The Cell Cycle PDF

Introduction  The continuity of life from one cell to another is based on the reproduction of cells via cell division.  This division process occurs as part of the cell cycle (the life of a cell from its origin in the division of a parent cell until its own division into two).  The division of...

Introduction  The continuity of life from one cell to another is based on the reproduction of cells via cell division.  This division process occurs as part of the cell cycle (the life of a cell from its origin in the division of a parent cell until its own division into two).  The division of a unicellular organism (e.g. Amoeba) reproduces an entire organism, increasing the population.  Cell division is also central to the development of a multicellular organism that begins as a fertilized egg or zygote. Multicellular organisms also use cell division to repair and renew cells that die normally or by accidents (blood cells from bone marrow). Cell division distributes the genetic material (DNA) to two daughter cells. Division is different among cells:. - Skin cells divide frequently. - Liver cells divide when needed (damage repair). - Nerve cells and muscle cells do not divide at all. Cell division distributes identical sets of chromosomes to daughter cells 3  A cell’s genetic information (genome) is packaged as DNA.  In prokaryotes, the genome is often a single long DNA molecule.  In eukaryotes, the genome consists of several DNA molecules.  A human cell must duplicate about 3 m of DNA and separate the two copies such that each daughter cell ends up with a complete genome.  DNA molecules are packaged into chromosomes.  Every eukaryotic species has a characteristic number of chromosomes in the nucleus.  Human somatic cells (body cells) have chromosomes. Human gametes (sperm or eggs) have 23 chromosomes, half the number in a somatic cell.   Each eukaryotic chromosome consists of a long, linear DNA molecule. 46  Each chromosome has hundreds or thousands of genes (the units that specify an organism’s inherited characters).  This DNA-protein complex (chromatin) is organized into a long thin fiber.  After the DNA duplication, chromatin condenses to form (chromosome). • • • • Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome’s DNA. The narrow region where the chromosomal strands connect is the called centromere. Later, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell during cell division. The process of the formation of the two daughter nuclei is called (mitosis) and is usually followed by division of the cytoplasm (cytokinesis). It occurs in somatic cells. Chromatid Chromatin Protein + DNA Sister chromatid Centromere Homologous Chromosome Chromosome  In the gonads, cells undergo a meiotic division, which yields four daughter cells, each with half the chromosomes’ number of the parent cell.  In humans, meiosis reduces the number of chromosomes from 46 to 23.  Each of us inherited 23 chromosomes from each parent: one set in an egg and one set in a sperm during meiosis.  gametes (eggs or sperm) are produced only in gonads (ovaries or testes).  The fertilized egg undergoes trillions of cycles of mitosis and cytokinesis to produce a fully developed multicellular human.  These processes continue every day to replace dead and damaged cell.  Fertilization fuses two gametes together and doubles the number of chromosomes to 46 again. Genes: The units that specify an organism’s inherited characters. Chromatin: A DNA-protein complex which is organized into a long thin fiber Chromosome: The package that is formed from a condensed, coiled and folded chromatin. Chromatids: Two sister arms (chromatids) formed from each duplicated chromosome. They contain identical copies of the chromosome’s DNA Centromere: The narrow region at which the chromosomal strands (Chromatids) are connected together. Mitosis: Is the division process which forms two daughter nuclei Cytokinesis: Is the division stage of the cytoplasm which usually follows mitosis. Ia Meiosis: A division process that occurs In the gonads, and yields four daughter cells, each with half the chromosomes of the parent. The mitotic phase alternates with interphase in the cell cycle: Phases of the Cell Cycle • • A. B. • The mitotic (M) phase of the cell cycle alternates ‫ تتبادل‬with the much longer interphase. – The M phase includes mitosis and cytokinesis . – Interphase accounts for 90% of the cell cycle. During interphase the cell prepares for division by producing cytoplasmic organelles and copying its chromosomes. Interphase has three sub-phases: 1. The G1 phase (“first gap”): cell is carrying out its everyday activities. 2. The S phase (“synthesis”): genetic material replicates itself, which allows the cell to contain enough material for 2 cells upon division 3. The G2 phase (“second gap”): cellular organelles are produced to allow for an adequate amount for the new cell being produced. Division phase (M). The cell starts the division process. The resulting daughter cells may then repeat the cycle again. O M A. Mitosis: usually includes five sub-phases :      Prophase, Prometaphase, Metaphase Anaphase Telophase • By late interphase (G2), the chromosomes have been duplicated but are loosely packed. • The centrosomes have been duplicated and begin to organize microtubules into an aster (“star”). 9 1) Prophase, the chromosomes are tightly coiled, with sister chromatids joined 10 together, The nucleoli disappear. The mitotic spindle begins to form and appears to push the centrosomes away from each other towards opposite ends (poles) of the cell. 2) Prometaphase, the nuclear envelope fragments and microtubules from one pole attach to one of two kinetochores (special regions of the centromere) while microtubules from the other pole attach to the other kinetochore. 3) Metaphase, the spindle fibers push the sister chromatids until they are all arranged at the imaginary plane equidistant between the poles, defining metaphase.  Anaphase, the centromeres divide, result in separating the sister chromatids. Each is then pulled 11 toward the pole to which it is attached by spindle fibers. By the end, the two poles have equivalent collections of chromosomes.  Telophase, the cell continues to elongate as free spindle fibers from each centrosome push off each other. 1) 2) 3) Two nuclei begin to form, surrounded by the fragments of the parent’s nuclear envelope. Chromatin becomes less tightly coiled. Cytokinesis, begins as the division of the cytoplasm occurs. B. The cytokinesis: is the division of the cytoplasm:  Cytokinesis (division of the cytoplasm) typically follows mitosis. 14  Contraction of the cell pinches the cell into two new cells 15 Cell Cycle Interphase Division process (M) Cytokinesis Mitosis G1 S Prophase G2 Prometaphase Metaphase Anaphase Telophase Fertilization and Meiosis alternate in sexual life cycles  A life cycle of an organism is the generation-to-generation sequence of stages in its reproductive history.  It starts at the conception of an organism until it produces its own offspring.  In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.  These homologous chromosome pairs carry genes that control the same inherited characters.  A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern. The Karyotype: It is a display of an individual’s chromosomes those are arranged according to size and shapes 18 Chromosomes (sex and autosomes)  An exception to the rule of homologous chromosomes is found in the sex chromosomes, the X and the Y.  The pattern of inheritance of these chromosomes determine an individual’s sex.   Human females have a homologous pair of X chromosomes (XX). Human males have an X and a Y chromosome (XY).  The other 22 pairs are called autosomes. • We inherit one chromosome of each homologous pair from each parent. – The 46 chromosomes in a somatic cell can be viewed as two sets of 23, a maternal set and a paternal set. • Sperm cells or ova (gametes) have only one set of chromosomes 22 autosomes and an X or a Y.  A cell with a single chromosome set is called haploid.  For humans, the haploid number of chromosomes is 23 (n = 23).  A haploid sperm reaches and fuses with a haploid ovum.  These cells fuse (syngamy) resulting in fertilization.  The fertilized egg (zygote) now has a diploid set of chromosomes from the maternal and paternal family lines.  The zygote and all cells with two sets of chromosomes are diploid cells 46 (2n = 46). • As an organism develops from a zygote to a sexually mature adult, the zygote’s genes are passes on to all somatic cells by mitosis. • Gametes, which develop in the gonads, are not produced by mitosis. • Instead, gametes undergo the process of meiosis in which the chromosome number is halved. Behavior of Chromosome Sets in the Human Life Cycle Fertilization restores the diploid condition by combining two haploid sets of chromosomes.  Fertilization and meiosis alternate in sexual life cycles.  Gametes:  produced by meiosis,  are the only haploid cells.  undergo no divisions themselves, but fuse to form a diploid zygote that divides by mitosis to produce a multicellular organism r Ing o Meiosis (Reduction Division) Reduces chromosome number from diploid to haploid : 22 in  Many steps of meiosis resemble steps mitosis.  Both are preceded by the replication of chromosomes.  However, in meiosis, chromosomes replicate once followed by two consecutive cell divisions, meiosis I and meiosis II, which results in four daughter cells.  Each final daughter cell has only half chromosomes number (haploid).  Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice.  The first division (meiosis I) separates homologous chromosomes.  The second (meiosis II) separates sister chromatids. 2- Meiosis A)- Meiosis I: 1)- interphase the chromosomes are replicated to form sister chromatids. 2)- Prophase I, the chromosomes condense and homologous chromosomes pair up to form tetrads. • Homologous chromosomes attached together (synapsis). – Chromatids of homologous chromosomes are crossed (at chiasmata) and segments of the chromosomes are exchanged (Crossing Over). 3)- Metaphase I, the tetrads are all arranged at the metaphase plate.  Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other. 4)- Anaphase I, the homologous chromosomes separate and are pulled toward opposite poles. 5)- Telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole.  Each chromosome consists of linked sister chromatids.  Cytokinesis follows B)- Meiosis II 1)- Prophase II a spindle apparatus forms, attaches to kinetochores of each sister chromatids, and moves them 26 around. 2)- Metaphase II, the sister chromatids are arranged at the metaphase plate. 3)- Anaphase II, the centromeres of sister chromatids separate and the separate sisters chromatids travel toward opposite poles. 4)- Telophase II, separated sister chromatids arrive at opposite poles.  Nuclei are formed around the chromatids.  Cytokinesis separates the cytoplasm.  At the end of meiosis, there are four haploid daughter cells. Meiosis Division (Reduction Division) Occurs in two steps A)- Meiosis I B)- Meiosis II - Separate homologous chromosomes. - No further replication of chromosomes. - Results in 2 cells with replicated -Occurs in the newly resulting cells from Meiosis I. chromosomes. a O (4 haploid cells) It occurs mainly in sex gonads to form Gametes (sperms and ova) Each of the resulting cells has half number of chromosomes of the original cell (23 in human). Thus, it is called Reduction Division Crossing over Chiasma Recombinant Chromosomes Crossing over -Occurs during prophase I. -The two homologous chromosomes joint together very closely. -Two non-sister chromatids of the homologous chromosomes are crossed over at a chiasma point and exchange corresponding segments. -The resulting chromosomes are called “recombinant chromosomes”. -It is important in genetic variation in sexual life cycle. Crossing over  3 steps cross over  breakage of DNA parts  re-fusing of DNA parts  New combinations of traits  What are the advantages of crossing over in sexual reproduction? Sexual life cycles produce genetic variation among offspring  32 Three mechanisms contribute to genetic variation: 1) independent assortment 2) crossing over 3) random fertilization 1)- Independent assortment: of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate. – There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.  Independent assortment alone would find each individual chromosome in a gamete that 33 would be exclusively maternal or paternal in origin. 2)- Crossing over: Homologous portions of two nonsister chromatids exchange places, producing recombinant chromosomes which combine genes inherited from each parent. 3)- The random fertilization: it adds to • the genetic variation arising from meiosis. Any sperm can fuse with any egg. Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells. 34 Comparison between Mitosis and Meiosis  The chromosome number is reduced by half in meiosis, but not in mitosis.   Mitosis produces daughter cells that are genetically identical to the parent and to each other. Meiosis produces cells that differ from the parent and each other. • Three events, unique to meiosis, occur during the first division cycle. 1. During prophase I, homologous chromosomes pair up in a process called synapsis. – Later in prophase I, the joined homologous chromosomes are visible as a tetrad. – At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged. – Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement. 2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.  In humans, you would see 23 tetrads. 3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell.  Sister chromatids remain attached at the centromere until anaphase II.  The processes during the second meiotic division are virtually identical to those of mitosis.