The Process of Meiosis - Essay Example

Instructions: Meiosis Meiosis also known as reduction division is a type of cell division where the cells divide toproduce cells containing half the number of chromosomes. This type of cell division constitutes a very important part of sexual reproduction and all eukaryotes reproducing sexually have meiosis as a part of cell division. However, the prokaryotes i.e. the bacteria do not reproduce by meiosis The process of meiosis is divided into two segments; Meiosis I and Meiosis II. Meiosis I is similar to mitosis and consists of Prophase I, Metaphase I, Anaphase I and Telophase I. When the first division comes to an end, there is the formation of two daughter cells and each contains 23 chromosomes and has undergone crossing over. Crossing over is the exchange of genetic material between two chromosomes. Meiosis II consists of the same stages. However the end stage produces four daughter cells each having a different genetic make-up. All four cells convert into sperms in males while in females only one matures as egg or ovum. As mentioned earlier, meiosis plays a very important role in sexual reproduction by causing genetic variations in the offspring. It reduces the number of chromosomes in the parent cells which are egg and sperm, collectively called as gametes or sex cells. Because of this reduction each cell contains only one set each. A process of independent assortment takes place where each allele gets different chromosome varying at any specific locus. Secondly, it also allows crossing-over which results in genetic mix-up between the two parental chromosomes and we see genetic variations in the offspring. Asexual and Sexual Reproduction Asexual reproduction is defined as a type of reproduction where the organism produces an exact copy of itself without any genetic variation or involvement of any other individual. Different organisms reproduce asexually in various manners. For instance bacteria reproduce by binary fission and the yeast by budding. Fragmentation, spore formation and vegetative reproduction are some other forms of asexual reproduction. Sexual reproduction, on the other hand, is a process where two parent organisms create an offspring that has a combination of genetic material from both the parents. Most of the animals and plants reproduce sexually. However there are certain organisms that reproduce both sexually and asexually like star fish, sea anemones, slime molds and aphids. Organisms that reproduce asexually can create descendents in large numbers but since the offspring have very few DNA variations they are all equally vulnerable to diseases. However sexually reproducing organisms undergo genetic variations and the species are stronger in withstanding the harsh environment. The sexual organisms also produce lesser amount of offspring. Because of this reason the organisms that reproduce through both ways, switch towards sexual reproduction under unfavorable conditions. This helps them in acquiring the genetic variations that facilitate them in adapting to the surroundings. However sexual reproduction is an energy requiring process in contrast to asexual reproduction which requires minimal amount of energy. To explain the extensive use of sexual reproduction by organisms, George C. Williams used the example of lottery tickets. Sexual reproduction is like buying few tickets of different numbers which increases the chance of winning. While asexual is like buying loads of tickets with same numbers. This theory is not considered now because of newfound evidences. History of Genetics History of genetics can be traced back to 1866 when the work of Gregor Johann Mendel on pea was published and his theory was recognized as Mendelian Inheritance. He was the first to study the genetic inheritance patterns in the peas and the fact that they followed a certain rule. After this breakthrough in the world of genetics different such theories came into sight. One considerable work was done in 1900 by Hugo de Varies, Carl Correns and Erich von Tschermak and was referred to as the rediscovery of Mendel. In 1915 another specimen was used to study the Mendelian theory which was the house fly or Drosophila melanogaster. Using this species, the geneticists made amendments and improvements in the Mendelian theory and presented it in 1925 which was widely approved and acknowledged. Once the scientists got hold of basic concepts and ideas related to patterns of genetic inheritance they started making advancements in the characteristics of gene. The existence of DNA as a composition of chromosomes and genes present on the chromosomes was demonstrated and proved through certain experiments in 1940s and 1950s. More and more organisms were studied to observe the double helical structure of DNA which was discovered in 1953, and these molecular breakthroughs gave existence to the complex molecular genetics. After discovery and detailed study of DNA, the next area of focus was the nucleic acids and proteins making up the structure of DNA. Chemists invented various techniques of sequencing and studying the genetic code which revolved around the conversion of information in DNA into proteins by the organisms. Genetic engineering was another turning point in the history of genetics which allowed the scientists to control gene expression. 20th century brought into sight some major genetic projects and breakthroughs. Mendelian Inheritance Mendelian inheritance was put forward by Gregor Mendel considered as the father of genetics in 1850s. Mendelian inheritance is defined as a group of rules that explain about the patterns of genetic inheritance. His objects of observation were pea plants where he carried out a series of experiments to observe the inheritance traits. He bred pea plants with different traits and then observed the appearance of dominant and recessive characteristics in the descendants. Scientists later found out that the rules can also be applied to humans as well. He laid the foundation of modern genetics. Mendelian inheritance follows two basic scientific rules. First is the Law of segregation. According to this law, each gamete gets only one of the factors (genes) from the pair. During meiosis, the genes in the pair split up and each of the sex cells gets one gene. These genes are the determining feature of the offspring traits and are exclusively hereditary. The second law is referred to as the Law of independent assortment. This law focuses on the independent separation of the alleles during gamete formation. For this independent assortment it is necessary for the genes to be on different chromosomes. During separation of chromosomes in gamete formation they are segregated haphazardly and hence the gamete may have chromosomal portions from any side of the parent. But during crossing-over, different parts are exchanged between pairs of chromosomes resulting in genetic variations. As a result every offspring has a different genetic make-up excluding identical twins. There are certain limitations applied to the Mendelian Laws which includes that they do not hold true for all living things. Only organisms participating in sexual reproduction and holding diploid chromosomes can be considered for these laws. Hence the laws cannot be applied to bacteria and other asexually reproducing plants and animals. Punnett Square Punnett Square was originally designed by Reginald C. Punnett. This is a diagrammatic presentation of the cross breed experiment and also shows the outcomes. The Punnett square gives all the combinations that can be made with one allele from the maternal side and one allele form the paternal side. This gives an idea that what can be the possible genotypes of all the offspring of one couple. Both monohybrid and dihybrid crosses can be presented through Punnett square. A capital letter presents the dominant allele whereas the lower-case letter denotes the recessive allele for example Yy. If both the parent organisms have the same genotype of Yy then the possible offspring genotypes will be YY, Yy and yy. A dihybrid cross is used when more than one gene patterns are to be calculated. Unlike monohybrid cross where 4 combinations are calculated, here a total of sixteen combinations are received. If the pea plant has a genotype of YyXx where Y allele represents shape and X represents color each can form four independent combinations Xy, XY, xY, xy. These four combinations are then cross-matched to find out the total sixteen genotypes. The Punnett Square gives provides for a ratio of 3:1 in monohybrid cross whereas the ratio for a dihybrid cross is 9:3:3:1. This is on the basis of two laws of segregation and independent assortment proposed by the Mendelian Inheritance. However certain divergences can be observed in some situations. For instance the alleles that are on the same chromosome or the parent chromosome do not have one pair of the gene. Incomplete dominance, co-dominance, imprinted alleles and epistasis also shows deviations in Punnett Square results from the Mendelian Inheritance. Genetic Disorder Genetic disorder is defines as a disease which occurs due to mutations in genes or chromosomes present before birth. Genetic disorders can either be heritable of sporadic. The heritable ones are passed on from the parents while the sporadic ones occur due to new mutations on the genetic make-up of the child. Some cancers like leukemia, breast cancers, and lymphomas can also be caused by genetic mutations in some people. Genetic disorders are broadly classified into single gene disorders and multifactorial or polygenic disorders. The single gene disorders, as the name indicates, are caused by mutations in a single gene. According to the type of allele and type of chromosome affected they are termed as dominant or recessive and autosomal or sex-linked genetic disorders, respectively. In the dominant disorders any one mutated allele can result into the appearance of the disease while in recessive disorders both the alleles should be mutated. In autosomal genetic diseases the somatic chromosomes are mutated and in sex-linked the sex chromosome genes have abnormalities. The sex linked disorders can either be X-linked or Y-linked. Since only the males inherit the Y chromosomes the father passes the disease to every male offspring. A female X-linked recessive mother passes the disease to 50% of her male offspring and 50% of females are carriers. While an X-linked recessive father doesn’t pass the disease to his sons, only the daughters will be carriers. X-linked dominant disorders affects the males more as they only have one X chromosome. Multifactorial or polygenic disorders are caused as a result of multiple gene mutations. Environmental factors also play a very importance role in their occurrence. These disorders are more commonly familial and are inherited. Some examples include heart disease, Diabetes, asthma, autoimmune disorders, cancers, obesity and infertility. Transformation Transformation is a process by which a living cell directly takes up genetic material from its surroundings and undergoes genetic modifications. Along with transformation, conjugation and transduction are another three similar processes. Transformation usually occurs in bacteria and such bacteria are known as competent bacteria. Transformation process can either occurs naturally or can also be induced artificially. Transformation can be successfully utilized to transfer the genetic material from bacteria to non-bacterial cells like that of animals and plants. However, “transfection” is described as insertion of DNA into eukaryotic cells. Frederick Griffith was the first bacteriologist to demonstrate transformation in Streptococcus pneumonia. With this process he showed the conversion of the non-virulent strains into virulent forms. In 1944, Avery, MacLeod and McCarty made some improvements in the experiment and explained this conversion into virulent strains due to uptake of genetic material from the heat-killed virulent strains. Until 1972 transformation was not readily accepted. But when Cohen did a successful experiment on Escherichia Coli where he transformed the bacteria with calcium chloride, transformation became a very famous procedure among bacteriologists. After bacteria, plants and animals were also tested for transformation. In 1982, the first transgenic mouse was created where growth hormone was injected into mouse embryo artificially. A very famous example of artificial competence is Agrobacterium mediated transformation. This was first discovered in 1907 this bacterium was discovered which caused tumors in plants through Ti plasmid particles. The plasmid genes were removed and replaced with newer ones and hence, the self-chosen genes were inserted into plants through the Agrobacterium. Read More