Genetics 2

• In eukaryotes, transmission of genetic material from one generation of cells to the next involves mitosis and meiosis
• Meiosis leads to the production of gametes
• Mitosis leads to the production of 2 cells, each with the same number of chromosomes as the parent cell
• Cell structure is closely tied to genetic function
• There are 2 main types of cells:

  • Prokaryotic (bacteria, archaea)
  • Eukaryotic (protists, plants, fungi, animals)
• All cells share some common features:

  • Plasma membrane
  • DNA
  • Ribosomes
• The cell is surrounded by a plasma membrane
• Plants have a cell wall composed mainly of cellulose
• Bacterial cells have peptidoglycan on their cell wall
• DNA in the nucleus is complexed with an array of acidic and basic proteins into thin fibers. During the non-divisional phases of the cell cycle, these fibers are uncoiled and dispersed into chromatin.
• Chromatin fibers coil and condense to form chromosomes during mitosis and meiosis
• Centrioles in the cytoplasm are located in a specialized region called the centrosome in animal cells. Centrioles organize spindle fibers for movement of chromosomes during meiosis and mitosis.
• Chromosomes exist in homologous pairs in diploid organisms
• Somatic cells (body cells) of a given species have a specific number of chromosomes

  • They present as homologous pairs
  • Humans: 46 chromosomes (23 homologous pairs)
• Homologous chromosomes are similar, they carry genes for the same inherited characteristics. They are not identical, and may carry different versions of the same gene.
• Each diploid organism contains 2 copies of each gene
• The members of each pair of genes need not be identical. Alternative forms of the same gene are called alleles
• Meiosis converts the diploid number (2n) of chromosomes to the haploid number (n)
• Gametes contain a haploid set of chromosomes
• Fusion of two gametes at fertilization results in a diploid zygote
• Sex-determining chromosomes are usually not homologous, yet behave as homologs in meiosis
• Mitosis partitions chromosomes into dividing cells

  • Genetic material is partitioned to daughter cells during nuclear division (karyokinesis)
  • Cytoplasmic division (cytokinesis) follows
• The cell cycle is composed of interphase and mitosis
• Interphase includes:

  • S phase, during which DNA is synthesized
  • Two gap phases (G1 and G2)
• G0 is a point in the G2 phase where cells withdraw from the cell cycle and enter a nondividing but metabolically active state
• Mitosis has discrete stages:

  • Prophase
  • Prometaphase
  • Metaphase
  • Anaphase
  • Telophase
• Prophase

  • During prophase, the centrioles divide and move apart
  • The nuclear envelope breaks down
  • Chromosomes condense and become visible
  • Sister chromatids are connected at the centromere
• Prometaphase - During prometaphase, the chromosomes move to the equatorial plane of the cell
• Metaphase

  • The centromeres/chromosomes are aligned at the equatorial plane
  • Spindle fibers bound to kinetochores associated with centromeres are responsible for chromosomes movement
• Anaphase 

  • Sister chromatids separate from each other and migrate to opposite poles during anaphase
  • The separated sister chromatids are called daughter chromosomes
• Telophase: the main events during telophase are

  • Cytokinesis
  • Uncoiling of the chromosomes
  • Re-formation of the nuclear envelope
• Mitosis partitions chromosomes into diving cells, producing daughter cells with a full diploid complement of chromosomes
• Significance of mitosis

  • Two daughter cells, same kind
  • Growth
  • Replacement of damaged cells
  • Multiplication in unicellular organisms
• Meiosis reduces the amount of genetic material by one-half to produce haploid gametes or spores containing one member of each homologous pair of chromosomes
• Meiosis I is a reduction cell division, Meiosis II is an equational division
• DNA synthesis occurs during interphase before the beginning of meiosis I but doesn’t occur again before meiosis II
• Meiosis I and II each have:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• The first meiotic subdivision: Prophase I
• Prophase I has 5 sub-stages, each with specific events

  • Leptonema
  • Zygonema
  • Pachynema
  • Diplonema
  • Diakinesis
• At the completion of prophase I, the centromeres of each tetrad structure are present on the equatorial plate