Cell Continuity.docx

Cell Continuity Cell continuity is the ability of cells to divide and survive from one generation to the next. All cells contain chromosomes, thread like structures made of DNA and protein, which contain all the genetic information that is passed from one generation to the next. The somatic (normal...

Cell Continuity Cell continuity is the ability of cells to divide and survive from one generation to the next. All cells contain chromosomes, thread like structures made of DNA and protein, which contain all the genetic information that is passed from one generation to the next. The somatic (normal body) cells of advanced organism contain two sets of chromosomes, one from the ‘mother’ and one from the ‘father’. This is described as diploid and can be shortened to 2n, where n stands for a complete set of chromosomes. The total number of cells in a diploid cell is called the diploid number. In Humans this is 46, made up of 23 pairs. All cells in the human body are diploid except for the gametes (Sperm and Egg Cells). These cells are referred to as haploid, meaning they have only one set of chromosomes. Haploid is shortened to n. Cell Cycle (1) The cell spends up to 90% of its life in the non-dividing state known as Interphase. Interphase is split into three sections in sequence. - The cell grows to its normal size, makes proteins, and carries out its other everyday functions. While this is happening, cell organelles such as mitochondria and chloroplasts replicate. - The DNA replicates. - The cell makes final preparations for division (mitosis). Mitosis Mitosis is the division of a nucleus into two genetically identical copies of itself. - A parent cell produces 2 daughter cells - There are the same number of chromosomes in parent/daughter cells (human cells have 46 chromosomes) - Cells are genetically identical. Mitosis has four stages: Prophase, Metaphase, Anaphase and Telophase. This can be remembered by the mnemonic ‘I party Monday and Tuesday’ **Know stages in detail and be able to draw** Interphase - DNA replication: The single stranded chromosomes duplicate i.e., sister chromatids are made, held together by the centromere. DNA replication is important to pass DNA(genes) on to new generations of cells unchanged. - Cell organelles e.g., mitochondria and centrioles are replicated. - Energy is built up to carry the process through. Prophase - The chromatin shortens so that individual chromosomes become visible. - Nucleolus and nuclear membrane disappear. - Centrioles separate to opposite sides of cell. Metaphase - Spindle formed. - Chromosomes line up along the equator and attach themselves to spindle fibres at their centromeres. Anaphase - Spindle fibres contract. - Chromosomes split at centromere, sister chromatids separate and are pulled to opposite poles. The chromatids are now called daughter chromosomes. Telophase - The chromosomes elongate reverting to chromatin. - Two nuclear membranes form. - Nucleoli reform. - Centrioles replicate Cytokinesis Cytoplasm splits in two. In an animal cell the cell membranes constrict (a cleavage furrow is formed) to form 2 cells. In a plant cell vesicles migrate to the equator. These vesicles contain all the cellulose and other chemicals required to make the cell wall and cell membrane. The vesicles fuse to form a cell plate. The cell plate becomes the middle lamella, and the cell walls and membrane are laid down on each side of it. (2) Function of Mitosis - Growth: increasing the number of cells in an organism e.g., fertilised egg becomes an adult - Cell replacement: repair of damaged or inefficient cells e.g., skin, blood corpuscles. - Asexual reproduction e.g., budding in yeast, binary fission in bacteria/Amoeba, vegetative reproduction e.g., cuttings in flowering plants, spore formation in Rhizopus. - Formation of gametes e.g., in flowering plants. Meiosis Meiosis is the division of a diploid nucleus producing four haploid genetically different nuclei. (3) Functions - Gamete formation: Production of sperm and egg cells in humans - Pollen grain formation: Spore formation in flowering plants - Maintains diploid number: Ensures that gametes are haploid so that when they fuse to form a zygote it will have the correct number of chromosomes - Increases variation: Independent assortment produces great genetic variation, which allows evolution to occur Cancer **Define and give 2 causes** Cancer is characterised by the body’s loss of control of the normal regulation of mitosis. Normally, cells divide to produce more cells only when the body needs them. If cells divide when new ones are not needed, they form a mass of excess tissue, called a tumour. It these cells stay together they are called benign tumours e.g., warts (caused by a virus), ‘skin tags’ (small blobs of raised skin) and most breast tumours. These tumours can be removed surgically. If they invade nearby organs or travel through the lymphatic or bloodstream to new body sites, they are called malignant tumours or cancer. Cancer cells can also break away to form new tumours. The spread of cancer is called metastasis. Causes A carcinogen is any chemical, biological or physical agent that could possibly be a cause of cancer e.g., tobacco smoke, asbestos, hydrocarbons in tars, radiation such as UV light, X-rays, and uranium, as well as certain viruses. Viruses work by taking over the nucleus of the cell they are attacking. Treatments Surgery, radiation therapy (burns out the cancer), chemotherapy (to slow down mitosis) or hormone therapy. Exam Questions 2014 – HL – Section A – Question 5 5. Indicate whether the following statements are true (T) or false (F) by placing a tick () in the appropriate box in each case. (a) Single-celled organisms use meiosis for asexual reproduction. False (b) In telophase of mitosis, a cleavage furrow forms in plant cells. False (c) When a cell is not dividing it is said to be in prophase. False (d) The nuclear membrane disappears in the early part of mitosis. True (e) Centromeres give rise to the nuclear spindle. False (f) Mitosis is a source of variation. False (g) In multicellular organisms mitosis is primarily used for growth. True 2013 – HL – Section C – Question 11 (c) Answer the following questions, which relate to events in the cell cycle. (i) What name is applied to the period of the cell cycle in which division is not taking place? Interphase (ii) Give a cellular process that occurs during this period in which the nucleus is not dividing. Gene expression (iii) Draw a labelled diagram to show the position of the chromosomes during metaphase of mitosis in a nucleus in which 2n = 6. (iv) 1. State a function of one of the structures, other than chromosomes, that you have labelled in your diagram of metaphase. Spindle fibres - pull the chromosomes to the poles of the cell during anaphase 2. How does the structure carry out this function? Spindles fibres are composed of contractile proteins that contract and shorten, thereby pulling on the chromosomes. (v) What term is used for the group of disorders in which control has been lost over the rate of mitosis? Cancer 2011 – HL – Section A – Question 2 Use your knowledge of mitosis to answer the following questions: (a) What is the role of mitosis in single-celled organisms? Asexual Reproduction (b) What medical term is used for the group of disorders in which certain cells lose normal control of mitosis? Cancer (c) Suggest a possible cause of one of the group of disorders referred to in (b). Ultraviolet Light (d) Name the stage of mitosis in which the chromosomes are located at the equator of the cell and before they begin to separate. Metaphase (e) To what are the chromosomes attached in the stage of mitosis referred to in (d)? Spindle Fibres (f) Towards the end of mitosis, in what type of cell does a cell plate form? Plant Cell (g) Give one way in which mitosis differs from meiosis. Mitosis produces 2 daughter cells, Meiosis produces 4 daughter cells References Sciencefacts.net Genome.gov Wikipedia.com