Internal features
Membrane-bound Several membrane-bound organelles
organelles absent present, including mitochondria,
Golgi body, endoplasmic reticulum
and chloroplasts (in plants & algae)
Ribosomes smaller (70S), Ribosomes larger (80S), free in
free in cytoplasm cytoplasm or attached to membranes
Respiratory enzymes Respiratory enzymes
bound to plasma membrane located in mitochondria
Cell wall
Usually based on When present, based on
peptidoglycan (not Archaea) cellulose or chitin
External features
Cilia (реснички) absent Cilia may be present
Flagella (жгутики), if present, Flagella, if present, have complex
composed of flagellin. (9 + 2)structure. Provide ‘whiplash’
Provide rotating motility motility
Pili may be present Pili (пили, фимбрии или ворсинки) absent
Outside layer (slime layer, Pellicle present in some types
capsule,glycocalyx) present (пелликула, слой под плазмалеммой у простейших)
in some types
Lecture 1.4 Сytoplasmic inheritance
Intracellular mechanisms of multicellular organisms’ genetic processes research. Nucleus and cytoplasm role in heredity relationship
Plan
1. Cytoplasmic heredity research
2. Cytoplasmic carriers of genetic information
3. Plastid’s heredity. Cytoplasmic male sterility
4. Chromosomal genes and plasmogenes role in organism’s characteristics formation
5. Epigenetic inheritance
1. Cytoplasmic heredity research
There are many indications that cytoplasmic elements do play important roles in cellular development and heredity.
Elements which have all properties of genes exist in the cytoplasm. Cytoplasm contain an auxiliary set of genes which are separated from the main nuclear hereditary material, they are possibly relics of some ancient system of cellular organization which has been almost abandoned (ликвидировать) in the course of evolution.
Already in the first years of genetics as a science formation the facts were known, which show that certain traits inheritance is associated with non-chromosomal cell components and does not obey the laws of Mendel. In 1908–1909 K. Correns and E. Baur independently described variegation (variegated leaves) in four o’clock plants and snapdragons, which is inherited through the cytoplasm.
In 1937, French geneticist Lerite discovered mutation of flies Drosophila melanogaster, causing their death in an atmosphere of carbon dioxide - CO2. Mutation was not inherited by Mendel, but by peculiar way: through the cytoplasm and not only through the maternal line. In a small percentage of cases there was a regular transfer of sensitivity to CO2 through males. Sensitivity can be transmitted by injection of hemolymph (it is a fluid in the circulatory system of arthropods, analogous to the blood in vertebrates). In these cases, a sign inheritance was unstable. Sensitivity to CO2 was associated with reproduction in flies’ cells of RNA -containing virus. Oogonia of females contain 10-40 viral particles and mature oocytes – up to 10 million particles. Virus «sigma» is typical facultative element, and mutation of viral and flies’ genomes lead to complex behaviors of this system. Increased temperature blocks the replication of virus particles. If females and males during gametogenesis will be for a few days at a temperature of 30 °C, then the offspring of such heat-treated flies will be free from virus and resistant to CO2.
If Euglena (lat. Euglena viridis) loses plastids it cannot restore them, despite the preserved nucleus. Most species of Euglena have about 100 photosynthesizing chloroplasts within the cell, which enable them to feed by autotrophy, like plants. In the darkness they can also take nutriments heterotrophically, like animals, plastids do not multiplied, although Euglena cells continue division. In a few generations individuals without chloroplasts appear, their offspring also haven’t chloroplasts.
There are plasmids in bacteria cytoplasm – circular DNA fragments, which are located separately from the main DNA molecule of bacterial cell. In some eukaryotic cells, such as yeast, additional DNA molecules are also found, which can ensure the stability of yeast against toxic substances.
These are examples of cytoplasmic inheritance, which for a long time have been considered as some departure from the laws of Mendel.
A characteristic feature of the cytoplasmic heredity is inheritance from a mother. At gametes fusion only the sperm nucleus with genetic material gets into the egg. Zygote receives cytoplasm and its contents only from the mother.
2. Cytoplasmic carriers of genetic information
Genetic material of chromosomes (genome) corresponds to the plasmon, which includes all cytoplasmic genetic material. Plasmogenes or extra chromosomal heredity material carriers are located in cytoplasm compartments. They determine the development of some cell signs, capable of doubling, are distributed between daughter cells during mother cell division.
Types of non-nuclear cell hereditary constituents:
plasmids
mitochondria
plastids
centrosomes or cellular center
small cytoplasmic particles such as ribosomes
surface structures, including basal bodies (kinetosomes).
Centrosome (Greek. σῶμα – body) or cell center is non-membrane organelle that serves as the main microtubule organizing center (MTOC) of eukaryotic cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883 and was described and named in 1888 by Theodor Boveri (“special organ of cell division”). Centrosome in many living organisms (animals and some protozoa) contains a pair of centrioles, cylindrical structures at right angles to each other. Each centriole (length 0.2-0.8 microns) is formed by nine triplets of microtubules arranged in a circle.
In the interphase of the cell cycle centrosomes are associated with the nuclear membrane. In prophase of mitosis the nuclear membrane breaks down, centrosome divides. Centrioles apart to the poles, defining the axis of cell division spindle. Microtubules growing from child centrosomes, fastened the other end to the kinetochores at chromosome centromeres, forming the division spindle.
In addition to participation in nucleus division, centrosome plays an important role in the formation of flagella and cilia (жгутиков и ресничек). Centrioles are located in centrosome, there are an organizing centers for flagella microtubules.
A basal body is also called a basal granule or kinetosome (Greek. kinetós – moving and soma – body). This is an organelle originated from a centriole. They are structurally the same, each containing 9 microtubule triplet of helicoidal configuration forming a hollow cylinder coated by membrane. In addition to proteins, carbohydrates and lipids, kinetosome contains DNA and RNA and is capable of self-protein biosynthesis and self-reproduction.
Basal body (kinetosome) is specifically the bases for cilia and flagella that extend out of the cell, deep in the thick of ectoplasm. Fibrils are a direct continuation of flagella peripheral fibrils. Flagella are basically attached to the cell membrane from a basal granule.
It is possible that the cytoplasmic heredity is also based on long-lived mRNA molecules or on selective RNA transcription from the maternal chromosome genes only.
Two forms of cytoplasmic inheritance are the best studied: plastid inheritance and cytoplasmic male sterility.
Plastids are major organelles found in eukaryotic cells of plants, algae and some unicellular organisms. They are the site of manufacture and storage of important chemical compounds used by the cell. Plastids often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's color. They are coated by two membranes and possess a double-stranded DNA molecule, which is circular, like that of prokaryotes.
In plants, plastids may differentiate into several forms, depending upon which function they play in the cell. Undifferentiated plastids (proplastids) may develop into any of the following variants:
Chloroplasts green plastids: for photosynthesis;
Chromoplasts/coloured plastids: for pigment synthesis and storage
Gerontoplasts: control the dismantling of the photosynthetic apparatus during senescence
Leucoplasts/colourless plastids: for monoterpene synthesis; leucoplasts sometimes differentiate into more specialized plastids:
Amyloplasts: for starch storage and detecting gravity
Elaioplasts: for storing fats
Proteinoplast/aleuronoplasts: for storing and modifying proteins
A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and proteins. The two membranes have different properties. Because of this double-membraned organization, mitochondrion has five distinct parts:
the outer mitochondrial membrane,
the intermembrane space (the space between the outer and inner membranes),
the inner mitochondrial membrane,
the cristae space (formed by infoldings of the inner membrane), and
the matrix (space within the inner membrane).
The human mitochondrial genome is a circular DNA molecule of about 16 kilobases. It encodes 37 genes: 13 for subunits of respiratory complexes I, III, IV and V, 22 for mitochondrial tRNA (for the 20 standard amino acids, plus an extra gene for leucine and serine), and 2 for rRNA. One mitochondrion can contain two to ten copies of its DNA.
As in prokaryotes, there is a very high proportion of coding DNA and an absence of repeats. Mitochondrial genes are transcribed as multigenic transcripts, which are cleaved and polyadenylated to yield mature mRNAs.
Not all proteins necessary for mitochondrial function are encoded by the mitochondrial genome; most are coded by genes in the cell nucleus and the corresponding proteins are imported into the mitochondrion.
The exact number of genes encoded by the nucleus and the mitochondrial genome differs between species. In general, mitochondrial genomes are circular. it lacks introns, as is the case in the human mitochondrial genome; however, introns have been observed in some eukaryotic mitochondrial DNA, such as that of yeast and protists, including Dictyostelium discoideum
Diagram of a mitochondrion Two mitochondria from mammalian
lung tissue displaying their matrix
and membranes as shown by
electron microscopy
3. Plastid’s heredity. Cytoplasmic male sterility
In many cases, changes in plastids structure and function are associated with mutations in chromosomal genes. In maize, barley and other crops numerous chlorophyll mutation are studied. They inherit by the rules of Mendel. Often, however, the inheritance of such changes is not subject to the laws of the Mendel, and it may only be explained on the basis of cytoplasmic inheritance.
Four o’clock plants (Mirábilis jalápa) may have variegated leaves. On one plant there are as completely green branches as branches with leaves with green parts and parts with clear stripes and spots. The flowers on the green branches of the variegated plants, no matter what pollen they are pollinated, provide seeds that always grow as normal green plants. Seeds from the branches without green color give colorless sprouts without chlorophyll. Seeds from the variegated shoots give offspring mixed in various ratios, consisting of green, variegated and colorless plants. A similar phenomenon was observed in variegated plants of snapdragons (Antirrhínum), pelargonium (Pelargonium), plantain (Plantágo, подорожник), evening primrose (Oenothera biennis, энотера, ослинник, примула вечерняя).
These facts can be explained by assuming that the variegated plants have two types of plastids: normal with chlorophyll and abnormal, those are not able to form chlorophyll. During the multiplication of the normal plastids normal offspring forms, but from abnormal – abnormal (white) plastids. Variegated mother plant produces three types of eggs: with green, colorless and mixed plastids. Since sperm cells do not contain plastids, the number of different progeny plants is determined by random plastids distribution during macrosporogenesis. After green maternal plant pollination only normal offspring with green leaves appears, after abnormal/colorless – just abnormal phenotype, regardless of the paternal form.
Cytoplasmic male sterility is total or partial male sterility associated with plant biology as the result of specific nuclear and mitochondrial interactions. Male sterility is the failure of plants to produce functional anthers (пыльники), pollen, or male gametes, the cause of which is the presence of a particular mutation in mitohondrione, i.e. in mitochondria genome. Plant fertility is restored completely or partially in the presence of a dominant allele of the nuclear fertility restorer gene. First this phenomenon is described by Marcus M. Rhodes in maize, now it is described in petunias, cabbage, sunflower and other plants. Cytoplasmic male sterility is characterized by so-called maternal inheritance.
4. Chromosomal genes and plasmogenes role in organism’s characteristics formation
Cell is a single integrated system, which determines traits transmission and reproduction in the offspring as the result of nuclear components (genes of chromosomes) interaction with the cytoplasm.
Most of the mitochondrial proteins encoded by the nuclear genes inherit according Mendel rules, other proteins encoded by mitochondrial DNA, which are transmitted only through the maternal line. Genes of cellular respiration enzymes found in mitochondria, as well as genes of resistance to several adverse effects.
Photosynthesis is associated with cytoplasmic structures of the cell –plastids and contained therein pigment chlorophyll. Plastids formation and function are caused by hereditary factors and by influence of external conditions (mainly light, in darkness the chlorophyll in the plastids is not form). During division of the cell containing abnormal plastids daughter cells form with the same plastids. This sign is transmitted only through the maternal line, and, therefore, it is not associated with the chromosomes, but with cytoplasm. Thus, the most important property of the cell – its ability to photosynthesis – is determined by the interaction of chromosomal genes, cytoplasmic structural elements, and the external environment.
5. Epigenetic inheritance
Epigenetics is the study of epigenetic inheritance patterns – the inheritance of the active or inactive state of individual genes (or gene groups) without a change or loss of the encoded in them genetic information.
Epigenetic changes are stored in a number of cell divisions, and can be transmitted to the next generation during meiosis. However, they do not cause any change in the DNA sequence and lead to differential gene expression.
The phenotype of the cell or organism as a whole depends on which genes are transcribed; inheritance of transcriptional genes status can lead to epigenetic effects. There are several levels of gene expression regulation, the first is remodeling of chromatin – DNA and associated proteins – histones – complex. Chromatin remodeling may be initiated by histone amino acids post-translational modification, for example, phosphorylation, ubiquitination, acetylation, deacetylation, methylation, sumoylation, as well as chemical modification of nitrogenous bases, for example, methylation of cytosine in DNA. In most cases, histone methylation or deacetylation leads to the suppression of gene activity, especially the methylation of its promoter regions, and demethylation – to it activation.
Сумоилирование (sumoylation) [англ. s(mall) u(biquitin-related) mo(difier) – малый убиквитин-родственный модификатор] – посттрансляционная модификация белка, заключающаяся в ковалентном связывании эпсилон-аминогруппы лизина, расположенной на С-конце полипептида, с белком SUMO, представляющим из себя компонент убиквитиновой системы.
Heritable information transfer or change during meiosis or mitosis, is not based on the DNA sequence can also occur with the prion proteins, and proteins.
Prions (from the English Proteinaceous infectious particles – infectious protein particles) – a new class of infectious agents, pure protein containing, no nucleic acids, causing severe diseases of central nervous system in humans and some higher animals (so-called "slow infection").
Prion protein with anomalous three-dimensional structure can directly catalyze the structural transformation of the normal cellular homologous protein to a similar (prion) protein, connecting the target protein and changing its conformation. Typically, prion protein state is characterized by the convertion of protein α-helix into β-layer. Prions are the only known infectious agents that multiple without nucleic acids participation. They carry out the only known way of information transfer from protein to protein.
Same Genes but a Different Kink in the Tail
Geneticist Emma Whitelaw (University of Sydney, Australia) studies phenomena in mice similar to those seen in human monozygotic twins, whereby genetically identical individuals can look or behave very differently. In a litter of mice that had all stably inherited a transgene at a specific locus, some mice expressed the transgene, but others didn't. This variable gene expressivity in genetically identical animals suggested that some kind of epigenetic mark had occurred differentially between individuals by chance.
For all these mice, epigenetic marks responsible for variable expressivity are inherited between generations. There are mice with kinky tails; the degree of kinkiness varies among genetically identical littermates.
Lecture 1.5
Cell cycle and cell division. Types of sexual reproduction
Plan
Chromosomes morphology
Cell cycle and cell division
Mitosis
Meiosis
Comparison of mitosis and meiosis
Types of sexual reproduction
Apoptosis
1. Chromosomes morphology
A chromosome is an organized structure of DNA and protein that is found in cells. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. DNA contains many genes, regulatory elements and other nucleotide sequences. The main functions are storage and transfer of genetic information.
Chromosomes are not visible in the cell’s nucleus–not even under a microscope – when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. During mitotic cell division, when there is condensation of chromatin, chromosomes are visualized well, so the morphology of eukaryotic chromosomes is studied at metaphase of mitosis. Chromosomes can be in two structural and functional states: in the condensed (spiral) and de-condensed (de-spiral). In interphase chromosomes are partially or fully de-condensed (in this state synthetic processes are active); only chromatin granules can be observed. Chromatin is chromosomal material formed by DNA double helix and associated with histone and non-histone proteins. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.
Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.
Centromere is responsible for contact with the spindle microtubules and for chromosome movement during cell division. In the centromeric regions of chromosomes satellite DNA is localized, which is represented by 171 base pairs (bp) that are tandem repeated. Strands include up to 3 million bp. Kinetochore is protein structure on the centromere of eukaryotic chromosome to which the spindle fibers attach during cell division. Internal region of kinetochore tightly associated with centromeric DNA and external region interact with microtubule spindle.
At the beginning of mitosis rapidly growing and immediately disintegrating microtubules actively "groping" cytoplasm in search of the kinetochore. Sheaf of microtubules attached to a single kinetochore is called K-bundles. With the progress of evolution and complexity of organisms, the number of microtubule-curing unit for kinetochore apparently increased: in yeast species Saccharomyces cerevisiae one microtubule is attached to one kinetochore, in Schizosaccharomyces pombe – from two to four, in flies Drosophila melanogaster – from four to six, in mammals – 20 or more.
Fig. Fluorescent electron micrograph of the cell division with mitotic spindle
Telomeres are the distal parts of linear chromosomes, which are composed of DNA sequences that are repeated. In humans, telomeres of chromosome are formed by thousands of TTAGGG repeated sequences (more than 3–20 kb). These chromosome areas have the same properties as heterochromatin, namely the presence of highly repeated sequences, the ability to form associates, inactivation of genes in this zone.
Telomeres play an important role in maintaining the stability of chromosomes. Back in the 30th of the XX century Barbara McClintock demonstrated that telomeres protect chromosomes from degradation, preventing telomere-telomere adhesion of chromosome and dicentrics formation.
Specific telomere elements required for sister chromatids division in mitosis and start of DNA replication. It is believed that in certain shortening of telomeres the cell cycle stops and cell aging.
Classification of chromosome on the basis of their relative length and centromere placement is offered by Patau. According to it distinguish 3 groups of chromosomes: metacentric (centromere is located in the center of the chromosome) submetacentric (centromere slightly shifted towards the end of the chromosome) and acrocentric (centromere is located in the distal part of the chromosome).
acrocentric metacentric sub metacentric
Fig. Schematic representation of metacentric, sub metacentric and acrocentric chromosomes
The satellites of human acrocentric chromosomes contain nucleolar organizer regions. In these regions, a large number of ribosomal RNA (rRNA) is located. rRNA genes often form tandem pairs that are organized in clusters. Each cluster corresponds to a nucleolar organizer, there are about 10 nucleolar organizer in humans.
Centromeres, telomeres and nucleolar organizer are functionally different parts of chromosome.
2. Cell cycle and cell division
The cell cycle is a period of the cell existence from the time it is generated by the division of a progenitor cell to the time it divides or dies is called the cell division cycle or just the cell cycle.
T
he
prokaryotic
growth
cycle is a relatively simple process. Prokaryotic organisms replicate
genetic material, grow until reaching a critical size for some time
and then divide, only to repeat the process over and over again. The
frequency of replication cycles is determined by the rate of cell
growth. Completion of the replication cycle is coordinated with
division of the cell into two, it is called binary fission, during
which daughter DNA molecules are separated; this is ending of cell
cycle.
Throughout the cell cycle bacterial chromosome never changes its length and thickness by spiralization, and microtubule system does not involved in daughter chromosomes separation (microtubules are not found in prokaryotes).
The rate of growth and division in bacterial cells is greatly dependent on environmental factors, such as nutrient availability and temperature. Under optimal environmental conditions, bacterial populations can grow exponentially in size at very rapid rates, increasing from several individuals to several million (106) or billion (109) individuals in hours or days. Duplication rate for E. coli vary from 18 to 180 minutes.
The eukaryotic cell cycle
The duration of the cell cycle varies from 2 to 3 h in a single-celled organism like yeast Saccharomyces cerevisiae to around 24 h in a human cell grown in a culture dish. During this period the cell doubles in mass, duplicates its genome and organelles, and divides these between two new progeny cells.
Interphase occupies about 90 % of the cell cycle and is a period of synthesis and growth, during which the cell doubles in mass but without displaying obvious morphological changes. The process of DNA replication takes place in the synthetic, or S, phase, of the cell cycle. The chromosomes are actually separated during mitosis or M phase. The S and M phases are separated by two gap phases, or G-phases, G1 and G2, in which normal processes of cellular activity and metabolism is. Together, G1, S and G2 constitute interphase. Once interphase is complete, the cell enters mitosis (M phases), which is a brief period of profound structural changes. The focal point of mitosis is the behavior of the chromosomes.
Fig. The eukaryotic cell cycle
3. Mitosis
Mitosis itself is only one of several phases of the eukaryotic cell cycle. Mitosis is designed to produce two progeny cells each containing an identical set of the progenitor cell’s chromosomes. Mitosis is divided into five stages, each of which is characterized by changes in the appearance of the chromosomes and their organization with respect to a cellular structure, called the mitotic spindle, which is responsible for their segregation. The stages of mitosis are shown diagrammatically in Figure. One pair of chromosomes is shown, one chromosome originating from the father and one from the mother.
Fig. Stages of mitosis
Prophase. The first evidence of mitosis in most cells is the compaction of the threads of chromatin that existed through interphase into compact chromosomes that are visible in the light microscope. As the chromosomes compact, each can be seen to be paired structures composed of two chromatids. This is the visible effect of the DNA molecules having been replicated in interphase. Chromosome condensation reduces the chance of long DNA molecules becoming tangled (запутанный) and broken. Each chromosome has a constriction called the kinetochore, a structure that forms around a region rich in satellite DNA called the centromere. The kinetochore is the point of attachment of the chromosome to the spindle. At the same time as the chromosomes are condensing within the nucleus, the centrosomes, which lie on the cytoplasmic side of the nuclear envelope, begin to separate to establish the mitotic spindle.
Prometaphase. At the breakdown of the nuclear envelope, the chromosomes become free to interact with the forming spindle. Microtubule assembly from the centrosomes is random and dynamic. The growing ends of individual microtubules make chance contact with and are captured by the kinetochores. Because of the random nature of these events, the kinetochores of chromatid pairs are initially associated with different numbers of microtubules, and the forces acting upon each chromosome are unbalanced. Initially, therefore, the spindle is highly unstable and chromosomes make frequent excursions toward and away from the poles. Gradually, a balance of forces is established and the chromosomes become aligned at the equator, with the kinetochores of each member of a chromatid pair oriented toward opposite poles.
Metaphase. Metaphase is the most stable period of mitosis. The system is at steady state with the chromosomes lined up half facing in one direction and half the other. The metaphase spindle consists of two major groups of microtubules: those connecting the chromatids to the poles and a second group extending from each pole toward the other. Others overlap at the spindle equator. All chromatids become attached to the spindle, all of the chromosomes enter anaphase together.
Anaphase. The trigger for the separation of the paired chromatids and the start of their journey to the spindle poles is the degradation of the protein cohesin, which acts as the glue holding the pairs of chromatids together. In anaphase A the microtubules holding the chromosomes shorten, pulling the chromosomes to the spindle poles. The chromosomes move as a “V” with the kinetochores, at which the force for chromosome movement is applied, leading the way. In contrast, in anaphase B the microtubules that overlap at the spindle equator lengthen, extending the distance between the poles. Compared to other forms of cell motility, the movement of chromosomes at anaphase is extremely slow, less than 1 μm per minute.
Fig. Anaphase only begins when all chromatids are correctly attached to spindle microtubules.
Telophase. This stage is the reversal for many of prophase events; the chromosomes de-condense, the spindle disassembles, the nuclear envelope reforms, Golgi apparatus and endoplasmic reticulum reform, and the nucleolus reappears. Each progeny nucleus contains one complete copy of the genome from the father and one copy from the mother.
Cytokinesis. During the last stages of telophase, the cell itself divides. In animal cells, a cleavage furrow (борозда) made of actin and myosin constricts the middle of the cell; in plants, a structure called the phragmoplast forms at the equator of the spindles where it directs the formation of a new cell wall.
Duration of mitosis. Actually mitosis often occurs relatively quickly. The average duration is 1–2 hours; this is only about 10 % of the cell cycle time. In animal cells mitosis usually occurs quickly and lasts in average 30–60 minutes, while in plant cells the average duration of mitosis is 2–3 hours. The most intensive mitosis occurs in embryonic cells (10–40 minutes in cleaving eggs).
Duration of mitosis depends on several factors: the size of a dividing cell, its ploidy, number of nucleus. The frequency of cell division is also dependent on the degree of cell differentiation and its specific functionality. Thus, the neurons cells or of human skeletal muscle cells are not divided at all, the liver cells are usually divided every one or two years, and some of the intestinal epithelial cells are divided more than two times a day.
Rate of cell division depends on the environmental conditions, particularly temperature. Higher ambient temperature within physiological limits increases the rate of mitosis; it may be explained by the laws of chemical reactions kinetics.
Fig. Events during mitotic stages
4. Meiosis
Meiosis, maturation or reduction division, makes sexual reproduction possible. Meiosis produces haploid cells, which contain just one member of every chromosome pair, a single round of DNA synthesis is followed by two nuclear divisions. In all animals, specialized germ cells in the reproductive organs undergo meiosis to produce haploid gametes (sperm and egg), which then fuse during sexual reproduction to create new diploid embryos. Plants, fungi, and some protists also perform meiosis. In plants, meiosis creates a multicellular haploid organism, called a gametophyte. Gametes are produced by mitosis of the gametophyte, which then fuse to form the embryo.
Two meiotic divisions are separated by interkinesis (it may be absent). The first division has very long, differentiated for stages prophase, prophase II and metaphase II may be absent, doubling the DNA occurs just before the first division.
Meiosis mixes maternal and paternal genes to give new combinations of traits through two mechanisms: (1) independent assortment of chromosomes at both of the meiotic divisions; and (2) exchange of chromosomal regions by crossing over.
There are three types of meiosis:
zygote, or initial (many fungi and algae) – occurs immediately after fertilization and results in the formation of haploid thallus or mycelium,
gamete, or terminal (all multicellular animals and some lower plants) – occurs in the genital organs and leads to gametes formation,
spore, or intermediate (higher plants) – occurs before flowering and results in haploid gametophyte formation
Walther Flemming (1843 – 1905) was a German biologist and a founder of cytogenetics. Flemming investigated the process of cell division and the distribution of chromosomes to the daughter nuclei, a process he called mitosis. Together with August Weismann's (1834–1914) he discovered meiosis in animals (1882), suggested cell theory with Theodor Schwann (1808–1890) and Matthias Schleiden's (1804–1881), developed the first genetic maps with Alfred Sturtevant's (1866–1945).
Eduard Adolf Strasburger (1844 – 1912) was a Polish-German professor, one of the most famous botanists of the 19th century. He came up with one of the modern laws of plant cytology: "New cell nuclei can only arise from the division of other nuclei" and originated the terms cytoplasm and nucleoplasm. Together with Walther Flemming and Edouard van Beneden he elucidated chromosome distribution during cell division, in 1888 discovered meiosis in plants.
Meiosis
Each meiotic division (known as meiosis I and II) involves the formation of a meiotic spindle and the same sequence of prophase, prometaphase, metaphase, anaphase, and telophase (Figure).
Stages of meiosis
One set of chromosomes is from the father and another from the mother. Meiosis is often extended process, in different organisms can last for months or even years, mainly due to lengthy prophase of meiosis I, prophase I. Prophase I differs from mitotic prophase in that the paternal and maternal chromosomes (each composed of two chromatids) line up side by side. Prophase I is the hardest stage of meiosis. By morphological changes of chromosomal material it is divided into leptotene, zygotene, pachytene, diplotene and diakinesis.
Leptotene begins with appearance in meiotic cell of chromosomal material as thin filaments, which coil and begin to shrink.
Zygotene begins with pairs of homologous chromosomes conjugation with exceptional accuracy and orderliness. Each point of one chromosome is located closely with morphologically identical point of corresponding homologous chromosome. These pairs are bivalents.
Pachitene is characterized by bivalent coiling continuing.
There are two longitudinal (продольный) grooves in each chromosome associate. One of them is a gap that has emerged due to the convergence of two-chromosome homologues. It is reduction groove. Along this fissure during meiotic anaphase I division of each bivalent may occur. Perpendicular to the reduction slit of bivalent there is another gap. It is equation groove. In the second meiotic division each chromosome of two chromatids may be divided into two daughter chromosomes along this groove. It may be opposite situation – in the first division chromatids may be divided, and in the second one – chromosomes may be divided.
Continued bivalent coiling is accompanied by chromomere formation. A chromomere (idiomere) is one of the serially aligned beads or granules of eukaryotic chromosome, resulting from local coiling of continuous DNA thread. Any change in the structure of chromosomes causes a change in chromomeres location on bivalent.
Diplotene starts with homologous chromosomes in bivalents reciprocal repulsion. However, their isolation from one another is impossible because of happened crossing-over between chromatids. So, along each bivalent x-like formations appear which are called chiasmas. It is believed that the number of chiasmas corresponds to the number of crossings that occurred between chromatids of homologous chromosomes.
Diakinesis is characterized by chiasmas terminalization. It starts in centromeric parts of chromosomes. Chiasmas gradually move to terminal areas of bivalent. By the end of diakinesis bivalents remain united among themselves only by terminal chiasmas. Therefore bivalent take the form of circulary structures. Meiotic spindle is formed, nucleolus and nuclear membrane disappear.
As the cell progresses to metaphase I, the maternal/paternal chromosome pairs line up along the metaphase plate. Centromeric parts of paired chromosomes are directed to the opposite poles.
At anaphase I, the homologous pairs separate (complete chiasmas terminalization), but the paired chromatids remain attached and journey together to the poles. дочірні клітини, які утворились внаслідок такого поділу, матимуть лише половинну, тобто – гаплоїдну кількість хромосом.
Telophase I starts after chromosomes achieve the poles. Chromosomes are compact, partially despiralized. The two progeny nuclei and nucleoli are formed. Cytokinesis occurs.
Two newborn cells are located next to one another (dyad) and almost immediately enter meiosis II. Interkinesis is very short-term, has not stages.
Prophase II is often so brief as to be undetectable. Meiotic spindle is formed, nucleolus and nuclear membrane disappear.
Metaphase II and anaphase II are similar to mitotic.
In metaphase II chromosomes actively move in the equatorial plane of the cell, and then their centromeres connected with spindle threads.
In anaphase II chromatids finally separate to give haploid gametes (sperm or eggs) and move to opposite poles. Each gamete contains only one copy, derived from either the father or the mother. Тепер кожна дочірня хроматида набуває статуту хромосоми
Telophase II is the last meiotic stage. Chromosomes despiralized completely. Nucleus and nucleolus form, centrioles doubles. Except nuclear membrane, cell walls form that separate protoplasm of daughter cells.
There are some important distinctions of male and female meiosis in animals. In males meiosis produces four equal sized haploid cells called spermatids, each subsequently developing into a spermatozoon. In females, both meiotic divisions are asymmetric, resulting in one large cell that survives, the oocyte, and three small cells, called polar bodies, that are discarded.
5. Comparison of mitosis and meiosis
Both somatic cells and premeiotic germ cells have two copies of each chromosome (2n), one maternal and one paternal. In mitosis, the replicated chromosomes, each composed of two sister chromatids, align at the cell center in such a way that both daughter cells receive a maternal and paternal homolog of each chromosome.
During the first meiotic division each replicated chromosome pairs with its homologous partner at the cell center; this pairing off is referred to as synapsis. One replicated chromosome of each morphologic type then goes into one daughter cell, and the other goes into the other cell in a random fashion. The resulting cells undergo a second division without intervening DNA replication, with the sister chromatids of each morphologic type being apportioned to the daughter cells. Each diploid cell that undergoes meiosis produces four haploid (1n) cells.
Fig. Distribution of genetic material during mitosis
Fig. Distribution of genetic material during meiosis
6. Types of sexual reproduction
Sexual reproduction is a process that creates a new organism by combining the genetic material of two organisms. It occurs both in eukaryotes and in prokaryotes. A key similarity between them is that DNA originating from two different individuals (parents) join so that homologous sequences are aligned with each other, and this is followed by exchange of genetic information (a process called genetic recombination). After the new recombinant chromosomes are formed they are passed on to progeny.
There are two main processes during sexual reproduction in eukaryotes: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original chromosomes number. During meiosis, the chromosomes of each pair usually cross over to achieve homologous recombination.
Fertilization (also known as conception, fecundation and syngamy) is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside (external fertilization). The entire process of development of new individuals is called reproduction.
The autogamy, the production of gametes by the division of a single parent cell, is the fertilization between two daughter gametes of the same gametocyte. Typically the nucleus of the diploid or tetraploid gametocyte divides without DNA synthesis and complete cytokinesis (separation of daughter cells). The resulting cell then behaves like a zygote resulting from fertilization. Autogamy may be found in species where cross pollination cannot be assured.
Self-fertilization or endogamy occurs in bisexual organisms, including most flowering plants, numerous protozoans, and many invertebrates. Fusion of female and male gametes derived from genetically similar sources, usually the same species.
Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilization, development of unfertilized eggs into new individuals. The word parthenogenesis comes from the Greek parthenos, meaning "virgin", and genesis, meaning "birth". The offspring are clones of the mother and hence are usually female. Parthenogenesis occurs naturally in many plants, some invertebrate animal species and a few vertebrates. This type of reproduction has been induced artificially in a few species including fish and amphibians.
Some species reproduce exclusively by parthenogenesis while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis or cyclical parthenogenesis. Facultative parthenogenesis occurs when a species that normally reproduces sexually undergoes asexual reproduction. This is in contrast to obligate parthenogenesis, where the females reproduce exclusively by asexual means.
Facultative parthenogenesis is a response to a lack of a viable male. A female may undergo facultative parthenogenesis if a male is absent from the habitat or if it is unable to produce viable offspring. This behavior has been documented in sharks, komodo dragons and a variety of domesticated birds. Facultative parthenogenesis has also been seen in crustaceans and decapods in nature. In fact, parthenogenesis has been documented in over 70 different species.
Obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means. There are over 80 species of unisex reptiles, amphibians and fishes in nature, for which males are no longer a part of the reproductive process; a male is not needed to provide sperm to fertilize the egg. A female will produce an ovum with a full set (two sets of genes) provided solely by the mother.
This form of asexual reproduction is thought to be a serious threat in some cases to biodiversity for the subsequent lack of gene variation and potentially decreased fitness of the offspring.
7. Apoptosis
An adult human is made up of about 30 trillion (3·1013) cells, all of which originate from a single fertilized egg. If this first cell divides into 2, the 2 progeny cells into 4, and so on, it would take only about 45 rounds of division to produce the number of cells required to make an adult human. In fact, cell division occurs constantly through our lifetimes, such that we generate a new complete set of 3·1013 cells every 2 weeks. The reason that multicellular organisms do not become infinitely large is because the proliferation of cells is balanced by cell death. Cells die for two quite different reasons. One is accidental, the result of mechanical trauma or exposure to some kind of toxic agent, and often referred to as necrosis. This is the only type of death seen in unicellular organisms. The other type of death is deliberate, the result of an built-in suicide mechanism known as apoptosis or programmed cell death. The two types of cell death are quite distinct. In cells that are injured, ATP concentrations fall so low that the Na+/K+ ATP-ase can no longer operate, and therefore ion concentrations are no longer controlled. This causes the cells to swell and then burst. The cell contents then leak out, causing the surrounding tissues to become inflamed. Cells that die by suicide on the other hand shrink, and their cell contents are packaged into small membrane-bound packets called blebs. The nuclear DNA becomes chopped up into small fragments, each of which becomes enclosed in a portion of the nuclear envelope. The dying cell modifies its plasma membrane, signaling to macrophage, which respond by engulfing the blebs and the remaining cell fragments and by secreting cytokines that inhibit inflammation. The changes that occur during apoptosis are the result of hydrolysis of cellular proteins by a family of proteases called caspases (short for cysteine-containing, cleaving at aspartate). All the cells of our body contain caspases, but they are normally locked in an inactive form by an integral inhibitory domain of the protein. Proteolysis cleaves the inhibitory domain off, releasing the active caspases. The advantage to the cell of this strategy is that no protein sythesis is required to activate the apoptotic pathway–all the components are already present. Thus, for example, if a virus infects a cell and takes over all protein synthesis, the cell can still commit suicide and hence prevent viral replication (Figure).
The complex control systems that regulate the decision to die or survive
Lecture 6,7
Genetic analysis methods
Plan
hybridological method
genealogical method
twins method
population and statistical method
cytogenetic methods
biochemical and other methods