Genetic Linkage PDF

GENETIC LINKAGE Genes on the same chromosome are called linked genes and are said to belong to a linkage group. The number of linkage groups in an organism equals the haploid number of chromosomes. (Humans would have 23 linkage groups). Partial linkage in the sweet pea A group of researchers disc...

GENETIC LINKAGE Genes on the same chromosome are called linked genes and are said to belong to a linkage group. The number of linkage groups in an organism equals the haploid number of chromosomes. (Humans would have 23 linkage groups). Partial linkage in the sweet pea A group of researchers discovered the first exception to the law of independent assortment of gene pairs. PPLL X ppll purple flower and long pollen X red flowers and short pollen. All F1 plants had purple flowers and long pollen indicating that purple flower color was dominant to red flower and long pollen was dominant to short pollen. Partial linkage occurs when homologous chromosomes exchange corresponding parts during meiosis through the process called crossing-over. Morgan’s linkage experiments with Drosophila To explain the recombinants, Morgan proposed that in 36.9% of the meiosis an exchange of genes had occurred between the two X chromosomes of the F1 females. Since males are hemizygous no genetic exchange occurs between the non-homologous X and Y chromosomes. In all cases the parental phenotypic classes were the most frequent while the recombinant classes occurred much less frequently. Morgan’s general conclusion was that during segregation of alleles at meiosis, certain ones tend to remain together because they lie near each other on the same chromosome.  Gene recombination In the 1930’s Creighton and McClintock did experiments with corn. The process of a chromosome segment breaking off from one chromosome and reattaching to another is called translocation. Cytologically distinguishable features such as these are called cytological markers; the genes involved are called genetic markers or gene markers. LOCATING GENES ON CHROMOSOMES: MAPPING TECHNIQUES Crossing-over takes place on prophase I of meiosis and involves only two of the four chromatids for a given crossing-over event. Genetic experiments can be used to determine the relative position of genes on chromosomes in eukaryotic organisms. This process is called genetic mapping. Genetic mapping is the creation of a Genetic map (linkage map). Two different ways of mapping are distinguished. Genetic mapping uses: 1. Classical genetic techniques (e.g. pedigree analysis or breeding experiments) to determine sequence features within a genome. Testcross (unknown X homozygous recessive) is better because the distribution of phenotypes is the result of segregation events in only one of the parents. The other parent contributes only recessive alleles to the progeny. 2. Modern molecular biology techniques for the same purpose is usually referred to as physical mapping. Loci or locus is the position of a gene on a genetic map.  Morgan thought that the characteristic crossover frequencies for linked genes might be related to the physical distance separating the genes on the chromosomes.  Morgan’s student suggested that the percentage of recombinants (produced by crossovers in gamete production) could be used as a quantitative measure of the genetic distance between two gene pairs on a genetic map.  This distance is measured in map units (mu). A crossover frequency of 1% between two genes equals 1 map unit (centi-Morgan cM). BACTERIAL GENETICS MAPPING GENES IN BACTERIA  Escherichia coli is an extensively used bacterium in genetics and molecular analysis. (Because it can be grown on a simple, defined medium and can be handle with simple microbiological techniques). E. coli is found most commonly in the large intestines of most animals including humans.  Bacterial cells are relatively small compared with eukaryotic cells. Its cytoplasm is full of ribosomes while the central nucleoid region contains the genetic material, a single circular chromosome consisting of DNA. There is no nuclear membrane.  Genetic material can be transferred between bacteria by three main processes: transformation, conjugation, and transduction. In each case: 1. Transfer is unidirectional; 2. There is no true diploid zygote; 3. Only genes included in the circular chromosome will be stably inherited. For these reasons genetic transfer processes are different from those in eukaryotes.  BACTERIAL TRANSFORMATION DNA from a donor strain is extracted and purified. The extraction process breaks the DNA into small linear double-stranded fragments, and this genetic material is then added to a suspension of recipient bacteria with a different genotype. The recipients whose phenotypes are changed by a recombination event are called transformants. Any bacterial strain can be the donor strain and any strain can be the recipient. Only if there are genetic differences between donors and recipients will transformants be obtained. Some bacteria such as Bacillus subtilis can be easily transformed in a test tube. Wild type E. coli, however, is not readily transformable because the bacterium produces enzymes that rapidly degrade incoming DNA. Cells are treated chemically to make the membrane more permeable to DNA. Such cells are called competent cells. In terms of mechanism there are two types of bacterial transformation: 1. Natural transformation in which bacteria are naturally able to take up DNA and be genetically transformed by it. (Bacillus subtilis) 2. Engineered transformation in which bacteria have been genetically altered to enable them to take up and be genetically transformed by added DNA. (E. coli) Using transformation it is possible to determine gene linkage, gene order and map distance. If co-transformation occurs at a frequency that is substantially higher than the product of the two single-gene transformation, the two genes must be close together. Gene order can also be determined from co-transformation data. XYZ a.Write the gene order on a chromosome if cotransformation occurs between genes U & R; E & N; L & A; R & E; A & U, but never between L & R; N & A; R & A; N & U. b.Write the gene order on a chromosome if cotransformation occurs between genes I & S; C & H; R & I; T & S; A & T; H & R, but never between C & I; H & S; R & T; H & A.  CONJUGATION IN BACTERIA Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. Conjugation is a mechanism of horizontal gene transfer as are transformation and transduction although these two other mechanisms do not involve cell-to-cell contact.  TRANSDUCTION IN BACTERIA Transduction is the process by which genetic material is transferred between bacterial strains by bacterial viruses called bacteriophages (phages), which are virus that infect bacteria. The structure of all phages is simple. A phage contains its genetic material (either DNA or RNA) in a single chromosome surrounded by a coat of protein molecules.  THE STRUCTURE OF GENETIC MATERIAL DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are macromolecules composed of smaller building blocks called nucleotides. Other macromolecules (large molecules – polymers) include proteins, carbohydrates, and fats. The nature of genetic material: DNA & RNA Some molecule had to carry the genetic information from generation to generation. Geneticists postulated that the material responsible for inheritable traits would have to have three principal characteristics: 1. It must contain, in a stable form, the information for an organism’s cell structure, function, development and reproduction. 2. It must replicate accurately so that progeny cells have the same genetic information as the parental cell. 3. It must be capable of variation. Without variation (such as through mutation and recombination), organisms would be incapable of change and adaptation, and evolution could not occur. In the 1940’s two scientists obtained evidence from experiments that genes function by controlling the synthesis of specific enzymes (this was called the one gene – one enzyme hypothesis) It was in the middle of the century that experiments showed that the genetic material of living organisms consisted of one of two types of nucleic acid. The chemical composition of DNA & RNA Both DNA and RNA are polymeric molecules made up of monomeric units called nucleotides. Each nucleotide consists of three distinct parts:  A pentose (5-carbon sugar) sugar.  A nitrogenous (nitrogen-containing) base.  A phosphate group. NUCLEOTIDE Because they can be isolated from nuclei, and because they are acidic, these macromolecules are called nucleic acids. For RNA the pentose sugar is ribose, and for DNA the sugar is deoxyribose. The nitrogenous bases fall into two classes, the purines and the pyrimidines. In DNA the purines are adenine (A) and guanine (G), and the pyrimidines are thymine (T) and cytosine (C). In RNA the thymine is replaced by uracil (U). In DNA and RNA, bases are always attached to the 1’ carbon of the pentose sugar by a covalent bond. The phosphate group (PO42-) is attached to the 5’ carbon of the sugar in both DNA and RNA. The sugar plus base only is called a nucleoside, so a nucleotide is also called a nucleoside phosphate. Because of the differences (sugar and base) the two nucleic acids have different chemical and biological properties. For example the presence of 2’ OH enables the RNA to be degraded with alkali, while DNA is resistant to that treatment. NUCLEOTIDE To form polynucleotides of either DNA or RNA, nucleotides are linked together by a covalent bond between the phosphate group (which is attached to the 5’ carbon of the sugar ring) of one nucleotide and the 3’ carbon of the pentose sugar of another nucleotide. These 5’-3’ phosphate linkages are called phosphodiester bonds. These bonds are relatively strong hence the repeated sugar-phosphate-sugar-phosphate backbone of DNA and RNA is a stable structure. It was in 1953 that James Watson and Francis Crick proposed a model for the physical and chemical structure of the DNA molecule. This model consists of two polynucleotide chains wound around each other in a right-handed (clockwise) helix. Other DNA structures DNA can exist in different forms, depending on the conditions. When humidity is relatively high, DNA is in what is called the B form (the form proposed by Watson and Crick). When the humidity is relatively low, DNA is found in the A form. Both of these DNA forms involve right-handed helices. In A-DNA the base pairs are tilted and pulled away from the axis of the double helix. In B-DNA the helix axis passes through the base pairs which are, themselves, oriented perpendicular to that axis. Another form, which is the most intriguing, is the Z-DNA because of its totally unexpected structure and because of the evidence for its rare existence in cells. Z-DNA has a left- handed helix and a zig-zag sugar-phosphate backbone. Whether Z-DNA exist within cells is currently debatable. B-DNA is the form typically found in cells.  THE STRUCTURAL CHARACTERISTICS OF EUKARYOTIC CHROMOSOMES It differs substantially from prokaryotic chromosomes The eukaryotic chromosome complement A fundamental difference between prokaryotic and eukaryotic is that prokaryotic have one chromosome, while most eukaryotes have a diploid number of chromosomes in almost all somatic cells. The karyotype A complete set of all the metaphase chromosomes in a cell is called its karyotype. For most organisms, all cells have the same karyotype. However, the karyotype is species-specific, so a wide range of number, size and shape of metaphase chromosomes is seen among eukaryotic organisms. A knowledge of the size, overall morphology, and banding patterns of chromosomes permits geneticists to identify certain chromosome aberrations that correlate with congenital abnormalities or dysfunction. This can be caused from error in cell division such as chromosome nondisjunction. In a human karyotype, the chromosomes are numbered for easy identification. The largest pair of homologous chromosome is designated as 1, the next largest 2 etc. 1 through 22 are called autosomes. The sex chromosomes constitute pair 23. Even though in humans they don’t fit properly in the size scale. The X chromosome is a large metacentric chromosome, and the Y chromosome is a much smaller acrocentric chromosome.