1. Genes & Development
Packet #26

2. Introduction Development Commitment Cell differentiation
The process by which descendents of a single cell specialize and organize into a complex organism
Commitment
The official series of overt changes in cellular biochemistry, and function.
The cell commits to a certain fate.
Two stages
Specification
When cell is capable of differentiating autonomously (independent of the surroundings) when placed in a neutral environment (Petri dish or test tube)
Determination
When cell is capable of differentiating when placed into autonomously when placed into another region of the embryo.
Cell differentiation
The development of specialized cell types

4. Introduction II Differential Gene Expression
The cause of differences among various cell types.
Has three postulates
Every cell nucleus contains the complete genome established in the fertilized egg
Unused genes in differentiated cells are not destroyed or mutated, and they retain the potential for being expressed
Only a small percentage of the genome is expressed in each cell
Hence a small portion of RNA is synthesized specified for that cell type

5. Introduction III Nuclear Equivalence Totipotency
All nuclei, with a few exceptions, of differentiated somatic cells of an organism are identical to each other and to the nucleus of the single cell from which they descended.
Genomic rearrangement
Gene amplification
Totipotency
Capability of cells to direct the development of an entire organism

6. Introduction IV Stem cells
Have the ability to produce differentiated descendents, yet retain the ability to divide to maintain the stem cell population.
Totipotent stem cells
Give rise to all cell types
Pluriopotent stem cells
Give rise to many, but not all, types of cells in an organism
Not capable of forming a functional organism.

7. Categories of Genes

8. Maternal Effect Genes
Genes expressed in the mother’s ovaries that produce mRNA’s that are ultimately placed in different regions of the developing egg
Establish the polarity of in the embryo.
Axis specification
Proteins, a type of morphogen, produced from the mRNA transcribed from the Maternal Effect Genes
Bicod & Hunchback
Regulate the production of anterior structures of the embryos.
Nanos & Caudal
Regulates the formation of the posterior structures of the embryos.
These proteins may behave as transcription factors that turn on HOX genes ultimately leading to the separation of the anterior from the posterior end

9. Morphogen Definition
A soluble substance, chemical agent, that affects differentiation and specification of any totipotent cell.

10. Segmentation Genes
The proteins produced, as a result of the transcription of the Maternal Effect Genes, affect a class of genes called the segmentation genes.
Genes responsible for the transition from cell specification to cell determination.
In drosophila, divide the early embryo into a repeating series of segmental primordia along the anterior-posterior axis.
Divide the embryo into 14 parasegments.

11. Classes of Segmentation Genes
Gap Genes
Respond to the concentrations of the material effect gene proteins
The first set of segmentation genes to act
Gap gene proteins define specific regions of the embryo
Begin organization of the body into anterior, middle and posterior regions
Gap gene proteins activate and repress the pair-rule genes

12. Classes of Segmentation Genes II
Pair Rule Genes
Proteins of pair-rule genes subdivide the broad gap gene regions, mentioned previously, into parasegments
Segment Polarity Genes
Responsible for maintaining certain repeated structures within each segment
Allowing for cell fates to be firmly established.

13. Homeotic Genes--Fruit Flies
Regulated by gap, and pair rule, genes.
Responsible for specifying the “identity” of each segment
Wings
Legs

14. Homework Assignment
What are the impacts of mutations in the Gap genes, pair rule genes and the homeotic genes.

15. Cloning
Nuclear Transplantation
Dolly The Sheep