Cell and Nuclear Division PDF

D2.1 Cell and nuclear division MULTICELLULAR ORGANISM The effect of cell size on the efficiency of diffusion: As a cell grows larger its surface area to volume ratio becomes smaller. The rate at which materials enter or leave a cell depends on the surface area of the cell. A cell t...

D2.1 Cell and nuclear division MULTICELLULAR ORGANISM The effect of cell size on the efficiency of diffusion: As a cell grows larger its surface area to volume ratio becomes smaller. The rate at which materials enter or leave a cell depends on the surface area of the cell. A cell that becomes too large may not be able to take in essential materials or excrete waste substances quickly enough. DIFFERENTIATION MULTICELLULAR ORGANISM Large organisms are multicellular. They consist of many cells. Cells have is unit of live that has distinctive properties. Division of labour: different types of cells - different tissues - different functions. Genome: a entire set of genes of an organism STEM CELLS Stem cells are defined as cells that have the capacity to divide and to differentiate along different pathways. Stem Cells https://www.youtube.com/watch?v= vvrlpVPxYiE Cell division All organisms need to produce new cells, for growth, maintenance and reproduction. One cell divides into two. The cell that divides is called the mother cell and those produced from it are daughter cells. The mother cell disappears as an entity in the process, unlike reproduction by animal parents. There is strong evidence for the theory that new cells are only ever produced by division of a pre-existing cell. D2.1.4 Roles of mitosis and meiosis in eukaryotes Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. (thread) Coiled DNA / roll / condense (Chromosome) Unroll (Chromatin) Chromatin Chromosomes: DNA molecules are coiled have proteins attached in eukaryotes Condensation: chromosomes become short and fatter by a complex process known as supercoiling. The two parts of the chromosome are called sister chromatids. They are held together at one point by a structure known as a centromere. Cell cycle The cell cycle is the sequence o events between one cell division and the next. It has two main phases: interphase and cell division. Interphase consists of three phases: G1 phase S phase G2 phase Interphase steps G1: DuringG1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps. S: InS phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase. G2: Duringthe second gap phase, or G 2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G 2 ends when mitosis begins. DNA replication as a prerequisite for both mitosis and meiosis MTOC microtubule organizing centres Chromosomes are moved using microtubules. A microtubule is a hollow cylinder of tubulin proteins that can be rapidly assembled or disassembled. 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. kinetochore At the same time, protein structures called kinetochores are assembled on the centromere of each chromatid. The kinetochore acts as a microtubule motor by removing tubulin subunits from the attached ends of the microtubules. This shortens the microtubules linking the kinetochores to the poles, putting them under tension. Kinetochore The phases of mitosis Mitosis:: where the nucleus divides to form two genetically identical nuclei. Phases: Prophase (pro=before). Metaphase (meta=after) Anaphase (ana =up). Telophase (telos=finally). Microscopy Prophase Metaphase Anaphase Telophase  Anaphase Prophase Metaphase Cytokinesis is the division of the cytoplasm to form two cells In plant cells a new cell wall is formed In animal cells the plasma membrane at the equator is pulled inwards until it across the equator of the cell, with meets in the centre of the cell, dividing it plasma membrane on both sides.This in two. (contractile proteins: actin and divides the cell in two. Daughter cells myosin). brings cellulose to the equator. Equal and unequal cytokinesis In many cases, cytokinesis divides the cytoplasm of the mother cell into equal halves. This happens in a growing root tip. Root growth is due to enlargement and division of cells arranged in columns. Cytoplasm is sometimes divided unequally. Small cells produced by unequal division can survive and grow if they receive a nucleus and at least one of each organelle that cannot be assembled from components in the cell. For example: mitochondria, chloroplast 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 Occasionally a plant cell divides and its chloroplasts all pass into one of the two Yeast cells reproduce asexually in a process daughter cells. These leaves from show areas called budding.The nucleus divides by mitosis. of cells without chloroplasts. Oogenesis in humans The production of both sperm and eggs in humans starts with two divisions of a mother cell. During sperm production, the cytoplasm is divided equally in the first and second divisions, resulting in four, equally sized small cells, each of which develops into a mature sperm. Usually only one egg cell (oocyte) is produced at a time, with enough stored food to sustain the developing embryo. There is therefore unequal division of cytoplasm during oogenesis. Videos Real Microscope Mitosis https://www.youtube.com/watch?v=L61Gp_d7evo Kinetochore and Mitosis https://www.youtube.com/watch?v=IvJrDsRuWxQ Animal Mitosis (Time Lapse) https://www.youtube.com/watch?v=2LBpF71GZbk  MEIOSIS Is the process that halves chromosomes number and allows a sexual life cycle with fusion of gametes. The haploid number of gametes is represented by the letter n so the diploid number is 2n. In meiosis, a diploid nucleus divide twice to produce four haploid nuclei. The DNA is replicated before the first division. Interphase Crossing over (Prophase I) https://www.youtube.com/watch?v=uTTKi4Pcsf0 https://www.genome.gov/ genetics-glossary/Meiosis Meiosis and genetic variation 1. Random orientation of pairs of homologous chromosomes in Metaphase I. 2. Crossing over during Prophase I 3. Species that reproduce sexually thus generate genetic variation by meiosis and by a random fusion of gametes. (fertilization) Non-Disjunction and Down Syndrome In Anaphase chromosome should separate and move to opposite poles. Sometimes chromosomes moves to the same pole. This can happen in meiosis I or II. Trisomy: three chromosomes of one type instead of two. Down Syndrome or trisomy 21. https://www.youtube.com/watch?v=CgGm_hUFiY0 ONLY HL Cyclins and cell cycle control A group of proteins called cyclins is used to ensure that tasks are performed at the correct time and that the cell only moves on to the next stage other cycle when it is appropriate. Cyclins bind to enzymes called cyclin-dependent kinases.These kinases then become active and attach phosphate groups to other proteins in the cell. The attachment of phosphate triggers the other proteins to become active and carry out tasks specific to one of the phases of the cell cycle. There are checkpoints in the cycle to hold cells until it is appropriate for them to progress to the next phase. These checkpoints are also used to ensure that cells stop dividing when there has been enough cell proliferation in a tissue Consequences of mutations in genes that control the cell cycle Control of the cell cycle ensures that tissues Mutagens increase the chance of tumour have enough cells, but not too many. formation: Sometimes control is lost in an individual cell Mutagenic chemicals substances that because of mutations to its genes. are “definitely carcinogenic” Tumour: is a group of cells that increases in high-energy radiation such as X-rays number exponentially without the normal and ultraviolet controls. Oncogene is a mutated gene that has the potential to cause cancer. There are two groups of genes that can change a normal cell into a tumour cell if they mutate. Proto-oncogenes Tumour-suppressor genes regulate expression of genes concerned prevent cell proliferation. with cell proliferation have roles that control the cell cycle or cell function as brakes at checkpoints in the division cell cycle is concerned with growth factors or are needed for DNA repair to correct errors receptors for them. in replication and thus reduce mutation rates Proto-oncogenes can mutate into has roles in programmed cell death oncogenes, which actively promote cell (apoptosis) within cells where there has proliferation and are genetically dominant. been irreparable DNA damage. https://www.genome.gov/genetics-glossary/Oncogene Mutations to tumour-suppressor genes increase the risk of tumour formation if the gene product no longer functions properly. Cancer Primary tumour: cell divides repeatedly, cell attached as single mass, classified as benign, not cancer Metastasis: spread of tumour cells, cells may invade neighbouring tissues, transport route such as blood or lymph, cells that have metastasized continue dividing and develop into secondary tumours. They are classified as malignant or cancer Mitotic index The mitotic index is the ratio between the number of cells in mitosis in a tissue and the total number of observed cells. Mitotic index = number of cells in mitosis / total number of cells Used by doctors to predict how rapidly a tumour will grow in cancer.