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CYTO311: CYTOLOGY WEEK 2: CYTOGENETICS:DEFINITION OF TERMS 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO GENETICS AND GENES...

CYTO311: CYTOLOGY WEEK 2: CYTOGENETICS:DEFINITION OF TERMS 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO GENETICS AND GENES THE RNAs Genetics - the study of inherited traits and their variation Messenger RNA (mRNA) – carries the instructions for protein synthesis to the sites of Heredity - the sum of all biological processes protein synthesis by which particular characteristics are transmitted from parents to their offspring. Ribosomal RNA (rRNA) – combines with Genes - the units of heredity, which is the transmission of inherited traits. Genes can be found on the nucleic acid DNA The Nucleic Acids RNA – important molecule in protein synthesis proteins to form ribosomes Both DNA and RNA are polymers of repeating Transfer RNA (tRNA) – RNA that delivers amino called nucleotides. acids to the sites of protein synthesis Genes, Chromosomes and Genomes Genome – it is the complete set of genetic instructions characteristic of an organism, including protein-encoding genes and other DNA sequences Chromosomes – structures that are a product of DNA coiling in association with proteins Genes, the segment of DNA strand that encodes for the production of different proteins, may have different variants. These There are four different nucleotides present in a are called alleles. DNA molecule. The various sequence combinations of these bases ultimately encode genetic information. Mutations, Phenotype, and Genotype Alleles are products of mutations. Mutations - defined as any heritable change in the DNA sequence and are the source of all genetic variation Phenotype – observable traits or features of an organism (alleles that are expressed) Genotype – set of alleles for a given trait carried by an organism (alleles that are present)  Mitosis and Meiosis Mitosis – a type of cell division in which one somatic cell give rise to two new ones Meiosis – cell division involved in the production of gametes Terms in genetics can be confusing sometimes… Chromosome…chromatin…chromatid… Genes…genomes…genetics… Genotype…phenotype  CYTO311: CYTOLOGY WEEK 2: INTRODUCTION TO CYTOGENETICS 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO Charles Darwin - proposed that existing species arose by INTRODUCTION TO CYTOGENETICS descent with modification from Branches of Genetics ancestral species. - formulated the theory of natural Classical genetics selection - refers to the study of the laws of hereditary - natural selection states that transmission in living organisms. individuals with heritable traits that - it began with Mendel’s study of inheritance in allow them to adapt to their garden peas. environment are better able to survive and reproduce than those with less adaptive traits. Population genetics - The study of genes in populations of animals, Gregor Mendel plants, and microbes provides information on past - published a paper describing how migrations, evolutionary relationships and extents traits are passed from one of mixing among different varieties and species, generation to the other, utilizing and methods of adaptation to the environment. pea plants as models - proposed that traits are passed Cytogenetics from parents to offspring in a – branch of genetics that studies the structure of predictable manner the DNA within the cell nucleus. It studies the - he further concluded that each trait in pea number and morphology of the chromosomes. plants is controlled by a pair of factors (which we now call genes) and that members of a Molecular genetics gene pair separate from each other during - the study of the molecular structure of DNA, its gamete formation (the formation of egg cells cellular activities (including its replication), and its and sperm) influence in determining the overall makeup of an organism. Chromosome Theory of Inheritance Early History of Genetics and Theories of Inheritance Heredity Hippocratic School of Medicine (500–400 B.C.) – - is dependent on the genes contained in the proposed that “humors” served as bearers of traits structures called chromosomes. The chromosomes were contributed to the Aristotle (384–322 B.C.)- generative power of male individual by the gametes. semen resided in a “vital heat” contained within it Diploid number (2n) – the characteristic number William Harvey – proposed the theory of of chromosomes a eukaryote has in most of its epigenesis cells Preformationism – states that the fertilized egg Chromosomes in diploid cells exist in pairs called contains a complete miniature adult called a homologous chromosomes homunculus Cell theory vs Spontaneous Generation Charles Darwin  Chromosomes behave differently during the two It is a single-stranded molecule that contains uracil forms of cell division, mitosis and meiosis. (U) in place of thymine. In mitosis, the chromosomes are copied and Gene Expression: From DNA to Phenotype distributed to each daughter cell. Both cells obtain The genetic information in the DNA is expressed a diploid set of chromosomes. to form a functional gene product, which in most cases, a protein. In meiosis, the cells receive only one chromosome from each chromosome pair, and the resulting In eukaryotic cells, the process begin in the nucleus number of chromosome is called the haploid with transcription. number (n). The mRNA produced then moves to the cytoplasm Finally, the chromosomal theory of inheritance and migrates to the ribosomes. states that “inherited traits are controlled by genes residing on chromosomes faithfully transmitted The synthesis of protein under the direction of the through gametes, maintaining genetic continuity mRNA is called translation. from generation to generation.” Information encoded in mRNA (the genetic code) Chemical Nature of Genes consists of linear series of nucleotide triplets Scientists tried to identify which component of the (codon). chromosomes carries genetic information Each codon is complementary to the information The major chemical component of chromosomes stored in DNA and specifies the insertion of a were DNA and proteins. specific amino acid into a protein. Structure of DNA and RNA Protein assemble is accomplished with the aid of tRNAs. DNA is a long, ladder-like macromolecule that twists to form a double helix. Proteins and Biological Function Proteins perform diverse biological functions Each strand of the molecule is made up of nucleotides. Enzymes, the largest category of proteins, serve as biological catalysts The four types of nucleotides found in DNA are: A (adenine), G (guanine), C (cytosine), T (thymine) Other types are critical components of cells and organisms The DNA ladder are exact complements of each other, so that the double helix consist of A=T and Some carry essential molecules (hemoglobin), G=C base pairs. regulate body processes (protein hormones e.g., insulin), take part in muscle (actin and myosin) and connective tissue (collagen) A protein’s shape and chemical behavior are determined by its linear sequence of amino acids, which in turn is dictated by the stored information in the DNA of a gene that is transferred to RNA, which then directs the protein’s synthesis. Once a protein is made, its biochemical or structural properties play a role in producing a phenotype. The RNA is also made up of nucleotides but When mutation alters a gene, it may modify or contains a different sugar than DNA. even eliminate the encoded protein’s usual function and cause an altered phenotype.  Recombinant DNA Technology Genomics, Proteomics and Bioinformatics Researchers discovered restriction enzymes that There have been efforts to decode each gene in the could be used to cut any organism’s DNA at genome and establish its function specific nucleotide sequences, therefore producing a reproducible set of DNA fragments. Genomics – study of genome. It studies the structure, function, and evolution of genes and Soon researchers discovered ways on how to insert genomes the DNA fragments into carrier DNA molecules (vectors) to form recombinant DNA molecules. Proteomics – identifies the set of proteins present in a cell under a given set of conditions, and studies The recombinant DNA will be transferred into their functions and interactions bacterial cells to produce thousand of copies, or clones. Bioinformatics – subfield of information technology used to store, retrieve and analyze the The cloned DNA fragments can be isolated from massive amount of data generated by genomics the bacterial host cells. and proteomics Such fragments can be used to isolate genes, study Model Organisms in Genetic Studies their organization and expression, and to study their nucleotide sequence and evolution Principles of inheritance described by Mendel were universal among plants and animals Geneticists gradually came to focus attention on small number of organisms, including the fruit fly (Drosophila melanogaster) and the mouse (Mus musculus) Reasons for using small number of organism: (1) genetic mechanisms were the same in most organisms (2) these organisms had characteristics that made them especially suitable for genetic research. They were easy to grow, had relatively short life cycles, produced many offspring, and their genetic analysis was fairly straightforward. Biotechnology The use of recombinant DNA technology and other They were called model organisms molecular techniques to make products is called biotechnology. Genetics, Ethics and Society Genetics and its applications in biotechnology are The use of recombinant DNA technology to developing much faster than the social genetically modify crop plants has revolutionized conventions, public policies, and laws required to agriculture. regulate their use. Biotechnology has also changed the way human There are many genetics related issues, including proteins for medical use are produced. concerns about prenatal testing, genetic discrimination, ownership of genes, access to and Biotechnology-derived genetic testing is now safety of gene therapy, and genetic privacy. available to perform prenatal diagnosis of heritable disorders and to test parents for their status as heterozygous carriers of more than 100 inherited disorders.  CYTO311: CYTOLOGY WEEK 3: CELLS 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO CELLS CHEMICAL CONSTITUENTS OF CELLS Cells Cells - are composed of macromolecules ➔ are highly varied and highly organized important in biological processes. structures Major Groups of These Substances: ➔ their forms and functions are dependent on carbohydrates, lipids, proteins and nucleic the genetic expression by each cell type acids. o Bone, blood, nerve and muscle cells o Carbohydrates - provide energy are somatic cells, also called as body o Lipids - form membranes and hormones, cells provide insulation and store energy o Proteins - have many diverse functions in o Somatic cells have two copies of the the body, and are important in blood genome and are said to be diploid clotting, nerve transmission, muscle contraction, and immunity, while others o Sperm and egg cells have only one serve and catalysts copy of the genome and are said to be o Most important in genetics are the nucleic haploid acids DNA and RNA CELL STRUCTURES AND ORGANELLES PROKARYOTES VS EUKARYOTES All cells are surrounded by a plasma Prokaryotes lack nucleus as opposed to the membrane, a covering that defines cell nucleated cells of the eukaryotes boundary Specialized organelles are also present in The plasma membrane actively controls the eukaryotic cells movement of materials in and out of the cell Most animal cells have glycocalyx or cell coat The glycocalyx provides biochemical identity at the surface of cells, and the components of the coat that establish cellular identity are under genetic control The nucleus is a membrane bound structure that houses the DNA, which is complex with protein into thin fibers During the nondivisional phases of the cell cycle, the fibers are uncoiled and dispersed into chromatin Page | 1  During mitosis and meiosis, chromatin fibers 3. intermediate filaments. coil and condense into chromosomes The nucleolus, present inside the nucleus, is They are distinguished by: where ribosomal RNA is synthesized o protein type o diameter o how they aggregate into larger structures MICROTUBULES Microtubules - are long and hollow - it provide many cellular movements - composed of a pair of protein called tubulin - they form the cilia, which are hair-like The remainder of the cell within the plasma structures membrane, excluding the nucleus, is called as the Cytoplasm and includes a variety of MICROFILAMENTS organelles Microfilaments Endoplasmic Reticulum = appears smooth in - these are long, thin rods composed of places where it serves as site for synthesis of many molecules of the protein actin fatty acids and phospholipids, and in other - solid and narrower than microtubules, they places, appears rough as it is studded with enable cells to withstand stretching and ribosomes compression Ribosomes = are sites of protein synthesis, - they also help anchor one cell to another guided by the information contained in the mRNA INTERMEDIATE FILAMENTS Mitochondria = provide energy by breaking Intermediate Filaments down nutrients from food. The energy liberated - they have diameters intermediate between from food is captured and stored in the bonds those of microtubules and microfilaments present in a molecule called adenosine - they are abundant in skin and nerve cells triphosphate (ATP) - in actively dividing skin cells, it forms a Centrioles = are a pair of complex structure strong inner framework that firmly attaches that are located in a specialized region called cells to each other and to the underlying the centrosome. tissue (Matrix Junctions) These are associated with the organization of spindle fibers that function in mitosis and meiosis. The organization of spindle fibers by the centrioles plays an important role in the movement of chromosomes during cell division CYTOSKELETON Cytoskeleton = is a meshwork of protein rods and tubules that molds the distinctive structures of a cell, positioning organelles and providing three- dimensional shape. Three Major Types of Elements: 1. microtubules, 2. microfilaments Page | 2  CYTO311: CYTOLOGY WEEK 5: MEIOSIS AND CHROMOSOME MORPH 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO Only in the second division (meiosis II) are the sister MEIOSIS & CHROMOSOME MORPHOLOGY chromatids pulled apart and segregated (as in mitosis) to produce haploid daughter nuclei. MEIOSIS Produces four haploid nuclei, each of which contains either the As in mitosis, meiosis is maternal or paternal copy of each chromosome, but not both preceded by a process of DNA replication that converts each PROPHASE I: chromosome into two sister ➔ Subdivided into the following five chromatids. phases based on chromosomal Meiosis is the form of behaviour eukaryotic cell division that ➔ Leptotene, Zygotene, Pachytene, produces haploid sex cells or Diplotene and Diakinesis. gametes (which contain a ➔ Take hours in yeasts, days in mice, single copy of each and weeks in higher plants chromosome) from diploid It is during early prophase I that the cells (which contain two homologs begin to associate along copies of each chromosome). their length in a process called pairing The process takes the form of As prophase progresses, the homologs become more one DNA replication followed closely juxtaposed, forming a four-chromatid structure by two successive nuclear called a bivalent and cellular divisions DNA double-strand breaks are formed at several locations (Meiosis I and Meiosis II). in each sister chromatid, resulting in large numbers of DNA In these species, the recombination events between the homologs reproductive cycle ends when Lead to reciprocal DNA exchanges called crossovers, a sperm and egg fuse to form where the DNA of a chromatid crosses over to become a diploid zygote, which has the potential to form a new individual. continuous with the DNA of a homologous chromatid THE KEY FEATURES OF MEIOSIS ARE AS FOLLOWS: Leptotene - the homologs condense and pair Meiosis involves two sequential cycles of nuclear and cell Zygotene - the synaptonemal complex begins to assemble at division called meiosis I and meiosis II but only a single cycle sites where the homologs are closely associated and of DNA replication. recombination events are occurring Meiosis I is initiated after the parental chromosomes have Bivalent or Tetrad - the complex formed by a pair of synapsed replicated to produce identical sister chromatids at the S homologous chromosomes phase. Pachytene - the assembly process is complete, and the Meiosis involves pairing of homologous chromosomes and homologs are synapsed along their entire lengths. During this recombination between them. stage bivalent chromosomes now clearly appears as tetrads. Four haploid cells are formed at the end of meiosis II. Meiotic Diplotene - the disassembly of the synaptonemal complexes events can be grouped under the following phases: and the concomitant condensation and shortening of the chromosomes Crossover events between nonsister chromatids can be seen as inter-homolog connections called chiasmata (singular chiasma) MEIOSIS INCLUDES TWO ROUNDS OF CHROMOSOME SEGREGATION: The first of these divisions (meiosis I) segregates the homologs. The duplicated paternal and maternal homologs pair up alongside each other and become physically linked by the process of genetic recombination. In the first meiotic anaphase, duplicated homologs are pulled apart and segregated into the two daughter nuclei.  CROSSING OVER METAPHASE II Crossing Over ➔ At this stage the chromosomes align at the equator and the - is the exchange of genetic microtubules from opposite poles of the spindle get material between two attached to the kinetochores of sister chromatids. homologous chromosomes. - is also an enzyme-mediated ANAPHASE II process and the enzyme ➔ It begins with the simultaneous splitting of the centromere involved is called of each chromosome (which was holding the sister recombinase. chromatids together), allowing them to move toward Recombination between opposite poles of the cell homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of TELOPHASE II: crossing over. ➔ Meiosis ends with telophase II, in which the two groups of The final stage of meiotic prophase I is diakinesis. chromosomes once again get enclosed by a nuclear This is marked by terminalisation of chiasmata. envelope; cytokinesis follows resulting in the formation of During this phase the chromosomes are fully condensed and the tetrad of cells i.e., four haploid daughter cells. meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase. METAPHASE I ➔ The bivalent chromosomes align on the equatorial plate ➔ The microtubules from the SIGNIFICANCE OF MEIOSIS opposite poles of the spindle Meiosis attach to the pair of homologous - is the mechanism by which conservation of specific chromosomes. chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the ANAPHASE I process, per se, paradoxically, results in reduction of ➔ The homologous chromosomes chromosome number by half. separate, while sister chromatids - it also increases the genetic variability in the population of remain associated at their centromeres organisms from one generation to the next. o Variations are very important for the process of evolution. TELOPHASE I ➔ The nuclear membrane and nucleolus MEIOSIS FREQUENTLY GOES WRONG reappear, cytokinesis follows and this is Mistakes are especially common in human female meiosis, called as diad of cells. which arrests for years after diplotene: meiosis I is completed The stage between the two meiotic only at ovulation, and meiosis II only after the egg is fertilized. divisions is called interkinesis and is generally short lived. Such chromosome segregation errors during egg development are the most common cause of both spontaneous abortion Interkinesis is followed by prophase II, (miscarriage) and mental retardation in humans a much simpler prophase than When homologs fail to separate properly—a phenomenon called prophase I. nondisjunction— the result is that some of the resulting haploid gametes lack a particular chromosome, while others have more MEIOSIS II than one copy of it. PROPHASE II Upon fertilization, these gametes form abnormal embryos, most ➔ Meiosis II is initiated immediately after cytokinesis of which die. ➔ Meiosis II resembles a normal mitosis Segregation errors during meiosis I increase greatly with ➔ The nuclear membrane disappears by the end of prophase II advancing maternal age. ➔ The chromosomes again become compact.  CHROMOSOMES There are 46 chromosomes in every somatic cell of a human being. Of which 22 pairs (44) are autosomes 23rd pair (XX or XY) are sex chromosomes. The DNA molecule may be circular or linear, and can be composed of 100,000 to 10,000,000,000 nucleotides in a long chain. Typically, eukaryotic cells (cells with nuclei) have large linear chromosomes and Prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. CHROMOSOME NOMENCLATURE International System for Human Cytogenetic Nomenclature “Mother cells” are diploid (2n) (ISCN) has been developed by the Standing Committee on Human Cytogenetic Nomenclature MITOSIS – VS – MEIOSIS (SUMMARY DETAILS) The pair of non-sex chromosomes (autosomes) are serially numbered, 1 to 22, as nearly as possible in descending order of length. Identification of the chromosomes is based on size, position of centromere and other morphological features. Chromosome short arms are called p (petit) and long arms q (queue). FOUR TYPES OF CHROMOSOMES BASED UPON THE POSITION OF THE CENTROMERE 1) Metacentric - In this type of chromosome the centromere occurs in CHROMOSOME MORPHOLOGY the center and all the four chromatids are of equal length. 2) Submetacentric - In this type of chromosome the centromere is a little away from the center and therefore chromatids of one side are slightly longer than the other side. 3) Acrocentric - In this type of chromosome the centromere is located closer to one end of chromatid therefore the chromatids on opposite side are very long. - A small round structure, attached by a very thin thread is observed on the side of shorter chromatid. - The small round structure that is a part of the chromatid is termed as satellite. 4) Telocentric - In this type of chromosome the centromere is placed at one end of the chromatid and hence only one arm. - Such telocentric chromosomes are not seen in human cells. * Telomeres - long regions of repetitive non-coding DNA that cap chromosomes to stop replication; undergo partial degradation (i.e., become shorter) each time a cell undergoes division TWO WAYS IN REPRESENTING CHROMOSOMES 1. SEM: Chromosomes - uncondensed in nucleus, upper right  CYTO311: CYTOLOGY WEEK 5: MITOSIS 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO MITOSIS MITOSIS Prophase Prometaphase DNA, GENES AND CHROMOSOMES Metaphase An organism’s DNA, containing arrays of genes, is Anaphase organized into structures called chromosomes Telophase Chromosomes serve as vehicles for transmitting genetic information TWO MAJOR PROCESSES INVOLVED IN THE INTERPHASE GENETIC CONTINUITY OF NUCLEATED CELLS: The interphase is note devoted solely to cell’s growth o Mitosis and normal function. It is also when the cell o Meiosis replicates the DNA on each chromosome DNA synthesis occurs before the cell enters mitosis. MITOSIS AND MEIOSIS This period is called the S phase Mitosis - leads to production of two cells with the There are two periods during interphase in which same number of chromosomes as the parent cell there is NO synthesis of DNA. Meiosis - reduces the genetic content and the One occurs before and one after the S phase number of chromosomes by half (G1/gap1 and G2/gap 2, respectively) This reduction in the number of chromosomes is important in the production of sex cells or gametes During G1, S phase, and G2, intensive metabolic activity, cell growth and cell differentiation are CELL DIVISION evident. By the end of G2, DNA has been replicated and the The process of mitosis is important in all eukaryotic cell volume has doubled. organisms The cell enters mitosis (M). The genetic material is partitioned into daughter cells Following mitosis, continuously dividing cells repeat during nuclear division (karyokinesis) this cycle (G1, S, G2 then M) over and over The chromosomes must be replicated and then accurately partitioned This is followed by cytoplasmic division (cytokinesis) At a point during G1, all cell follow one of two paths It partitions the cell volume inti two parts and then They either encloses each new cell in a distinct plasma (1) withdraw from the cycle, become quiescent, membrane and enter the G0 stage, or (2) become committed to proceed through G1 and complete the cycle. THE CELL CYCLE Cells that enter the G0 remain viable and Many cell divisions transform a fertilized egg into a metabolically active but are not proliferative full-grown adult Cell Cycle - is a series of events that describe the The chromosomes are not visible during the sequence of activities a cell prepares for division and interphase then divides Instead, the nucleus is filled with chromatin fibers formed from the uncoiling and dispersal of the chromosomes after the previous mitosis PROPHASE The centrioles migrate to opposite ends of the cell After centriole migration, the centrosomes, are responsible for organizing cytoplasmic microtubules into spindle fibers that run between the poles The nuclear envelope break down and disappear, and STAGES OF THE CELL CYCLE the nucleolus disintegrates INTERPHASE The diffuse chromatin fibers condenses, and the G1 chromosomes become visible S phase G2  The chromosomes appear as a double structure split Migration is made possible by the binding of spindle longitudinally except at a single point of constriction, the fibers to the chromosome’s kinetochore, an assemble of centromere. multilayered plates of proteins associated with the The two parts of each chromosome are called sister centromere. chromatids because the DNA contained in each of This structure forms on opposite sides of each paired them is genetically identical centromere, in intimate association with the two sister The sister chromatids are held together by protein chromatids. complexes called cohesion ANAPHASE The shortest stage of mitosis, the anaphase, whose events are critical to the chromosome distribution during mitosis During this phase, the sister chromatids of each chromosome, separate from one another (an event PROMETAPHASE AND METAPHASE described as disjunction), and pulled to opposite ends At the completion of metaphase, each centromere is of the cell. aligned at the metaphase plate with the chromosome As these events proceed, each migrating chromatid is arms extending outward in a random array. now referred to as a daughter chromosome PROPHASE IMPORTANT: Even though one cannot see the chromatids in the interphase because the chromatin is uncoiled and dispersed in the nucleus, the chromosomes are already double structures, which becomes apparent in late prophase. PROMETAPHASE AND METAPHASE The chromosomes migrate to the equatorial plane (also The steps that occur during anaphase are critical in called the metaphase plate) providing each subsequent daughter cell with an identical The metaphase plate is the midline region of the cell, a set of chromosomes. plane that lies perpendicular to the axis established by the spindle fibers TELOPHASE Prometaphase refers to the period of chromosome At the beginning of telophase, two complete sets of movement chromosomes are present, one on each pole Metaphase is the chromosome configuration following Cytokinesis then occur, partitioning the cytoplasm in migration order to produce two new cells from one Animal cells undergo constriction of the cytoplasm, producing the cell furrow that is characteristic of newly divided cells.  In each new cell, the chromosomes begin to uncoil In each new cell, the chromosomes begin to uncoil and become diffuse chromatin once again The nuclear envelope reforms, and the spindle fibers disappear The nucleolus re-forms and become visible At the completion of telophase, the cell enters the interphase  CYTO311: CYTOLOGY WEEK 8: MEIOSIS AND CHROMOSOME MORPH 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO THE KEY FEATURES OF MEIOSIS ARE AS FOLLOWS: MEIOSIS & CHROMOSOME MORPHOLOGY Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle MEIOSIS of DNA replication. ***Extra notes: Meiosis I is initiated after the parental chromosomes have In meiosis, we produce replicated to produce identical sister chromatids at the S haploid containing one phase. chromosome from Meiosis involves pairing of homologous chromosomes (sister homologous pair. chromatids from paternal and maternal) and recombination The sister chromatids are between them. separated to produce four Four haploid cells are formed at the end of meiosis II (contains haploid daughter cells. This 23 chromosomes). Meiotic events can be grouped under the will undergo single DNA following phases: replication from the start Then, the homolog chromosome pair will undergo crossover in which there is a genetic exchange between the paternal and maternal chromosome. MEIOSIS INCLUDES TWO ROUNDS OF CHROMOSOME After that, it will segregate. SEGREGATION: However, this chromosome is The first of these divisions (meiosis I) segregates the homologs. genetically different from the The duplicated paternal and maternal homologs pair up original chromosome because alongside each other and become physically linked by the it undergoes process of genetic recombination (or crossing over). crossover/genetic exchange In the first meiotic anaphase, duplicated homologs are pulled The essence of meiosis is to have recombination or genetic apart and segregated into the two daughter nuclei. exchange of chromosome coming from parent cells to have Only in the second division (meiosis II) are the sister genetic variation for more chances of survival of organism chromatids pulled apart and segregated (as in mitosis) to The sister chromatids will separate from each other that will produce haploid daughter nuclei. result to four haploid daughter cell. o Each of the cell will contain 23 chromosomes These daughter cells are genetically different from its original Produces four haploid nuclei, each of which contains either the chromosome. It contains either paternal or maternal maternal or paternal copy of each chromosome, but not both chromosome but they cannot contain both of these chromosome PROPHASE I: ➔ Chromosomes are condense As in mitosis, meiosis is preceded by a process of DNA ➔ Once it get ready for cell replication that converts each chromosome into two sister division, chromatin fibers will chromatids. undergo “condensation” Meiosis is the form of eukaryotic cell division that produces ➔ Term in Prophase: haploid sex cells or gametes (which contain a single copy of each a) Synapsis – movement of chromosome) from diploid cells (which contain two copies of homologous towards each each chromosome). other; pairing of The process takes the form of one DNA replication followed by chromosomes two successive nuclear and cellular divisions (Meiosis I and ▪ The replicated Meiosis II). paternal chromosome and replicated maternal In these species, the reproductive cycle ends when a sperm and chromosome will undergo pairing depends on the egg fuse to form a diploid zygote, which has the potential to form length a new individual. ▪ Sister chromatids will pair and form ”bivalent” or “tetrads”  ▪ Tetrads – has four sister chromatids of paired Diplotene - the disassembly of the synaptonemal complexes homologous chromosomes (bivalent chromosome will unbutton to each other but the point ➔ Subdivided into the following five phases based on of attachment / chiasmata will remain attached) and the chromosomal behaviour concomitant condensation and shortening of the chromosomes ➔ Different Substages of Prophase I: Crossover events between nonsister chromatids can be seen a) Leptotene as inter-homolog connections called chiasmata (singular b) Zygotene chiasma) c) Pachytene Diakinesis – nuclear membrane and nucleolus completely d) Diplotene disappear e) Diakinesis ➔ Has a variable in the duration. It could take hours, days, or weeks (depends on organism). ➔ Take hours in yeasts, days in mice, and weeks in higher plants It is during early prophase I that the homologs begin to associate along their length in a process called pairing As prophase progresses, the homologs become more closely juxtaposed, forming a four-chromatid structure CROSSING OVER (paired homologous chromosomes) called a bivalent / tetrads Chiasmata DNA double-strand breaks are formed at several locations in each sister chromatid, resulting in large numbers of DNA recombination events between the homologs Lead to reciprocal DNA exchanges called crossovers, where the DNA of a chromatid crosses over to become continuous with the DNA of a homologous chromatid o Where the genetic exchange from the paternal and maternal chromosomes undergo Crossing Over Leptotene - the homologs condense and pair - is the exchange of genetic material between two homologous o Chromatid fibers will condense to make it short and better chromosomes. visualization of chromosome - non-identical means it will come from mother and father Zygotene - the synaptonemal complex begins to assemble at - is also an enzyme-mediated process and the enzyme involved sites where the homologs are closely associated and is called recombinase. recombination events are occurring Recombination between homologous chromosomes is o The synaptonemal complex form bivalent or tetrads kaya completed by the end of pachytene, leaving the chromosomes magkakadikit na linked at the sites of crossing over (or chiasmata). o These complex help the chromosomes to stick with one another during recombination The final stage of meiotic prophase I is diakinesis. Bivalent or Tetrad - the complex formed by a pair of synapsed This is marked by terminalisation of chiasmata. homologourecs chromosomes During this phase the chromosomes are fully condensed and the o Bivalent or Tetrads indicates that both homologs meiotic spindle is assembled to prepare the homologous duplicated chromosomes for separation. Pachytene - the assembly process is complete, and the By the end of diakinesis, the nucleolus disappears and the homologs are synapsed along their entire lengths. During this nuclear envelope also breaks down. stage bivalent chromosomes now clearly appears as tetrads. Diakinesis represents transition to metaphase. Chiasma – point of recombination of chromatids from mother and father METAPHASE I ➔ The bivalent chromosomes align on the equatorial plate ➔ Do random arrangement of maternal and paternal chromosome (for genetic variation)  ➔ How to identify metaphase I:  Chromosome is aligned in the equatorial plate  Bivalent is still present and touch each other (point of chiasmata)  Double-stranded chromosome is still present ➔ The microtubules from the opposite poles of the spindle During (?) meiosis I, there is recombination. attach to the pair of homologous chromosomes. Recombination – Crossing over of alleles in homologous chromosomes ANAPHASE I Chiasma – Where “genetic exchange” happens; not ➔ The homologous chromosomes separate, while sister homologous pair but a “non-identical chromatids” chromatids remain associated at their centromeres ➔ Purpose: Have independent reassortment or genetics inherent characteristics ➔ Resulted to separation of bivalent and independent assortment ➔ How to identify anaphase I:  Double-stranded chromosomes are pulled into Chiasma point of attachment = between mother and father opposite pole chromosome (genetic exchange)  Recombination is visible (genetic exchange happens) Result to = recombinant chromosome  Chromosome separated from its partner Chiasma Recombinant chromosome: Recombination Recombinant chromosome Recombinant chromosome – genetically variable to its TELOPHASE I parent/original chromosome (nagkaroon na kasi ng ➔ Partition of cells into two parts recombination; ➔ The nuclear membrane and nucleolus reappear, cytokinesis Reason why we are different from our biological parents, follows and this is called as diad of siblings: Because there is a recombination of chromosome cells. between our father and mother ➔ Separates cytoplasm Probability of combinations: About billions probability of the ➔ Two non-identical individual cell or half combination of different genes (so, we are totally different from the genetic information our parents. siblings, identical twin) The stage between the two meiotic From the beginning, before meiosis cell division proceed, it divisions is called interkinesis and is starts with “diploid” cells generally short lived. At the end of meiosis I, we have two haploid cells Interkinesis is followed by prophase II, a much simpler prophase than prophase I. Each haploid cells contain chromosome from homologous pair o Interkinesis – short phased in cell division but no Obtain two non-identical individual cells that contain half of activities involved genetic information Interkinesis – between meiosis I and II; there is no activities on ***Review about Meiosis I: here Meiosis I – there is single DNA replication. You have chromosome from father and mother. There is a “synapses” MEIOSIS II wherein movement of two homologous chromosome to each PROPHASE II other or pairing of homologous chromosome resulted to = ➔ Meiosis II is initiated immediately after cytokinesis Bivalent or Tetrads ➔ Meiosis II resembles a normal mitosis (however, NO DNA Tetrads – four sister chromosomes of paired homologous REPLICATION happens here because in meiosis, there is chromosomes only single DNA replication and that happens on Meiosis I) ➔ The nuclear membrane disappears by the end of prophase II ➔ The chromosomes again become compact.  ➔ No crossing over unlike in Prophase I ➔ Chromosomes are not homologous partner because they are genetically different from each other because of recombination during Prophase I ➔ There is a spindle fiber formation and centrioles move to opposite side TELOPHASE II: Recombinant ➔ Meiosis ends with telophase II, in chromosome which the two groups of chromosomes - not genetically once again get enclosed by a nuclear identical to parent envelope (nuclear membrane started chromosome to form); cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells. METAPHASE II ➔ At this stage the chromosomes align at the equator At the end of Meiosis II, every single cell has (equatorial plane) and the microtubules from opposite only one chromosomes. poles of the spindle get attached to the kinetochores of End product of Telophase II: Four Unidentical Cell or Four sister chromatids (for pulling up sister chromatids). Unidentical Haploid Cell ➔ Purpose: For random arrangement of recombinant Each cells contain single stranded chromosomes chromosomes ➔ Can see double stranded chromosomes align at the equator If nagkaroon ng error during Meiosis: If too much formation, or ➔ Spindle fiber is attached to the centromere not separated = nagkakaroon ng abnormality during the Meiosis II SIGNIFICANCE OF MEIOSIS Meiosis - is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half (number of chromosome is ANAPHASE II reduced by half but it INCREASES GENETIC VARIABILITY) ➔ It begins with the simultaneous splitting of the centromere because of recombination or crossing over process of each chromosome (which was holding the sister Recombination or Crossing Over Process chromatids together), allowing them to move toward - It also increases the genetic variability in the population of opposite poles of the cell organisms from one generation to the next. ➔ Recombinant chromosomes separate so, there is a - Variations are very important for the process of evolution. separation of chromatids ➔ Purpose: Separate the recombinant chromosome, forming MEIOSIS FREQUENTLY GOES WRONG a single-stranded chromosome Mistakes are especially common in human female meiosis, ➔ Spindle fibers are contracting which arrests for years after diplotene: Meiosis I is completed ➔ Double stranded will become single stranded Spindle fiber only at ovulation, and Meiosis II only after the egg is fertilized. chromosomes here! o In Meiosis I, genetic exchange happens o In Egg cell, same lang yung age ng egg cell sa age ng female o As age of mother increases, nagkakaroon ng “segregation error” that happens in Meiosis II after the Chromatids (called before!) but centromere pulled apart so, they “egg is fertilized” are now “individual chromosome from one another” o Segregation error - can lead to spontaneous abortion, miscarriage, etc (mas mataas magkaroon ng risk of End of anaphase II: abnormality kapag above 40s na yung mother) Such chromosome segregation errors during egg development are the most common cause of both spontaneous abortion (miscarriage) and mental retardation in humans  When homologs fail to separate properly—a phenomenon called nondisjunction— the result is that some of the resulting haploid gametes lack a particular chromosome, while others have more than one copy of it. o There is some abnormalities happen because of the lack of particular genes resulting to inability to produce some PROTEINS o Sometimes, there is an increase in that genes that can result to abnormalities. Upon fertilization, these gametes form abnormal embryos, most of which die. o Fertilization is a potential development of new individual. If nagkaroon ng error in segregation in fertilization, it can result to abortion or discontinuation of development of individual “Mother cells” are diploid (2n) Segregation errors during meiosis I increase greatly with advancing maternal age. MITOSIS – VS – MEIOSIS (SUMMARY DETAILS) Chromosome number of mitosis: 46 Chromosome number of meiosis: 23 Meiosis in human completes after sexual maturity: kaya may recommended child-bearing age for women CHROMOSOME MORPHOLOGY ***Review: Mitosis - copy of the parent cell; genetically identical with each other - result: two diploid cell Meiosis - single replication during the Meiotic Division I - there is crossing over that resulted to “Recombinant Chromosome” - during Meiotic Cell Division, one of the chromatids will go to one cell - result: four haploid cell (unidentical cells, because it contains only one strand of recombinant chromosome) - genetically different from its parent cell - reason of “genetic variation” because (sperm cell + egg cell) >>> ilang variation ang pwedeng maganap sa sperm cell; tapos ilang variation din ang pwedeng maganap sa egg cell (so we are unique!)  - A small round structure, attached by a very thin thread is observed on the side of shorter chromatid. - The small round structure that is a part of the chromatid is termed as satellite. CHROMOSOMES There are 46 chromosomes in every somatic cell of a human 4) Telocentric being. Of which 22 pairs (44) are autosomes 23rd pair (XX or XY) - In this type of chromosome the are sex chromosomes. centromere is placed at one end of the The DNA molecule may be circular or linear, and can be chromatid and hence only one arm. composed of 100,000 to 10,000,000,000 nucleotides in a long - Such telocentric chromosomes are not chain. seen in human cells. Typically, eukaryotic cells (cells with nuclei) have large linear chromosomes and Prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. CHROMOSOME NOMENCLATURE International System for Human Cytogenetic Nomenclature (ISCN) has been developed by the Standing Committee on Human Cytogenetic Nomenclature The pair of non-sex chromosomes (autosomes) are serially numbered, 1 to 22, as nearly as possible in descending order of length. Identification of the chromosomes is based on size, position of centromere and other morphological features. Chromosome short arms are called p (petit) and long arms q (queue). * Telomeres - long regions of repetitive non-coding DNA that cap FOUR TYPES OF CHROMOSOMES BASED UPON THE POSITION chromosomes to stop replication; undergo partial degradation (i.e., OF THE CENTROMERE become shorter) each time a cell undergoes division 1) Metacentric - In this type of chromosome TWO WAYS IN REPRESENTING CHROMOSOMES the centromere occurs in 1. SEM: Chromosomes the center and all the four - uncondensed in nucleus, upper right chromatids are of equal length. 2) Submetacentric - In this type of chromosome the centromere is a little away from the center and therefore chromatids of one side are slightly longer than the other side. 3) Acrocentric - In this type of chromosome the centromere is located closer to one end of chromatid therefore the chromatids on opposite side are very long.  CYTO311: CYTOLOGY WEEK 9: MODEL ORGANISMS 1ST SEMESTER | S.Y 2023-2024 Instructor: PROF. HOLLY GRACE O. ESPIRITU, RMT, MS MICRO o To easily replace the model organism or change the Model Organisms methods to come up with a better and more effective 01. Definition experimental product* 02. Characteristics o Time = money* 03. Type of Model Organisms Cab be breed in large numbers 04. Strengths and Limitations of Various Model Organisms o Just like bacteria that can replicate within 18-24 Animal handling is important to avoid animal cruelty hours* during experimentation. To make sure that methods are Readily available and inexpensive maintenance safe, will not cause harm to the animals, and for the safety Similar genes or similar-sized genomes to humans of the scientist.* Tractability to experimental methodology What is a model organism? Model Organisms A model organism is a non-human species that is extensively studied to understand basic biological phenomena, with the Bacteria expectation that discoveries made in the model organism can be extrapolated to other species, Including humans. The geneticist is gradually focusing on small number of organisms such as fruit fly and mouse* As science progresses, biobanking has been developed. Yeast – eukaryotes* Biobanking is an institution that houses or stores a specific bacteria that has specific characteristics that can be used on experiment because the scientist want to monitor that characteristics or to test products against that characteristic to know the effect of the experiment on that specific organism.* Biobanking uses ultra-low temperature. This uses negative Arabidopsis thaliana – plant* ultra-low freezer. Biobanking do lyophilization which turns bacteria into a powdered form* Used to study biological processes* Cannot use human because the effect of the experiment is still unknown* Caenorhabditis elegans – To know if the product is safe for humans* nematodes* Has a characteristic of same organs or genes that is comparable to humans* Drosophila melanogaster – Once the experimental product is approved or did not cause fruit fly* any harm to model organism, try the experimental product to another model organism then proceed to human. This process is called as clinical trial* o US FDA (America) Danio rerio o EU FDA (Europe) o FDA (PH) Although the product has been approved by international FDAs, should have clinical trial in the Philippines* o Exception for COVID-19 (due to not enough Chick Embryo resources, etc.) o First and second clinical trials for COVID-19 vaccine are done on Africans. (human guinea pigs). Example of model organisms are mouse, bacteria, nematodes, yeast cell, plants, insects, and fish. Mouse Bacteria under the microscope Typical Characteristics of Model Organisms Small adult size o Bigger model organism requires bigger facility* Rapid development with short life cycles - Can grow within 18-24 hours* o Effects should be observed immediately*  possible to trace the lineage of every one of its Bacteria as a Model Organism approximately 1000 constituent cells The foundations of molecular biology were based on studies of bacteria. C. elegans Life Cycle and Research o Way before, bacteria is used as a model in genetics* o Serratia marcescens is a pigmented organism* o Bacteria has chromosomes. Bacteriophage can be used as a vector to inject nucleic acid in the host cell then it will divide. The nucleic acid injected can be copied that can be harvested for the future use* Antibiotics Recombinant DNA technology o Can correct chromosome of the host’s DNA* - Easily propagates* A. Developmental and Cell biology B. Neurobiology o Especially in the nervous system* C. Aging D. Human disease studies Fruit fly (Drosophila) Yeast as a Model Organism - A versatile model organism that has been used extensively Eukaryotic system. for biomedical research. Signaling molecules and cell cycle are nearly similar. o Low-maintenance, small, and easy to propagate* Good model system to understand many human diseases o Used in biology for genetic study. They breed fruit fly including cancer and observe for its eyes?* Ease of genetic manipulation allows its use for analyzing - Easy-to-manipulate genetic system and can be used to and functionally dissecting gene products from other study development, physiology and behavior. eukaryotes. - Biological complexity comparable to that of a mammal o Easy to manipulate since it is also eukaryotic which - Many organ systems in mammals have well-conserved is comparable to human* homologues in Drosophila Has provided new insights into Last decade four Nobel prizes were awarded for forms of cancer, neurodegenerative diseases, behavior, discoveries involving yeast. immunity, aging, multigenic inheritance, and development. o Studies the effect of chemicals on cancer cells and the present gene on the cancer patient* o It studies the gene of cancer patient to know if the medication is applicable or if it will be effective because there are certain genes that makes the medicine not effective* o Clinical trial of CA drug or chemotherapy drug undergoes testing to check for its eligibility and ensure its high chance of curing the cancer patient* Life cyle of Drosophila Caenorhabditis elegans (nematode) - One of the best characterized multicellular animal at the level of genomics, genetics, embryology o This nematode is chosen for the study of development and function of the nervous system because the C. elegans’ nervous system has only few cells and its cells are well studied* - Its genome is fully sequenced o Genes present are known and well studied* C. elegans is unique in that it can be grown and genetically manipulated with the speed and ease of a micro-organism while offering the features of a real animal C. elegans has a full set of organ systems, has complex sensory systems, shows coordinated behavior, and it is Within 5 days of its life cycle, there will be adult drosophila*  Chick Embryo Development Danio rerio (zebrafish) - Small size, short life cycle, ease of culture, and ability to readily produce mutations relevant to human health and disease o Used to study vertebrate development* o Small and reproduce rapidly* - The embryonic development can be seen through its transparent egg and closely resembles that of higher vertebrates o Larva is transparent making it easier for scientists to observe* Mus musculus (Mouse) - Other shared features with humans include blood, kidney, and optical systems - Closest mammalian model organism to humans - In addition, its genome is model or of the mouse and Genes that code for proteins responsible for carrying out human genomes, which is valuable in identification of key vital biological processes in both the human and the vertebrate genes. mouse share a high degree of similarity. - Development in ex vivo. Therefore, the mouse has already proven extremely useful in development, genetic, and immunology studies - Entire initial development is transparent. Transgenics and KO's possible - 48hrs is enough for the development of most of the organ o Trangenics is one or more genes that can be altered systems. or removed and observe for its effect. This is also o Within 48 hours, all common vertebrates the method to understand if these genes are specifically body features are observable* responsible for a specific disease or once these genes are not formed on one individual, does it affect the development or function of the body or the lacking of this genes may cause disease* o Knockout gene (KO) is almost the same with transgenics. This disables a particular gene and understand the effect if this will occur.* A great system for studying and understanding human disease, as well as a mechanism for investigating new treatment strategies in ways that cannot be done in humans Arabidopsis thaliana (Thale cress) Within 5 days, zebrafish can already swim and look for food* Chick Embryo - Small flowering plant - Has a small genome relative to other plants and is easily grown under laboratory conditions - Amenable to some genetics particularly generation of transgenics - Allows insight into numerous features of plant biology, including those of significant value to agriculture, energy, environment, and human health - The chick embryo provides an excellent model system for studying the development of higher vertebrates Relative Advantages and Limitations of Model Organisms wherein growth accompanies morphogenesis. - Can be used for cultivation of viruses such as flu and ORGANISM ADVANTAGES LIMITATIONS development of medicine* Fruit Fly Genome sequenced Embryological In reproductive theory, they used chick embryo and they Excellent genetics manipulations observed that an organism were previously an embryo RNAi effective difficult and this is also used for its blood circulation* Fast generation time Targeted gene Morphogenesis: is the developmental cascade of Second site suppressor/ disruption still pattern formation. Which one is formed first? -- from enhancer screens difficult although embryo yung eyes ba, arms ba, etc.* Powerful molecular possible Morphogenesis is a pattern formation and body plant techniques establishment resulting to final form of organism* Genes can be easily cloned  Transgenic animals easily generated Model Organisms Used to Study Human Diseases Targeted misexpression of genes in space and TABLE 1.2 time Model Organisms Used to Study Human Diseases Mosaic analysis: Organism Human Diseases determine where gene E. coli Colon cancer and other cancers acts S. cerevisiae Cancer, Werner syndrome Laser ablation of single