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CYTOGENETICS COMPILED TOPIC NOTES TOPIC 1: ORIGIN AND IMPORTANCE OF CYTOGENETICS 1.1: Cytogenetics and Its Branches What is Cytogenetics? Cytogenetics is the study of chromosomes and the related disease states cause by abnormal chromosome number and/or structure. Th...

CYTOGENETICS COMPILED TOPIC NOTES TOPIC 1: ORIGIN AND IMPORTANCE OF CYTOGENETICS 1.1: Cytogenetics and Its Branches What is Cytogenetics? Cytogenetics is the study of chromosomes and the related disease states cause by abnormal chromosome number and/or structure. The cell is the basic structural and functional unit of all organisms. The chromosomes that cell contain are the containers of the hereditary factors which we call as the “gene”. Chromosomes in cells that are abnormal in number and size have an abnormal effect to what the organism will eventually become. Cytogenetics if dissected is from two branches of science: Cytology- study of cells Genetics- study of heredity Therefore, CYTOGENETICS tries to understand how cells produce what organism will eventually look like including the genetic disorders it has. In modern times, cytogenetics or genetics itself has reached the molecular level. This resulted to biochemical and molecular genetics. What are the fields of Genetics? 1. Transmission Genetics- A scientist working in the field of transmission genetics examines the relationship between the transmission of genes from parent to offspring and the outcome of the offspring’s traits. a. Example: How can two brown-eyed parents produce a blue-eyed child? b. The following questions are under transmission genetics: i. How are chromosomes transmitted during cell division and gamete formation? ii. What are the common patterns of inheritance for genes? 2. Molecular Genetics- The goal of molecular genetics is to understand how the genetic material works at the molecular level. It is understanding the molecular features of DNA and how these features underlie the expression of genes. a. The following questions are under Molecular genetics: i. What is the composition and conformation of chromosomes? ii. How is the genetic material copied? iii. How is gene expression regulated so it occurs under the appropriate conditions, in the appropriate cell type, and the correct stage of development? iv. What is the molecular nature of mutation? 3. Population Genetics- this field helps us understand how process such as natural selection have resulted in the prevalence of individuals that carry particular alleles. Population geneticists are particularly interested in genetic variation and how that variation is related to an organism’s environment. In this field, the frequencies of alleles are of central importance. a. The following are questions under Population Genetics: i. Why are two or more different alleles of a gene maintained in a population? ii. What factors alter the prevalence of alleles within a population? iii. What are the contributions of genetics and environment in the outcome of a trait? iv. How do genetics and environment influence quantitative traits, such as size and weight? *Natural Selection- the process whereby organism better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution. (Definition from Oxford Dictionary) 1.2: The History of Cytogenetics Historical Development By the middle of the 19th century, the universality of cell division as the central phenomenon in the reproduction of organisms was established, and Virchow expressed it in the famous aphorism “Omnis cellula e cellula”. From this time on, the study of cells and of heredity and evolution converged, as was stated by Wilson: “Heredity appears as a consequence of genetic continuity of the cells by division” CMBS MLS114: CYTOGENETICS- TOPIC NOTES A.Y. 2020-2021 Page 1 of 46 Notable Persons and Discoveries: Van Beneden, Flemming, Strasburger, Boveri and others- observed germ cells which supported the theory of the continuity of the germ plasm proposed by Weisman in 1883. o GERM THEORY states o that the transference of hereditary factors from one generation to the next takes place through the continuity of what he called ‘germ plasm’, located on the sex elements (Sperm and Egg), and not through somatic cells. Discovery of Fertilization in Animals- foreseen by O. Hertwig but observed directly by H. Fol (1879) Discovery of Fertilization in Plants- observed by Strasburger.; both discovery led to the theory that the cell nucleus is the bearer of the physical basis of heredity Roux- postulated that chromatin, the substance of the nucleus that constitutes the chromosome, must have a linear organization Weismann- stated that hereditary units are disposed along the chromosomes in an orderly manner Gregor Mendel- discovered the Fundamental Laws of Heredity in 1865, but at that time cytologic changes produced in the sex cells were not sufficiently known to permit an interpretation of the independent segregation of hereditary characters. For this and other reasons, little attention was paid to Mendel’s work until the botanists Correns, Tschermack and De Vries in 1901 independently rediscovered Mendel’s Law 1865/ 1866 Gregor Mendel published his investigations into inheritance of pea plants 1890 Theodor Boveri suggested that chromosomes are involved with inheritance 1900 Walter Sutton observed chromosomes in grasshopper cells 1900/1901 Mendel's work was rediscovered by three scientists: Hugo De Vries, Erich von Tschermack, and Carl Correns 1902 Archibald Garrod discovered that some diseases must be inherited 1903 Sutton and Boveri, working independently, suggested that each egg of sperm cell contains only one of each chromosome pair 1905 Edmund Beecher Wilson and Nettie Stevens, working independently, proposed that certain chromosomes determine sex. They show that a single Y chromosome determines maleness, and two copies of the X chromosome determine femaleness 1906 Bateson gave the term ‘genetics’ 1909 Wilhelm Johannsen used the term 'gene' to describe the carrier of heredity, 'genotype' to describe an organism's genetic make-up, and 'phenotype' to describe an organism's outward appearance 1910 Thomas Hunt Morgan proved that genes are carried on chromosomes. He also showed that some characteristics are carried on the sex chromosome 1911/1913 Alfred Henry Sturtevant mapped the genes o the fruit fly’s sex chromosome 1912 Sir William Henry Bragg and his son discover that X-rays can be used to study the molecular structure of simple crystals, such as salt 1926 Morgan published the ‘Theory of the Gene’ 1928 Frederick Griffith discovered 'transformation' in bacteria 1944 Oswald Avery, Colin MacLeod, and Maclyn Mccatty used bacteria to show that DNA is the hereditary material 1949 Erwin Chargaff finds that the amounts of adenine and thymine in DNA are about the same, as area the amounts of guanine and cytosine 1953 James Watson and Francis Crick proposed that the DNA molecule is a double- stranded helix 1963-1966 Marshall Nirenberg and Heinrich Matthaei work out the genetic code 1977 DNA from virus is sequenced for the first time by Frederick Sanger, Walter Gilbert and Allan Maxam, working independently 1983 Kary Mullis discovered the Polymerase Chain Reaction (PCR), enabling lengths of DNA to be multiplied 1987 Rebecca Cann, Mark Stoneking, and Allan Wilson analyze mitochondrial DNA in different human races. They declared that humans have a common ancestor who lived 200,000 years ago CMBS MLS114: CYTOGENETICS- TOPIC NOTES A.Y. 2020-2021 Page 2 of 46  1989 The first Human gene is sequences by Francis Collins and Lap-chee Tsui. It is the gene that cause cystic fibrosis 1990 The Human Genome Project is launched 1993 Cystic fibrosis became the first genetic disease to be treated using gene therapy 1995 The genome of H.influenzae is sequenced. This is the first complete genome of an organism 2000 First draft sequences of human genome are released at the same time by the Human Genome Project and Celera genomics 2003 The Human Genome Project is successfully completed on 14th of April Reference: Camara, J.S and Oclay, A. (2012). Cytogenetics: Principles and Application. Dagupan City: Space Browser Publishing p.4-5 TOPIC 2: CYTOLOGICAL BASIS OF HEREDITY 2.1: Origin and Importance of Cytogenetics The body of all living organisms (bacteria, blue green algae, plants and animals) except viruses has cellular organizations and may contain one or many cells. The organisms with only one cell in their body are called unicellular organisms (e.g bacteria, blue green algae, some algae, Protozoa). The organisms having many cells in their body are called multicellular organism (e.g most plants and animals). Any cellular organisms may contain only one type of cell from the following types of cells: A. Prokaryotic cells; B. Eukaryotic Cells *The term prokaryotic and Eukaryotic were suggested by Hans Ris in the 1960’s. PROKARYOTIC CELLS: From Gr. pro=primitive or before; karyon=nucleus; they are small, simple and most primitive. They are probably the first to come into existence about 3.5 billion years ago; essentially a one- envelop system organized in depth. It consists of central nuclear components (viz., DNA molecule, RNA Molecule and nuclear proteins) surround by cytoplasmic ground substance; the cytoplasm of a prokaryotic cell lacks in well defined cytoplasmic organelles; nuclear envelope; nucleoli, cytoskeleton, centrioles and basal bodies. Ex. Bacteria EUKARYOTIC CELLS: From Gr., eu=well; karyon= nucleus; they have evolved from the prokaryotic cells and the first eukaryotic (nucleated) cells may have arisen 1.4 billion years ago (Vidal, 1983); essentially two envelope systems and they are much larger than prokaryotic cells. Secondary membranes envelop the nucleus and other internal organelles; The eukaryotic cells are true cells which occur in the plants and animals. CMBS MLS114: CYTOGENETICS- TOPIC NOTES A.Y. 2020-2021 Page 3 of 46