Chapter 21: The Genetic Basis of Development

From single cell to multicellular organism: A. Embryonic development = cell determination  differentiation  morphogenesis One week

-apical meristems (stems and roots) in plants are perpetual embryonic regions, continually growing.

Researchers use the “model organisms” to study development: 1. Mus musculus = mouse 2. Drosophila melanogaster = fruit fly 3. Caenorhabditis elegans = nematode 4. Danio rerio = zebrafish 5. Arabidopsis thaliana = common wall cress These are model organisms because they have: 1. readily observable embryos 2. short generation times 3. relatively small genomes 4. preexisting knowledge of organism

The C. elegans have 959 somatic cells. Researchers have mapped out exactly how they develop into adulthood. These are transparent worms. This is a cell lineage; a fate map, showing what cells are destined to become.

Cells differentiate based on which genes are turned on and which genes are turned off. Different types of cells in an organism have the same DNA. This is called “genomic equivalence.” Can differentiated cells ever become a whole new organism? In many plant species, you can take differentiated cells and create a new plant. This is called “Totipotency.” The new organism becomes a clone of the parent plant.

Nuclear Transplantation: Differentiated cells from animals usually fail to become a new organism. The ability of the transplanted nucleus to support develop- ment depends on the age of the donor an Embryo  tadpole develops Tadpole  <2% develop

Cloning the first mammal: Dolly, the sheep: In 1997, Ian Wilmut cloned an adult sheep by nuclear trans- plantation:

Mammary cells “starved” so that cells go into G0 phase. Mammary nuclei are implanted into egg that has been denucleated. Egg will grow in culture and then when an embryo is formed, it will be implanted surgically into the uterus of a sheep.

After 256 tries, Dolly was born. Dolly died after 6 years of life due to a lung disease; she also had arthritis. However, before her death, she was able to have 4 offspring.

Stem cells: Unspecialized cells that that continually reproduce, and under specific conditions, differentiate into specialized cells.

Stem cells are multipotent, or pluripotent, which means they can become many types of cells.

Stem cell research is of great interest because with it, we may be able to repair damaged organs by adding healthy, new cells. a. Parkinsons: Brain cells b. Diabetics: Pancreatic cells http://www.sumanasinc.com/webcontent/ anisamples/generalscience/stemcells.html

Determination leads up to differentiation in cells. As an organism develops, cells become committed to its final state – it is “determined.” The first sign of differentiation is when mRNA for specific proteins are made.  code for that cell’s “tissue-specific proteins.”

“Master Control/Regulatory Genes” commit the cell.

How are the “master control gene” turned on?  Maternal cytoplasm in the egg contains maternal proteins, mRNA, and organelles, not equally spread out in the egg: These maternal molecules are called “cytoplasmic determinants.” Unevenly distributed, they will turn on certain genes.

Cells can synthesize signal molecules that turn on genes in neighboring cells. This is called induction. Pattern formation: the development of a spatial organization in which the tissues and organs of an organism are all in their characteristic places.

Pattern formation in the fruit fly: “Positional Information” tells a cell where it is located relative to the body axes and to neighboring cells. They are molecules that determine how the cell will respond to molecular signals.

Mitosis takes place without cytokinesis After the 10th division, the nuclei migrate outward. After the 13th division, plasma membrane forms around each nuclei.

HIERARCHY OF GENE ACTIVITY IN DROSOPHILA DEVELOPMENT (A cascade of gene activations sets up the segmentation pattern) (Basic subdivisions along the A-P axis) (Segments in pairs)

B.After segment polarity genes are turned on, The homeotic genes direct the identity of body parts (antennae, legs, and wings develop on appropriate segments). All homeotic genes of Drosophila include a 180-nucleotide sequence called the homeobox, which specifies a 60-amino-acid homeodomain. An identical or very similar sequence of nucleotides (often called Hox genes) are found in many other animals, including humans.

Mutations in hox genes:

5´ 3´ anterior posterior

Programmed Cell Death: “Apoptosis” Required for normal development. It is controlled by apoptosis genes, “suicide genes.” Examples: -Hands and feet -Neurons; surplus cells eliminated -Endometrium at the start of menstrual cycle

Regulated by changes in the activity of proteins