biology4alevel600

1. Growth of a multicellular organism is a result of parent cells dividing to produce genetically identical daughter cells. 2. During cell division the nucleus divides first, followed by division of the whole cell. 3. Division of a nucleus to produce two genetically identical nuclei is achieved by the process of mitosis. 4. Mitosis is used in growth, repair, asexual reproduction and cloning of cells during an immune response. 5. Although a continuous process, mitosis can be divided for convenience into 4 phases: prophase, metaphase, anaphase and telophase. The phase between successive nuclear and cell divisions is called interphase. Replication of DNA takes place during interphase so that the new cells will each have identical DNA. 6. The period from one cell division to the next is called the cell cycle. It has four stages or phases: G1 is a growth stage, S (for synthesis) is when the DNA replicates, G2 is a second growth stage, and nuclear and cell division. G1, S and G2 are collecti...

Each cell contains genes that help to control when it divides . Cells divide by mitosis only when required . When receives signals from neighbouring cells, it responds by dividing or not dividing. If this control goes wrong, cells may not divide when they should (growth does not take place, or wounds do not heal) or they may divide when they should not (a tumour may form). 1. Cancer and uncontroled cell division   In cancer: genes that control cell division mutate --> cell divide over and over again, forming an irregular mass of cells. In malignant tumour: some of cancer cells may break off and start to form new tumours elsewhere in the body. Several genes must mutate before a cell becomes cancerous. This can happen just by chance.  The risk is increased by factors that cause mutation (carcinogenic factors):          - ionising radiation (from X ray and radioactive sources emitting  α, β or γ  radiation)       ...

Mitosis is a nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosome. Significance of mitosis  production of geneticlly identical cells : It keeps the chromosome number constant and genetic stability in daughter cells, so the linear heredity of an organism is maintained. growth:  a single cell divides repeatedly to produce all the cells in the adult organism repair of tissue and  cell replacement:  produce new cells to replace ones that have been damaged ( repair and generation of lost parts)  or worn out ( healing of wouds). asexual reproduction:  a single parent gives rise to genetically identical offspring Strictly speaking, mitosis is division of the nucleus of the cell. After this, the cell itself usually divides as well ( cytokinesis ). The cell cycle The cell cycle is the continuous cycle of  growth  and  mitotic division . It has 2 major phases...

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each  chromosome  is made up of  DNA  tightly coiled many times around proteins called  histones  that support its structure. Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during  cell division  and is then visible under a microscope.  Each chromosome has a constriction point called the  centromere,  which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes. A  chromatid  is 1 of the 2 identical strands of DNA that make...

5.1 Replication and division of nuclei and cells      5.2 Chromosome behaviour in mitosis When body cells reach a certain size they divide into two. Nuclear division occurs first, followed by division of the cytoplasm. The mitotic cell cycle of eukaryotes involves DNA replication followed by nuclear division. This ensures the genetic uniformity of all daughter cells. Learning Outcomes Candidates should  be able to: 5.1 Replication and division of nuclei and cells  During the mitotic cell cycle, DNA is replicated and passed to daughter cells. Stem cells in bone marrow and the skin continually divide by mitosis to provide a continuous supply of cells that differentiate into blood and skin cells. a) describe the structure of a chromosome, limited to DNA, histone proteins, chromatids, centromere and telomeres b) explain the importance of mitosis in the production of genetically identical cells, growth, cell rep...

• Replication and division of nuclei and cells • Understanding of chromosome behaviour in mitosis Learning Outcomes Candidates should be able to: (a) explain the importance of mitosis in the production of genetically identical cells, growth, repair and  asexual reproduction; (b) outline the cell cycle, including growth, DNA replication, mitosis and cytokinesis; (c) [PA] describe, with the aid of diagrams, the behaviour of chromosomes during the mitotic cell cycle and  the associated behaviour of the nuclear envelope, cell membrane, centrioles and spindle (names of the  main stages are expected); (d) explain how uncontrolled cell division can result in the formation of a tumour and identify factors that can  increase the chances of cancerous growth; (e) explain the meanings of the terms haploid and diploid (see section 5) and the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction (note: descriptions of homologous chromosomes are not ...