Nucleus and Phases of Cell Cycle PDF

8 Nucleus and phases of cell cycle ILOs By the end of this lecture, students will be able to 1. Correlate structure of different components to the nucleus to its function. 2. Differentiate between functional forms of the chromatin. 3. Interpret structural organization...

8 Nucleus and phases of cell cycle ILOs By the end of this lecture, students will be able to 1. Correlate structure of different components to the nucleus to its function. 2. Differentiate between functional forms of the chromatin. 3. Interpret structural organization of the chromosome. 4. Discuss the nuclear and cellular changes during the phases of the cell cycle The Cell Nucleus: Introduction The nucleus contains a blueprint for all cell structures and activities, encoded in the DNA of the chromosomes. It also contains the molecular machinery to replicate its DNA and to synthesize and process the three types of RNA; ribosomal (rRNA), messenger (mRNA), and transfer (tRNA). (Are there DNA in the cell outside the nucleus?) The nucleus does not produce proteins; the numerous protein molecules needed for the activities of the nucleus are imported from the cytoplasm. Structure of the nucleus as seen by LM The nucleus frequently appears as a rounded or elongated structure, usually in the center of the cell (Figure.1A). Its main components are the nuclear envelope, chromatin, nucleolus, and nuclear matrix (Figure.1B). The size and morphological features of nuclei in a specific normal tissue tend to be uniform. In common hematoxylin and eosin-stained preparations; the nucleus, however, appears intensely stained dark blue or black. (Why?) Ultrastructure of the nucleus Three components are recognized: 1. Nuclear Envelope Electron microscopy shows that the nucleus is surrounded by two parallel membranes separated by a narrow space called the perinuclear cisterna. Together, the paired membranes and the intervening space make up the nuclear envelope. (Fig 1B) Polyribosomes are attached to the outer membrane, showing that the nuclear envelope is a in continuity with the endoplasmic reticulum. (Why?) At sites at which the inner and outer membranes of the nuclear envelope fuse, there are gaps, the nuclear pores (Figure 1B), that provide controlled pathways between the nucleus and the cytoplasm. Because the nuclear envelope is impermeable to ions and molecules of all sizes, the exchange of substances between the nucleus and the cytoplasm is made only through the nuclear pores. Ions and Page 1 of 5 molecules with a diameter up to 9 nm pass freely through the nuclear pore without consuming energy. But molecules and molecular complexes larger than 9 nm are transported by an active process, mediated by receptors, which uses energy from adenosine triphosphate (ATP). 2. Chromatin o Chromatin is composed mainly of coiled strands DNA bound to basic proteins (histones). o The basic structural unit of chromatin is the nucleosome (Figure 2), which consists of a core of four types of histones, wrapped around DNA base pairs. (What is the role of histones?) o Linker DNA; An additional DNA segment forms a link between adjacent nucleosomes, and another type of histone is bound to this DNA. This organization of chromatin has been referred to as "beads-on-a-string." Nonhistone proteins are also associated with chromatin, but their arrangement is less well understood. o Functional forms of Chromatin; in nondividing nuclei, is in fact the chromosomes in a different degree of uncoiling. According to the degree of chromosome condensation, two types of chromatin can be distinguished with both the light and electron microscopes (Figure 3). Heterochromatin (Gr. heteros, other, + chroma, color), which is electron dense, appears as coarse granules in the electron microscope and as basophilic clumps in the light microscope. It represents the inactive form of chromatin and acts as a reserve in less active cells. Euchromatin is the less coiled portion of the chromosomes, visible as a finely dispersed granular material in the electron microscope and as lightly stained basophilic areas in the light microscope. It represents the active form of chromatin and more abundant in active cells. The proportion of heterochromatin to euchromatin accounts for the light-to-dark appearance of nuclei 0in tissue sections as seen in light and electron microscopes. The intensity of nuclear staining of the chromatin is frequently used interpret the functional state of the nucleus. (How?) Figure 2 Nucleosome structure Page 2 of 5  Figure 3 Electron micrograph of a nucleus showing the heterochromatin (HC) and euchromatin (EC). Unlabeled arrows indicate the nucleolus-associated chromatin around the nucleolus (NU). Arrowheads indicate the perinuclear cisterna. Underneath the cisterna is a layer of heterochromatin, the main component of the so-called nuclear membrane seen under the light microscope. X 26,000. Careful study of the chromatin of mammalian cell nuclei reveals a heterochromatin mass that is frequently observed in female cells but not in male cells. This chromatin clump is the sex chromatin and is one of the two X chromosomes present in female cells. The X chromosome that constitutes the sex chromatin remains tightly coiled and visible, whereas the other X chromosome is uncoiled and not visible. Evidence suggests that the sex chromatin is genetically inactive. The male has one X chromosome and one Y chromosome as sex determinants; the X chromosome is uncoiled, and therefore no sex chromatin is visible. In human epithelial cells, sex chromatin appears as a small granule attached to the nuclear envelope. The cells lining the internal surface of the cheek are frequently used to study sex chromatin. Blood smears are also often used, in which case the sex chromatin appears as a drumstick-like appendage to the nuclei of the neutrophilic leukocytes. 3. Nucleolus o The nucleolus is a spherical structure (Figure 17-5) that is rich in rRNA and protein. It is usually basophilic when stained with hematoxylin and eosin. o Significance of the nucleolus; it contains DNA that codes for rRNA (type of RNA present inside ribosomes). The nucleolus is the site of synthesis of ribosomal subunits (to be explained later). Ribosomal proteins, synthesized in the cytoplasm, become associated with rRNAs in the nucleolus; ribosome subunits then migrate into the cytoplasm. Heterochromatin is often attached to the nucleolus (nucleolus-associated chromatin), but the functional significance of the association is not known. The rRNAs are synthesized and modified inside the nucleus. In the nucleolus they receive proteins and are organized into small and large ribosomal subunits, which migrate to the cytoplasm through the nuclear pores. (Figure 17-4) Page 3 of 5  4. Nucleoplasm: The protoplasm within the nucleus, consisting of a fluid portion, a proteinaceous matrix, and various ribonucleoproteins particles. Nuclear & cellular changes during cell cycle: Cell cycle The cell cycle is a series of events within the cell that prepare the cell for dividing into two daughter cells. Phases of the cell cycle: I- Interphase; long period of time during which the cell increases its size and content and replicates its genetic material. It includes three stages: a) G1 (gap) phase, synthesis of macromolecules essential for DNA duplication begins & cell growth. b) S (synthetic) phase, DNA is duplicated. c) G2 phase, the cell undergoes preparations for mitosis. II- Mitosis, a shorter period of time during which the cell divides its nucleus and cytoplasm, giving rise to two daughter cells. The cell cycle may be thought of as beginning at the conclusion of the telophase stage in mitosis, after which the cell enters interphase. (Figure 6) The interphase Gap 1 Daughter cells formed during mitosis enter the G1 phase. During this phase, the cells synthesize RNA, regulatory proteins essential to DNA replication, and enzymes necessary to carry out these synthetic activities. Thus, cell growth occurs restoring cell size to normal. The centrioles (a cell organelle involved in cell division) begin to duplicate themselves, a process that is completed by the G2 phase. S Phase During the S phase, the synthetic phase of the cell cycle, the genome is duplicated. The cell now contains twice the normal complement of its DNA. Autosomal cells contain the diploid amount of DNA before the synthetic (S) phase of the cell cycle that becomes doubled after S phase in preparation for cell division. Page 4 of 5 G2 Phase The gap 2 phase (G2 phase) is the period between the end of DNA synthesis and the beginning of mitosis. During the G2 phase, the RNA and proteins essential to cell division are synthesized, the energy for mitosis is stored. Duplication of centrioles and formation of the needed microtubules are completed. DNA replication is analyzed for possible errors, and any of these errors is corrected. Cells that become highly differentiated (What is meant by differentiation?) after the last mitotic event may stop to undergo mitosis either permanently (e.g., neurons, muscle cells) or temporarily (e.g., peripheral lymphocytes) and return to the cell cycle at a later time. Cells that have left the cell cycle are said to be in a resting stage, the G0 (outside) phase, or the stable phase. Figure 6. The cell cycle in actively dividing cells. Nondividing cells, such as neurons, leave the cycle to enter the G0 phase (resting stage). Other cells, such as lymphocytes, may return to the cell cycle. Page 5 of 5 32 Meiosis ILOs By the end of this lecture, students will be able to 1. Correlate chromosomal changes to different phases of meiosis. 2. Interpret role of crossing over in genetic variations. 3. Interpret the different types of chromosomal abnormalities. 4. Differentiate between aneuploidy and polyploidy. 5. Differentiate between balanced and unbalanced karyotypes. 6. Correlate the phenotypic outcome with types of chromosomal aberrations. Meiosis Meiosis is a special type of cell division resulting in the formation of gametes (spermatozoa or ova) whose chromosome number has been reduced from the diploid (2n) to the haploid (1n) number. Meiosis begins at the conclusion of interphase in the cell cycle. It produces the germ cells- the ova and the spermatozoa. This process has two crucial results: 1. Reduction in the number of chromosomes from the diploid (2n) to the haploid (1n) number, ensuring that each gamete carries the haploid amount of DNA and the haploid number of chromosomes. 2. Recombination of genes, ensuring genetic variability and diversity of the gene pool Meiosis is divided into two phases: I- Meiosis I, or reductional division (first event): Homologous pairs of chromosomes line up, members of each pair separate and go to opposite poles, and the cell divides; thus, each daughter cell receives half the number of chromosomes (haploid number). II- Meiosis II, or equatorial division (second event): The two chromatids of each chromosome are separated, as in mitosis, followed by migration of the chromatids to opposite poles and the formation of two daughter cells. These two events produce four cells (gametes), each with the haploid number of chromosomes and haploid DNA content. Page 1 of 3  In gametogenesis, when the germ cells are in the S phase of the cell cycle preceding meiosis, the amount of DNA is doubled to 4n but the chromosome number remains at 2n (46 chromosomes). Meiosis I 1. Prophase I: It begins after the DNA has been doubled to 4n in the S phase. Prophase of meiosis I lasts a long time. Homologous pairs of chromosomes approximate each other and condense. The most significant event in prophase I is formation of chiasmata (crossing over sites) as random exchange of genetic material occurs between homologous chromosomes. 2. Metaphase I: is characterized by lining up of homologous pairs of chromosomes, each composed of two chromatids, on the equatorial plate of the meiotic spindle. 3. Anaphase I: Homologous chromosomes migrate away from each other, going to opposite poles. 4. Telophase I: The chromosomes reach the opposing poles, nuclei are re-formed and cytokinesis occurs, giving rise to two daughter cells. Each cell possesses 23 chromosomes, the haploid (1n) number, but because each chromosome is composed of two chromatids, the DNA content is still diploid. Figure 2. Phases of meiosis d; double amount of DNA, s; haploid amount of DNA Page 2 of 3 Meiosis II (equatorial division) occurs without DNA synthesis and proceeds rapidly through four phases and cytokinesis to form four daughter cells each with the haploid chromosome number It is subdivided into prophase II, metaphase II, anaphase II, telophase II, and cytokinesis The chromosomes line up on the equator, the kinetochores attach to spindle fibers, followed by the chromatids migrating to opposite poles, and cytokinesis divides each of the two cells. Outcome of meiosis II: 1. Results in a total of four daughter cells from the original diploid germ cell. Each of the four cells contains a haploid amount of DNA and a haploid chromosome number. 2. The cells are genetically distinct because of reshuffling of the chromosomes and crossing over. Thus, every gamete contains its own unique genetic complement. Page 3 of 3