From Single Cell To Multicellular Organism

  1. From a single fertilized egg, a multicellular organism must emerge. The core processes of growth and development from a zygote to an adult are:
    • cell division
    • cell differentiation
    • morphogenesis- the physical processes that gives the organism its form and shape
  2. Development in animals differ from development in plants because:
    1. Animal morphogenesis requires the movement of cells to certain areas and regions to aquire the correct shape.
    2. Plants are not restricted by certain periods of growth and can continuously grow using apical meristems, which are unspecialized cells at the tips of roots and shoots that allow the plant to grow further.
  3. Model organisms are organisms selected for the study of a characteristic that spans over a large group.
    • examples include: frog, Drosophila, nematode C. elegans, mouse, zebrafish, and the Arabidopsis
    • In the nematode C. elegans, an entire cell lineage map has been traced. A cell lineage map is basically a directory that shows where each cell during development wound up in the end in the organism.

Differential Gene Expression

  1. It is believed that most cells in an organism carry the same genetic information. This is known as genetic equivalence. This leaves gene expression to represent all the differences in the cells of a multicellular organism.
  2. Cloning is the use of a somatic cell of an organism to generate an identical organism.
    1. Under cell culture, the roots of a carrot plant can develop into a genetically identical carrot plant.
    2. Cells that can retain and maintain a zygotes ability to differentiate and develop into a whole multicellular organism are referred to as totipotent.
  3. A generalized breakdown of the cloning experiment:
    1. Retrieve an egg cell or unfertilized egg
    2. Destroy its nucleus
    3. Insert target cell's nucleus
    4. Observe the development
      • In many cases, this experiment resulted in failure.
      • Researchers then concluded that during differentiation, the nuclei does change (DNA methylation, etc.) and that changes occur that may be irreversible.
  4. The first successful mammal clone was a lamb named "Dolly". The method was a little different.
    1. Culture of udder cells in nutrient poor medium to arrest the cell at the G° or G 0 phase of interphase. This apparently allowed for the nucleus to dedifferentiate and become totipotent in a sense.
    2. Nucleus from donor egg cell removed and the udder cell nucleus was inserted.
    3. Grown in culture and then implanted in the uterus of the mother.
    4. Out of many experimental trials, Dolly emerges.
      • Other mammalian clones follow soon after.
      • Researchers then discovered that the DNA of clones are often unproperly methylated, which results in defects and earlier death.dolly.gif
  5. Stem Cell Research is a very important field of study on development. It can act as permanent treatment for permanent injuries (like nerve damage) and for disorders/diseases (like diabetes). This is because stem cells are undifferentiated somatic cells.
    • They can come from a developing embryo.
    • They can come from the bone marrow.
  6. Differentiation occurs when the cell begins to synthesize specific proteins. When the cell initiates this process, it is referred to as becoming "determined", meaning it is becoming specialized.
  7. Some important factors during development to remember are:
    • Cytoplasmic determinants are substances that exist in the egg cell cytoplasm. These substances originated from the mother and play a key role in regulating gene expression in a developing embryo.
    • Induction is the signaling of nearby cells to express or to stop expressing certain proteins, which affects differentiation.

Genetic and Cellular Mechanisms of Pattern Formation

  1. Positional information are molecular cues that determine the location in a body of specific cells, which leads to spatial organization known as pattern formation.
  2. Maternal effect genes are genes that reside in the mother, but its products affect the development of the embryo.
    • Cytoplasmic determinants are an example.
    • Mutations in such genes can lead to embryonic deaths/lethals/deformities.
    • They set the orientation or polarity of the egg.
  3. Morphogens are substances that establish the axes and form of a developing embryo.
    • They form gradients in the cell, which triggers segmentation genes since they act as transcriptional factors. Segmentation genes establish the basic segments that will emerge as specialized structures.
      • Gap genes- lead to a general division of the body; establishes the anterior-posterior axis
      • Pair-rule genes- defines the pattern of pairs of segments and appendages
      • segment polarity genes- sets the anterior-posterior axis for each segment.
  4. The homeotic genes are the regulatory genes that define the identity and specific structure of segments.
    • The homeobox (Hox genes) are a nucleotide sequence that appears in most animals, which shows evolutionary conservation.
      • They are associated with development.
      • Generates a homeodomain in proteins, which can then bind to DNA and act as a transcription factor.
  5. Induction is used in some animals to stimulate development.
    • It forms certain organs.
    • The concentration of inducers will have certain effects.
    • Signal transduction of inducers results in a response that helps differentiate the cells.
  6. Differentiation in cells may involve apoptosis, or cell death. At times, cells may have to be removed, so certain genes are triggered and expressed to kill certain cells.
    • The mitochondria plays an interesting role in apoptosis. When the cell needs to be removed, the mitochondria is attacked by certain proteins to cause its outer membrane to leak out apoptosis genes' promoting factors, like cytochrome c.
    • Apoptosis is important for the operation of the nervous system, the immune system, and during morphogenesis.
  7. In plants, organ identity genes are like homeotic genes in that they determine what floral organ arises from a whorl, which is part of the floral meristem.
    • Induction is primarily used in this case.