genetics week 2
genetics week 2 Biol 3451
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This 16 page Class Notes was uploaded by UNT_Scientist on Saturday September 10, 2016. The Class Notes belongs to Biol 3451 at University of North Texas taught by Robert Curliss Benjamin in Fall 2016. Since its upload, it has received 24 views. For similar materials see Genetics in Biology at University of North Texas.
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Date Created: 09/10/16
Genectics Week 2 UNT 2.2 Chromosomes Exist in Homologous Pairs in Diploid Organisms o Chromosomes exist in homologous pairs in diploid organisms o Somatic cells (body cells) of a given species have a specific number of chromosomes Present as homologous pairs E.g.: Humans: 46 chromosomes (23 homologous pairs) o Homologous chromosomes are similar Carry genes for the same inherited characteristics (Fig. 2.4) They are not identical May carry different versions of the same gene o Each diploid organism contains two copies of each gene o The members of each pair of genes need not be identical o Alternative forms of the same gene are called alleles o Meiosis converts the diploid number (2n) of chromosomes to the haploid number (n) Reduce the genetic load in half Create genetic variation within the gametes o Gametes contain a haploid set of chromosomes o Fusion of two gametes at fertilization results in a diploid zygote back to 2n o Sexdetermining chromosomes are usually not homologous (Figure 2.4) yet behave as homologs in meiosis For example, in humans there is an X and a Y chromosome Called X and y because the size of the cells is different If they are the same size sex chromosomes they are called z and w o 2.3 Mitosis Partitions Chromosomes into Dividing Cells Mitosis cuts chromosomes into cells Genetic material is partitioned to daughter cells during nuclear division (karyokinesis) Cytoplasmic division (cytokinesis) follows Interphase and cell cycle The cell cycle is composed of interphase and mitosis Interphase includes: Sphase, during which DNA is synthesized Two gap phases (G1 and G2) (Fig. 2.5) G0 is a point in the G1 phase where cells withdraw from the cell cycle and enter a nondividing but metabolically active state Mitosis has discrete stages: prophase the centrioles divide and move apart, the nuclear envelope breaks down, and chromosomes condense and become visible Sister chromatids are connected at the centromere prometaphase the chromosomes move to the equatorial plane of the cell metaphase the centromeres/ chromosomes are aligned at the equatorial plane Spindle fibers bound to kinetochores associated with centromeres are responsible for chromosome movement anaphase Sister chromatids separate from each other and migrate to opposite pole The separated sister chromatids are called daughter chromosomes telophase cytokinesis uncoiling of the chromosomes reformation of the nuclear envelope (see Figure 2.7) o 2.4 Meiosis Reduces the Chromosome Number from Diploid to Haploid in Germ Cells and Spores Meiosis reduces the amount of genetic material by onehalf to produce haploid gametes or spores containing one member of each homologous pair of chromosomes that are virtually identical Overview Meiosis I is a reductional division Goes from 4n to 2n Meiosis II is an equational division (Figure 2.9) Goes from 2n to n DNA synthesis occurs during interphase before the beginning of meiosis I but does not occur again before meiosis II Meiosis I and II each have prophase, metaphase, anaphase, and telophase stages Doesn't bother with prometaphase Use the Roman numerals of prophase to know what you are in Prophase 1 Meiosis 1 Prophase 2 Meiosis 2 Prophase Mitosis The first meiotic division: Prophase I (don't need to know all the names just what is going on) Prophase I has five sub stages, each including specific events (see Figure 2.10): leptonema Chromosomes appear as long, single threads, unassociated with one another zygonema Homologous chromosomes pair with one another, gene by gene, over the entire length of the chromosomes. The pairing of the homologous chromosomes is called synapsis. Each pair of homologous chromosomes is known as bivalent. pachynema Each paired chromosome (bivalent) becomes shorter and thicker and splits into two sister chromatids except at the region of the centromere. They are also called tetrads. During the synapsis and tetrad formation, an exchange of genetic material between nonsister chromatids occurs. It is called crossing over. diplonema diakinesis At the completion of prophase I, the centromeres of each tetrad structure are present on the equatorial plate During this process, each of the two nonsister chromatids is cut using an enonuclease at identical points An interchange of broken chromatid segments takes place in between the two nonsister chromatids of the same tetrad Each broken chromatid segment unites with the nonsister chromatid of its own tetrad by the help of an enzyme called ligase Metaphase I Shortest phase Tetrads align on the metaphase plate INDEPENDENT ASSORTMENT OCCURS: Orientation of homologous pair to poles is random Variation Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations Meiosis I During meiosis I, the centromeres holding each pair of sister chromatids together do not divide One pair of each tetrad is pulled toward each pole Meiosis significantly increases the level of genetic variation due to crossing over during meiosis I and independent assortment This is not mentioned in text but very important for generating variation in a sexually reproducing population Metaphase I, Anaphase I, and Telophase I Homologous chromosomes separate and move toward the poles Sister chromatids remain attached at their centromeres Duplicated chromosomes reach the poles. Each pole now has haploid set of chromosomes Meiosis II During meiosis II, the sister chromatids in each dyad are separated to opposite poles Each haploid daughter cell from meiosis II has one member of each pair of homologous chromosomes o 2.5 The Development of Gametes Varies in Spermatogenesis Compared to Oogenesis (not important to know) The development of gametes varies in spermatogenesis compared to oogeneis Male gametes are produced by spermatogenesis in the testes Female gametes are produced by oogenesis in the ovary (Figure 2.12) Spermatogenesis The primary spermatocyte undergoes meiosis I to produce two secondary spermatocytes, which undergo meiosis II to produce a total of four haploid spermatids Oogenesis During oogenesis, the four daughter cells do not receive equal cytoplasm The cell that receives the most cytoplasm undergoes both meiosis I and II and develops into the ovum The cytoplasmdeficient polar bodies produced at meiosis I and II do not undergo further division o 2.6 Meiosis Is Critical to the Successful Sexual Reproduction of All Diploid Organisms Meiosis is critical to the successful sexual reproduction of all diploid organisms The mechanism of meiosis is the basis for the production of extensive genetic variation Gametes receive either the maternal or the paternal chromosome from each homologous pair of chromosomes An organism can produce 2n (where n represents the haploid number) combinations of chromosomes in gametes Crossing over adds further genetic variation because chromosomes become a mixture of maternally and paternally derived DNA In many fungi, the predominant stage of the life cycle is haploid The life cycle in multicellular plants alternates between a diploid sporophyte stage and a haploid gametophyte stage (Figure 2.13) Meiosis and fertilization are the bridge between these two stages o 2.7 Electron Microscopy Has Revealed the Physical Nature of Mitotic and Meiotic Chromosomes Electron microscopy has revealed the physical nature of mitotic and meiotic chromosomes Chromosomes are visible only during mitosis and meiosis because the chromatin fibers that make up chromosomes coil and condense in these stages Electron microscopic observations of mitotic chromosomes in varying states of coiling led to postulation of the foldedfiber model (Figure 2.14c) Chapter 3 Mendelian genetics o 3.1 Mendel Used a Model Experimental Approach to Study Patterns of Inheritance Mendel used a model experimental approach to study patterns of inheritance Mendel chose the garden pea as his model system because: it is easy to grow It has truebreeding strains it has controlled matings: selffertilization or crossfertilization Flowers can fertilize by themselves and you can fertilize the flowers if you wanted it grows to maturity in one season In one growing season you could get multiple growths it has observable characteristics with two distinct forms Using seven visible features, each with two contrasting traits (Figure 3.1), and truebreeding strains, Mendel kept accurate, quantitative records of his experiments The results were unappreciated during his lifetime He was rediscovered in the turn of the century Mendel's postulates were eventually accepted as the basis for Mendelian, or transmission, genetics Don't forget that during this whole time Mendel is a monk so when he isn't working on his experiments he is praying o 3.2 The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation The monohybrid cross reveals how one trait is transmitted from generation to generation Monohybrid crosses involve a single pair of contrasting traits The original parents are the P1 generation, and their offspring are the F1 generation Offspring arising from selfing (selffertilizing) the F1 generation are the F2 generation In the F1 generation of a monohybrid cross, all of the plants have just one of the two contrasting traits Purple with white flowers You will only have one “win” or be the dominate trait In the F2 generation, 3/4 of the plants exhibit the same trait as the F1 generation, and 1/4 exhibit the contrasting trait that disappeared in the F1 generation 3/4 dominate Purple 1/4 recessive White To explain these results, Mendel proposed the existence of "particulate unit factors" for each trait He suggested that these factors (now called genes) are passed unchanged from generation to generation, determining various traits expressed by each individual plant Mendel's monohybrid crosses were not sex dependent For example, it did not matter whether a tall male plant pollinated a dwarf female plant, or vice versa. The results were the same either way This is called a reciprocal cross Mendel proposed three postulates of inheritance: Unit factors exist in pairs Genetic characters are controlled by unit factors existing in pairs in individual organisms The flowers have a purple fat or and a white factor and the purple beats white because the purple was passed on In the pair of unit factors for a single characteristic in an individual, one unit factor is dominant and the other is recessive In the pair of unit factors for a single characteristic in an individual, one unit factor is dominant and the other is recessive The paired unit factors segregate (separate) independently during gamete formation The paired unit factors segregate (separate) independently during gamete formation There is a 50/50 chance that each pollen that the flower makes will be purple or white because of gamete formation The parental plants are the P generation PP purple or pp white Their hybrid offspring are the F1 generation Pp and Pp A cross of the F1 plants forms the F2 generation PP Pp Pp pp Cause 3/4 of the flowers to be Purple 1/4 to be white For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different A homozygous genotype has identical alleles A heterozygous genotype has two different alleles Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene The genotype is the genetic makeup of an individual The phenotype is the physical expression of the genetic makeup A Punnett square allows the genotypes and phenotypes resulting from a cross to be visualized easily (Figure 3.3) Testcross Looks at one trait testcross is a way to determine whether an individual displaying the dominant phenotype is homozygous or heterozygous for that trait o 3.3 Mendel's Dihybrid Cross Generated a Unique F2 Ratio Mendel's dihybrid cross generated a unique F2 ratio A dihybrid cross involves two pairs of contrasting traits (Figure 3.5) The product law can be used to predict the frequency with which two independent events will occur simultaneously Mendel's fourth postulate: Independent assortment Mendel's fourth postulate states that traits assort independently during gamete formation all possible combinations of gametes will form with equal frequency A Punnett square of a dihybrid cross is shown in Figure 3.7 Note the 9:3:3:1 dihybrid ratio o 3.4 The Trihybrid Cross Demonstrates That Mendel's Principles Apply to Inheritance of Multiple Traits The trihybrid cross demonstrates that mendel's principles apply to inheritance of multiple traits Trihybrid crosses involving three independent traits show that Mendel's rules apply to any number of traits The forkedline (branched diagram) method is easier to use than a Punnett square for analysis of inheritance of larger number of traits Some simple mathematical rules shown in Table 3.1 apply in working genetics problems o 3.5 Mendel's Work Was Rediscovered in the Early Twentieth Century Mendel's work was rediscovered in the early twentieth century Mendel suggested that heredity resulted in discontinuous variation, as opposed to the existing continuous variation hypothesis of his time—in which offspring were thought to be a blend of the parental phenotypes The chromosomal theory of inheritance The chromosomal theory of inheritance proposed that the separation of chromosomes during meiosis could be the basis for Mendel's principles of segregation and independent assortment o 3.6 The Correlation of Mendel's Postulates with the Behavior of Chromosomes Provided the Foundation of Modern Transmission Genetics Independent assortment leads to extensive genetic variation The chromosomal theory of inheritance proposed that the separation of chromosomes during meiosis could be the basis for Mendel's principles of segregation and independent assortment o 3.7 Independent Assortment Leads to Extensive Genetic Variation (not much of the focus) Laws of probability help to explain genetic events A major consequence of independent assortment is the production of genetically dissimilar gametes Genetic variation results from independent assortment and is very important to the process of evolution The probability of two independent events occurring at the same time can be calculated using the product law The probability of both events occurring is the product of the probability of each individual event The sum law is used to calculate the probability of a generalized outcome that can be accomplished in more than one way The sum law states that the probability of obtaining any single outcome, where that outcome can be achieved in two or more events, is equal to the sum of the individual probabilities of all such events When one event depends on another, the likelihood of the desired outcome is the conditional probability The binomial theorem can be used to calculate the probability of any specific set of outcomes among a large number of potential events o 3.8 Laws of Probability Help to Explain Genetic Events ChiSquare analysis evaluates the influence of chance on genetic data Chance deviation from an expected outcome is diminished by larger sample size ChiSquare Calculations and Null Hypothesis When we assume that data will fit a given ratio, we establish what is called the null hypothesis—so named because it assumes that there is no real difference between the measured values (or ratio) and the predicted values (or ratio) The apparent difference can be attributed purely to chance Chisquare (2) analysis is used to test how well the data fit the null hypothesis Table 3.3 shows the steps in 2 calculations for the F2 generation of a monohybrid cross Chisquare analysis requires that the degree of freedom (df) be taken into account, since more deviation is expected with a higher degree of freedom (Figure 3.11) The degree of freedom is equal to n – 1, where n is the number of different categories into which each datum point may fall o 3.9 ChiSquare Analysis Evaluates the Influence of Chance on Genetic Data o 3.10 Pedigrees Reveal Patterns of Inheritance of Human Traits
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