Unit 14: Cell biology. 14 4 Cell division The process of cell division is responsible for a new life being made. When an egg is fertilised it divides over and over again to eventually make a baby. Children and adults continue to rely on the process of cell division for growth and repair. Cells renew themselves all the time, for example, in the gut cells are replaced at a very fast rate. Cell division can, however, also cause terrible medical problems such as cancer. If a cell divides uncontrollably, a mass of tissue known as a tumour can form. If the tumour is malignant the person will need to undergo treatment for cancer to remove the tissue mass. On successful completion of this topic you will: understand the growth and development of cells (LO3). To achieve a Pass in this unit you need to: compare the processes of mitosis and meiosis (3.1) explain the events of the cell division cycle (3.2) explain the control of the cell division cycle (3.3) explain how multicellular organisms develop by a process of growth and differentiation (3.4). 1
1 Mitosis During mitosis two genetically identical nuclei are formed from one parent cell. In this process genetic material is carefully copied and chromosomes are replicated and separated. Mitosis has four distinct phases prophase, metaphase, anaphase and telophase and, in humans, produces cells that are diploid and contain 23 pairs of chromosomes. The cell cycle The cell cycle describes the steps that the parent cell goes through in order to produce two new, identical cells. Each time a cell divides it goes through all the stages of mitosis plus interphase (see Figure 14.4.1). Interphase Interphase has three stages, G 1, S and G 2 : G 1 is the initial growth phase and this occurs shortly after the cell has been made. These new cells are small with little cytoplasm but consist of a fully mature nucleus. During the initial growth phase the volume of the cytoplasm increases as protein synthesis commences and the organelles required by the cell also increase in number. A cell enters the S phase after the G 1 phase when it is going to divide. During the S phase DNA replication occurs, copying exactly the genetic instructions present in the cell s nucleus. The restriction point is the point at which the replication starts and, when the replication is completed, the cell is described as restricted and will now follow the steps of cell division and complete the cell cycle. G 2 takes place after the S phase but before mitosis. During this phase proteins for cell division are synthesised. This is the second growth phase but it is shorter than G 1. Maturation-promoting factor (MPF) enables the G 2 phase to move into mitosis by phosphorylating proteins needed for mitosis. MPF must be activated by protein kinase and cyclins they cause phosphorylation by adding phosphate groups where necessary. This coordinates the movement from one stage of the cell cycle into the next. Mitosis: prophase There are two genetically identical chromatids, known as sister chromatids and joined by a centromere, present during prophase. The nuclear envelope disappears and the centrioles move to opposite sides of the nucleus to form the protein spindle. Mitosis: metaphase During metaphase the chromatid pairs line up on the equator of the spindle by the centromere. 2
Mitosis: anaphase The sister chromatids are pulled apart during anaphase they split at the centromere and are now single structures called chromosomes. As the spindle fibres shorten, these chromosomes travel to opposite poles. Each chromosome is identical to the original one from the parent cell. Mitosis: telophase A new nuclear envelope is formed during telophase, around each set of chromosomes. The spindle disappears and the chromosomes start to uncoil. Cytokinesis Cytokinesis is the splitting of the cell itself into two genetically identical daughter cells. Figure 14.4.1: The movement of chromosomes during prophase, metaphase, anaphase, telophase and cytokinesis. Prophase Metaphase Anaphase Telophase Cytokinesis 2 Meiosis Meiosis is the process of cell division where haploid gametes (sex cells) are produced. These cells contain half the number of chromosomes in a diploid cell and are not genetically identical because the genes have been mixed around. This is why offspring are not identical to their parents (see Figure 14.4.2). Meiosis has two phases, meiosis I and II. These phases consist of the same stages as mitosis but each stage occurs twice. During interphase before meiosis I the 46 chromosomes replicate so there are 92 chromatids. Meiosis I Prophase I The chromosomes come together to form homologous pairs called a bivalent and the paternal and maternal chromatids attach at the chiasmata. During this phase sections of genetic material (alleles different versions of the same gene) may swap places. This is called crossing over and it plays an important role in genetic diversity within a population. The spindle starts to form. Metaphase I Bivalents are randomly lined up along the equator of the spindle fibres, attaching by the centromeres. Random assortment of the bivalents also plays an important part in genetic diversity within a population. 3
Anaphase I The homologous chromosomes are pulled apart and travel to opposite poles. The chiasmata separate but the centromeres stay intact. If any crossovers have occurred, donated DNA will remain attached to the new centromere so chromatids may carry a mixture of DNA from both parents. Telophase I Two new nuclear envelopes develop around each pole and cytokinesis occurs producing two non-identical haploid daughter cells. Meiosis II Prophase II The newly formed nuclear envelope breaks down and the spindle fibres start to form. Metaphase II The chromosomes are randomly lined up on the equator of the protein spindle by the centromeres. Anaphase II The chromatids are separated at the centromere and pulled to the opposite poles by the shortening of the protein fibres. Telophase II In this final stage nuclear envelopes form around the four new haploid daughter nuclei. Figure 14.4.2: The movement of chromosomes during the stages of meiosis, with the final product being four daughter cells. Prophase II Metaphase II Anaphase II Telophase II Cytokinesis 4
Genetic diversity Genetic variation within populations allows natural selection to favour the fittest individuals, which is the basis of evolution. Genetic variation arises by mutation, and mutations can spread quickly in populations by recombination during sexual reproduction. This can occur through the random assortment of chromosomes and/or crossing over during meiosis. Link Genetic diversity is also discussed in Unit 7: Molecular biology and genetics. Apoptosis Cell death occurs quickly in all multicellular organisms it is programmed to happen to ensure it is organised and tidy. Apoptosis occurs after cells have gone through approximately 50 mitotic divisions. Enzymes break down the cytoskeleton of the cell and the cell membrane breaks off into small bits called blebs. The cell breaks into vesicles and phagocytosis takes place to ensure all cellular remains are removed. Take it further Find out more about controlling the cell cycle by accessing the journal article: Cyclin-dependent protein kinases: Key regulators of the eukaryotic cell cycle (Nigg, E. A., 1995), Bioessays, 17: 471 480. doi: 10.1002/bies.950170603. Checklist In this topic you should now be familiar with the following ideas about cell division: cells divide in order that an organism can grow, and replace and repair cells mitosis produces two identical diploid cells meiosis produces four non-identical haploid cells genetic diversity is caused by crossing over and independent assortment that occurs during meiosis apoptosis is programmed cell death. Activity Access the PowerPoint slide titled Stages of mitosis and meiosis and label the stages based on the position of the chromosomes. 5
Key terms Cleavage: A series of rapid cell divisions early in embryonic development that does not lead to cell growth. Zygote: A diploid cell produced when two gametes fuse. Morula: A mass of cells produced after a series of cellular divisions early in embryonic development. Cell junction: The site at which two cells join together. Gastrulation: The process where the blastocyst is reorganised and continues to develop. Stem cells: Undifferentiated cells. Pluripotent: Describes a cell that can differentiate to form all cells of the body but not most of the extraembryonic membranes. Totipotent: Describes a cell that can differentiate to form all cells of the body and all the extra-embryonic membranes. 3 Growth and differentiation Foetal development Approximately 24 hours after fertilisation the first cleavage division occurs. Cells that make up a zygote are continually copied producing a mass of cells called a morula. As the morula moves from the oviduct to the uterus, it changes to form a blastocyst. There is no growth in the size of the embryo at this point; the divisions create cells that are smaller than the original cell because it is restricted within a glycoprotein shell called a zona pellucida. Cell junctions are found in developing blastocysts these are structures found between two cells that are joined together. Proteins on the surface of cells called cell adhesion molecules (CAMs) bind with other cells to join them together in the process called cell adhesion. Tight cell junctions in the blastocyst close to the outer surface create a seal to isolate the embryo from the external medium and gap junctions enable communication between cells of the epithelium surrounding the fluid-filled cavity. When the blastocyst reaches the uterus it hatches out of the shell. Cell adhesion molecules play an important part because they allow the trophectoderm cells in the outer layer to stick to the cells lining the uterus. This stage is known as implantation and usually occurs 8 10 days after ovulation. Rapid growth of the embryo must now take place in order for body plan features to develop. The body plan of the embryo starts to develop during gastrulation the blastocyst changes from a single-layered structure into a three-layered structure known as the gastrula. Early in gastrulation the primitive streak is formed. This is the structure that creates a mirror image or bilateral symmetry of the body. During gastrulation a group of epithelial cells called the germ layers differentiate to form major structures and organs. The three germ layers are the ectoderm, mesoderm and endoderm: The ectoderm differentiates to form the nervous system, brain and tooth enamel; it also forms some of the linings in the body, for example mouth, anus and nostrils. The mesoderm differentiates to form muscle, the cartilage of the ribs and vertebrae, blood vessels and connective tissue. The endoderm differentiates to produce the one-cell-thick lining of the digestive system and respiratory system, and organs needed for digestion. Stem cells Stem cells are self-renewing undifferentiated cells that can also give rise to other cell types. The range of cell types that can be produced from stem cells depends on the stage of development. Embryonic stem cells are pluripotent, which means they can give rise to all of the cells in the body, but not to the membranes and structures surrounding the embryo, such as the foetal part of the placenta. Only very early cells that exist just after fertilisation can do this, and they are described as totipotent. 6
Figure 14.4.3: The different ways stem cells can specialise into blood cells, cardiac muscle and nerve cells. Key terms Multipotent: Describes a cell that can differentiate into a number of distinct cell types. Differentiated: A cell that has reached its final stage of structural and functional specialisation. Link Transcriptional factors for gene expression are also discussed in Unit 7: Molecular biology and genetics. Adult stem cells are found in tissues such as bone marrow, adipose tissue and even the lining of the nose. These stem cells are restricted in the types of cells they can produce and are described as multipotent. Cord blood stem cells are isolated from umbilical cord tissue. They can differentiate into many different kinds of blood cells. Stem cells can be used later in life to help reverse degenerative diseases and repair damaged tissue. Differential gene expression The differentiation of cells reflects the expression of genes encoding specialised proteins (e.g. globin proteins in developing red blood cells, keratin in skin cells, or myosin in muscle cells). The process of differentiation therefore involves the activation of cell-type specific genes that encode the proteins necessary for each cell type to function properly. Case study The NHS has been praised by health ministers for the success of the new cord blood donation centre in London. By January 2012, 140 new mothers had donated their umbilical cord blood since the centre opened in November 2011. Blood that is present in the placenta and umbilical cord after a baby is born is the cord blood, which could prove useful in the future. It contains plentiful stem cells that can help cancer patients. For more information visit: http://www.nhsbt.nhs.uk/cordblood/. 7
Checklist In this topic you should now be familiar with the following ideas about growth: stem cells are cells that are not differentiated there are three types of stem cells: embryonic stem cells, adult stem cells and cord blood stem cells specialised cells will have specific characteristics that are due to the proteins produced depending on the genes expressed in that particular cell cells that make up a zygote are continually copied and produce a morula. the morula moves from the oviduct to the uterus, and it changes into a blastocyst. Further reading http://www.embryology.ch/genericpages/moduleembryoen.html Twyman, R.M. (2002) Instant Notes in Developmental Biology, Taylor and Francis, Abingdon, UK Acknowledgements The publisher would like to thank the following for their kind permission to reproduce their photographs: Corbis: Steve Gschmeissner / Science Photo Library; Science Photo Library Ltd: Carol & Mike Werner / Visuals Unlimited, Inc. 7 All other images Pearson Education Every effort has been made to trace the copyright holders and we apologise in advance for any unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any subsequent edition of this publication. 8