parental and these will be a bout equal thus independent assortment This also means that if you were to graph recombination frequency RF vs genetic distance it would show a maximum of 05 for RF This is also the RF that is observed in independent assortment equal numbers of parental and recombinant means 05 recombinant
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