History of Genetics PDF

traits are passed in predictable ways. He concluded that each trait is controlled by genes and separates during gamete...

traits are passed in predictable ways. He concluded that each trait is controlled by genes and separates during gamete formation (the formation of egg cells and sperm). Mendel's COVERAGE: work forms the foundation for genetics, the study of 1. History & Origin of Genetics & heredity and variation. Cytogenetics 2. Three general areas of Genetics. Mendel's Hybridization Research Phase: 3. Gregor Mendel in the field of Genetics. Started in 1856 with hybridization experiments with 4. Importance of Genetics & Cytogenetics garden pea. in the field of medicine. Continued until 1868, becoming abbot of a monastery. 5. The Cell theory Successful in providing evidence about inheritance due to 6. Different Techniques in Genetic study elegant design and analysis. Used easy-to-grow pea plant and seven visible features. Obtained true-breeding strains from local seed merchants. Success attributed to selection of suitable organism, limited examination, and accurate quantitative records. William Bateson coined the term "genetics" in 1905, Analysis led to development of transmission genetics derived from the Greek words "genetikos" and "genesis," principles. meaning generative and origin. MILESTONE DISCOVERIES IN THE GENETICS Genetics is a field of science that includes the study of inheritance and genetic variations by investigating the 1842: Wilhelm von Nageli, a Swiss botanist, observed the DNA, genes, genome, chromosome and other components plant cell. of it. 1866: Mendel’s research work published under the title of “experiments on plant hybridization.” HISTORY OF GENETICS 1869: Friedrich Miescher discovered the nucleic acid. 1888: Waldeyer identified the chromosome present in the Gregor Johann Mendel's work on pea plants in 1866 cell. introduced Mendelian Inheritance. 1889: Richard Altmann purified DNA from the protein. 1905: William Bateson coined the term “genetics”. 1900s: The Rediscovery of Mendel 1908: discovery of Hardy-Weinberg’s law. Hugo de Vries, Carl Correns, and Erich von Tschermak 1910: Morgan T, explained that the genes are located on the rediscover Mendel in 1900. chromosomes. Also, they By 1915, Mendelian genetics principles were applied to experimented on Drosophila Melanogaster and determined various organisms, including Drosophila melanogaster. the nature of sex-linked traits. Thomas Hunt Morgan and his team developed the 1923: Griffith F, experimented on bacteria and postulated Mendelian model, widely accepted by 1925. that the DNA is the genetic material. Mathematicians developed population genetics statistical 1953: Watson and Crick identified the structure of DNA. framework, integrating genetic explanations into evolution studies. BRANCHES OF GENETICS 1940s and 1950s: Investigations of Gene Physical Nature Experiments identified DNA as the portion of chromosomes holding genes. Discovery of double helical structure of DNA in 1953 marked the transition to molecular genetics. 1960s and 1970s: Sequencing Nucleic Acids and Proteins Chemists developed sequencing techniques for nucleic acids and proteins. Regulation of gene expression became a central issue in the 1960s. Molecular Genetics By 1970s, gene expression could be controlled and - Interdisciplinary field studying DNA and genes' structure manipulated through genetic engineering. and function. In late 20th century, biologists focused on large-scale - Uses techniques like Polymerase chain reaction and DNA genetics projects. sequencing. MENDEL’S WORK ON TRANSMISSION OF TRAITS Cytogenetics - Sub-branch of genetics studying inheritance through Gregor Johann Mendel was born in 1822 an Augustinian chromosomal analysis. monk, conducted experiments on pea plants and found that ANDREA RARANG - Screens for structural and numerical chromosomal Any of the molecular genetic experiment divided into the 4 abnormalities. sub-steps: Human Genetics 1. Separation of molecule - Study of genetic alteration and its role in disease development. 2. Purification of molecules Preimplantation Genetics 3. Processing of molecule - Characterizes or profiles the genetic composition of the embryo before implantation. 4. Detection of molecule -Screens high-risk pregnancy. Separation extracts DNA or mRNA from cell debris, Clinical Genetics purifies it using a kit or alcohol, and then processes it for downstream applications. - Involves studying disease root, adverse effects, and inheritance pattern. PCR (polymerase chain reaction) amplifys millions of copies of a DNA segment in vitro, temperature-dependent. Plant Genetics - Study of genetic variation and chromosomal abnormalities divided into three steps: in plants.  Denaturation: the double-stranded DNA denatured into Microbial Genetics a single-stranded one. - Applies the study of genes, genotypes, and gene expression  Annealing: the Sequence-specific DNA primer of microorganisms. binds/anneal to its complementary sequence on single- stranded DNA. Population Genetics  Extension: the Taq DNA polymerase amplifies the - Interdisciplinary field studying genetic difference within DNA using the 3′ end of the primer. and between populations or individuals. Epigenetics - Study of alterations in an organism caused by gene expression. Physiological Genetics - Study of physiological characteristics like sex differentiation, blood group factor, and sickle cell anemia. Biochemical Genetics Study of the chemistry of DNA, gene, chromosome, RNA, and related biomolecules. DNA cloning DNA cloning is a traditional method for the synthesis of Quantitative Genetics DNA. Using a cloning vector our sequence of interest can be - A branch of population genetics studying continuously synthesized by the bacterial artificial chromosome. The varying phenotypes. method is time-consuming and not so accurate. It includes tedious steps like cell culture and isolation. Behavioral Genetics - Study of the behavioral phenotypes of an organism DNA sequencing: governed by genetic factors The method of reading the sequence of DNA using a computational tool is called DNA sequencing. In this TECHNIQUES USED IN THE STUDY OF GENETICS method, we can actually analyze any variation or new mutation in our sequence of interest using the fluoro-labeled Molecular Genetics Techniques Overview: dNTPs.  Screening pathogenic mutations.  Detecting SNPs, minor deletions, or duplications at Conclusion: DNA level.  Conducting gene expression studies.  Utilizing advanced DNA sequencing for new variations or mutations.  Techniques include polymerase chain reaction, gene cloning, DNA sequencing, and DNA quantification. ANDREA RARANG|  In a broader sense, genetics is a study of genotype, its related phenotype, and alterations in the genome. Using genetics tools, nowadays, the diagnosis of inherited diseases is a common medical practice.  The Human Genome Project was completed in the year 2013. Now we have the entire genomic sequence of human. We can use this data for the identification of new mutations and alterations.  Further, genomic data of so many other organisms are now available which is used for identification and characterization of different organisms and species. Fluorescence in Situ Hybridization (FISH): COVERAGE: A molecular cytogenetic technique developed in the 1980s. 1. Cytogenetics, Karyotyping, Flourescent In-su Uses fluorescent probes to detect specific DNA sequences hydridization & DNA microarray analysis on chromosomes. 2. Applications of Cytogenetics. Fluorescent microscopy helps locate the probe's location 3. Nature of Chromosomes. on chromosomes. 4. Sutton’s Chromosomal Theory of Inheritance. Used in genetic counseling, medicine, and species identification. 5. Importance of Meiosis in the principles of inheritance. 6. Explain the process of homologous recombination, or crossing over. 7. How chromosome maps are created. CYTOGENETICS Cytogenetics is a sub-branch of genetics that studies inheritance through chromosomal analysis using techniques like karyotyping staining, banding, and FISH, screening for structural and numerical abnormalities. Major deletions and duplications can only be detected using DNA Microarray: FISH (Fluorescent in situ hybridization) or DNA microarray A collection of microscopic DNA spots attached to a solid techniques. surface. Used to measure gene expression levels or genotype Karyotyping: multiple genome regions. Pairing and ordering all chromosomes for a genome-wide Each spot contains probes, short sections of a specific snapshot. DNA sequence. Prepared using standardized staining procedures. Probe-target hybridization detected and quantified by Identifies structural features for each chromosome. fluorophore-, silver-, or chemiluminescence-labeled targets. Analyzed by clinical cytogeneticists to detect genetic First computerized image-based analysis published in 1981, changes. invented by Patrick O. Brown. Can reveal changes in chromosome number associated with anueploid conditions. Some of the structural and numerical anomalies are enlisted in the table below ANDREA RARANG Disease Abnormality Cytogenetic indication The centromere's location gives chromosomes its Down syndrome Numerical Trisomy 21 characteristic shape and can help identify specific genes. Klinefelter One extra X chromosome Genes and Chromosomes syndrome Numerical in males (XXY). Genes code for RNA and protein molecules required by an Philadelphia Translocation between organism. syndrome Structural chromosome 9 and 22 Chromosomes in human is linear while circular in bacteria monosomy in female, Turner syndrome Numerical single X chromosome. Eukaryotic Chromosomes Neuroblastoma Structural Chromosome 1p deletion All chromosomes are stored inside the nucleus in eukaryotes. Humans inherit one set of chromosomes from their mother Human Genetics: and a second set from their father. Study of genetic alteration and its role in disease Most human cells contain 46 chromosomes, 22 pairs of development. autosomes, and two sex-determining chromosomes. Uses cytogenetic, molecular genetics, phylogenetic, population genetics, and clinical genetic methods. Diploid and Haplotype Cells Study of disease inheritance pattern, severity, and potential Cells with two sets of chromosomes are called diploid. for consecutive generation inheritance. Haplotype cells contain half the chromosomes of diploid Useful in cancer screening, prognosis, and diagnosis. cells. Preimplantation Genetics: Characterizes or profiles the genetic composition of Differences between Human Chromosomes embryos before implantation. Chromosomes are often observed as X-shaped structures. Covers the genetic profile of oocytes or sperm before fertilization. Major application: screening high-risk pregnancy. Prevents selective abortions by analyzing pre-embryonic cells for molecular or cytogenetic analysis. CHROMOSOMAL THEORY OF INHERITANCE Gregor Mendel, the father of modern genetics, studied heredity in 1843. With advancements in microscopic Any of the genetic abnormality or disease can be identified prior to techniques, scientists like Theodor Boveri and Walter Sutton implantation. Although the field has some most promising applications, re-evaluated Mendel's model, focusing on chromosome the PIGD still under the pre-clinical phase. behavior during mitosis and meiosis. This led to the Chromosomal Theory of Inheritance, identifying Clinical Genetics: chromosomes as the genetic material responsible for Studying disease root. Mendelian inheritance. Identifying adverse effects. Understanding inheritance patterns. NATURE OF CHROMOSOME Chromosome Structures DNA is packaged into chromosomes in the nucleus of each cell. Chromosomes are not visible in the cell's nucleus when not dividing. During cell division, DNA becomes more tightly packed, making chromosomes visible under a microscope. The short arm of the chromosome is labeled the “p arm.” Sutton and Boveri: (a) Walter Sutton and (b) Theodor Boveri are The long arm of the chromosome is labeled the “q arm.” credited with developing the Chromosomal Theory of Inheritance, which states that chromosomes carry the unit of heredity (genes). Centromere Each chromosome has a constriction point called the centromere, dividing it into two sections. ANDREA RARANG The Chromosomal Theory of Inheritance was consistent Synapsis, the pairing and interaction between homologous with Mendel’s laws and was supported by the following chromosomes, organizes homologs for migration to separate observations: daughter cells. When synapsed, homologous chromosomes undergo  During meiosis, homologous chromosome pairs reciprocal physical exchanges of DNA at their arms, known migrate as discrete structures that are independent of as homologous recombination or "crossing over." other chromosome pairs.  The sorting of chromosomes from each homologous Experimental Results pair into pre-gametes appears to be random. Heterozygous individuals with dominant maternal alleles  Each parent synthesizes gametes that contain only and recessive paternal alleles produced gametes with equal half of their chromosomal complement. frequencies.  Even though male and female gametes (sperm and If genes were unlinked, genotypes Ab and aB were egg) differ in size and morphology, they have the nonparental types resulting from homologous recombination. same number of chromosomes, suggesting equal Morgan and his colleagues found that when heterozygous genetic contributions from each parent. individuals were crossed to a homozygous recessive parent,  The gametic chromosomes combine during both parental and nonparental cases occurred. fertilization to produce offspring with the same chromosome number as their parents. The Chromosomal Theory of Inheritance was proposed long before direct evidence of traits being carried on chromosomes. Thomas Hunt Morgan, using Drosophila melanogaster, conducted experiments to support this theory. He identified the gene for eye color as X-linked, the first X- linked trait to be identified. This discovery was made after crossing normal flies with red eyes and mutated flies with white eyes, revealing that males are hemizygous, having only one allele for any X-linked characteristic. Morgan's study on inheritance patterns of unlinked and linked genes reveals three types of chromosomes: (a), (b), (c), and (d). Unlinked genes are located on different chromosomes, while linked genes are on the same chromosome. Morgan observed a recombination frequency of 17% for fruit fly wing length and body color, while a crossover frequency ranges from 0% to 50%. GENETIC MAP Eye Color in Fruit Flies: In Drosophila, the gene for eye color is located Alfred Sturtevant's Chromosome Map Creation on the X chromosome. Red eye color is wild type and is dominant to white eye color. Created the first "chromosome map" in 1913. Assumed genes were serially ordered on threadlike GENETIC LINKAGE & DISTANCES chromosomes. Hypothesized recombination between two homologous Mendel's Trait Inheritance Model chromosomes could occur anywhere along the chromosome Mendel's model suggests traits are inherited independently. length. Morgan identified a 1:1 ratio between a segregating trait Hypothesized alleles far apart dissociated more during and the X chromosome. meiosis due to larger recombination region. Random segregation of chromosomes is the physical basis Alleles close to each other were likely to be inherited of Mendel's model. together. Linking genes disrupt Mendel's predicted outcomes. Correlation between average crossovers and recombination Each chromosome can carry many linked genes, allowing frequency. individuals to have more traits than chromosomes. Divided genetic map into centimorgans (cM), with a Morgan's laboratory observations suggest alleles on the recombination frequency of 0.01 corresponding to 1 cM. same chromosome are not always inherited together. HOMOLOGOUS RECOMBINATION Frans Janssen's 1909 Study on Chromosome Exchange Janssen observed chiasmata, the point where chromatids interact and exchange segments, before the first division of meiosis. He suggested that alleles become unlinked when chromosomes physically exchange segments. ANDREA RARANG Genetic Maps: This genetic map orders Drosophila genes on the basis of Karyotype and Genetic Abnormalities recombination frequency. Karyotype reveals genetic abnormalities with too many or too few chromosomes per cell. Sturtevant's Linear Map of Genes Down Syndrome and Turner Syndrome are examples of Genes can range from perfectly linked (recombination these abnormalities. frequency = 0) to perfectly unlinked (recombination Geneticists can identify large DNA deletions or insertions, frequency = 0.5). like Jacobsen Syndrome. Perfectly unlinked genes correspond to frequencies Karyotype can identify translocations, where a segment of predicted by Mendel to assort independently in a dihybrid genetic material breaks from one chromosome and cross. reattaches to another. Recombination frequency of 0.5 indicates 50% of Translocations are implicated in certain cancers, including offspring are recombinants and 50% are parental types. chronic myelogenous leukemia. This allowed calculation of distances between several Karyotype allows geneticists to visualize chromosomal genes on the same chromosome. composition, confirming or predicting genetic abnormalities in offspring before birth. IDENTIFICATION OF CHROMOSOME Summary: Karyotypes: Sutton and Boveri's Chromosomal Theory of inheritance Depict the number, size, and abnormalities of suggests chromosomes are the vehicles of genetic heredity. chromosomes in an organism. Chromosome behavior involves segregation, independent Form the basis of cytogenetics and are the primary method assortment, and occasionally linkage. for detecting chromosomal abnormalities in humans. Sturtevant's method assesses recombination frequency and Cytologists photograph chromosomes and create a infers relative positions and distances of linked genes. karyogram or ideogram to view an individual's karyotype. Alleles on the same chromosome are inherited together, while homologous recombination biases alleles towards Chromosome Identification: independent assortment. In a given species, chromosomes can be identified by their number, size, centromere position, and banding pattern. Key Points: Autosomes or “body chromosomes” are generally A normal human karyotype contains 23 pairs of organized in order of size from largest to smallest. chromosomes: 22 autosomes and 1 sex chromosome. Chromosome 21 is shorter than chromosome 22, The short arm of a chromosome is the p arm, while the discovered after Down syndrome's naming as trisomy 21. long arm is the q arm. The X and Y chromosomes are not autosomes and are To observe a karyotype, cells are collected, arrested in referred to as the sex chromosomes. metaphase, preserved, and stained with dye. A karyotype can visualize abnormalities in chromosomes, Karyotyping: such as incorrect number of chromosomes, deletions, Mendel's experiments were performed without the tools insertions, or translocations of DNA. commonly used today. Karyotyping is a powerful cytological technique that Lesson 3: MITOSIS & MEIOSIS identifies traits characterized by chromosomal abnormalities from a single cell. The chromosomes are stained with dyes to visualize COVERAGE: distinct and reproducible banding patterns. 1. Parts & Function of a Cell The Giemsa staining results in approximately 400–800 2. Discuss Cytogenetics, Karyotyping, bands arranged along all of the 23 chromosome pairs. fluorescent In-Situ hybridization and DNA Chromosomes are further identified based on size and microarray analysis centromere location. 3. Nature of chromosome. 4. Mitosis and meiosis. 5. Importance of Meiosis in the Principles of Inheritance. Genetic continuity is crucial for the survival of living organisms. DNA, composed of chromosomes, is the genetic material that directs the metabolic activities of cells. Two major processes, mitosis and meiosis, maintain genetic A human karyotype: This karyotype is of a male human. Notice that continuity in nucleated cells. Mitosis divides hereditary homologous chromosomes are the same size, and have the same components into daughter cells, while meiosis reduces centromere positions and banding patterns. A human female would have genetic content and chromosomes by half. Both processes an XX chromosome pair instead of the XY pair shown. are essential for sexual reproduction without doubling the amount of genetic material in each new generation. ANDREA RARANG Chromosomes are visible only during these processes, while chromatin unfolds and uncoils during non-division. A second division occurs, dividing the chromosomes into 23 rods in each of the 4 cells. CELL STRUCTURE IS CLOSELY TIED TO GENETIC FUNCTION Recombination, the exchange of DNA pieces, occurs during meiosis, leading to greater genetic diversity and Electron Microscope Analysis of Cell Structures individual uniqueness. Cells are highly organized structures dependent on specific genetic expression. MITOSIS Components like nucleolus, ribosome, and centriole are involved in genetic processes. The Forms of DNA Mitochondria and chloroplasts contain unique genetic information. Cells are surrounded by a plasma membrane, controlling movement of materials. Plant cells have a cell wall, primarily composed of cellulose. Chromosome. After DNA replicates, it forms chromosomes like the one shown here. 1. Chromatid, 2. Centromere, 3. short arm, 4. long When a cell divides, DNA condenses into a chromosome, consisting of two identical copies called sister chromatids. These sister chromatids are joined at a centromere. The process of mitosis occurs during cell division, where the two sister chromatids separate and move to opposite poles of the cell. Mitosis occurs in four phases: prophase, metaphase, anaphase, and telophase, and is crucial for the formation of two daughter cells. PROPHASE Cellular Reproduction and Division Mitosis is the process of a cell splitting into two Prophase is the longest phase of mitosis. genetically identical copies. Chromatin condenses into chromosomes during prophase. The nuclear envelope breaks down. DNA is condensed into rod-like structures known as Centrioles near the nucleus separate and move to opposite chromosomes, which stick together in the middle. cell poles. Centrioles ensure new cells contain complete After the nuclear membrane breaks down, chromosomes chromosomes. line up in a neat row at the center of the cell. A spindle forms between centrioles, composed of microtubule-made fibers. Chromosome pairs split and move apart toward opposite poles of the cell before dividing into two genetically identical daughter cells. Sex cells, sperm and egg, undergo meiosis, a process where DNA in sex cells must undergo another round of division. Meiosis produces four genetically different cells containing half of the genetic material. METAPHASE The mother cell copies its DNA molecules and condenses them into rods (chromosomes). Spindle fibers attach to centromeres of sister chromatids during metaphase. Chromosomes pair up, line up at the center of the cell for Sister chromatids line up at cell equator. the first round of division. Spindle fibers ensure sister chromatids separate and move to daughter cells during cell division. ANDREA RARANG  Some spindles grow longer than centromeres, elongating the entire cell. ANAPHASE Karyokinesis (or mitosis) is divided into five stages— prophase, prometaphase, metaphase, anaphase, and Sister chromatids separate and centromeres divide. telophase. We should note that this is a continuous process Shortening of spindle fibers pulls sister chromatids apart. and that the divisions between the stages are not discrete. One sister chromatid moves to one cell pole, other to The pictures at the bottom were taken by fluorescence opposite pole. microscopy (hence, the black background) of cells At end of anaphase, each cell pole has a complete set of artificially stained by fluorescent dyes: blue fluorescence chromosomes. indicates DNA (chromosomes) and green fluorescence indicates microtubules (spindle apparatus). MEIOSIS Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. It produces genetically diverse individuals, unlike asexual reproduction which produces identical clones. TELOPHASE The process involves meiosis, a cell division that halves the number of chromosomes, resulting in offspring with Chromosomes decondense into stretched-out chromatin. traits of both parents but not identical to either parent. Mitotic spindles depolymerize into tubulin monomers. Cytokines assembly for each daughter cell. Prior to meiosis, the cell's DNA is replicated, generating Nuclear envelopes form around chromosomes. chromosomes with two sister chromatids. Nucleosomes appear within nuclear area. Homologous chromosomes, or homologs, are similar in size, shape, and genetic content and contain the same genes. Sexual reproduction is the primary method of reproduction for most multicellular organisms, including almost all animals and plants. Fertilization joins two haploid gametes into a diploid zygote, the first cell of a new organism, entering G1 of the CYTOKINESIS first cell cycle. Final stage of cell division in eukaryotes and prokaryotes. Cytokinesis involves cytoplasm splitting and cell division. Process varies in plant and animal cells. Animal cells: parent cell's plasma membrane pinches inward, forming two daughter cells. Plant cells: cell plate forms along parent cell's equator, new plasma membrane and cell wall forms. Meiosis is a cell division process that reduces the number of chromosomes by half, producing haploid cells. It occurs in certain special cells of an organism, such as gamete- producing cells within the gonads. During meiosis, homologous chromosomes pair up and exchange genetic material, forming recombinant chromosomes. This process ANDREA RARANG increases genetic variation and allows offspring to have different alleles and genes than their parents. Meiosis I and II occur in four phases: prophase, metaphase, anaphase, and telophase, which are similar to mitosis, the division of the nucleus during routine cell division. Meiosis I Prophase I: Breaks down the nuclear envelope, causing chromosomes to condense. Centrioles move to opposite cell poles, forming a spindle. Homologous chromosomes pair up, unique to prophase I. Crossing-over occurs during prophase I. Summary of Difference Between Mitosis and Meiosis Metaphase I: Mitosis results in two identical nuclei to the original Spindle fibers attach to paired chromosomes, forming a nucleus. line along the cell's equator. Meiosis produces four nuclei with half the original cell's In metaphase of mitosis and meiosis II, sister chromatids chromosomes. line up along the equator. In animals, meiosis only occurs in cells that give rise to sex cells (gametes). Anaphase I: Spindle fibers shorten, and chromosomes of each pair separate. One moves toward one cell pole, the other towards the opposite pole. Telophase I and Cytokinesis: Spindle breaks down, new nuclear membranes form, cell cytoplasm divides, and two haploid daughter cells result. Meiosis II: Prophase II breaks down, spindle forms in each daughter cell, and centrioles separate. Metaphase II lines up sister chromatids along the cell's equator. Anaphase II separates sister chromatids and moves to DIFFERENCE BETWEEN MITOSIS & MEIOSIS opposite poles. ANDREA RARANG   The joining together of a sperm and egg during fertilisation returns the number of the chromosomes to 46.  Cells that undergo meiosis go through the cell cycle, including the S phase, so the process begins with chromosomes that consist of two chromatids just as in mitosis.  Meiosis consists of meiosis I and meiosis II. In meiosis I, homologous chromosomes are separated into different nuclei.  This is the reduction division; chromosome number is divided in half. Meiosis II is very similar to mitosis; chromatids are separated into separate nuclei.  As in mitosis, it is spindle fibres that “pull” the chromosomes and chromatids apart in meiosis.  The end result of meiosis is four cells, each with one complete set of chromosomes instead of two sets of chromosomes. SIMILARITIES BETWEEN MITOSIS AND MEIOSIS  Both mitosis and meiosis take place in the cell nuclei, which can be observed under a microscope.  Both mitosis and meiosis involve cell division.  Both the processes occur in the M-phase of the cell cycle. In both cycles, the typical stages are prophase, metaphase, anaphase and telophase.  In both cycles, synthesis of DNA takes place. MITOSIS OVERVIEW Conclusion  Mitosis is a continuous process of cell division which occurs in all types of living cells. The difference between Mitosis and Meiosis is quite  Mitosis involves four basic phases – prophase, apparent. They are two very different processes that have metaphase, anaphase and telophase. two different functions. Meiosis is required for genetic  Mitosis is the process where the division of cell variation and continuity of all living organisms. Mitosis, on occurs by asexual reproduction. the other hand, is focused on the growth and development of  In mitosis, the nuclear membrane is broken down, cells. Meiosis also plays an important role in the repair of spindle fibres (microtubules) attach to the genetic defects in germline cells. chromatids at the centromere and pull apart the chromatids.  When the chromatids reach separate ends of the cells, the spindle fibres disintegrate and a nuclear membrane rebuilds around the chromosomes making two nuclei.  Each nucleus is identical to the original nucleus as it was in G1 phase. MEIOSIS OVERVIEW  Meiosis is the form of nuclear cell division that results in daughter cells that have one-half the chromosome numbers as the original cell.  In organisms that are diploid, the end result is cells that are haploid. Each daughter cell gets one complete set of chromosomes, i.e., one of each homologous pair of chromosomes.  In humans, this means the chromosome number is reduced from 46 to 23.  The germ cells undergo meiosis to give rise to sperm and eggs. ANDREA RARANG