Life Sciences I Cell Biology Cell Reproduction PDF

Life Sciences I Cell biology Cell Reproduction www.lsmu.lt Cell Reproduction The cell's reproductive cycle is termed the cell cycle. Each complete cycle ends with cell division (mitosis) and yields two daughter cells. During mitosis, a parent cell divides and ea...

Life Sciences I Cell biology Cell Reproduction www.lsmu.lt Cell Reproduction The cell's reproductive cycle is termed the cell cycle. Each complete cycle ends with cell division (mitosis) and yields two daughter cells. During mitosis, a parent cell divides and each of the two daughter cells receives a chromosomal set identical to that of the parent cell. The chromosomes replicated during the preceding S phase are distributed to the daughter cells. The long period between mitoses (the G1, S, and G2 phases) is also commonly called interphase. The events of mitosis are subdivided into four major stages. 2 Sequence of events during the cell cycle. A) Immediately following mitosis (M) the cell enters a gap phase (G1). At this point many cells will undergo cell arrest (G0 phase). G1 is followed by a DNA synthesis phase (S) a second gap phase (G2) and back to mitosis. B) Stages of mitosis are highlighted. Citation: General Principles & Energy Production in Medical Physiology, Barrett KE, Barman SM, Brooks HL, Yuan JJ. Ganong's Review of Medical Physiology, 26e; 2019. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=204290215&bookid=2525 Accessed: July 10, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 3 Cell cycle of a generalized eukaryotic cell. The positions of major checkpoints and the associated proteins are indicated. Citation: Chapter 3. The Nucleus & Cell Cycle, Paulsen DF. Histology & Cell Biology: Examination & Board Review, 5e; 2010. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=563&sectionid=42045296 Accessed: July 08, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 4 Mitosis and interphase. Early views of cellular reproduction focused on detectible structural changes occurring during mitosis. The apparently inactive phase between successive mitoses seemed a resting period and was dubbed the interphase. Yet, even in rapidly dividing cells, the duration of mitosis is brief compared with the length of interphase. Both mitosis and interphase are complex and important cell-cycle components and each has been divided into steps to facilitate our understanding. 5 6 7  The G1phase of interphase follows the telophase of mitosis. A gap is a period during which no DNA synthesis occurs, as indicated by the fact that no radiolabeled thymidine (3H-thymidine) is incorporated into the cell's DNA. RNA and protein syntheses do occur during the gap phases, and each daughter cell grows to the parent's size. G1, typically the longest phase of the cycle, is also the most variable in length among different cell types. In rapidly dividing (e.g., embryonic and neoplastic) cells, G1 is shorter and the transition to subsequent phases is continuous. More-differentiated cells may withdraw from the cycle in G1 and enter a phase called G0, in which preparations for mitosis are suspended in favor of specialized functions. G0 cells, unable to reenter the cycle (e.g., muscle, nerve), are said to be terminally differentiated. Other cells in G0 (e.g., hepatocytes, fibroblasts) reenter the cell cycle in response to growth factors encountered during or after an injury. 8  During the S (synthesis) phase, DNA synthesis and replication occur, as indicated by 3H-thymidine uptake. The amount of DNA in the cell doubles during S-phase. The centrioles often self-duplicate during this stage. During G2 (gap 2), the final preparations for cell division occur; these include repair of DNA damage, synthesis of tubulin for the spindle apparatus, and ATP accumulation for the energy-expensive mitosis. Very little synthesis occurs during mitosis. 9  Steps in cell division (mitosis) Mitosis is a brief, and continuous process. Structural changes observed during this complex process have been used to divide it into four successive phases: prophase, metaphase, anaphase, and telophase. 10  Phases of the cell cycle and mitosis Mitotic phase Cell cycle Description of events Interphase G1, S, G2 DNA doubling occurs Prophase M Nuclear envelope degenerates; spindle forms Metaphase M Chromosomes align at cell equator Anaphase M Duplicated chromosomes separate Telophase M Chromosomes to poles, cytoplasm divides 11 Phases of mitosis. Citation: The Nucleus, Mescher AL. Junqueira’s Basic Histology: Text and Atlas, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=190276489&bookid=2430 Accessed: July 10, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 12  During prophase, chromatin coils to form chromosomes. As the nucleolar organizer DNA coils into its respective chromosomes, the nucleoli disintegrate. The nuclear envelope remains intact. The two-centriole pairs migrate to opposite poles of the cell, cytoplasmic microtubules depolymerize, and the mitotic spindle apparatus begins to assemble between the centriole pairs. Microtubule polymerization, in preparation for spindle formation, causes the ER and Golgi complex to disintegrate into a multitude of vesicles. 13  Metaphase During metaphase, lamin phosphorylation promotes nuclear envelope disintegration. Chromosomes line up at the cell equator between the centriole pairs, and each chromosome splits lengthwise to form a pair of sister chromatids. Each chromosome has a centromere (late-replicating DNA, or kinetochore) to which certain microtubules of the spindle apparatus attach. 14 Metaphase 15  Metaphase Mitotic spindle and metaphase chromosomes. Citation: The Nucleus, Mescher AL. Junqueira’s Basic Histology: Text and Atlas, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=190276489&bookid=2430 Accessed: July 10, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 16  Anaphase During anaphase, replication of kinetochore DNA allows the sister chromatids to separate and move to opposite poles of the now-elliptical cell along the mitotic spindle. The centromere leads, with the chromatin dragging behind, often in a V shape. Other microtubules extending from the opposing centrioles meet and overlap without binding to the chromosomes. Continued polymerization and interaction between these microtubules helps push the centrioles apart. Thus chromatid translocation involves both pushing and pulling actions as well as molecular motor proteins on the spindle microtubules. 17 Anaphase 18  Telophase During telophase, the chromosomes begin to uncoil. Nucleoli and nuclear envelopes reappear as components of two separate nuclei at opposite ends of the cell. Nuclear envelope reassembly involves the dephosphorylation of nuclear lamins. A purse-string constriction, formed by microfilaments band beneath the plasma membrane, appears at the equator. Tightening of the constriction eventually divides the cytoplasm and organelles between the daughter cells, a process termed cytokinesis, which signals the end of mitosis. 19 20  During cytokinesis at the end of telophase, constriction of this ring produces a cleavage furrow and progresses until the cytoplasm and its organelles are divided into two daughter cells, each with one nucleus. 21  MITOSIS and MEIOSIS At the time of each somatic cell division (mitosis), the two DNA chains separate, each serving as a template for the synthesis of a new complementary chain. DNA polymerase catalyzes this reaction. One of the double helices thus formed goes to one daughter cell and one goes to the other, so the amount of DNA in each daughter cell is the same as that in the parent cell. The life cycle of the cell that begins after mitosis is highly regulated and is termed the cell cycle. The G1 (or Gap 1) phase represents a period of cell growth and divides the end of mitosis from the DNA synthesis (or S) phase. Following DNA synthesis, the cell enters another period of cell growth, the G2 (Gap 2) phase. The ending of this stage is marked by chromosome condensation and the beginning of mitosis (M stage). 22 Mitotic cells in adult tissues. Citation: The Nucleus, Mescher AL. Junqueira’s Basic Histology: Text and Atlas, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=190276489&bookid=2430 Accessed: July 10, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 23  MITOSIS and MEIOSIS In germ cells, reductive division (meiosis) takes place during maturation. The net result is that one of each pair of chromosomes ends up in each mature germ cell; consequently, each mature germ cell contains half the amount of chromosomal material found in somatic cells. Therefore, when a sperm unites with an ovum, the resulting zygote has the full complement of DNA, half of which came from the father and half from the mother. The term “ploidy” is sometimes used to refer to the number of chromosomes in cells. Normal resting diploid cells are euploid and become tetraploid just before division. Aneuploidy is the condition in which a cell contains other than the haploid number of chromosomes or an exact multiple of it, and this condition is common in cancerous cells. 24 Mitosis and meiosis. Citation: The Nucleus, Mescher AL. Junqueira’s Basic Histology: Text and Atlas, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=190276489&bookid=2430 Accessed: July 10, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 25  MEIOSIS Meiosis, from the Greek word meioun, meaning "to make small," refers to the specialized process by which germ cells divide to produce gametes. Because the chromosome number of a species remains the same from one generation to the next, the chromosome number of germ cells must be reduced by half during meiosis. To accomplish this feat, meiosis, unlike mitosis, involves a single round of DNA replication followed by two rounds of cell division. Meiosis also differs from mitosis in that it involves a process known as recombination, during which chromosomes exchange segments with one another. 26 https://microbenotes.com/meiosis/  MEIOSIS As a result, the gametes produced during meiosis are genetically unique. Two divisions, meiosis I and meiosis II, are required to produce gametes. Meiosis I is a unique cell division that occurs only in germ cells; meiosis II is similar to a mitotic division. Before germ cells enter meiosis, they are generally diploid, meaning that they have two homologous copies of each chromosome. Then, just before a germ cell enters meiosis, it duplicates its DNA so that the cell contains four DNA copies distributed between two pairs of homologous chromosomes. 27 https://microbenotes.com/meiosis/  28 https://mrleehamber119.wordpress.com/2020/02/20/ch-2-images/meiosis-i-and-ii-2/  Meiosis I Interphase Just like mitosis, meiosis also consists of a preparatory phase called interphase. The interphase is characterized by the following features : The nuclear envelope remains intact, and the chromosomes occur in the form of diffused, long, coiled, and indistinctly visible chromatin fibers. The DNA amount becomes double. Due to the accumulation of ribosomal RNA (rRNA) and ribosomal proteins in the nucleolus, the size of the nucleolus is significantly increased. In animal cells, a daughter pair of centrioles originates near the already existing centriole and, thus, an interphase cell has two pairs of centrioles. In the G2 phase of interphase, there is a decisive change that directs the cell toward meiosis, instead of mitosis. At the beginning of the first meiotic division, the nucleus of the dividing cell starts to increase in size by absorbing the water from the cytoplasm, and the nuclear volume increases about three folds. 29  Meiosis I 30 Figure: Phases of Meiosis I. Image Source: Wikipedia (Ali Zifan).  Prophase I Prophase I is the longest stage of the meiotic division. It includes the following substages: Leptotene In the leptotene stage, the chromosomes become even more uncoiled and resemble a long thread-like shape, and they develop bead-like structures called chromomeres. The chromosomes at this stage remain directed towards centrioles, so the chromosomes in the nucleus appear like a bouquet in the animal cell. Therefore, this stage is also called the Bouquet Stage. 31 https://microbenotes.com/meiosis/  Prophase I Zygotene or Synaptotene The zygotene stage begins with the pairing of homologous chromosomes, which is called synapsis. The paired homologous chromosomes are connected by a protein-containing framework called a synaptonemal complex. The synaptonemal complex helps to stabilize the pairing of homologous chromosomes and to facilitate recombination or crossing over. The synapsis might begin at one or more points along the length of the homologous chromosomes. Synapsis might start from the ends of the chromosomes and continues towards their centromeres (proterminal synapsis), or it might start at the centromere and proceed towards the ends (procentric pairing). In some cases, the synapsis occurs at various points of the homologous chromosomes (random pairing). 32 https://microbenotes.com/meiosis/  Prophase I Pachytene In this stage, the pair of chromosomes become twisted spirally around each other and cannot be distinguished separately. In the middle of the pachytene stage, each homologous chromosome splits lengthwise to form two chromatids, but they continue to be linked together by their common centromere. The chromosomes at this point are termed bivalent because it consists of two visible chromosomes, or as a tetrad because of the four visible chromatids. This stage is particularly crucial as a critical genetic phenomenon called “ crossing over” takes place in this stage. The crossing over involves redistribution and mutual exchange of hereditary material between two homologous chromosomes. The enzyme endonuclease breaks the non-sister chromatids at the place of crossing over. After the breaking of chromatids, the interchange of chromatid segments takes place between the non-sister chromatids of the homologous chromosomes. Another enzyme, ligase, binds the broken chromatid segments with the non-sister chromatid. The process of mutual exchange of chromatin material between one non-sister chromatid of each homologous chromosome is known as the crossing over. 33 https://microbenotes.com/meiosis/  Prophase I Diplotene The synaptonemal complex appears to be dissolved, leaving chromatids of the paired homologous chromosome physically joined at one or more localized points called In diplotene, chiasmata move towards the end of chromosomes in a zip like a manner. Diakinesis In this stage, the bivalent chromosomes become more condensed and uniformly distributed in the nucleus. At this point, the nuclear envelope breaks down, and the nucleolus disappears. Further, the chiasmata reach the end of the chromosomes, and the chromatids remain attached until metaphase. 34 https://microbenotes.com/meiosis/  Metaphase I Metaphase I consists of spindle fiber attachment to chromosomes and chromosomal alignment at the equator. During metaphase I, the spindle fibers are attached with the centromeres of the homologous chromosomes, which are directed towards the opposite poles. Anaphase I At anaphase I homologous chromosomes are separated from each other, and due to the shortening of chromosomal fibers or microtubules, each homologous chromosome with its two chromatids and undivided centromere move towards the opposite poles of the cell. Because during the chiasma formation, one of the chromatids has changed its counterpart, therefore, the two chromatids of a chromosome are not genetically identical. 35 https://microbenotes.com/meiosis/  Telophase I The onset of telophase I is defined by the movement of a haploid set of chromosomes at each pole. The nuclear envelope is formed around the chromosomes, and the chromosomes become uncoiled. The nucleolus reappears and, thus, two daughter nuclei are formed. Cytokinesis I In animals, cytokinesis occurs by the constriction of the cell membrane while in plants, it occurs through the formation of the cell plate, resulting in the creation of two daughter cells. 36 https://microbenotes.com/meiosis/  Meiosis II In the second phase of the meiotic division, the haploid cell divides mitotically and results in four haploid cells. This division is also known as the homotypic division. This division does not include the exchange of the genetic material and the reduction of the chromosome number as in the first meiotic division. 37 Figure: Phases of Meiosis II. Image Source: Wikipedia (Ali Zifan). Meiosis II consists of the following steps: Prophase II In prophase II, each centriole divides, resulting in two pairs of centrioles. The centrioles move towards the opposite poles and the nuclear membrane, and the nucleolus disappears. Metaphase II During metaphase II, the chromosomes get arranged on the equator of the cell through the spindle fibers. The centromere divides and, thus, each chromosome produces two daughter chromosomes. The spindle apparatus is attached to the centromere of each chromosome. 38 https://microbenotes.com/meiosis/ Anaphase II The daughter chromosomes move towards the opposite poles due to the shortening of chromosomal microtubules and the stretching of interzonal microtubules of the spindle. Telophase II The chromatids migrate to the opposite poles and now known as chromosomes. The endoplasmic reticulum forms the nuclear envelope around the chromosomes, and the nucleolus reappears due to the synthesis of ribosomal RNA. Cytokinesis II The process of cytokinesis is identical to cytokinesis I resulting in the division of cytoplasm for each of the four daughter cells formed. 39 https://microbenotes.com/meiosis/  In summary, meiosis and mitosis share many aspects of chromatin condensation and separation, but differ in key ways: Mitosis is a cell division that produces two diploid cells. Meiosis involves two cell divisions and produces four haploid cells. During meiotic crossing over, new combinations of genes are produced and every haploid cell is genetically unique. Lacking synapsis and the opportunity for DNA recombination, mitosis yields two cells that are the same genetically. 40 https://microbenotes.com/meiosis/ Essential differences between mitosis and meiosis within seminiferous tubules Mitosis Meiosis 1 cell division, 2 daughter cells 2 cell divisions, 4 daughter cells Chromosome number maintained Chromosome number halved No pairing, chromosome homologs Synapse of homologs, prophase I No crossovers >1 crossover per homolog pair Centromeres divide, anaphase Centromeres divide, anaphase II Identical daughter genotype Genetic variation in daughter cells 41