Автор: Пользователь скрыл имя, 28 Марта 2013 в 10:41, реферат
The сhromosome theory of heredity has established a leading role of nuclei and containing in it chromosomes in the phenomena of heredity. But at the same time in the early years of formation of the science of genetics was know the facts, which show that the inheritance of certain traits associated with non-chromosomal components of the cell and does not obey to Mendel laws, based on the distribution of chromosomes during meiosis.
The concept of cytoplasmic inheritance.
Forms of cytoplasmic heredity.
The plastids heredity.
Cytoplasmic male sterility.
Extranuclear inheritance of organelles.
Extranuclear inheritance of parasites.
Types of extranuclear inheritance.
Emerging Discoveries in cytoplasmic inheritance.
Сytoplasmic inheritance
Plan
The сhromosome theory of heredity has established a leading role of nuclei and containing in it chromosomes in the phenomena of heredity. But at the same time in the early years of formation of the science of genetics was know the facts, which show that the inheritance of certain traits associated with non-chromosomal components of the cell and does not obey to Mendel laws, based on the distribution of chromosomes during meiosis.
In 1908 - 1909 years. C. Correns and simultaneously E. Baur described the variegation in four-o'clock plant and snapdragon, which is inherited through the cytoplasm(Fig.1) In subsequent years similar observations have been made on other sites. All are correct interpreted as examples of cytoplasmic inheritance, but at least they have long regarded simply as individual deviations from the laws of Mendel.
Fig.1: Leaf variegation in Mirabilis Jalapa
Further study of the phenomena of heredity led to the need to establish not only the mechanism of gene transfer of chromosomes from one generation of organisms to another, but also how these genes control the processes of cell metabolism and the development of certain characters and properties. Therefore, the cell was seen as a single integrated system, which defines the transmission and reproduction traits in the offspring of resulting from the interaction nucleus components (genes of chromosomes) and the cytoplasm, which can be illustrated by acquiring of its ability to photosynthesize. Photosynthesis is associated with cytoplasmic structures of the cell – plastids and contained therein pigment chlorophyll. The formation and function of plastids are caused by hereditary factors and the influence of external conditions (mainly light, without which the chlorophyll in the plastids is not formed). Mutations in certain chromosomal loci can be partially or completely disrupt the formation of plastids and they contain chlorophyll. These so-called chlorophyll mutations are inherited, strictly obeying the laws of Mendel. But abnormal (white) plastids can form in cells with a normal set of genes, and in good light. This feature is not inherited by the rules of Mendel. During cell division, comprising said abnormal plastids formed daughter cells with the same plastids, but when crossed this trait is transmitted only through the maternal line, and, therefore, it is not associated with the chromosomes, but with the cytoplasm. Thus, the most important property of the cell - its ability to photosynthesize - is determined by the interaction of genes of chromosome structural elements of the cytoplasm and the external environment.
Genetic material of chromosomes (genome) corresponds to the plasmon, which includes all the genetic material of the cytoplasm. Like genes of chromosomes in the structural elements of the cytoplasm - plastids, Basal bodies, mitochondria, centrosomes and plasmides is material carriers of extrachromosomal heredity - plasminogen. They can determine the development of some signs of a cell, capable of doubling their reproduction, during the dividing of the mother cell they are distributed between daughter cells.
It is possible that the cytoplasmic heredity is also due to long-lived molecules mRNA and the selective transcription of mRNA molecules only from genes of maternal chromosomes. The best studied two forms of cytoplasmic inheritance: plastid and cytoplasmic male sterility.
The cytoplasm genetic continuity of organelles was first established for plastids. Many plant species are individuals deprived of color, or those in which the leaves have some unpainted areas of tissue. Their cells have no visible plastids or contain plastids not able to form chlorophyll. Plants deprived of greenness - albinos are not viable and usually die in the phase of the seedlings. But some parts of the tissue without a green color develops in the green leaf, feeding on through normal tissue, which supply their by products of photosynthesis.
In many cases, changes in the structure and function of plastids associated with mutations in one of the chromosomal gene. In maize, barley and other crops studied numerous chlorophyll mutation is inherited by the rules of Mendel. Often, however, the inheritance of such changes are not subject to the laws of Mendel, and it can only be explained on the basis of the concept of genetic continuity of plastids. Using electron-microscopic and autoradiographic methods was proved the existence of plastid DNA-containing areas. They are specific ribosome. Green plastids are able to synthesize DNA, RNA, and protein.
The plants of four-o'clock plant have a variegated variety. At one and the same plant, along with green branches are branches with leaves on which green tissue alternates with colorless stripes and spots. The flowers on the green branches of the variegated plants no matter what pollen pollinate them, provide seeds that always grow normal green plants. Seeds from the branches, the leaves which have no green color, give unpainted non-green sprouts. From seed, knot in variegated shoots, give mixed in various ratios offspring, consisting of green, variegated plants and unpainted.
A similar phenomenon was observed in variegated plants of snapdragons, pelargonium, evening primrose, plantain. These facts can be explained by assuming that the variegated plants have two types of plastids: normal and abnormal, is not able to form chlorophyll. During the multiplication of the normal are formed normal plastids, but from abnormal - abnormal (white) plastids. Of the ovule, which includes both types of plastids by mitotic divisions formed egg carrying only white or both simultaneously plastids.
One-sided, only through the maternal line, the transfer of the traits associated to the example of reciprocal crosses variegated plants and normal green plants. Variegated plant, if it is taken as the parent form, forms three types of eggs: with green, white and mixed plastids. Because of his father's sperm green leaves plant plastids do not contain such crossing will mixed offspring, in which a number of different plants will be determined by the random of the distribution of plastids during macrosporogenesis. In backcross green plant will form an egg with green plastids. Variegated plants are fertilized by sperm, they will give offspring, consisting only of plants with green leaves. Consequently, for reciprocal crosses between normal green plants or flowers with normal green variegated shoots and flowers from plants or shoots carrying abnormal plastids, plastid type and the nature of the offspring is determined by the parent form. Normal mother plant provides only normal offspring, and abnormal - just abnormal phenotype regardless of the paternal form.
Many species of plants with bisexual flowers and occasionally monoecious there are few individuals with sterile male generative organs. These facts were known to Darwin. He regarded them as a kind of tendency to move from monoecious to dioecious, which in evolutionary thought better. Thus, the formation of individuals with male sterility, is a natural phenomenon of evolution. Male sterility was first discovered by K.Correns in 1904 at the garden plants summer savory. In 1921, B.Batson found it in linen, in 1924 a geneticist D. Jones - for onions, in 1929 A.I.Kuptsov - in sunflower. Later it was found that male sterility is widely spread among flowering plants. The mutations that cause male sterility are described now in most crop plants.
Male sterility is the absence of pollen or her incapable of. Genetic male sterility genes could be due to the sterility of the nucleus and the interaction of genes and nuclear plazmogenes. According to the two types of male sterility: the nuclear, or gene, and cytoplasmic. Nuclear sterility is caused by mutations of chromosomal genes ms. Due to the fact that the genes of sterility are recessive and fertility genes are dominant, and the type of inheritance of sterility from crossing sterile plants with fertile F1 all plants are fertile (msms x MsMs Msms), and F2 is split on the fertile and sterile forms against 3:1 in the next generation, the number of sterile plants from such crosses decreases continuously. It was being developed methods using genetic techniques to produce sterile hybrids of cotton, sunflower and other crops.
To explain the causes of cytoplasmic sterility were put forward three hypotheses. One of them, known as viral, links the emergence of male sterility with a viral infection that can be transmitted through sexual reproduction through the cytoplasm of the egg. The second hypothesis considers the emergence of CMS as a result of non-compliance of the cytoplasm and nuclei of different species in the distant hybridization. Indeed, in some cases, such as when crossing wheat Triticum aestivum with Tr Aestivum, there are forms to CMS. However, many cultures found CMS is not related to a distant hybridization. Therefore, the greatest recognition, is now has a hypothesis that considers the emergence of CMS as a result of specific mutations plazmogenes.
As a result of learning and generalization of experimental data on the inheritance of male sterility rise to the notion that this property is due to the interaction of the cytoplasm of chromosomes and genes, collectively known as the genetic system. Cytoplasm, which determines pollen sterility, called Cyts (sterile cytoplasm), and the cytoplasm, giving plants with fertile pollen-Cytn (normal cytoplasm). There are localized in the chromosomes of a dominant gene RF (from the initial letters of restoring fertility-restoring fertility), which, without changing the structure and specificity of sterile cytoplasm, at the same time prevent its manifestation. Sterile cytoplasm exerts its effect only in combination with recessive alleys of this gene. Therefore, only the combination of Cyts rfrf may make development of sterile pollen. Fertile pollen is formed on the basis of normal cytoplasm combinations Cytn RfRf, Cytn Rfrf and Cytn rfrf and on the basis of sterile cytoplasm in combination Cyts RfRf and Cyts Rfrf. Thus inheritance CMS maternal possible only in crossing plants Cyts rfrf x Cytn rfrf Cyts rfrf (sterility is fixed).Crossbreeding Cyts rfrf x Cytn (s) RfRf all plants will fertile, a complete restoration of fertility.
The immediate cause of the forms with a CMS, some scientists considered a violation of protein synthesis as a result of mutations in the nucleus, leading to incorrect microsporogenesis other degeneration of pollen grains attributed to disruption in supply of sterile anthers of plants.
Of course, chloroplasts are not the only DNA-containing organelle inherited through gamete cytoplasm. Mitochondria also contain DNA, and they show similar patterns of uniparental and biparental inheritance. In fact, because most animals lack chloroplasts, the main form of cytoplasmic inheritance in animals is via mitochondrial DNA, which is known as mtDNA.
The first people to capture images of DNA “fibers” in mitochondria were Margit and Sylvan Nass in 1963. With a powerful electron microscope, they were able to zoom in on part of a mitochondrion and take a photograph at a magnification of 150,000X. In doing so, the Nasses noticed a black threaded material inside mitochondria that would disappear after exposure to an enzyme (deoxyribonuclease) that specifically dissolved DNA. This was the first visual and chemical evidence that DNA existed in mitochondria.
Since the Nass experiments, interest in mtDNA has expanded greatly. Scientists now know that within a single cell, there can be thousands of mitochondria, and each mitochondrion contains from two to ten copies of mtDNA. During cell division, the mitochondria aggregate randomly into progeny cells. This means that each cell can theoretically contain a different mixture of normal mtDNA and mutated mtDNA, which can in turn generate a variety of phenotypes (Fig.2). But are these mtDNA mutations anything more than a sideshow to the main events of nuclear function? Indeed they are. Mutations in mitochondria can have very serious effects, and they are even the basis for several diseases. Mitochondria’s crucial role as the main producer of ATP within cells means that malfunctions in these organelles are truly bad news.
Fig.2 : Cytoplasmically inherited characteristics frequently exhibit extensive phenotypic variation because cells and individual offspring contain various proportions of cytoplasmic genes. Mitochondria that have wild-type mtDNA are shown in red; those having mutant mtDNA are shown in blue.
Extranuclear inheritance of mitochondria has been tracked in families that carry defective mitochondrial genes. Some of the diseases caused by defective mitochondria particularly affect muscle tissue, as muscle uses ATP and mitochondria are cellular producers of this substance. One such condition is an inherited disorder called progressive external ophthalmoplegia (PEO). Spelbrink et al. (2001) analyzed multiple pedigrees of families affected by PEO, and they analyzed the mitochondrial genes along with the inheritance pattern of the disease. Eventually, the researchers noticed that the mtDNA of those individuals affected with PEO showed many more deleted sequences than the mtDNA of unaffected individuals. Next, the team tried to figure out what these deletions meant for mitochondrial function. After taking blood samples from 12 different families affected with PEO, they extracted mtDNA from blood lymphocytes and screened it for common patterns in affected and unaffected individuals. They found that PEO carriers had 11 different deletions in the coding sequence for a mitochondrial protein that appeared to be involved in mtDNA replication. They named this protein Twinkle, because it is distributed all over mitochondria like a constellation pattern in the night sky. The investigators concluded that Twinkle is likely a DNA helicase protein involved in maintaining the integrity of mtDNA as it replicates. Therefore, although Twinkle is encoded by nuclear DNA, it affects the transcription of mtDNA (Spelbrink et al., 2001). In other words, when it comes to PEO, both nuclear and mitochondrial DNA are involved in the inheritance of a single disease.
Extranuclear transmission of viral genomes and symbiotic bacteria is also possible. An example of viral genome transmission is perinatal transmission. This occurs from mother to fetus during the perinatal period, which begins before birth and ends about 1 month after birth. During this time viral material may be passed from mother to child in the bloodstream or breastmilk. This is of particular concern with mothers carrying HIV or Hepatitis C viruses. Examples of cytoplasmic symbiotic bacteria have also been found to be inherited in organisms such as insects and protists.
Three general types of extranuclear inheritance exist. These are vegetative segregation, uniparental inheritance and biparental inheritance.
• Vegetative segregation results from random replication and partitioning of cytoplasmic organelles. It occurs with chloroplasts and mitochondria during mitotic cell divisions and results in daughter cells that contain a random sample of the parent cell’s organelles. An example of vegetative segregation is with mitochondria of asexually replicating yeast cells.
• Uniparental inheritance occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring. A classic example of uniparental gene transmission is the maternal inheritance of human mitochondria. The mother’s mitochondria are transmitted to the offspring at fertilization via the egg. The father’s mitochondrial genes are not transmitted to the offspring via the sperm. Very rare cases which require further investigation have been reported of paternal mitochondrial inheritance in humans, in which the father’s mitochondrial genome is found in offspring. Chloroplast genes can also inherit uniparentally during sexual reproduction. They are historically thought to inherit maternally, but paternal inheritance in many species is increasingly being identified. The mechanisms of uniparental inheritance from species to species differ greatly and are quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species.
• Biparental inheritance occurs in extranuclear genes when both parents contribute organellar DNA to the offspring. It may be less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only a fraction of the time. An example of biparental mitochondrial inheritance is in the yeast, Saccharomyces cerevisiae. When two haploid cells of opposite mating type fuse they can both contribute mitochondria to the resulting diploid offspring.
At first glance, the inheritance of nonnuclear genes appears to follow a random pattern of cytoplasmic matter separation. But as we examine this mode of inheritance more closely, new patterns emerge that betray that far more complex processes affect the transfer and maintenance of the organelle genome. It has also become clear that mtDNA may have a close relationship with nuclear genes, and that the integrity of mtDNA may be related to actions coordinated by the cell nucleus. Overall, nonnuclear inheritance is characterized by random patterns of distribution in progeny that appear to follow an entirely different set of rules we are only beginning to understand. In fact, some inheritance patterns are not always due strictly to DNA, nuclear or otherwise. For example, the direction of coiling in snails is determined by a nonuniform distribution of cytoplasmic factors in the early embryo. Early cell divisions result in irregular distribution of these factors in the embryonic cells, which has been linked to the inheritance of left- or right-handed coiling.
Together, all cellular sources of DNA and the intracellular factors that are inherited from parent to offspring interact to influence the heredity of traits. The complexity of all these parts working together makes inheritance relevant even today. Mendel principles helped guide the way for understanding the basic inheritance of alleles, but the complexity of how genetic, epigenetic, and environmental factors intertwine to control distinct phenotypes continues to be explored by scientists every day.
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