D2.1.1-2.1.7 Cell Biology PDF

Continuity and change D2.1.1 Generation of new cells in living organisms by cell division All organisms need to produce new cells, for growth, maintenanceand...

Continuity and change D2.1.1 Generation of new cells in living organisms by cell division All organisms need to produce new cells, for growth, maintenanceand reproduction. They do this by cell division. One cell divides into two. The divides is called the mother cell and those produced from it are daughtercells The mother cell disappears as an entity in the process, unlike reproductionby animal parents. There is strong evidence for the theory that new cells are only ever produced by division of a pre-existing cell. The implications of this theory are profound. If we consider the trillionsof cells in our bodies, each one was formed when a pre-existing cell divided intwo. We can trace this back to the original cell—the zygote that was the startofour individual lives, produced by the fusion of a sperm and an egg. Sperm and egg cells were produced by cell division in our parents.The origins of all cells in our parents' bodies goes back to the zygote from which they developed and then on through all previous generations of human ancestors.If we accept that humans evolved from pre-existing ancestral species, we cantrace the origins of cells back through hundreds of millions of years to the earliestcells on Earth. This means there is a continuity of life from its beginnings to the cellsin our bodies todayc A Figure 3 Attempts are being made by BaSyC, a research group in the Netherlands,to builda new living cell from individual lifeless components. This flying-cell artwork representsthechallenges of the still unachieved endeavour 610  Cells D2.1.2 Cytokinesis as splitting of cytoplasm ina parent cell between daughter cells Incytokinesis,the cytoplasm of a cell is divided between two daughter cells. It isa partof cell division, along with nuclear division by mitosis or meiosis. The processof cytokinesis can begin as soon as chromosomes have separated and arefarenough apart to ensure that none of them ends up in the wrong cell. All thecytoplasm and its contents of the mother cell are shared out between the daughtercells. Plant and animal cells carry out cytokinesis differently. Inanimalcells, the plasma membrane is pulled inwards around the equator of the cell toforma cleavage furrow. This is accomplished using a ring of contractile proteins immediatelyinside the plasma membrane, usually at the equator. The proteins are actinand myosin and are similar to those that cause contraction in muscle. When the cleavagefurrow reaches the centre, the cell is pinched apart into two daughter cells. Inplant cells, microtubules are built into a scaffold straddling the equator, which is used to assemble a layer of vesicles. The vesicles fuse together to form plate- A Figure4 A starfishzygote has just shapedstructures. With the fusion of more vesicles, two complete layers of divided for the firsttime, to produce a two- membraneare formed across the whole of the equator of the cell. They become cell embryo theplasma membranes of the two daughter cells adjacent to the new dividing walls.They are connected to the existing plasma membranes at the sides of the cell, completing the division of the cytoplasm. bundles of microtubules scaffold the formation of a new cell wall across the equator with new plasma membrane on eitherside Figure 5 Construction of a new plant cell walls across the equator of a diving cell new cell wall completed across the equator vesicles vesicles complete aligned fuse to double membrane plasmodesmata on the form forming formed equator plates 611  Continuity and change substances to be brought is for pectins and other in The next stage in plants between the two new membranes. exocytosis vesicles and deposited by This the new cell walls. Both daughter cells that will link then forms the middle lamella deposit it by exocytosis adjacent to the and bring cellulose to the equator middle result, each cell builds its own cell wall across the equator. lamella. As a Figure6 These cells are from a growing onion root. Cell A is large enough to divide but is in the early stages of mitosis so cannot yet divide Cell B has nearly completed mitosis and is already constructing a wall across the equator, with the microtubule scaffolding visible Label C points to where the previous cell division occurred to produce cells A and B from a mother cell D2.1.3 Equal and unequal cytokinesis the mother cell into equal In many cases, cytokinesis divides the cytoplasm of growing root tip. Root growth is due to enlargement halves. This happens in a columns. The cells in a column all differentiatein and division of cells arranged in equally when they divide. the same way, so cytoplasm is apportioned Cytoplasm is sometimes divided unequally. Smali cells produced by unequal division can survive and grow if they receive nucleus and at least one of each organelle that cannot be assembled components in the cell. For example, mitochondria can only be produced division ofa pre-existing mitochondrion, so there must be at least ore mitochondrion in a daughter are budding in yeast cell for it to be viable. Two examples of unequal division and oogenesis in humans. Figure 7 Nuclei in these onion root cells have been stained red. New cell walls have mostly divided cytoplasm equally, as at X; but in some cases, the division was unequal, as at Y 612  Cells Figure8 Occasionally a plant cell divides and its chloroplasts all pass into one of the two daughter cells. The other cell with no chloroplasts can never regain them and it produces more cells without chloroplasts when it divides. These leaves were collected from one plant and show areas of cells without chloroplasts yeast Buddingin reproduce asexual's in a process called budding. The nucleus divides A small outgrowth of the mother cell is formed. It receives one of the b,mitosts.. but small share of the cytoplasm. A dividing wall is constructed, two cells. The snnall cell then splits away, leaving a scar where it eparatingthe larger cell. Yeast cells carry out this budding process repeat- wasattachedto the do not have to double in size between each division. edtyand Oogenesisin humans -neproductionof both sperm and eggs in humans starts with two divisions of mothercell. During sperm production, the cytoplasm is divided equally in and second divisions, resulting in four, equally sized small cells, each of into a mature sperm. *hichdevelops large numbers of sperm are needed in humans, usually only one egg at a time, with enough stored food to sustain the cell(oocyte)is produced developing embryo. There is therefore unequal division of cytoplasm during oogenesis.The first di€sion produces one large cell with nearly all the cytoplasm A Figure 9 There are many different does not develop further.Only the large cell carries species of yeast. These are Komagataella andasmallpolar body unequal division of the cytoplasm again resulting phaffii which can use methanol as a carbon outthesecond division source if glucose is not available. Several inonelarge cell and small polar body. The large cell develops into a fertilization. cells are budding nature oocyte that is cytoplasm of oocyte polar body zona pellucida human oocyte is visible. coat (zona pellucida) around this Figure10 The protective jelly a tiny polar body that contains food stores. To the right is Thedensecytoplasm of the oocyte not survive cytoplasm because it has no role and will hasreceived a nucleus but very little 613  Continuity and change D2.1.4 Roles of mitosis and meiosis in eukaryotes If a cell divides without firstundergoing nuclear division, one daughter cell has the nucleus and the other one would be anucleate (without a nucleus) O Anucleate cells cannot synthesize polypeptides, so they cannot grow or maintain themselves. They have limited lifespans. For example, red blood cells, which have no nucleus, survive for about 120 days, To produce extra nuclei before cell division, cells undergo either mitosis or meiosis. These two types of nuclear division have differentroles, so most organisms use both during their lifecycle. Mitosis—continuity Meiosis—change O Mitosis is used to produce genetically Meiosis is used to halve the chromosome identical cells. 2n is the diploid number number from diploid (2n) to haploid (n) of chromosomes. In humans, n = 23. and to generate genetic diversity division of a cell with only one nucleus produces one nucleated and one anucleate cell Cells produced using mitosis Cells produced using meiosis have the same number of have half as many chromosomes oo chromosomes as the parent cell, as the parentcell. Division so the chromosome number is of a nucleus with two sets of maintained. chromosomes (diploid) resultsin Cells produced by mitosis have nuclei with only one set (haploid). the same genes as the parent cell, This is essential to produce so mitosis maintains the genome. hapioid gametes fromdiploid This ensures that every cell in a germ cells in sexual life cycles. O multicellular organism has all the Pairs of genes in a diploid genes that it needs. It also ensures mother cell are dealt randomly that the cells in an individual are to daughter cells, so thereare genetically identical, preventing an almost limitless numbersof to produce two daughter cells, problems such as tissue rejection. possible combinations. Meiosis each with a nucleus, the mother Mitosis allows a successful therefore generates variation cell's nucleus must first be genome to be inherited without and genetic diversity,allowing divided changes by offspring in asexual evolution by naturalselection. A Figure 11 Nucleated and anucleate reproduction. cells produced from cell division 614  Cells 02.1.5 DNA replication as a prerequisite mitosis and for a both meiosis preparing for nuclear division Acellthatis by mitosis or meiosis replicates all the This ensures that each daughter cell DNA. produced receives it to perform any function a full complement ofgenes,allowing required. An earlier was that a single centromere held hypothesis, nowfalsified, the chromatids together when it divided, allowing the chromatids until anaphase, to separate. Beforereplication, the DNA within the nucleus exists as long single molecules calledchromosomes. After replication, there are pairs of identical DNA molecules. These identical DNA molecules are still considered to be part of the samechromosome and they are held together by loops of a protein complex, calledcohesin. The cohesin loops are not cut until the start of anaphase during mitosisor meiosis. Earlyinterphase Prophase Metaphase A Figure 13 The number and structure Anaphase of chromosomes in a species can be studied by staining cells in mitosis with a pigment that binds to DNA and then by burstingthe cells on a microscope slide so the chromosomes spread out. The Indian muntjac, Muntiacus muntjak (top) has the smallest number of chromosomes per body cell among mammals (2n = 6) and the Viscacha rat, Tympanoctomysbarrerae (bottom) has the largest number (2n = 102) singlechromatid chromosome microtubules cohesin loops have beforeDNA with two attached to been cut, so sister replication.During chromatids centromeres chromatids can interphase it he!d together pull on chromatids separate and be wouldbe much by cohesin but cohesin holds pulled to opposite moreelongated Icons, as them together poles in prophase of mitosis A Figure 12 Chromosomes, chromatids and cohesion loops WhenDNA is in an elongated state, chromosomes are too narrow to be seen witha light microscope. They gradually become shorter and fatterduring theearlystages of mitosis or meiosis and are then visible. Eventuallyeach chromosomecan be seen to have two strands, called chromatids. Each chromatidcontains a single very long DNA molecule, produced by DNA replicationfrom an original molecule. The two strands in a chromosome are thereforeknown as sister chromatids and they are genetically identical. Strands ondifferentchromosomes are non-sister chromatids and do not usually have identicalgenes. Figure 13 shows chromosomes consisting of pairs of sister chromosomesin two species of mammal. 615  Continuityand change D2.1.6 Condensation and movement of chromosomes as shared features of mitosis and meiosis During mitosis and meiosis, chromosomes are moved to opposite poles of the cell, so they can become part of separate nuclei (Figure 14). The DNA molecules in these chromosomes are immensely long. For example, the average length in human chromosomes is more than 50,000 pm and the nucleus is less than 5 pm wide. To separate and move molecules as elongated as this withoutknots tangles or breaks they must be packaged into much shorter structures.This condensation of chromosomes and their subsequent movement is thereforean essential feature of mitosis and meiosis. in mitosis and the 2nd in the 1stdivision of meiosis division of meiosis sister homologous chromosomes chromatids separate and (each with two sister chromatids) are moved to opposite poles are moved to opposite poles opposite poles equator of cell A Figure 14 Movement of chromosomes during mitosisand meiosis Chromosomes are condensed by being made shorter An initial shortening is carried out by wrapping the double helix of DNA histone proteins to form nucleosomest and linking the nucleosomes together nere are several more stages to condense the chromosomes but they are not yet fully understood—this is an active research field. Chromosomes are moved using microtubules. A mic:otubule is a hollow cylinderof tubulin proteins that can be rapidly assembled or disassembled. During interphase, microtubules serve a variety of functions including acting as a cytoskeleton. Some of these microtubules are disassembled in the early stages of mitosis and are reassembled by microtubule organizing centres (MTOCs) at the poles of the cell, which link tubulin molecules together. Microtubules are assembled that reach the equator of the cell, forming a spindle-shaped array. At the same time, protein structures called kinetochores are assembled on the centromere of each chromatid. Some of the growing microtubules link up with these kinetochoresand some attach to other microtubules from the opposite pole. 616  Cells there are two types of tubulin in microtubules, both arranged in a helix cetubulin ß-tubulin 24 nm microtubules are at 2 nm the DNA the next pair of hollow and rigid like double helix is narrower tubulin subunits will steel scaffolding than a microtubule be added here poles figure 15 Microtubules can be changed in length by adding tubulins at one end, or removingthem from the other end Thekinetochore acts as a microtubule motor by removing tubulin subunits from theattached ends of the microtubules. This shortens the microtubules linking thekinetochores to the poles, putting them under tension. Initially, in mitosis thechromatidsdo not move because loops of cohesin hold them together. As soonas the cohesin hes been cut, shortening of the spindle microtubules by the kinetochorescauses sister chromatids to move to opposite poles. In meiosis, homologous chromosomes are initially held together by knot-like structures calledchiasmata, buy en these have slid to the ends of the chromosomes, movementto opposit- r oles can begin. D2.1.7 Phases of mitosis Mitosisrequires a choreographed sequence of actions. These are usuallyconsidered as (our phases. Prophase—thestarting phase with condensation of chromosomes (pro = before). Metaphase—the phase after condensation with chromosomes released from the nucleus (meta = after). Anaphase—a brief phase during which the chromosomes are moved up to A Figure 16 Fluorescentstains have been poles from the equator (ana = up). used to reveal the position in anaphase of DNA (blue), kinetochores(red) and Telophase—the final phase in which nuclei reform and chromosomes telomeres which form the ends of the decondense (telos = finally). chromosomes (green) 617  Continuity and change The chromosomes are Interphase chromatin (dispersed cytoplasm chromosomes dispersed through the inside the nucleus) nucleus so are not individually discernible. To prepare for mitosis, all of the DNA is replicated and each chromosome then consists of two very elongated chromatids containing identical DNA. plasma nuclear membrane membrane Interphase—preceding mitosis The chromosomes condense Prophase by packing the DNA tightly sister kinetochore into thicker, shorter structures. chromatids attached to the This is a protracted process held together centromere that continues throughout by loops of of the prophase. cohesin chromatid Towards the end of prophase microtubules grow from structures at the poles of the cell called microtubule organizing centres (MTOCs) microtubules spindle to form a spindle-shaped array organizing microtubules linking the poles of the cell. centre (MTOC) At the end of prophase the nuclear membrane breaks Prophase do„vn. Metaphase Microtubules growing chromosomes chromosomes {rcm the poles attachto the aligned on fully of each chromatid. the equator condensed Siste! chromatids within each chromosome become attached to opposite poles. The spindle microtubules kinetochore are put under tension to test spindle whether the attachment is microtubules correct. If the attachment is cytoplasm not MTOC correct, the chromosomes separated from cannot yet be pulled to either nucleus by a pole due to cohesin loops. nuclear membrane the At the end of metaphase, Metaphase on chromosomes are aligned the equator of the cell. 618  Cells Anaphase Cohesin loops that have kinetochore removes tubulin held the sister chromatids subunits to shorten spindle microtubule and pull genetically together are now cut, so the chromosome to the pole identical chromatids become separate chromosomes chromosomes. (formerly sister chromatids) Microtubules link each moving to chromosome to one of the opposite poles. A kinetochore shortens poles the microtubules, pulling the chromosome to the pole. At the end of anaphase all the chromosomes have reached chromosome arms trail behind in the viscous the poles but have not started to cytoplasm decondense. Anaphase Telophase At each pole the cytokinesis has chromosomes chromosomes are pulled into started with are now a tight group near the MTOC a furrow where enclosed and a nuclear membrane the plasma inside a reforms around them. membrane is nuclear pulled in membrane The chromosomes around the decondense and spread out equator to form dispersed chromatin inside the nucleus. By this stage of mitosis the cell is usuallyalready dividing its cytoplasm and the two daughter cells produced by chromosomes this enter interphase again. are decondensing Telophase Interphase Genes in the decondensed chromosomes each chromosomes can be consisting of a single transcribed and the mRNA DNA molecule are translated to synthesize dispersed throughout proteins that the cell needs. the nucleus The cell grows usually cytokinesiscompleted doubling in size before the with the cytoplasm and next mitosis. plasma membrane pinched apart cytoplasm is very active Interphasefollowing mitosis 619