Chapter 21: The Genetic Basis of Development

I. 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.

B.Researchers use the “model organisms” tostudy development:1. Mus musculus = mouse2. Drosophila melanogaster = fruit fly3. Caenorhabditis elegans = nematode4. Danio rerio = zebrafish5. Arabidopsis thaliana = common wall cress

These are model organisms because they have: 1. readily observable embryos

2. short generation times3. relatively small genomes4. preexisting knowledge of organism

C.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.

II. Cells differentiate based on which genes areturned on and which genes are turned off.

A.Different types of cells in an organism havethe same DNA. This is called “genomic equivalence.”

B.Can differentiated cells ever become a whole new organism?

1.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.

Differentiated cells from animals usuallyfail to become a new organism.The ability of the transplanted nucleus to support develop-ment depends on the age of the donor anEmbryo tadpole developsTadpole <2% develop

Nuclear Transplantation:

C.Cloning the first mammal:1.Dolly, the sheep: In 1997, Ian Wilmut

cloned an adult sheep by nuclear trans-plantation:

1.Mammary cells“starved” so thatcells go into G0 phase.

2.Mammary nucleiare implanted into egg that hasbeen denucleated.

3.Egg will grow inculture and thenwhen an embryois formed, it willbe implanted surgically into theuterus of a sheep.

After 256 tries,Dolly was born.Dolly died after6 years of life due to a lungdisease; she also had arthritis.However, beforeher death, she was able to have4 offspring.

D.Stem cells: Unspecialized cells that that continually reproduce, and under specificconditions, differentiate into specialized cells.

1.Stem cells are multipotent, or pluripotent,which means they can become many typesof cells.

2.Stem cell research is of great interest because with it, we may be able to repairdamaged organs by adding healthy, newcells.a. Parkinsons: Brain cellsb. Diabetics: Pancreatic cells

http://www.sumanasinc.com/webcontent/anisamples/generalscience/stemcells.html

E.Determination leads up to differentiation in cells.1.As an organism develops, cells become

committed to its final state – it is “determined.”

2.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.

F.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:

1.These maternalmolecules are called“cytoplasmic determinants.” Unevenly distributed,they will turn oncertain genes.

2.Cells can synthesize signal molecules thatturn 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.

III.Pattern formation in the fruit fly:A.“Positional Information” tells a cell where it

is located relative to the body axes and toneighboring cells. They are molecules that determine how the cell will respond to molecular signals.

Mitosis takes placewithout cytokinesis

After the 10th division,the nuclei migrateoutward. After the 13th

division, plasmamembrane forms aroundeach nuclei.

HIERARCHY OF GENE ACTIVITY IN DROSOPHILA DEVELOPMENT

(A cascade of gene activations sets up the segmentation pattern)

(Basic subdivisionsalong 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).

1.All homeotic genes of Drosophila include a 180-nucleotide sequence called the homeobox, which specifies a 60-amino-acid homeodomain.

a.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´

anteriorposterior

C.Programmed Cell Death: “Apoptosis”Required for normal development. It is controlled by apoptosis genes, “suicide genes.”1.Examples:

-Hands and feet-Neurons; surplus cells eliminated-Endometrium at the start of menstrual cycle

Regulated by changes in the activity of proteins