2.3

• The cell cycle is a regulated process in which cells divide into two identical daughter cells.
• Stages of cell cycle

  1. Interphase

  2. Mitosis or meiosis (nuclear division)

  3. Cytokinesis (cytoplasmic division)
• Outline what happens during Interphase:

  1. G1: cell synthesises proteins for replication e.g. tubulin for spindle fibres & cell size doubles

  2. S: DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere

  3. G2: organelles divide
• Stages of mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Outline what happens during Prophase:

  1. Chromosomes condense, becoming visible. (X-shaped: 2 sister chromatids joined at centromere)

  2. Centrioles move to opposite poles of cell (animal cells) & mitotic spindle fibres form

  3. Nuclear envelope & nucleolus break down = chromosomes free in cytoplasm
• Outline what happens during Metaphase:

  Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres
• Outline what happens during Anaphase:

  1. Spindle fibres contract = centromeres divide

  2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cell. (looks like 'V' shapes facing each other)

  3. Spindle fibres break down
• Outline what happens during Telophase:

  1. Chromosomes decondense, becoming invisible again

  2. New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome
• What happens during Cytokinesis?

  1. Cell membrane cleavage furrow forms

  2. Contractile division of cytoplasm
• Explain how to prepare a temporary root tip squash

  1. Place root in hydrochloric acid to halt cell division & hydrolyse middle lamella

  2. Stain root tip with a dye that binds to chromosomes

  3. Macerate tissue in water using mounted needle

  4. Use mounted needle at 45° to press down coverslip & obtain a single layer of cells. Avoid trapping air bubbles
• Procedure for a root tip squash experiment

  1. Prepare a temporary mount of root tissue

  2. Focus an optical microscope on the slide. Count total number of cells in the field of view and number of cells in a stage of mitosis

  3. Calculate mitotic index (proportion of cells undergoing mitosis)
• Name 2 dyes that bind to chromosomes

  • toluidine blue (blue)
  • acetic orcein (purple-red)
• What is Meiosis?

  A form of cell division that produces four genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as gametes
• What happens during Meiosis I?

  1. Homologous chromosomes pair to form bivalents

  2. Crossing over (exchange of sections of genetic material) occurs at chiasmata

  3. Cell divides into two. Homologous chromosomes separate randomly. Each cell contains either maternal or paternal copy
• What happens during Meiosis II?

  1. Independent segregation of sister chromatids

  2. Each cell divides again, producing 4 haploid cells
• What is Chromosome mutation?

  Involves section of a chromosome rather than 1 base. Types include: deletion, inversion, duplication, translocation
• What is a Translocation mutation?

  A base sequence from one chromosome fuses with a non-homologous chromosome
• Non-disjunction mutation


  Chromosomes fail to separate correctly in meiosis. Gametes have at least one more (polysomy) or one less (monosomy) chromosomes than normal
• Conditions caused by non-disjunction

  • trisomy-21 = Down's syndrome
  • X-monosomy of sex chromosomes = Turner's syndrome
• What is the purpose of mitosis?

  To produce 2 genetically identical daughter cells for:

  • growth
  • cell replacement / tissue repair
  • asexual reproduction
• Why is only the root tip used when calculating a mitotic index?

  Meristematic cells at root tip are actively undergoing mitosis
• How does meiosis produce genetic variation?

  • Crossing over during meiosis I
  • Independent assortment (random segregation) of homologous chromosomes and sister chromatids.

  Results in new combinations of alleles
• What is asexual reproduction?

  The production of genetically identical offspring from one parent through the process of mitosis
• What is chromosome translocation?

  It’s a mutation which is caused when part of a chromosome breaks off and then re-joins to a different chromosome.
• What is meant by crossing over?

  It’s the exchange of genetic material between two chromosomes in a bivalent.
• What is a diploid?

  Diploid is cells with two copies of each chromosome
• What is haploid?

  Haploid are cells with only one copy of each chromosome.
• What is meant by independent assortment?


  It’s a source of variation in meiosis where the bivalent chromosomes can line up either way around that metaphase plate