Lec 2

• Cell Size:

  • Prokaryotic organisms have a cell size ranging from 1 to 10 μm (LD)
  • Eukaryotic organisms have a cell size ranging from 10 to 100 μm (LD)
• Organelles:

  • Prokaryotic organisms have no organelles
  • Eukaryotic organisms have organelles present
• Nuclear Membrane:

  • Prokaryotic organisms have an absent nuclear membrane
  • Eukaryotic organisms have a present nuclear membrane
• Cell Division:

  • Prokaryotic organisms undergo binary fission
  • Eukaryotic organisms undergo mitosis and meiosis
• Cell Cycle:

  • The cell cycle 2 phases
  • The phases are:

  1. Interphase:

  • G1 phase involves chromosomes
  • S phase involves replication of DNA, resulting in sister chromatids
  • G2 phase involves DNA packaging into visible chromosomes.

  2. M-Phase:

  
  

  2.1. Mitosis

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase

  2.2. Meiosis

  • Prophase I : Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis
  • Metaphase I
  • Anaphase I
  • Telophase I
  • Prophase II, Metaphase II, Anaphase II, Telophase II
• Structure of a Chromosome:

  • Chromatin is composed of chromosomes, chromatids (1 strand chromosome), and sister chromatids
  • Nucleosomes are the basic building blocks of chromosomes, composed of a nucleosome core and DNA
  • Twisted around the nucleosome core is DNA
  • Linking of the nucleosome to the next nucleosome is called a "Linker DNA"
  • Chromosomes can have a Nucleolus Organizer Region (NOR) carrying the gene for ribosomal DNA. This is where the satellite is attached.
• Genome:

  • Genome is the complete set of genes/chromosomes from a male or female parent (haploid)
  • Basic chromosome number (n) is the haploid, while true diploid X = n
• Meiosis

  • Reductional Division
  • Prophase I:
  • Leptotene/Leptonema – thin thread stage
  • Zygotene/Zygonema - homologous pairing stage. o Formation of bivalent (II) chiasmata hold II together. Synaptonemal complex –like chiasmata but chromosomes does not overlapped with each other hence no cross-over occurred.
  • Pachytene/Pachyma - Crossing-over stage. Results to genetic recombination.
  • Diplotene/Diplonema - II separation starting at the centromere. Terminalization of the chiasmata
  • Diakinesis - II are evenly distributed. Best stage to establish chromosome number.
• Mitosis:

  • Equational division resulting in two identical cells

  o Stages:

  • Prophase – chromosomes are visibly composed of sister chromatids
  • Prometaphase – 6 chromosomes. 12 sister chromatids.
  • Metaphase – chromosomes align at the equatorial plane/plate.
  • Anaphase- separation of sister chromatids and their movement on the opposite poles. “Per Pole”. Once separated, now called chromosomes.
  • Telophase- Chromosomes regrouped at the poles. Same DNA content as they proceed to S phase (replication)
  • (Cytokinesis)-2 cells. 6 chromosomes per cell.
• Timing of Meiosis and Fertilization:

  • Gametic or terminal timing is before zygote and gametes formation (n) in animals
  • Zygotic or initial timing is after zygote formation in most fungi and some algae
  • Sporic or intermediary timing is in plants and some algae, between diploid and haploid stages
• Chromosome

  o   Folded chromatin fiber           

  o   Carries of the genes

  o   Dextran Sulfate /high salt can disrupt/destroy the nucleosome core. This will then release the DNA.

  • However, the protein scaffold in the chromosome will not be broken/destroyed as it is a non-histone protein.

  o   Some chromosomes will have a Nucleolus Organizer Region (NOR).

  • Carries the gene for ribosomal DNA
  • This is where satellite will be attached to. Attached in the nucleolus. 
  • Attachment of satellite and the site of centromere position is permanent hence, they can be used as cytological marker.
• • Metaphase I - alignment of the bivalent (II) at the equatorial plane
  • Anaphase I – separation of II into I (univalent). Reductional division. Moving to each poles.
  • Telophase I. Chromosomes re-grouped at opposite poles. Two haploid cells formed.
• o   Meiosis II – Equational division

  • Prophase II
  • Metaphase II – univalent aligning in the equatorial plane
  • Anaphase II – separation of sister chromatids
  • Four haploid cells (n) are formed
• Timing of meiosis and fertilization (syngamy)

  • Gametic or terminal: Animals. Before zygote and gametes formation (n)
  • Zygotic or initial: Most fungi and some algae. Happens after zygote formation. Will form diploid gametes 
  • Sporic or Intermediary: Plants and some algae. In between is the diploid stage (zygote formation) and the haploid stage (gametic).