BSC197 Lecture Notes
BSC197 Lecture Notes BSC197
Popular in Molecular and Cellular Basis of Life
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This 9 page Class Notes was uploaded by Brittany Notetaker on Sunday October 18, 2015. The Class Notes belongs to BSC197 at Illinois State University taught by Wade Nichols in Summer 2015. Since its upload, it has received 33 views. For similar materials see Molecular and Cellular Basis of Life in Biological Sciences at Illinois State University.
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Date Created: 10/18/15
BSC197 Lecture Notes 101215 101615 Cell Division 0 In unicellular organisms division of one cell reproduces the entire organism o Multicellular organisms depends on cell division for 0 Development from a fertilized cell egg 9 human 0 Growth number of cells rise 0 Repair 0 Cell division is an integral part of the cell cycle the life of a cell from formation to its own division 0 Most cell division results in daughter cells with identical genetic information DNA 0 A special type of division produces nonidentical daughter cells gametes sperm and egg cells Cellular Organization of the Genetic Material 0 All the DNA in a cell constitutes the cell s 0 A genome can consist of a single DNA common in prokaryotic cells or a number of DNA molecules common in eukaryotic cells 0 DNA molecules in a cell are packaged into DNA amp proteins 0 DNA is associated with histone proteins to form 0 The chromatic is further compacted and looped to form very dense chromosomes 0 Chromosome ends are referred to as 0 Each chromosome has a narrowing or pinched region called the centromere Chromosome Structure 0 The location of the centromere is used to describe specific chromosome structure o centromere is near the center of the chromosome 0 centromere is between center and end 0 centromere is near the telomere o centromere is at the telomere 0 Chromosomes may occur as unreplicated single chromosomes or as replicated chromosomes which have two sister chromatids connected at a single centromere 0 When in doubt count centromeres to get the number of actual chromosomes Cells and Chromosomes 0 Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus 0 nonreproductive cells cells that make up the body have two sets of chromosomes 0 reproductive cells sperm and eggs have half as many chromosomes as somatic cells 0 Eukaryotic cell division consists of o the division of the nucleus 0 the division of the cytoplasm 0 Gametes are produced by a variation of cell division called 0 Meiosis yields nonidentical daughter cells that have only one set of chromosomes half as many as the parent cell 0 Interphase about 90 of the cell cycle can be divided into subphases o G1 phase rst gap newly produced daughter cells expand in size to become more mature I Start functioning o S phase synthesis replicating DNA 0 G2 phase second gap cell recovery compare to running a few miles and recovering o The cell grows during all three phases but chromosomes are duplicated only during the S phase 0 Mitotic or M phase is divided into two subphases o Mitosis division of replicated DNA 0 Cytokinesis division of cytoplasm Stages of Mitosis o Mitosis is conventionally divided into five phases 0 replicated chromosomes thicken and shorten I Centrosomes moving outward o chromosomes continue to thicken I Spindles extend from centrosomes and bind to kinetochore of centromere chromosomes are pushed to midline of cell by spindles centromere splits I Spindles begin to shorten and pull chromosomes apart I Two sister chromatids are now two separate chromosomes 0 chromosomes are pulled to opposite ends of the cell I Cell is prepared to divide 0 Cytokinesis is well underway by late tel0phase 00 The Mitotic Spindle A closer look 0 The is an apparatus of microtubules that controls chromosome movement during mitosis 0 During prophase assembly of spindle microtubules begins in the centrosome o The centrosome replicates forming two centrosomes that migrate to opposite ends of the cell as spindle microtubules grow out from them 0 An a radial array of short microtubules extends from each centrosome 0 During prometaphase some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes 0 At metaphase the chromosomes are all lined up at the metaphase plate the midway point between the spindle s two poles o In anaphase sister chromatids separate and move along the microtubules toward opposite ends of the cell 0 The microtubules shorten by depolymerizing at their kinetochore ends Cytokinesis o In animal cells cytokinesis occurs by a process known as cleavage forming a o In plant cells forms during cytokinesis 101415 Lecture The Eukaryotic Cell Cycle is regulated by a Molecular Control System 0 The frequency of cell division varies with the type of cell 0 These cell cycle differences result from regulation at the molecular level The Cell Cycle Control System 0 The sequential events of the cell cycle are directed by a distinct cell cycle control system which is similar to a clock 0 The cell cycle control system is regulated by both internal and external controls 0 The clock has specific checkpoints where the cell cycle stops until a goahead signal is received For many cells the G1 checkpoint seems to be the most important one o If a cell receives a goahead signal at the G1 checkpoint it will usually complete the S G2 and M phases and divide o If the cell does not receive the goahead signal it will exit the cell switching into a nondividing state called the G0 phase Loss of Cell Cycle Controls in Cancer Cells 0 Cancer cells do not respond normally to the body s control mechanisms 0 They may make their own growth factor 0 They may convey a growth factor s signal without the presence of the growth factor 0 They may have an abnormal cell cycle control system Meiosis Overview Variations on a Theme 0 Living organisms are distinguished by their ability to reproduce their own kind is the scientific study of heredity and variation is the transmission of traits from one generation to the next is demonstrated by the differences in appearance that offspring show from parents and siblings Inheritance of Genes 0 Genes are the units of heredity and are made up of segments of DNA 0 Genes are passed to the next generation through reproductive cells called gametes sperm and eggs 0 Each gene has a specific location called a locus on a certain chromosome 0 One set of chromosomes is inherited from each parent Comparison of Asexual and Sexual Reproduction In asexual reproduction one parent produces genetically identical offspring by mitosis o Bacteria singlecell organisms A clone is a group of genetically identical individuals from the same parent In sexual reproduction two parents give rise to offspring that have unique combinations of genes inherited from the two parents 0 The sex chromosomes are called X and Y Human females have the homologous pair of X chromosomes XX Human males have one X and one Y chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23 one from the mother and one from the father 0 A diploid cell 2n has two sets of chromosomes For humans the diploid number is 26 2n46 A gamete sperm or egg contains a single set of chromosomes and is called a haploid n For humans the haploid number is 23 n23 Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg ovum the sex chromosome is X In a sperm cell the sex chromosome may either be X or Y Fertilization is the union of gametes the sperm and the egg The fertilized egg is called a zygote and has one set of chromosomes from each parent 0 The zygote produces somatic cells by mitosis and develops into an adult 0 O O O O 00 000000 In animals meiosis produces gametes which undergo no further cell division before fertilization Gametes are the only haploid cells in animals Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organisms Meiosis Reduces the Number of Chromosome Sets from Diploid to Haploid Like mitosis meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions called meiosis I and meiosis II The two cell divisions result in four daughter cells rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell In the first cell division meiosis I homologous chromosomes separate Meiosis I results in two haploid daughter cells with replicated chromosomes it is called the reductional division In the second cell division meiosis II sister chromatids separate Meiosis II results in four haploid daughter cells with unreplicated chromosomes it is called the equational division Meiosis I is preceded by interphase in which chromosomes are replicated to form sister chromatids 0 Division in meiosis I occurs in four phases 0 Prophase I Prophase I typically occupies more than 90 of the time required for meiosis Chromosomes begin to condense In synapsis homologous chromosomes loosely pair up aligned gene by gene In crossing over nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad a group of four chromatids Each tetrad usually has one or more chiasmata Xshaped regions where crossing over occurred 0 Metaphase I In metaphase I tetrads line up at the metaphase plate with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome 0 Anaphase I In anaphase I pairs of homologous chromosomes separate One chromosome moves toward each pole guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole o Telophase I and cytokinesis In the beginning of telophase I each half of the cell has a haploid set of chromosomes each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously forming two haploid daughter cells 0 Division in meiosis II also occurs in four phases 0 Prophase II In prophase II a spindle apparatus forms In late prophase II chromosomes each still composed of two chromatids move toward the metaphase plate 0 Metaphase II In metaphase II the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles o Anaphase II In anaphase II the sister chromatids separate The sister chromatids of each chromosome now more as two newly individual chromosomes toward opposite poles o Telophase II and cytokinesis In telophase II the chromosomes arrive at opposite poles I Nuclei form and the chromosomes begin decondensing Cytokinesis separates the cytoplasm I At the end of meiosis there are four daughter cells each With a haploid set of unreplicated chromosomes I Each daughter cell is genetically distinct from the others from the parent cell Meiosis II is very similar to mitosis Three events are unique to meiosis and all three occur in meiosis I o Synapsis and crossing over in prophase I Homologous chromosomes physically connect and exchange genetic information 0 At the metaphase plate there are paired homologous chromosomes tetrads instead of individual replicated chromosomes 0 At anaphase I it is homologous chromosomes instead of sister chromatids that separate Genetic Variation Produced in Sexual Life Cycles Contributes to Evolution Mutations changes in an organism s DNA are the original source of genetic diversity Mutations create different versions of genes called alleles Reshuf ing of alleles during sexual reproduction produces genetic variation The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation 0 Independent assortment of chromosomes 0 Crossing over 0 Random fertilization Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible When chromosomes assort independently into gametes is 2 Where n is the haploid number For humans n23 there are more than 8 million 223 possible combinations of chromosomes Crossing Over Crossing over produces recombinant chromosomes Which combine genes inherited from each parent Crossing over begins very early in prophase I as homologous chromosomes pair up gene by bene In crossing over homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum unfertilized egg The fusion of two gametes each with 84 million possible chromosome combinations from independent assortment produces a zygote with any of about 70 trillion diploid combinations Overview Genetics The blending hypothesis is the idea that genetic material from the two parents blends together like blue and yellow paint blend to make green The particulate hypothesis is the idea that parents pass on discrete heritable units genes Mendel documented a particulate mechanism through his experiments with garden peas Mendel Used the Scientific Approach to Identify Two Laws of Inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments Advantages of pea plants for genetic study 0 There are many varieties with distinct heritable features or characters such as ower color character variants such as purple or white owers are called traits o Mating of plants can be controlled 0 Each pea plant has spermproducing organs stamens and egg producing organs carpels o Crosspollination fertilization between different plants can be achieved by dusting one plant with pollen from another Mendel chose to trach only those characters that varied in an eitheror manner In a typical experiment Mendel mated two contrasting truebreeding varieties a process called hybridization The truebreeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals selfpollinate the F2 generation is produced The Law of Segregation When Mendel crossed contrasting truebreeding white and purple owered pea plants all of the F1 hybrids were purple When Mendel crossed the F1 hybrids many of the F2 plants has purple owers but some had white Mendel discovered a ratio of about three to one purple to white owers in the F2 generation Mendel reasoned that only the purple ower factor was affecting ower color in the F1 hybrids Mendel called the purple ower color a dominant trait and the white ower color a recessive trait Mendel observed the same pattern of inheritance in six other pea plant characters each represented by two traits What Mendel called a heritable factor is what we now call a gene Mendel s Model Mendel developed a hypothesis to explain the 31 inheritance pattern he observed in F2 offspring Four related concepts make up this model These concepts can be related to what we not know about genes and chromosomes The first concept is that alternative versions of genes account for variations in inherited characters 0 For example the gene for ower color in pea plants exists in two versions one for purple owers and the other for white owers The alternative versions of a gene are now called alleles The second concepts is that for each character an organism inherits two alleles one from each parent 0 Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosomes may be identical as in the truebreeding plants of Mendel s P generation Alternatively the two alleles at a locus may differ as in the F1 hybrids The third concept is that if the two alleles at a locus differ then one the dominant allele determines the organism s appearance and the other the recessive allele has one noticeable effect on appearance The fourth concept now known as the law of segregation states that the two alleles for a heritable character separate segregate during gamete formation and end up in different gametes Thus an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism Mendel s segregation model accounts for the 31 ratio he observed in the F2 generation of his numerous crosses The possible combinations of sperm and egg can be shown using a Punnett square a diagram for predicting the results of a genetic cross between individuals of known genetic makeup A capital letter represents a dominant allele and a lowercase letter represents a recessive allele Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Unlike homozygotes heterozygotes are not truebreeding Because of the different effects of dominant and recessive alleles an organism s traits do not always reveal its genetic composition Therefore we distinguish between an organism s phenotype or physical appearance and its genotype or genetic makeup In the example of ower color in pea plants PP and Pp plants have same phenotype purple but different genotypes The Testcross How can we tell the genotype of an individual with the dominant phenotype Such an individual must have one dominant allele but the individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype the mystery parent must be heterozygous The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in the cross were monohybrids individuals that are heterozygous for one character A cross between such heterozygous is called a monohybrid cross Mendel identified his second law of inheritance by following two characters at the same t1me Crossing two truebreeding parents differing in two characters produces dihybrids in the F1 generation heterozygous for both characters A dihybrid cross a cross between F1 dihybrids can determine whether two characters are transmitted to offspring as a package or independently Using the dihybrid cross Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Strictly speaking this law applies only to genes on different nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together
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