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Cell Division - Coggle Diagram

- - - - Examples:
        
        binary fission
        
        vegetative reproduction (runners and buds)
        
        budding of yeast cells
      - The only source of genetic variation of the products (daughter cells) is mutation, a spontaneous or induced change in the genetic material.
- - - - Require cell division to form new organisms (reproduction) this increase the number of individuals in a population
    - - Requires cell division to increase the number of cells in the body (growth) and to replace old or damaged cells (repair)
- - - - These pairs are called HOMOLOGOUS CHROMOSOMES (meaning same type)
      - One from your mother (maternal) and one from your father (paternal)
      - Each member of pair looks similar in length
      - The centromere of each chromosome is in similar position
      - Diploid Cells
        
        A cell that has pairs of homologous chromosomes is diploid.
        
        Each pair of homologous chromosomes has one chromosome from the mother and one from the father. Most of the cells in your body are diploid.
        
        A cell that has only one chromosome from each pair of homologous chromosomes is haploid
        
        Somatic Diploid Cells
        
        Somatic cells – form the body cells of eukaryotic multicellular organisms
        
        Somatic cells are described as diploid – there are 2 sets of chromosomes in the nucleus
        
        The number of chromosomes in a diploid cells is called the diploid number
        
        The chromosomes are arranged in pairs in a diploid cells, each pair consists of one maternal and one paternal chromosomes
        
        Humans have a diploid number of 46 and a haploid number of 23
        
        New somatic cells are produced in in mitosis, daughter cells are genetically identical
        
        Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 2n)
        
        These nuclei will possess two gene copies (alleles) for each trait
        
        All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
        
        Diploid cells are present in most animals and many plants
      - The maternal and paternal chromosome in a pair are called homologous chromosomes
        Homologous chromosomes have the same number and type of genes in the same location
      - Haploid Cells
        
        Haploid cells are gametes (sex cells), and contain only one member of each homologous pair of chromosomes
        
        So, each sex cell has half the number of chromosomes as the original parent cell and is called haploid.
        
        Label each line in this flow diagram as haploid or diploid
        
        How must meiosis be different to mitosis?
        
        Nuclei possessing only one set of chromosomes are haploid (symbolised by n)
        
        These nuclei will possess a single gene copy (allele) for each trait
        
        All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
        
        Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)
      - Human somatic cells contain 46 chromosomes…these are actually 23 pairs of chromosomes.
      - Homologous pairs contain genes for the same characteristics
    - - As sexually reproducing organisms receive genetic material from both parents, they have two sets of chromosomes (diploid)
      - To reproduce in turn, these organisms must create sex cells (gametes) with half the number of chromosomes (haploid)
      - When two haploid gametes fuse, the resulting diploid cell (zygote) can grow and develop into a new organism in each cell there are two sets of chromosomes (diploid)
    - - Gametes
        
        Gametes – are involved in reproduction
        
        Gametes are described as haploid – there is only 1 set of chromosomes in the nucleus
        
        The number of chromosomes in a haploid cell is called the haploid number
        
        Sperm and egg are the male and female gametes in humans
        
        Gametes are formed through a special type of cell division called meiosis
        
        Meiosis introduces genetic variations into the offspring so that all gametes are genetically different
    - - Crossing over during Prophase 1 of meiosis
        
        Occurs in Prophase 1 of Meiosis
        
        The homologous chromosomes (consisting of 2 identical sister chromatids) lie next to each other.
        
        The chromosomes are in contact at a number of points called chiasmata.
        
        Enzymes cut identical sites on both non-sister chromatids and sections of their DNA are swapped.
        
        This forms recombinants.
        
        Now new genetic information is found on the chromosomes…a new combination of maternal and paternal genes is created.
      - The independent assortment of homologous chromosomes at metaphase 1
        
        Occurs during Metaphase 1
        
        Homologous chromosomes (maternal & paternal) move to the equator.
        
        Here they line up randomly (sometimes called independently)
        
        There is an equal chance for a maternal chromosome to be on the left or right side of the equator.
        
        This means that each of the resulting cells will have unique combinations of maternal and paternal chromosomes
        
        Take time to consider the diagrams alongside, showing the different possibilities.
        
        Humans, with 23 pairs of chromosomes can produce 223 (about 8 million) different types of gametes.
        
        Little wonder that siblings look different !!
      - Fertilisation restores the diploid number, (each unique sperm meets each unique ova…..and a unique combination of parents genes starts off!)
      - Mutations
  - - - Prophase 1
        
        Chromosomes condense, forming identical sister chromatids. They pair up forming a bivalent. Crossing over can occur now
        
        The replicated chromosomes condense and the nuclear membrane is broken down at the start of prophase I
        
        Homologous chromosomes then pair up and exchange DNA in a process called crossing over
        
        Crossing over involves the exchange of DNA between the sister chromatids of the maternal and paternal chromosome.
        
        Crossing over:
        
        Chromatids on the maternal chromosome cross over with chromatids on the paternal chromosome crossover with chromatids on the paternal chromosome at a point of contact called the chiasmata (singular chiasma)
        
        Crossing over between a pair of homologous chromosomes produces four sister chromatids that are genetically unique.
        
        The new combinations of DNA created during crossover are a source of genetic variation in gametes
      - Metaphase 1
        
        Pairs of homologous chromosomes move to middle (equator). Independent assortment occur
        
        Important note…..unlike mitosis, the sister chromatids stay together in this division
        
        Pairs of homologous chromosomes are arranged randomly along the equator of the cell in 2 lines
        
        The assortment of the maternal and paternal chromosome randomly is called (independent assortment)
        
        Independent assortment is another source of genetic variation in the gametes
      - Anaphase 1
        
        Homologous pairs (randomly arranged at equator) now separate to opposite poles of the cell
        
        One replicated chromosome from each pair of homologous chromosomes is pulled to the opposite pole of the cell by contracting spindle fibres
        The parent cell begins to elongate in preparation for cytokinesis
      - Telophase and Cytokinesis 1
        
        Chromosomes are at poles, spindle breaks down
        
        Two haploid cells have been formed.
        
        Nuclear membrane are assembled around the replicated chromosomes in each of the dividing cells.
        
        The chromosomes de-condense and are no longer visible
        
        The parent cell divides into 2 daughter cells through cytokinesis
        
        In humans, at the end of meiosis I the parent diploid cells will have divided into 2 diplod daughter cells containing 46 chromosomes (Each Chromosome is genetically different but has all of the same type of genes but with some different versions (alleles)
        
        In Human diploid spermatocyte or oocyte contain 46 replicated chromosomes which divide to form two diploid daughter cells which both contain 46 chromosomes (see above ) at the end of meiosis I
    - - Prophase 2
        
        Note: replication does not occur
        
        New spindle forms
        
        The replicated chromosomes condense and are then visible under an optical microscope
        
        The nucleoli and nuclear membrane break down
        
        Centrioles (in animals) migrate to opposite poles of the cell
      - Metaphase 2
        
        Chromosomes (consisting of identical sister chromatids) are moved to middle (equator)
        
        Replicated chromosomes line up in a single line along the equator of the cell
        Spindle fibres are attached to the centromeres
      - Anaphase 2
        
        Sister chromatids are separated & move to opposite poles of cell
        
        Sister chromatids are pulled to opposite poles of the cell by contracting spindle fibres
        The cells begins to elongate
      - Telophase 2
        
        Spindles disappear, nuclear membranes re-forms, chromosomes de-condense forming chromatin
        
        4 HAPLOID CELLS ARE PRODUCED.
        
        Are they identical?
        
        Nuclear membranes are assembled around the chromosomes in each of the dividing cells
        
        The chromosomes de-condense and are no longer visible with an optical microscope
        
        Cytokinesis occurs in the same way as mitosis
        
        4 daughter cells are produced each containing half of the parents cells DNA – haploid
        
        The product of meiosis are haploid cells containing a single set of chromosomes
      - fertilisation
        
        Fertilisation is the fusion of gametes
        
        Restores the diploid number of chromosomes
        
        Fertilisation brings variation to reproduction as the new zygote has material from two different parents
        
        (You now know that each gamete has multiple opportunities to receive varied chromosomes….so each gamete brings a unique combination of parental DNA)
- - - - The replicated chromosomes are arranged along the equator (middle) of the cell with the centromeres attached to the ends of the spindle fibres
      - sister chromatids face opposite poles.
    - - The centromeres of the replicated chromosomes are broken down
      - The sister chromatids separate and are pulled to opposite poles (ends) of the cells by contracting spindle fibres.
      - The parent cells begins to elongate in preparation for cytokinesis
    - - New nuclear membrane are assembled around the sets of chromosomes in each of the dividing cells
      - Nucleolus reappear
      - Chromosomes un-coil (de-condense) reverting back to chromatin
      - Chromosomes are no longer visible with an optical microscope
      - Telophase is the final stage of mitosis (division of the nucleus)
    - - Early stages of prophase:
        
        The chromosomes condense into visible bodies. Each has 2 identical sister chromatids joined at a region called the centromere)
        
        centrioles divide to form 2 pairs which move to opposite poles (centrioles are made up of microtubules)
        
        In animal cells: the centrioles move towards opposite poles of the nucleus
        
        There are no centrioles in plant cells
      - The chromosomes in the eukaryotic cell are normally indistinct when viewed under a optical microscope
      - In middle stages of prophase:
        
        The chromosomes start to move towards the equator (middle) of the nucleus
        
        Nucleolus disappears
        
        Nuclear membrane is broken down releasing chromosomes into the cytoplasm of the cell
      - In late stages of prophase
        
        In animals: the centrioles reach the poles (end) of the cells and synthesis spindle fibres
        
        Spindle fibres are made of proteins called microtubules
        
        spindle apparatus forms (centrioles + spindle fibres)
        
        In plants: spindle fibres are produces without centrioles
        
        The spindle fibres attach to chromosomes
    - - The cytoplasm and its contents are divided between the two new daughter cells.
        
        Each daughter cell is genetically identical to each other and to the original parent cell.
        
        The second stage of cell division is called cytokinesis this begins once telophase finishes
        Cytokinesis is the division of the cytoplasm of the cell which results in the separation of the parent cell into daughter cell
        
        Animal Cells
        
        Cytokinesis is facilitated in animal cells by proteins called actin and myosin
        
        Actin and myosin proteins form a contractile ring at the equator of the parent cell
        
        The contractile ring is condensed forming an indentation called a cleavage furrow in the parent cell
        
        The cleavage furrow moves inward and as the contractile ring condenses during telophase and at the start of cytokinesis
        
        The contractile ring continues to condense, and the cleavage furrow deepens until the cell is physically cleaved (split into two)
        
        Plant Cells
        
        Cytokinesis is facilitated by vesicles that assemble at the equator of the parent cell
        
        The vesicles contain compounds for the synthesis of a cell wall and the call membrane
        
        The vesicles fuse during telophase forming a structure called the cell plate
        
        The daughter cells separate as the new cell wall and membrane are formed
- - - - the cell cycle
        
        An alternating cycle of cell division and enlargement is known as the cell cycle. The cell cycle is comprised of three main processes;
        
        Interphase (DNA replication)
        
        Mitosis (Nuclear Division)
        
        Cytokinesis (Division of the Cytoplasm)