Week 008 Module Perpetuation of Life PDF

MODULE OF INSTRUCTION Perpetuation of Life Plant and animal reproduction, as well as all movements and functions of living things, require energy to proceed. In the previous module, we learned a...

MODULE OF INSTRUCTION Perpetuation of Life Plant and animal reproduction, as well as all movements and functions of living things, require energy to proceed. In the previous module, we learned about bioenergetics, and we focused on how plants create photochemical energy from sunlight. We also learned extensively about the parts of the cell, the basic unit of living things, as well as their functions. In this module, we will learn about how life continues through reproduction. We will also learn about how hereditary materials are passed from parents to offspirings. Finally, we will also delve into genetic engineering in order to understand genetically modified organisms (GMOs) and their implications for our lives today. Plant Reproduction The reproduction of plants is important for the propagation of life on earth. Plants reproduce through three types: asexual, sexual, and vegetative. Asexual Reproduction In the asexual mode of reproduction, offsprings are produced from the vegetative unit produced by a parent without any fusion of sex cells or Manual Title 1 X.X Module Title gametes. In addition to this, only a single parent is involved and the offspring produced are genetically identical to the parent. There are also several types of asexual reproduction. Fission can be seen in unicellular organisms such as yeast or bacteria. The content of the parent cell divides into 2, 4, or 8 daughter cells. Accordingly, fission may be called binary (2) or multiple (4 or more). Each daughter cell that is newly formed grows into a new organism. Budding is bud-like growth formed on one side of the parent cell. As soon as the bud separates from the parent cell, it becomes a whole new organism (e.g. yeast). Fragmentation occurs in filamentous algae. It occurs as a result of accidentally breaking off a filament into many fragments. Each new fragment may give rise to a new organism through cell division (e.g. Spirogyra). Spore formation occurs in lower plants, such as pteridophytes and byrophytes. During this type of asexual reproduction, special reproductive units develop asexually on the body of the parent. These special reproductive units are called spores. These are microscopic units and are covered by protective wall. Once spores reach an 2 MODULE OF INSTRUCTION environment that is conducive to growth, they develop into new plant bodies (e.g. bread moulds, mosses, ferns). Vegetative Reproduction Vegetative reproduction involves the formation of new plants from a somatic, or vegetative cell, or buds or organs of the plant. Here, a vegeatitive part of the plant, such as the root, stem, leaf, or bud, is detached from the body of the parent and grows into a daughter plant that is independent. It is similar to asexual reproduction in that it only requires mitotic division. Thus, no gametic fusion occurs and daughter plants are exact genetic copies of their parents. Sexual Reproduction Sexual reproduction involves the fusion of female and male reproductive cells (gametes). These gametes are haploid, which means that they contain only half the genetic material (chromosomes) for a new organism to exist. The fusion of gametes is also called fertilization and it results in the production of diploid zygote. When the zygote undergoes further development, it gives rise to a new individual that is diploid. At the beginning stages of sexual reproduction, meiosis occurs. The offsprings are not genetically identical to their parents. Manual Title 3 X.X Module Title Reproduction in Lower Plants Two representative plants that are considered lower plants are Spirogyra (multicellular) and Chlamydomonas (unicellular). The unicellular algae, Chlamydomonas, is a haploid, unicellular algae that is found in freshwater ponds. The plant’s body is pear-shaped, and there are two flagella attached to the narrow end. Flagella are filaments found in flagilates. A large chloroplast is present. Towards the center of the organism, a nucleus is present. The chloroplast contains a single pyrenoid. The organism may undergo sexual or asexual reproduction. When it undergoes asexual reproduction, it is through zoospores. Consequently, its flagellae is lost and the organism becomes non- motile. The protoplasm divides mitotically and forms 4-8 zoospores. Each zoospore develops a cell wall and it also grows into an adult cell. The parent cell, however, does not exist anymore. During sexual reproduction of the Chlamydomonas, the cell again becomes non-motile by losing its flagella. The protoplasm also divides mitotically into 2,4,8,16, and 132 daughter cells. Each daughter cell then develops its own flagella and is released to the water by the rupture of the mother cell wall. Each daughter cell acts as a gamete. The gamete is morphologically identical (isogamous). Two gametes released from the mother cell fuse together. The contents of the 4 MODULE OF INSTRUCTION gametes then fuse and form a zygote (diploid). This is the only stage in the organim’s life cycle that is diploid. The zygotes then develop a thick wall around itself (zygospores). Then, the zoospore grows into a new organism. On the other hand, Spirogyra is a free-floating algae found in freshwater ponds. The body contains a row of rectangular cells that are joined end to end (filamentous alga). Each cell has a sparial ribbon- shaped chloroplast that contains many pyrenoids. The nucleus is present in the cental vacuole with support from cytoplasmic strands. It undergoes two types of reproduction: vegetative reproduction by fragmentation and sexual reproduction. Vegetative reproduction by fragmentation occurs first when filaments break into smaller fragments. Then, each fragment grows into a new organism by cell division. On the other hand, sexual reproduction occurs in the organism. Scalariform conjugation, which is when filaments conjugate to form a ladder-like appearance, start when two filaments lie very close to each other. The cells of the two filaments connect with each other through a conjugation tube. The contents of the cytoplasm of each cell rounds of to act as a separate gameter. The gamete from one cell (male) passes Manual Title 5 X.X Module Title into the conjugation tube towards the other cell (female). The contents of these two gametes fuse to form a diploid zygote. Reproduction in Angiosperms (Flowering Plants) Angiosperms may reproduce vegetatively or sexually. Sexual reproduction occurs by the fusion of male and female gametes that are present in the flower. Thus, the plant’s basic reproductive unit is the flower. Angiosperms can be classified according to the following:  Annuals: these plants live for only one year. The plants that produce seeds and flowers within just one season are termed as annuals (e.g. peas).  Biennials: plants that live for two seasons, and complete their life cycles within these two seasons. During the first year, the plant is in a vegetative state. In the second year, the plants produce flowers, fruits, or seeds and then they perish (e.g. radish).  Perennials: plants that live for several years. The vegetative state of these plants may last from one year to several years. In the year following their vegetative state, they produce flowers, seeds, or fruits (e.g. mangoes). 6 MODULE OF INSTRUCTION  Monocarpic: perennial plants that reproduce only once during their lifetime and then die (e.g. bamboo). Initiation of Flowering When the plant’s seed germinates, plantlets emerge from it. The young plant grows and continues to grow until it has a definites shape and size. The plant’s vegetative parts (root, stem, leaves) must be well-developed. This phase in the plant’s life cycle is known as the young of juvenile phase. After the plant completes vegetative growth, the plant then enters into the reproductive phase, or the adult phase. A vegetative shoot apex then tranforms into a floral apex, a reproductive part, and starts bearing flowers. The flowering stage may last from several days to several years. A juvenile shoot has a soft stem, and only bears a few leaves. The size and shape of the leaves remain the same. It does not respond to stimuli nor does it produce flowers. On the other hand, an adult shoot has well-developed stems and leaves. The size and shape of the leaves change. It also responds to stimuli and can produce flowers. Factors Affecting Flowering Manual Title 7 X.X Module Title The plant’s flowering is affected by light (photoperiodism) and by temperature (vernalisation). Vernasilation is when low temperatures occur, and this stimulates the early formation of flowers. On the other hand, photoperiodism is the response of the plant to the duration of dark and light per day. This determines its growth and flowering. The sex of a flower may be bisexual, which means that they have both carpels and stamens, or unisexual (having only a staminate or pistillate). The sexual determination of flowers may vary in dioceious species. However, sex determination may have a chromosomal basis. The plants may also exhibit different levels of substances required for growth. For instance, Cucumis, which bear male flowers, have high levels of gibberellin as compared to those that bear only female flowers. Gibberellin is a plant hormone that assists in growth and reproduction. When gibberellin is applied externally, the production of male flowers may be induced even in plants that are genetically female. Conversely, treating male plants with ethylene or auxin may induce the development of functional female flowers. The latter response has been seen in Cannabis. Parts of a Flower 8 MODULE OF INSTRUCTION A typical flower consists of four whorls which are located on a stalk (thalamus). Sepals comprise the calyx. Petals comprise the corolla. Additionally, stames comprise the androecium and pistils (gynoecium) consists of carpels. The two outer whorls are known as non-essential or accessory whorls because they do not play a part in the plant’s reproduction, although they aid indirectly. The two inner whorls, the androecium (male reproductive organ) and the gynoecium (female reproductive organ) are termed as essential whorls because they are the main components of the plant’s reproduction. Stamen, Microsporagia, and Pollen Grain The plant’s stamen consists of an anther that contains microsporagia, or four pollen sacs. These supported by a slender filament. Each sporangium contains masses of large cells. These cells show a prominent nucleus and abundant cytoplasm. These cells are also known as the sporangeous or the microspore mother cells. Each microsporangium is madeup of a distinct layers of cells when mature. The outer most layer is the epidermis. It has a middle layer of cells with thin walls. The innermost layer is the tapetum, which consists of large cells. The tapetum nourishes the developing grains of pollen. Manual Title 9 X.X Module Title Microspore mother cells undergo meiosis. Each mother cell produces four haploid microspores (diploid pollen grains) that are arranged in a tetrad. The Development of the Male Gametophyte The wall of the microspore consists of two principal layers. The outer layer is the exine and thin spaces (germ pores). The exine is made up of a durable substance called sporopollenin. The pollen tube grows out of the pollen grain through the germ pores. The inner layer is the cellulosic wall (the intine). The microspore moves towards the periphery. The cell then divides into a small generative cell and a large vegetative cell. At this stage, the pollens are released by the rupture of the stodium dehiscence of the anther. The pollen grain itself is not a male gamete. Rather, it produces the male gamete and is therefore a male gametophyte. The Development of the Female Gametophyte The main part of the ovule is bounded by two coverings (integuments). These integuments leave behind a small aperture, or opening. The ovule is attached to the ovary via a stalk, known as the furniclus. The basal part of this structure is the chalaza. 10 MODULE OF INSTRUCTION The female gamete’s gynoecium (pistil) represents its reproductive part. Each pistil is composed of a stigma, ovary, and style. The ovary contains one or more ovules (megasporangia), which act as future seeds. An ovule develops as a type of projection from the placenta in the ovary. It consists of integuments and nuclei. As the ovule grows, it becomes raised on the stalk, termed as furniculus. This is attached to the placenta on the other end. Within the nucleus, a single hypodermal cell becomes larger and it becomes the megaspore mother cell. This cell undergoes meiotic division, and then gives rise to four haploid megaspore cells. Usually, three of the megaspores degenerate, while one remains as the functional megaspore. Thus, 8 nuclei are formed as a result of this division. The enlarged structure, shaped like an oval and with 8 nuclei, is known as the embryo sac. The nuclei then migrate and form three groups. Cell membranes and nuclei develop around the nuclei, except the two at the center of the sac, which is now termed as the central cell. Vegetative Reproduction in Angiosperms The natural method of the vegetative reproduction of angiosperms starts with the underground modification of stems, such as ing ginger, potato, onion, and corn. These are provided with buds Manual Title 11 X.X Module Title which develop into a new plant and are therefore used to carry out vegetative propagation of the plant in the filed. Plants with subaerial modification, such as chrysanthemum and pistia, are also used for vegetative propagation. Artificial methods of vegetative reproduction include the use of cuttings, layering, and aerial layering. Animal Reproduction Animal reproduction is the process by which animals propagate on earth and it is also the process through which genetic materials are transferred to offspring. Animals, like plants, may reproduce through asexual or sexual means. Asexual reproduction is primarily employed by turnicates, protists, and cnidaria. However, it may also occur in the more complex animal species. Indeed, the formation of identical twins by the separation of two identicall cells in the early embryo is a form of asexual reproduction. Through mitosis, genetically identical cells are produced from one parent cell. This permits asexual reproduction to occur in protists by the organism’s division, called fission. Cnidaria commonly reproduce by budding, which is when a part of the parent’s body is separated from the rest and differentiates into a new organism. The new organism may become independent, or it may remain attached to the parent organism, forming a colony. 12 MODULE OF INSTRUCTION Sexual reproduction occurs when a new individual is formed from the union of two sex cells, or gametes. Gamates include the sperm and the egg. The union of these two produces a fertilized egg, or zygot. Through mitotic division, the zygote develops into a new organism. The zygote and the cells that it forms are diploid. This means that they contain both members of each pair of homologous chromosoms. The gametes are formed in the sex organs, or gonads (the testes and the ovaries), and are haploid. The process of sperm formation (spermatogenesis) and egg formation (oogenesis) are also included in the study of the reproduction of animals. Different Approaches to Sex Virgin birth, or parthenogenesis, is common in many species of arthropods. Some species are exclusively parthenogenic (all female), while others switch between generation. Another variation in the reproductive strategies used by animals is hermaphroditism. This is the case when one individual has both testes and ovaries. Tapeworms are hermaphroditic, and it is able to fertilize itself. However, most hermaphroditic animals require another organism to reproduce, such as in the case of two earthworms. There are also some deep sea fish which are hermaphrodites, meaning that they are both male and female at the same time. Numerous species of fish can change their sex, a Manual Title 13 X.X Module Title process which is called sequential hermaphroditism. The change from female to male is protogyny, while the change from male to female is protandry. Sex Determination In fish, there are conditions which cause changes in sex. In mammals, however, sex is already determined early in embryonic development. The reproductive systems of both males and females (humans) are identical during the first 40 days of embryonic development. During this time, the cells that will give rise to either ova or sperm move from the yolk sac to the embryonic gonads. These gonads can become testes in males and ovaries in females. For this reason, embryonic gonads are said to be indifferent. If the embryo is a male, it will posses a Y chromosome. If the embryo is a female, it will have no Y chromosomes. Recent evidence suggests that the sex-determining gene (SRY) appears to have been highly conserved during the evolution of vertebrate groups. Once the testes are formed in the embryo, they secrete testosterone and other hormones that will promote the development of the external genitalia of the male, as well as accessory reproductive organs. In other words, all embryos are females until they are masculanized by testosterone. Fertilization and Development 14 MODULE OF INSTRUCTION There are two types of fertilization: internal and external. External fertilization commonly occurs among organisms in the ocean, where water allows for the rapid dispersion of sperm or ova towards others of the same species. On the other hand, internal fertilization is common in terrestrial animals. Internal fertilization is the introduction of the male gamete into the female’s reproductive tract. Vertebrates that practice internal fertilization have three strategies:  Oviparity, which is found in some amphibians, fish, and some reptiles, is when the eggs are deposited outside the mother’s body after fertilization.  Ovoviviparity is commonly found in mollies, guppies, and mosquito fish. The fertilized eggs are retained within the mother in order to complete their development. The embryos still take all of their nourishment from the egg yolk. The young are thus fully developed when they hatch.  Viviparity is found in almost all mammals. The young develop within the mother and takes its nourishment directly from their mother’s blood, as opposed to egg yolks. Reproduction in Fish and Amphibians Manual Title 15 X.X Module Title In most species of bony fish (teleosts), the fertilization of eggs occurs externally. The eggs contain only enough yolk to sustain the developing embryo until it is ready to tach. The development of fish is rapid, and the young are able to find their own food source from a very young age. Although thousands of eggs are fertilized during a mating period, most of the eggs perish. In most cartilaginous fish, however, most fertilization is internal. The male introduces sperm into the female by means of a modified pelvic fin. In these vertebrates, the development of the young is viviparous. Amphibians use external fertilization in most cases. In these organisms, gametes from the males and females are released through the cloaca. Most amphibian eggs develop in the water. The time required for amphibians to develop is much longer than fish. However, amphibian eggs do not have a lot of yolk. Instead, the process of amphibian development is divided into embryonic, larval, and adult stages. Reproduction in Reptiles and Birds Most reptiles and birds are oviparous. That is, after their eggs have been fertilized, they are deposited outside of the mother’s body in order to complete their development. As with most animals that 16 MODULE OF INSTRUCTION fertilize internally, male reptiles have a penis that they use to introduce male gametes into the female’s reproductive tract. The shells of reptile eggs are leathery, and this allows for better withdstanding of environmental conditions. All birds practice internal fertilization, although most birds lack a penis. In some of the larger birds (e.g. ostriches, geese, and swans), the male cloaca can extend to form a false penis. As the eggs passes through the oviduct, the glands secrete the egg whites and the hard shells that distinguish bird eggs from reptilian eggs. Most birds are also homeotherms, meaning that they keep a stable body temperature. Thus, they often incubate their eggs after laying them to keep them warm. The young that emerges from bird eggs do not develop rapidly, and they need to be assisted and fed by their parents until they are ready to be independent. Bird and reptile eggs show the stark evidence for adaptation to land. These eggs are termed as amniotic eggs because the embryo that develops within the cavity filled with fluid is surrounded by a membrane called an amnion. The amnion is an extra-embryonic membrane and develop outside of the body of the embryo. Other extra-embyronic membranes include the chorion, the yolk sac, and the allantois. Manual Title 17 X.X Module Title Reproduction in Mammals The reproductive cycles of mammals differ greatly. Some are seasonal breeders that reproduce only once a year. Other have shorter reproductive cycles. Among those that have short reproductive cycles, females usually undergo the reproductive cycle, while males are more constant in their reproductive activity. Ovulation in females is the cyclic release of an egg from the ovary. Most mammals are fertile only at the time of ovulation. The period of sexual receptivity is called estrus, and the reproductive cycle is therefore called an estrous cycle. The estrous cycle of most mammals change according to the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These are secreted by the anterior pituitary gland and cause changes in egg cell development and hormone secretion in the ovaries. Like other mammals, humans and apes have an estrous cycle. However, unlike other mammals, humans and apes can mate anytime during their reproductive cycle. The most primitive mammals, the monotremes, are oviparous. The marsupials (e.g. kangaroos) give birth to offspring that are already completely developed. The placental mammals retain their young for a much longer period within the mother’s uterus. The fetuses 18 MODULE OF INSTRUCTION are nourished by the placenta, which is derived from the chorion and the uterine lining of the mother. The fetus derives its nutrients from the mother’s blood, since fetal and maternal blood vessels are in close proximity. Overview of Genetics The most fundamental characteristic of all living things is the ability to reproduce. All organisms gain their genetic material from their parents. Genetic information determines their structures and functions by directly influencing the synthesis of proteins. Genes and Chromosomes Gregor Mendel deduced the classical principles of genetics in 1865. He based his deductions on the results of breeding experiments with peas. Characteristics of the peas, such as seed color, could be predicted by Mendel through the determination of a pair of inherited factors. These inherited factors are now called genes. One gene copy, which ci termed as an allele, specifies a certain trait that is inherited from each parent. A gene is said to be dominant if it contains alleles for two colors, and only one color shows. For instance, breeding yellow and green peas yields yellow peas. In this case, the yellow is said to be dominant gene while green is said to be recessive. If Y designates Manual Title 19 X.X Module Title yellow and y designates green, then the genetic composition (genotype) of the peas is Yy, and their physical appearance (phenotype) is yellow. Mendelian genetics is the term for the deductions of Mendel. Shortly after, the role of chromosomes as carriers of genes was proposed. It was also realized that higher animals and plants have diploid cells, which contain two copies of each chromosome. Cell division in the form of meiosis involves the daughter cell inheriting only one member of each chromosome pair. Consequently, the sperm and egg are haploid cells at fertilization, and this creates diploid organisms. 20 MODULE OF INSTRUCTION Experiments on the fruit fly, Drosophila melanogaster, established most of the principles of genetics today. The fundamentals of genetic linkage, mutation, and the relationships between chromosomes and genes were elucidated. Genetic alterations were observed in Drosophila in the 1900s. These involved readily observable traits, such as eye color and wing shape. This experiment showed that there are traits which are inherited in pairs, which are said to be linked genes. Chromosomes exchange materials during meiosis, leading to the linked genes’ recombination. The frequency of recombination between two linked genes depends on their distance from each other on the chromosome. Thus, the frequency with which different genese recombine can be used for mapping their positions on chromosomes, which is known as genetic mapping. Manual Title 21 X.X Module Title Genes and Enzymes The first evidence for the existence of enzymes came in 1909, through the study of the disease called phenylketoneuria. The disease results from a genetic defect that results in problems with the metabolism of phenylalanine, an amino acid. This defect was hypothesized to result from a lack of enzymes needed to catalyze the metabolic reaction. Subsequently, this led to the suggestion that genes also specify the synthesis of enzymes. Understanding the chromosomal basis of heredity and the relationship between enzymes and genes did not itself provide a molecular 22 MODULE OF INSTRUCTION explanation for the gene. Chromosomes, aside from containing DNA, also contain proteins. The structure of DNA is three-dimensional. We owe our understanding of this structure to James Watson and Francis Crick, who formed the basis for present-day molecular biology. DNA is a polymer composed of four nucleic acid bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The former two are purines, while the latter two are pyrimidines. These bases are linked to phosphorylated sugars. The central model of the DNA is that it is double-helix with a sugar- phosphate backbone on the outside of the molecule. On the inside, bases are held together by hydrogen bonds that are formed between purines and pyrimidines on opposite chains. The amount of adenine is always equal to the amount of thymine, and the amount of guanine to that of cytosine. Due to this specific base pairing, two strands of DNA are complementary: each strand contains the bases that are required to specify the sequence of the other strand. Replication of DNA The discovery of complementary base pairing between DNA strands suggest that there is a molecular solution to the problem of how genetic material directs its own replication. Two strands of DNA can separate to serve as templates for a new strand. This would be Manual Title 23 X.X Module Title specified by base pairing. This process is called semiconservative replication, because one strand is conserved in the progeny DNA molecule. The enzyme that catalyzes DNA replication is DNA polymerase. The replication of DNA can either be bidirectional, going both forwards and backwards, or unidirectional, going only one direction. DNA polymerase adds nucleotides to the DNA chain in a specific direction, which is from 5’ to 3’. DNA Transcription and Translation Protein synthesis is directed by genes. When genes are defective, they produce defective proteins and this results in abnormalities such as albinism. There are two basic steps to the synthesis of protein. The first is the transcription of genes, which produces a messenger RNA (mRNA) molecule. The second step to protein synthesis is translation. 24 MODULE OF INSTRUCTION This is the portion of protein synthesis in which the mRNA molecule is translated into proteins. During transcription, the sequence of nucleotides in a gene in the DNA is copied to the corresponding sequence of nucleotides in mRNA. During translation, the sequence of nucleotides in the mRNA determines the sequence of amino acids in the proteins. DNA transcription is mediated by RNA polymerase. It separates the two strands of the double helix and constructs an mRNA molecule by adding nucleotides one at a time. The base-pairing rule summarizes which nucleotides pair with each other. Guanine pairs with cytosine, while adenine pairs with thymine. DNA translation determines the sequence of amino acids in the protein. The cell uses transfer RNA (tRNA) the bring the correct amino acid for each codon in the mRNA. Each tRNA has three nucleotides that form an anti-codon. The three nucleotides in the anti- codon are complementary to the three nucleotides in the mRNA codon for a specific amino acid. Amino acids are the building blocks of proteins. Manual Title 25