Lecture 2: Cell Division PDF

Lecture 2: Cell Division Dr./ Ahmed Mahdy Topics Discussed in this Lecture Chromosomes and Chromatin Cell Division and Reproduction Eukaryotic Cell Division and Cell Cycle 3 4 Chromosome Organization 6  8 Histon DNA Nucleo...

Lecture 2: Cell Division Dr./ Ahmed Mahdy Topics Discussed in this Lecture Chromosomes and Chromatin Cell Division and Reproduction Eukaryotic Cell Division and Cell Cycle 3 4 Chromosome Organization 6  8 Histon DNA Nucleosome Protein Chromatid Chromatin Solenoid Chromosome 7 8  Terminology - Phenotype: The appearance of a specific character or trait, which is dependent on genetic makeup, usually expressed in words, - e.g., “tall”, “dwarf”, “albino”. - Genotype: The genetic makeup of an individual for specific traits under consideration, usually expressed by a symbol, - e.g., DD (tall), dd (short), etc. 9  Terminology The structural pattern of the Core histones is known as the Histone fold domain. The 3D structure of Protein is determined by the amino acids sequence and the folding of proteins in their correct local structure is very important to perform their function. They are formed by three alpha helices which are connected with each other by two loops. Chromatin refers to a mixture of DNA and proteins that form the chromosomes found in the cells of humans and other higher organisms. Many of the proteins — namely, histones — package the massive amount of DNA in a genome into a highly compact form that can fit in the cell nucleus. Heterochromatin: The densely packed or compacted regions of chromatin. It is genetically inactive. Transcription is inhibited in heterochromatin because the DNA is packaged so tightly & inaccessible to the proteins of RNA transcription. Euchromatin: Less densely compacted chromatin in a transcriptionally active nucleus. It is uncoiled structures & allow RNA polymerases and regulatory proteins' to access to DNA. 10  Terminology A histone is a protein that provides structural support for a chromosome, the DNA wraps around complexes of histone proteins, giving the chromosome a more compact shape. Histones also play a role in the regulation of gene expression. Eight histone proteins can come together to make up a nucleosome. Histones are made up of high content of basic amino acids, Lysine and Arginine that give them a positive charge, and this help in binding with the negatively charged DNA. Histone octamer – It is formed by the complex of eight proteins that are present at the center of the Nucleosome core particle and these proteins play an important role in the packaging of DNA. Histone Octamer is made up of two copies of each of the histone proteins H2A, H2B, H3, and H4. 11  A nucleosome is like a tiny spool that DNA can wind around. So histones play an important part in keeping the genome organized and wrapped neatly within a cell. chain of the nucleosomes (consisting of Six nucleosomes) is wrapped into a 30nm spiral it will call a Solenoid. In simple words, we can say that it is a condensed chromatin fiber with a diameter of 30nm and play its role in DNA packaging. The solenoid structure of a chromatin is a an organization of nucleosomes in 30nm fibre. 12  A nucleosome is the basic repeating subunit of chromatin packaged inside the cell’s nucleus. In humans, about six feet of DNA must be packaged into a nucleus with a diameter less than a human hair, and nucleosomes play a key role in that process. A single nucleosome consists of about 150 base pairs of DNA sequence wrapped around a core of histone proteins. In forming a chromosome, the nucleosomes repeatedly fold in on themselves to tighten and condense the packaged DNA. 13 14  Heterochromatin Euchromatin 15 16  Genome is the complete set of genetic information in an organism. Chromatid is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids. A gene’s specific location along the length of chromosome is called the gene’s locus (plural, loci; meaning “place”). Our genome consists of the genes and other DNA that make up the chromosomes we inherited from our parents. 17  Chromosome structure - Chromosomes: are thread-like structures located inside the nucleus of animal and plant cells. - Humans have 46 chromosomes in their somatic cells. - Each chromosome consists of a single DNA double helix, coiled in association with histone & non-histone proteins. - A single chromosome includes several hundred to a few thousand genes. - Telomeres are hexameric DNA repeats (TTAGGG) found at the ends of chromosomes that serve to protect the chromosome from degradation and prevent adherence. - Centromeres serve as "handles:' which allow mitotic spindles to attach to the chromosome during cell division. 18 19 20 21 Types of chromosomes according to centromere location: 22  Kinetochore is a disc-shaped protein complex present in the centromere region of a chromosome, The inner layer of the kinetochore is associated with the centromere while the outer layer interacts with microtubules. It associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The number of bound microtubules to a kinetochore varies with the species. For example, human kinetochore binds approximately with 15 microtubules whereas kinetochore of Saccharomyces binds with only one microtubule. The kinetochore is a large proteinaceous structure that mediates interactions between chromosomal DNA and spindle-microtubule polymers 23 24 25 Measurements related to chromosomes 26  2.5%-24% N.B.; Telocentric: 0-2.4 % 27  Some definitions Cross, The deliberate mating of two parental types of organisms in genetic analysis. Autosome. The chromosomes which are not associated with sex are known as autosomes. Except the sex chromosomes (X) and (Y) other chromosomes are the autosomes. Sex chromosomes. Heteromorphic chromosomes that do not occur in identical pairs in both sexes in diploid organisms; in humans and fruit flies these are designated as X and Y chromosomes, respectively; in fowl, as the Z and W chromosomes. Barr body. A densely staining mass that represents an X-chromosome inactivated by dosage compensation. 28  Allele (Allelomorph): One of two or more forms that can exist at a single gene locus, distinguished by their differing effects on the phenotype. Alleles are genes controlling the same characteristic (e.g. hair colour) but producing different effects (e.g. black or red), and occupying corresponding positions on homologous chromosomes. 29 Cell Division and Reproduction  Methods of Reproduction ▪ Living organisms reproduce by two methods 1. Asexual reproduction – Offspring are identical to the original cell or organism – Involves inheritance of all genes from one parent – Prokaryotes reproduce asexually by binary fission. 2. sexual reproduction - Involves inheritance of unique sets of genes from two parents - Offspring are similar to parents, but show variations in traits  Asexual reproduction ❑ Asexual reproduction requires only one parent. ❑ Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. ❑ This type of reproduction occurs in prokaryotic unicellular microorganisms (bacteria) and in some eukaryotic single-celled and multi-celled organisms, animal like protists and yeast and some invertebrates. ❑ There are many types of asexual reproduction. Major types are: Fission, Budding, Fragmentation, Regeneration and Parthenogenesis. 32  Prokaryotes reproduce by binary fission Asexual reproduction ▪ Binary fission means “dividing in half” – Occurs in prokaryotic cells – Two identical cells arise from one cell – Single parent cell doubles its DNA, then divides into two cells. Usually occurs in bacteria. – Steps in the process: – A single circular chromosome duplicates, and the copies begin to separate from each other – The cell elongates, and the chromosomal copies separate further – The plasma membrane grows inward at the midpoint to divide the cells Prokaryotic chromosome Plasma membrane Cell wall 1 Duplication of chromosome and separation of copies Binary fission in Prokaryotes Continued elongation of the 2 cell and movement of copies 3 Division into two daughter cells  Variety of asexual reproduction in organisms ❑ Binary fission in Protists: e.g. Amoeba In unicellular eukaryotes, asexual reproduction consists of the division of the nucleus (karyokinesis) by mitosis, followed by the splitting of the cytoplasm (cytokinesis). Division (mitosis) yields 2 organisms, it may be longitudinal or transverse. The nucleus elongated and splits into 2 parts and then cytoplasm is divided into 2 parts creating 2 organisms. 35 36 ❑ Multiple fission or schizogony: e.g. Schizogony in Plasmodium Plasmodium is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects causing malaria. karyokinesis is repeated several times yielding multinucleate schizont. Then, cytoplasm is divided among daughter nuclei yielding a large number of daughter cells. Genome duplication not followed by the division of nucleus, a process known as endomitosis, results in an increase in the ploidy level of the nucleus. 37 A Sexual Reproduction in multicellular Organisms: ❑ Budding in Hydra (small animal) : Budding: Small growth on surface of parent breaks off, resulting in the formation of two individuals. Occurs in yeast and some animals (like the hydra). Hydra reproduces asexually by budding. The bud is a localized mass of mitotically dividing cells, develops into a small hydra, which detaches from the parent. 38  Hydra is exclusively a freshwater organism having different species. It is very small, just a half centimetre long. It is a cnidarian having a tubular body which is composed of a head, distal end and afoot at the end. Budding in hydra involves a small bud which is developed from its parent hydra through the repeated mitotic division of its cells. The small bud then receives its nutrition from the parent hydra and grows healthy. Growth starts by developing small tentacles and the mouth. Finally, the small newly produced hydra gets separated from its parent hydra and becomes an independent organism. 39  Yeasts are non-green, eukaryotic, single-celled microorganisms belonging to the kingdom fungus. They are generally larger than the bacteria and they typically measure 3-4 µm in diameter. Yeast cells reproduce asexually by an asymmetric division process called budding. In yeast, budding usually occurs during the abundant supply of nutrition. In this process of reproduction, a small bud arises as an outgrowth of the parent body. Later the nucleus of the parent yeast is separated into two parts and one of the nuclei shifts into the bud. The newly created bud divides and grows into a new cell. 40 ❑ Fragmentation: Organisms break into two or more fragments that develop into a new individual. Occurs in many plants, as well as some animals (like coral, sponges, and starfish). 41  Sea star for which an arm of the individual is broken off and regenerates a new sea star. Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams. If starfish has lost its arm. The fragment is growing into a new individual, while the parent regrows its lost arm. Fragmentation also occurs in annelid worms, turbellarians, and poriferans. Sponges, some flatworms, and certain types of fungi can also undergo fragmentation. 42 ❑ Regeneration in Planaria (small animal): Planarians (triclads) are free-living flatworms, found in freshwater, marine, and terrestrial habitats. Regeneration is the process of renewal, restoration, and tissue growth that makes genomes, cells, organisms. Planaria has the ability to renew lost or injured parts. Another example: Regeneration (mitotic division) of skin in human to repair skin wounds. 43  ❑ Parthenogenesis Parthenogenesis is a form of a sexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones) and diploid females (workers). If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced. Some animals that can undergo parthenogenesis include insects like bees and grasshoppers, lizards such as the komodo dragon, and very rarely in birds. 44  Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Some vertebrate animals, such as certain reptiles, amphibians, and fish, also reproduce through parthenogenesis. Two Komodo dragons, a bonnethead shark, and a blacktip shark have produced parthenogenic young when the females have been isolated from males. 45  ❑ Vegetative Propagation Vegetative propagation is an asexual method of plant reproduction that occurs in its leaves, roots and stem. This can occur through fragmentation and regeneration of specific vegetative parts of plants. o Types of Vegetative Propagation Natural Vegetative Propagation Artificial Vegetative Propagation 46 47  ❑ Natural vegetative propagation This occurs when plants grow and develop naturally without any human interference. Natural vegetative propagation can be enabled by the development of adventitious roots. Thus, new plants may emerge from the roots, stem and leaves of the parent plant. The vegetative plant structures arising from the stem are known as rhizomes, bulbs, runners, tubers, etc. The plants propagated vegetatively are given below: Stem: Runners grow horizontally above the ground. The buds are formed at the nodes of the runners. Roots: New plants emerge out of swollen, modified roots known as tubers. Buds are formed at the base of the stem. Leaves: Leaves of a few plants get detached from the parent plant and develop into new plants. Bulbs: Bulbs have an underground stem to which the leaves are attached. These leaves are capable of storing food. The centre of the bulb contains an apical bud that produces leaves and flowers. Shoots are developed from the lateral buds. 48  ❑ Artificial Vegetative Propagation This is a type of vegetative reproduction carried out by humans in the fields and laboratories. The most common types of vegetative reproduction occurring artificially include: Cutting, In this, a part of a plant, specifically a stem or leaf is cut and planted in the soil. These cuttings are sometimes treated with hormones to induce root development. The new plant is formed from the adventitious roots developing from the cutting. Grafting, In this, the cutting from some other plant is attached to the stem of a plant rooted in the ground. The tissues of the graft become integrated with the tissues of the rooted plant and develop as a single plant over time. Layering, In this, the stem of the plant is bent to the ground and covered with soil. Adventitious roots emerge from the plant parts covered with the soil. This attached stem with developing roots is known as a layer. Tissue Culture, In this, the plant cells from different parts of a plant are cultured in the laboratory to develop a new plant. This technique is helpful in increasing the number of rare and endangered plant species that are unable to grow under natural conditions. 49  ❑ Spore Formation Spores are haploid unicellular bodies that are produced as a result of sexual or asexual reproduction in eukaryotic organisms such as algae, bacteria, fungi and some plants. The process of formation of spores is referred to as sporogenesis. Spores are reproductive cells that are capable of giving rise to a new organism as compared to gametes that require to fuse with another gamete to give rise to new individuals. 50 ❑ Examples In fungi, the spores are formed on a reproductive knob-like structure known as the sporangium. They produce minute haploid spores that grow into new organisms in favourable conditions. E.g., Aspergillus, Penicillium. In bacteria, another type of spores called endospores are formed as a result of unfavourable conditions. It is a non-reproductive structure that is not a true spore. E.g., Bacillus, Clostridium. In plants, spore formation is the sole means of asexual reproduction. Plants follow an alternation of generation life cycle, where diploid sporophyte produces haploid spores and ultimately gives rise to haploid gametophyte. E.g., liverworts, mosses and hornworts. 51  Sexual Reproduction Sexual reproduction is a natural way of reproduction in humans, animals and the majority of plants. This type of reproduction is more complex and lengthy as compared to asexual reproduction. Moreover, reproducing sexually gives the benefit of variation and offsprings are unique. Sexual reproduction consists of a set of events and can be divided into three stages: Pre-fertilization, Fertilization, and Post-fertilization. 52 53  Pre-Fertilization This stage involves the events prior to fertilization. Gamete formation (gametogenesis) and transfer of gamete are the two processes that take place during this stage. Gametes are sex cells, which are haploid (23 chromosomes) in nature and are distinct in males and females. The male gamete is called sperm whereas female gamete is called ovum or egg. In every organism, these gametes are formed within special structures. Since female gamete is immobile, male gametes need to be transferred for fertilization. In plants, this is attained by pollination. Unisexual animals transfer gametes by sexual intercourse. Fertilization Once the haploid male and female gametes meet and fuse together to form a zygote, this is known as fertilization or syngamy. This can occur either outside the body called external fertilization or inside the body called internal fertilization. 54 Post-Fertilization Fertilization results in diploid zygote formation. Eventually, the zygote divides mitotically and develops as an embryo. This process is called embryogenesis. During embryogenesis, cell differentiates and modifies accordingly. Zygote development depends on the organism and its life cycle. Animals are classified into oviparous and viviparous based on whether the zygote develops outside or inside the body respectively. In angiosperms (flowering plants), the zygote develops into the ovary and ovary transforms into the fruit while ovules develop into seeds. 55 Comparing sexual vs asexual reproduction Sexual Asexual Requires 2 parents Requires 1 parent Single organism makes an exact Sperm fertilizes egg copy of itself Used by animals, flowering Bacteria, some plants and fungi, plants, some fungi few animals (sponges) Offspring are different from Offspring are identical to parent parents Provides genetic variation, but Fast and easy, but no genetic time-consuming variation 56 Eukaryotic Cell Division and Cell Cycle 57  Eukaryotic Cell Division and Cell Cycle The cell cycle is an ordered sequence of events for cell division. ▪ Cells divide when they reach a certain size. ▪ The cell cycle consists of two stages 1) Interphase: Includes G1, S, and G2 phases INTERPHASE during which cell contents are duplication. G1: first gap phase, growth and prepares for S-phase S (DNA synthesis) S: DNA synthesis phase, duplication of chromosomes, each becomes two sister G1 chromatids G2: second gap phase, growth and G2 preparation for division 2) Mitotic phase: (the M phase) involves mitosis and cytokinesis. Mitosis: division of the chromosomes Cytokinesis: division of cytoplasm The eukaryotic cell cycle  Eukaryotic chromosomes The chromosomes carry the genetic information. Eukaryotic chromosomes contain DNA and protein The chromosomes are so named because they may be stained by certain dyes When cells are not dividing, the genetic material is decondensed and is called chromatin When cells are dividing, the genetic material is condensed and is called chromosome Condensed Chromosome Decondensed Chromatin Chromosome Organization  Chromosomes, Mitosis and Meiosis Human chromosomes Human chromosomes karyotype metaphase spread  The large, complex chromosomes of eukaryotes duplicate with each cell division ▪ Early in the division process, chromosomes duplicate in S-phase. ▪ Each chromosome appears as two sister chromatids containing identical DNA molecules. ▪ Sister chromatids are joined at a narrow region called the centromere.  A Duplicated Chromosome Sister chromatids Chromosome duplication Sister chromatids Centromere Chromosome distribution To daughter cells Chromosome duplication and Electron micrograph distribution of a duplicated chromosome 64