Cell Continuity

Cell Continuity Cell continuity is the ability of cells to divide and survive from one generation to the next. The necessity for a cell to divide becomes apparent when it is considered that every cell must grow, repair and reproduce. There are two types of cell division, mitosis and meiosis. Mitosis is the most common type. It occurs in most animal (somatic) cells and in growth tips (meristems) of plants.

Chromosomes Structures in Nucleus, made of DNA & Protein Not dividing = Chromatin (long thin threads) When dividing = Chromatin forms a numbers of clearly distinguishable Chromosomes

1. Chromosomes are found in the nucleus 2. Chromosomes are made of DNA 3. Sections of chromosomes are called genes

Each organism has a distinct number of chromosomes, in humans, every cell contains 46 chromosomes. Other organisms have different numbers, e.g. a dog has 78 chromosomes per cell.

Haploid A Haploid cell has one set of each chromosome Haploid is symbolised by letter ‘n’ and number of chromosomes in the cell is given as n=2 or n = 3 etc. In humans, egg cells and sperm cells are haploid, n=23

Diploid A diploid cell is a cell that contains two copies of each chromosome. One set of chromosomes is donated from each parent. Chromosomes are in pairs in diploid cell, called homologous pairs. Diploid is symbolised as ‘2n’ 2n=total number of chromosomes in the cell. Body cells or Somatic cells such as blood, muscle cells etc are all diploid.

CELL CYCLE consists of three phases: Interphase, mitosis(nuclear division) and cytokinesis (cell division).

Interphase 1. Chromosomes are elongated, called chromatin When cells are not actively dividing they are said to be in interphase. Longest phase in cell cycle = 90% of a cells life is in interphase 1. Chromosomes are elongated, called chromatin 2.Cell very active in Interphase, produces new mitochondria, chloroplasts etc. and chemicals needed for growth. 3. In the later part of interphase, chromosomes produce identical copies of themselves, producing a chromosome with 2 strands (sister chromatids are made held together by a centromere). The 2 strands have identical genes.

Mitosis MITOSIS is the division of a nucleus into two genetically identical copies of itself i.e. (1) 1 parent cell produces 2 daughter cells (2) There are the same no. of chromosomes in parent/daughter cells (human cells have 46 chromosomes) (3) Cells are genetically identical.

A sentence to help you remember the 4 stages of mitosis I Party Monday And Tuesday

At end of Interphase, Chromosomes contract and become visible. Each chromosome appears as a duplicated strand. Fibres appear in cytoplasm. Nuclear membrane starts to break down.

Nuclear membrane broken down. Chromosomes thicken even more. Chromosomes line up across the centre or equator of the cell. Fibres attach to chromosomes. Each chromosome has 2 fibres attached, 1 from each side of the cell.

Fibres contract, chromosomes pulled apart. Chromosomes split at centromere, sister chromatids separate and are pulled to opposite poles. The cell has 8 chromosomes at this stage. The 4 chromosomes pulled to each pole have identical genes..

Spindle fibres break down. Nuclear membrane forms around each of the 2 sets of chromosomes. Nucleoli begin to re-form. Chromosomes elongate within each nucleus.

CYTOKINESIS (cleavage) Cytoplasm splits in two CYTOKINESIS (cleavage) Cytoplasm splits in two. In an animal cell the cell membranes constricts (a cleavage furrow is formed) to form 2 cells..

Plant Cells: At cell division, many vesicles gather at the equator of the cell. These contain cellulose and all the material needed to form new cell membranes and cell walls. The vesicles join to form a cell plate. New cell walls form on either side of the cell plate.

Parts of the chromosome involved in mitosis

Functions of Mitosis Single-celled organisms – to allow organisms to reproduce. e.g. budding in yeast, asexual reproduction in bacteria/Amoeba Multi-cellular organisms – for growth. 1. Growth - increasing the number of cells in an organism 2. Cell replacement - repair of damaged or inefficient cells e.g. skin, blood corpuscles.

Significance of mitosis: 1. Exact copies produced – important in growth and repair of tissues in that all cells are genetically identical 2. Asexual reproduction – mitosis produces offspring (a clone) that are identical to parent e.g. Amoeba or yeast. They all have the same advantages of the parent in surviving in the same habitat.

Cancer Rate of cell division (mitosis) is carefully controlled. Sometimes a cell or group of cells lose the ability to control the rate of cell division. Cancer is a group of disorders in which certain cells lose their ability to control both the rate of mitosis and the number of times mitosis takes place They form a mass of cells called a tumour which can be benign or malignant.

Benign tumours Benign means kind, they are not life threatening and do not invade other tissues. Eg warts, skin tags. Cells stop dividing after some time Malignant tumours Uncontrolled multiplication of abnormal cells Malignant tumours (cancers) invade other cells and move around the body Movement of these cells called Metastasis. Cancer cells divide indefinitely.

Causes of Cancer Caused when normal genes are altered to form cancer- causing genes called oncogenes Brought about by cancer causing agents called carcinogens eg cigarette smoke, asbestos fibres, ultraviolet radiation and some viruses Most cancers can be cured with Radiation (burn out cancer), Chemotherapy (Chemicals slow down mitosis) and surgery

Meiosis Is a form of nuclear division in which the daughter nuclei contain half the chromosome number of the parent nucleus.

Functions of Meiosis 2 functions in Multicellular Organisms Allows sexual reproduction without increasing the number of chromosomes in the offspring Allows new combinations of genes Occurrence of meiosis: In mammals, in the testes of the male and ovaries of the female. In angiosperms, the anther and ovule.