FBI101-6 Cell Division PDF

Assoc. Prof. Dr. Melis SÜMENGEN ÖZDENEFE 1 2  The continuity of life is based on the reproduction of cells, or cell division.  Cell division plays several important roles in life.  The division of one prokaryotic cell rep...

Assoc. Prof. Dr. Melis SÜMENGEN ÖZDENEFE 1 2  The continuity of life is based on the reproduction of cells, or cell division.  Cell division plays several important roles in life.  The division of one prokaryotic cell reproduces an entire organism.  The same is true of a unicellular eukaryote (Figure 12.2a). 3  Cell division also enables multicellular eukaryotes to develop from a single cell, like the fertilized egg that gave rise to the two- celled embryo in Figure 12.2b.  And after such an organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear or accidents. For example, dividing cells in your bone marrow continuously make new blood cells (Figure 12.2c). 4  The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells. (Our use of the words daughter or sister in relation to cells is not meant to imply gender.)  Passing identical genetic material to cellular offspring is a crucial function of cell division. 6  The reproduction of an assembly as complex as a cell cannot occur by a mere pinching in half; a cell is not like a soap bubble that simply enlarges and splits in two.  In both prokaryotes and eukaryotes, most cell division involves the distribution of identical genetic material—DNA—to two daughter cells. 7  What is most remarkable about cell division is the fidelity with which the DNA is passed along from one generation of cells to the next.  A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells. 8  A cell’s endowment of DNA, its genetic information, is called its genome.  Although a prokaryotic genome is often a single DNA molecule, eukaryotic genomes usually consist of a number of DNA molecules. 9  Yet before the cell can divide to form genetically identical daughter cells, all of this DNA must be copied, or replicated, and then the two copies must be separated so that each daughter cell ends up with a complete genome.  The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into structures called chromosomes, so named because they take up certain dyes used in microscopy (from the Greek chroma, color, and soma, body) (Figure 12.3). 10  Each eukaryotic chromosome consists of one very long, linear DNA molecule associated with many proteins.  The DNA molecule carries several hundred to a few thousand genes, the units of information that specify an organism’s inherited traits.  The associated proteins maintain the structure of the chromosome and help control the activity of the genes. 12  Together, the entire complex of DNA and proteins that is the building material of chromosomes is referred to as chromatin.  Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus. 13  For example, the nuclei of human somatic cells (all body cells except the reproductive cells) each contain 46 chromosomes, made up of two sets of 23, one set inherited from each parent.  Reproductive cells, or gametes—sperm and eggs—have half as many chromosomes as somatic cells, or one set of 23 chromosomes in humans. 14  The number of chromosomes in somatic cells varies widely among species:  18 in cabbage plants, 48 in chimpanzees, 56 in elephants, 90 in hedgehogs, and 148 in one species of alga. 15  When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each chromosome is in the form of a long, thin chromatin fiber.  After DNA replication, however, the chromosomes condense as a part of cell division:  Each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and so thick that we can see them with a light microscope. 16  Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome (Figure 12.4).  The two chromatids, each containing an identical DNA molecule, are initially attached all along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion.  Each sister chromatid has a centromere, a region containing specific DNA sequences where the chromatid is attached most closely to its sister chromatid. 17  Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one forming at each end of the cell.  Once the sister chromatids separate, they are no longer called sister chromatids but are considered individual chromosomes.  Thus, each new nucleus receives a collection of chromosomes identical to that of the parent cell (Figure 12.5). 19  Mitosis, the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm.  One cell has become two, each the genetic equivalent of the parent cell.  Mitosis and cytokinesis produced the 200 trillion somatic cells that now make up your body, and the same processes continue to generate new cells to replace dead and damaged ones. 21  In contrast, you produce gametes—eggs or sperm—by a variation of cell division called meiosis, which yields nonidentical daughter cells that have only one set of chromosomes, half as many chromosomes as the parent cell.  Meiosis in humans occurs only in the gonads (ovaries or testes). In each generation, meiosis reduces the chromosome number from 46 (two sets of chromosomes) to 23 (one set). 22  Fertilization fuses two gametes together and returns the chromosome number to 46, and mitosis conserves that number in every somatic cell nucleus of the new individual. 23  Mitosis is just one part of the cell cycle (Figure 12.6). In fact, the mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle.  Mitotic cell division alternates with a much longer stage called interphase, which often accounts for about 90% of the cycle. 24  During interphase, a cell that is about to divide grows and copies its chromosomes in preparation for cell division.  Interphase can be divided into subphases:  the G1 phase (“first gap”),  the S phase (“synthesis”), and  the G2 phase (“second gap”). 25  During all three subphases, a cell that will eventually divide grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.  However, chromosomes are duplicated only during the S phase.  Thus, a cell grows (G1), continues to grow as it copies its chromosomes (S), grows more as it completes preparations for cell division (G2), and divides (M). The daughter cells may then repeat the cycle. 26  Mitosis is broken down into five stages:  prophase, prometaphase, metaphase, anaphase, and telophase.  Overlapping with the latter stages of mitosis, cytokinesis completes the mitotic phase. 28 29  A nuclear envelope encloses the nucleus.  The nucleus contains one or more nucleoli (singular, nucleolus).  Two centrosomes have formed by duplication of a single centrosome.  Centrosomes are regions in animal cells that organize the microtubules of the spindle. 30  Each centrosome contains two centrioles.  Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed. 31  The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope.  The nucleoli disappear.  Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins (sister chromatid cohesion). 33  The mitotic spindle (named for its shape) begins to form. It is composed of the centrosomes and the microtubules that extend from them.  The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”).  The centrosomes move away from each other, propelled partly by the lengthening microtubules between them. 34  The nuclear envelope fragments.  The microtubules extending from each centrosome can now invade the nuclear area.  The chromosomes have become even more condensed.  Each of the two chromatids of each chromosome now has a kinetochore, a specialized protein structure at the centromere. 36  Some of the microtubules attach to the kinetochores, becoming “kinetochore microtubules,” which jerk the chromosomes back and forth.  Nonkinetochore microtubules interact with those from the opposite pole of the spindle. 37  The centrosomes are now at opposite poles of the cell. The chromosomes convene at the metaphase plate, a plane that is equidistant between the spindle’s two poles.  The chromosomes’ centromeres lie at the metaphase plate. For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. 39  Anaphase is the shortest stage of mitosis, often lasting only a few minutes.  Anaphase begins when the cohesin proteins are cleaved.  This allows the two sister chromatids of each pair to part suddenly. Each chromatid thus becomes a full-fledged chromosome.  The two liberated daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten.  Because these microtubules are attached at the centromere region, the chromosomes move centromere first. 42  The cell elongates as the nonkinetochore microtubules lengthen.  By the end of anaphase, the two ends of the cell have equivalent—and complete— collections of chromosomes. 43  Two daughter nuclei form in the cell.  Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system.  Nucleoli reappear.  The chromosomes become less condensed.  Any remaining spindle microtubules are depolymerized.  Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete. 46  The division of the cytoplasm is usually well under way by late telophase, so the two daughter cells appear shortly after the end of mitosis.  In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two. 47 51  Genes are the units of heredity and are made up of segments of DNA.  Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs).  Most DNA is packaged into chromosomes.  Humans have 46 chromosomes in their somatic cells, all cells of the body except gametes and their precursors.  A gene’s specific position along a chromosome is called the locus. 53  In asexual reproduction, a single individual passes all of its genes to its offspring without the fusion of gametes.  A clone is a group of genetically identical individuals from the same parent.  In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents.  0.5 mm Parent Bud (a) Hydra (b) Redwoods  A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.  Human somatic cells have 23 pairs of chromosomes. A karyotype is an ordered display of the pairs of chromosomes from a cell.  The two chromosomes in each pair are called homologous chromosomes, or homologs.  Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters. Application Technique Pair of homologous duplicated chromosomes Centromere 5 µm Sister chromatids Metaphase chromosome  The sex chromosomes, which determine the sex of the individual, are called X and Y.  Human females have a homologous pair of X chromosomes (XX).  Human males have one X and one Y chromosome.  The remaining 22 pairs of chromosomes are called autosomes.  Each pair of homologous chromosomes includes one chromosome from each parent.  The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father.  A diploid cell (2n) has two sets of chromosomes.  For humans, the diploid number is 46 (2n = 46)  In a cell in which DNA synthesis has occurred, each chromosome is replicated.  Each replicated chromosome consists of two identical sister chromatids.  Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Sister chromatids of one duplicated chromosome Centromere Two nonsister Pair of homologous chromatids in chromosomes a homologous pair (one from each set)  A gamete (sperm or egg) contains a single set of chromosomes and is haploid (n).  For humans, the haploid number is 23 (n = 23)  Each set of 23 consists of 22 autosomes and a single sex chromosome.  In an unfertilized egg (ovum), the sex chromosome is X. In a sperm cell, the sex chromosome may be either X or Y.  Fertilization is the union of gametes (the sperm and the egg).  The fertilized egg is called a zygote and has one set of chromosomes from each parent.  The zygote produces somatic cells by mitosis and develops into an adult.  At sexual maturity, the ovaries and testes produce haploid gametes.  Gametes are the only types of human cells produced by meiosis, rather than mitosis.  Meiosis results in one set of chromosomes in each gamete.  Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number. Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)  Like mitosis, meiosis is preceded by the replication of chromosomes.  Meiosis takes place in two consecutive cell divisions, called meiosis I and meiosis II.  The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis.  Each daughter cell has only half as many chromosomes as the parent cell. 69  Chromosomes duplicate during interphase.  The resulting sister chromatids are closely associated along their lengths.  This is called sister chromatid cohesion.  The chromatids are sorted into four haploid daughter cells. Interphase Pair of homologous chromosomes in diploid parent cell Chromosomes Pair of duplicated duplicate homologous chromosomes Sister Diploid cell with chromatids duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes  MEIOSIS I: Separates MEIOSIS II: Separates homologous chromosomes sister chromatids Telophase I Telophase II Prophase I Metaphase I Anaphase I and Prophase II Metaphase II Anaphase II and Cytokinesis Cytokinesis Centrosome Centromere (with (with centriole kineto- Sister chromatids Sister pair) chore) remain chroma- Chiasmata tids Meta- attached phase Spindle plate Sister Haploid chromatids daughter Homologous Cleavage separate cells Homo- chromo- furrow forming logous somes chromo- separate somes Fragments of nuclear Microtubules envelope attached to kinetochore  Division in meiosis I occurs in four phases ◦ Prophase I ◦ Metaphase I ◦ Anaphase I ◦ Telophase I and cytokinesis Prophase I  In early prophase I each chromosome pairs with its homolog and crossing over occurs.  X-shaped regions called chiasmata are sites of crossover. Metaphase I  In metaphase I, pairs of homologs line up at the metaphase plate, with one chromosome facing each pole.  Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad.  Microtubules from the other pole are attached to the kinetochore of the other chromosome. Anaphase I  In anaphase I, pairs of homologous chromosomes separate.  One chromosome of each pair moves toward opposite poles, guided by the spindle apparatus.  Sister chromatids remain attached at the centromere and move as one unit toward the pole. Telophase I and Cytokinesis  In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids.  Cytokinesis usually occurs simultaneously, forming two haploid daughter cells.  In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.  No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated.  MEIOSIS I: Separates homologous chromosomes Telophase I Prophase I Metaphase I Anaphase I and Cytokinesis Centrosome Centromere (with (with centriole kineto- Sister chromatids Sister pair) chore) remain chroma- Chiasmata Meta- attached tids phase Spindle plate Homo- Homologous Cleavage logous chromo- furrow chromo- somes somes Fragments separate of nuclear Microtubules envelope attached to kinetochore  Division in meiosis II also occurs in four phases ◦ Prophase II ◦ Metaphase II ◦ Anaphase II ◦ Telophase II and cytokinesis  Meiosis II is very similar to mitosis. Prophase II  In prophase II, a spindle apparatus forms.  In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate. Metaphase II  In metaphase II, the sister chromatids are arranged at the metaphase plate.  Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.  The kinetochores of sister chromatids attach to microtubules extending from opposite poles. Anaphase II  In anaphase II, the sister chromatids separate.  The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles. Telophase II and Cytokinesis  In telophase II, the chromosomes arrive at opposite poles.  Nuclei form, and the chromosomes begin decondensing.  Cytokinesis separates the cytoplasm.  At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes.  Each daughter cell is genetically distinct from the others and from the parent cell.  MEIOSIS II: Separates sister chromatids Telophase II Prophase II Metaphase II Anaphase II and Cytokinesis Sister Haploid chromatids daughter separate cells forming  After interphase the sister chromatids are held together by proteins called cohesins.  The nonsister chromatids are broken at precisely corresponding positions.  A zipper-like structure called the synaptonemal complex holds the homologs together tightly.  DNA breaks are repaired, joining DNA from one nonsister chromatid to the corresponding segment of another. 90  Pair of homologous DNA DNA chromosomes: breaks breaks Crossover Crossover Centromere Paternal Cohesins sister chromatids 1 Maternal 3 sister chromatids Chiasmata Synaptonemal complex forming 2 4  Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell.  Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell.  MITOSIS MEIOSIS Parent cell Chiasma MEIOSIS I Prophase Prophase I Homologous Chromosome Chromosome chromosome Duplicated duplication duplication pair 2n = 6 chromosome Individual Pairs of Metaphase Metaphase I chromosomes homologous line up. chromosomes line up. Anaphase I Sister Homologs Telophase I Anaphase Telophase chromatids separate. separate. Daughter cells of Sister meiosis I MEIOSIS 2n 2n chroma- II Daughter cells tids of mitosis separate. n n n n Daughter cells of meiosis II  SUMMARY Mitosis (occurs in both diploid Property Meiosis (can only occur in diploid cells) and haploid cells) DNA Occurs during interphase Occurs during interphase before replication before mitosis begins meiosis I begins Number of One, including prophase, Two, each including prophase, metaphase, divisions prometaphase, metaphase, anaphase, and telophase anaphase, and telophase Synapsis of Does not occur Occurs during prophase I along with homologous crossing over between nonsister chromosomes chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of Two, each genetically Four, each haploid (n); genetically different daughter cells identical to the parent from the parent cell and from each other and genetic cell, with the same number composition of chromosomes Role in the Enables multicellular animal Produces gametes (in animals) or spores animal or or plant (gametophyte or (in the sporophyte plant); reduces number plant body sporophyte) to arise from a of chromosomes sets by half and introduces single cell; produces cells genetic variability among the gametes or for growth, repair, and, in spores some species, asexual reproduction; produces gametes in the gametophyte plant  Three events are unique to meiosis, and all three occur in meiosis l ◦ Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information ◦ Homologous pairs at the metaphase plate ◦ Separation of homologs during anaphase I  Sister chromatid cohesion allows sister chromatids to stay together through meiosis I.  In mitosis, cohesins are cleaved at the end of metaphase.  In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids).  Meiosis I is called the reductional division because it reduces the number of chromosomes per cell. 97