lesson 2

• Chromosome

  Self-replicating genetic material that directs the activities and functions of the cells

  
• DNA (Deoxyribonucleic acid)

  Genetic material located inside a chromosome in the nucleus of the cell

  
• DNA

  • Helical structure consisting of two strands
  • Organized and packaged by proteins or histones to form different levels of chromosome packaging

  
• Chromosome packaging

  1. DNA helix forms nucleosomes

  2. Nucleosomes form 30-nanometer chromatin fiber (solenoid)

  3. Solenoid forms chromatin loops

  4. Chromatin loops condense to form mitotic chromosome (1,400 nm)

  
• Chromatids

  Two identical copies of a chromosome

  
• Centromere

  Constricted region that holds the two chromatids together
• Telomeres

  Protective caps at the ends of chromosomes
• Chromatin

  DNA strands attached and coiled looking like beads on a string, measuring up to 11 nm
• Chromatin fiber

  Series of chromatin molecules forming a 30-nanometer coiled nucleosomes called solenoid
• Chromatin loops

  Supercoiled chromatin fiber forming loops, leading to condensation of chromosomes up to 700 nm
• Mitotic/Metaphase chromosome

  Replicated and packaged/coiled DNA molecule, measuring up to 1,400 nm

  
• Chromosome

  • Chromatids - two identical halves of a replicated chromosome
  • Centromere - attachment point of chromatids, divides chromosome into "arms"
  • Short arm (p arm) - upper, usually shorter
  • Long arm (q arm) - lower, usually longer

  
• Number of chromosomes is characteristic of the species

  
• Chromosome number of selected organisms

  • Drosophila melanogaster (fruit fly) - 8
  • Canis familiaris (dog) - 78
  • Homo sapiens (man) - 46
  • Oryza sativa (rice) - 24
  • Zea mays (corn) - 20

  
• Interphase

  Interval between two cell divisions, where cell grows and DNA replicates
• Interphase

  1. G1 - cell grows, protein/RNA synthesis, mitochondria increase

  2. S - DNA synthesis, chromosome replication

  3. G2 - rapid cell growth, preparation for cell division
• Mitosis

  Cellular process where two nuclei and two cells are produced from one, with same chromosome number
• Stages of Mitosis

  1. Prophase - chromosomes coil, nuclear membrane breaks down

  2. Metaphase - chromosomes align at equatorial plane, spindle fibers attach

  3. Anaphase - sister chromatids separate, move to opposite poles

  4. Telophase - chromosomes at poles, new nuclear membranes form, cell divides

  
• Cell plate

  Forms between two new nuclei in plant cells during telophase, new cell wall forms
• Somatic cells

  Body cells with two complete sets of chromosomes (diploid)

  
• Gametes

  Sex cells with one set of chromosomes (haploid)
• Meiosis decreases chromosome number by half to produce gametes
• Meiosis I

  1. Prophase I - chromosome pairing, crossing over

  2. Metaphase I - homologous pairs align

  3. Anaphase I - homologous pairs separate

  4. Telophase I - two haploid daughter cells

  
• Meiosis II follows Meiosis I, producing four haploid daughter cells
• Leptotene

  Each chromosome is made up of sister chromatids. These are long threadlike structures which result from the replication of DNA during the Synthesis or S phase of the cell cycle.

  
• Zygotene

  The homologous chromosomes start to pair off through the process known as synapsis. Pairs of chromosomes that are similar in size and shape are called homologous chromosomes or tetrads.

  
• Pachytene

  1. The repeated coiling of chromosomes occurs resulting to its contraction and thickening making the homologous pair of chromosomes to be very close to each other

  2. Crossing over happens. Here, the exchange of segments between the sister chromatids of the homologous chromosomes occurs

  3. The exchanging process form a cross-linkage called a chiasma

  4. After crossing over, the sister chromatids of each chromosome may not be identical with each other based on the genetic material they contain

  
• Diplotene

  The two homologous chromosomes forming a tetrad begin to repel one another and move apart. They are held only by the chiasma.
• Diakinesis

  • Bivalents or homologous pair chromosomes distribute them evenly in the nucleus
  • The nuclear membrane breaks down and the nucleolus disappears
  • Chiasma moves towards the end, which is called terminalization
  • Chromatids remain attached only at the terminal chiasmata and enter the metaphase stage

  
• Metaphase I

  1. Spindle fibers from the centrosomes of each pole connect to bivalents or tetrads through the kinetochores

  2. Homologous chromosomes line up at the equatorial plane
• Anaphase I

  1. Spindle fibers begin to contract and separate the bivalent or tetrads

  2. Homologous chromosomes separate and migrate to each pole of the cell
• Telophase I

  1. Chromosomes decondense

  2. Nuclear membranes reform

  3. Cytokinesis or the cytoplasmic division occurs and two haploid (N) daughter cells with chromosomes with two sister chromatids are formed

  
• Prophase II

  1. Chromosomes (chromatids) condense to form metaphase chromosomes

  2. Nuclear membrane dissolves and nucleolus disappears

  3. Centrosomes move towards each pole of the cell

  
• Metaphase II

  1. Spindle fibers attach to chromatids at the kinetochores

  2. Chromosomes line up at the equatorial plane

  
• Anaphase II

  1. Spindle fibers shortened and separated the sister chromatids

  2. Chromatids that are now called chromosomes move towards each pole of the cell

  
• Telophase II

  1. Single-stranded chromosomes decondense

  2. Nuclear membrane and nucleolus reforms

  3. Cytoplasm divides (cytokinesis)

  4. Four haploid (n) daughter cells are formed

  
• Differences in Characteristics between Mitosis and Meiosis

  • Location
  • Chromosome number of parent cell
  • Chromosome number of daughter cell
  • Number of daughter cells produced
  • Number of nuclear divisions
  • Steps required to complete the cell division phase
  • Occurrence of synapsis
  • Occurrence of crossing over
  • Presence of chiasma
  • Kind of reproduction associated with

  
• Roles of Mitosis and Meiosis in the Cell Division

  • For somatic or body cell production
  • For asexual reproduction
  • For genetic stability
  • For the repair of damaged cells/tissues
  • For gametes or sex cell production
  • For sexual reproduction
  • For genetic diversity
  • Aids in the repair of genetic defects
• Stages of cell division

  • Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I
  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II
  • Interphase
• Genes consist of DNA