Genetics

• Cell Theory


  • All living things are composed of one or more cells
  • Cells are the smallest units of living organisms
  • New cells come only from pre-existing cells by cell division
• Cell division


  • For growth of the organism
  • Repair of tissues and organs that have been damaged
  • Maintenance to replace dying or dead cells
• Cell cycle checkpoints


  • Monitor growth to ensure the cycle continues when it should through a complex network of signals
  • Cancer is the result of uncontrolled, rapid cell division
• Cancerous cells


  Progress quickly from one cell division to the next, resulting in a mass of cells (don't take as long in interphase as they should) called a tumour
• Chromosomes


  Structure in nucleus containing DNA
• Sister chromatid


  One of two chromosomes that are genetically identical and held together at the centromere
• Centromere


  The region where two sister chromatids are held together in a chromosome
• Spindle fibre


  • A microtubule structure that facilitates the movement of chromosomes within a cell
• Centrosome


  • A structure that helps to form the spindle fibres
• Interphase


  The cell grows, develops into a mature, functioning cell, copies its DNA, and prepares for division
• G1 (Growth 1) phase


  The major period of growth for a cell, synthesizing many new molecules in preparation for the next phase
• S (Synthesis) phase


  The cellular DNA is replicated, DNA exists as uncondensed fibres called chromatin
• G2 (Growth 2) phase


  The cell synthesizes more molecules prior to mitosis and cell division
• Prophase


  1. The cell's chromatin condenses into chromosomes, each chromosome exists as two copies

  2. The nuclear membrane breaks down, and the nucleolus disappears

  3. Spindle fibres are formed from the centrosomes as they move apart to opposite poles of the cell
• Metaphase

  1. The spindle fibres guide the chromosomes to the equator of the cell

  2. The spindle fibres from opposite poles attach to the centromere of each chromosome
• Anaphase


  1. Each centromere splits apart, and the sister chromatids separate from each other

  2. The spindle fibres shorten, pulling the chromosomes to opposite poles of the cell
• Telophase


  1. The chromosomes start to unwind into strands of less-visible chromatin

  2. The spindle fibres break down, the nuclear membrane forms around the new set of chromosomes, and a nucleolus forms within each new nucleus
• Cytokinesis


  1. Division of the cytoplasm to complete the creation of two genetically identical daughter cells

  2. In animal cells, an indentation called a cleavage furrow forms in the cell membrane along the equator of the cell and pinches off, dividing the cytoplasm equally

  3. In plant cells a new structure called a cell plate forms between the two daughter nuclei, and cell walls then form on either side
• DNA


  Made up of two long strands that form a spiral shape called a double helix
• RNA


  Double stranded
• Chromatin


  DNA exists as strands of chromatin fibre, which condenses into distinct chromosomes
• Nucleotides


  • Helix has sugar + phosphate backbones, with bases that protrude inward at regular intervals
  • The four bases in DNA are adenine (A), guanine (G), thymine (T), and cytosine (C)
  • Adenine pairs with thymine and guanine pairs with cytosine (complementary base pairs)
• Genetic mutation


  A change in the nucleotide sequence of DNA
• Genome


  The complete DNA sequence in every cell of an organism
• DNA replication


  1. During interphase, the double helix unwinds, and each strand of DNA serves as a template for a new strand

  2. Each of the new double-stranded DNA molecules contains one original strand of DNA + one new strand of DNA
• Chromosomes


  • Somatic cells have 46 chromosomes organized into 23 pairs, one maternal and one paternal
  • Sex cells are X and Y, XX for females and XY for males
  • Autosomes are all the chromosomes except the sex chromosomes
• Homologous chromosomes


  Pairs of chromosomes that appear similar but NOT identical, in terms of their length, centromere location, and banding pattern
• Alleles


  Carry genes for the same traits (i.e. hair colour) at the same location but can carry different forms of the same gene
• Genes


  Sections of DNA that contain genetic information for the inheritance of specific traits
• Karyotype


  A person's chromosomes paired with homologous partners, from longest to shortest, with sex chromosomes last
• Sexual reproduction


  1. Involves two parents and leads to the production of genetically distinct offspring

  2. A male haploid gamete (sperm cell) + a female haploid gamete (egg cell or ovum) undergo fertilization, and the cell that results from this fusion of gametes is called a diploid zygote
• Gametes


  Contain single, unpaired chromosomes, are said to be haploid (n)
• Somatic cells


  Contain pairs of chromosomes, are said to be homologous diploid (2n)
• Meiosis I


  1. Prophase I: Each pair of homologous chromosomes lines up side by side, and crossing over can occur

  2. Metaphase I: The pairs of homologous chromosomes line up along the equator of the cell

  3. Anaphase I: The homologous chromosomes separate and move to opposite poles of the cell

  4. Telophase I: The homologous chromosomes begin to uncoil and the spindle fibres disappear, cytokinesis takes place and two haploid cells form
• Meiosis II


  1. Prophase II: No new chromosome replication occurs, the nuclear envelope breaks down, and the spindle apparatus forms

  2. Metaphase II: A haploid number of chromosomes line up at the equator, single file

  3. Anaphase II: The sister chromatids are pulled apart at the centromere by the spindle fibres, the chromosomes move toward the opposite poles

  4. Telophase II: A nuclear envelope forms around each set of chromosomes and cytokinesis occurs, producing four haploid daughter cells
• Mitosis


  Consists of only one set of division phases and produces two diploid daughter cells that are identical
• Meiosis


  Consists of two sets of divisions and produces four haploid daughter cells that are not identical, resulting in genetic variation/diversity
• Fertilization


  Haploid + haploid = diploid
• Unequal division of cytoplasm


  The cell that receives most of the cytoplasm after the first division continues to form a viable egg
• Fraternal twins


  May be no more alike than any other siblings