Chapter 20- Developmental patterning in eukaryotes 

Understand what it means that eukaryotic cells are genetically equivalent (except gametes)

differentiated cells are genetically equivalent

• mature plant and animals cells genetically equivalent (they contain the same genes)

• cell differences result from differential gene expression

Explain what it means that cells differentiate and what is controlling that on the genetic/chromosomal level.

cell differentiation: process by which cells become specialized in structure and function

cell differentiation is controlled primarily by gene expression: 

• all cells in the body (except for gametes) have the same DNA, meaning they share the same genetic information. but not all genes in the genome are turned on or expressed in every cell. 

• gene regulation: controlled by regulatory mechanisms

  • transcription factors: proteins that bind to specific DNA sequences near genes and either promote or inhibit transcription of those genes

  • epigenetic modifications: chemical changes to the DNA or the proteins around it (like histones) that affect how tightly or loosely the DNA is packed. 

  • non-coding RNAs: these RNA do not encode protein but play roles in regulating gene expression. 

differentiation pathways: certain cells receive signals from their environment (other cells, growth factors, or hormones). these signals activate specific transcription factors that direct the cell to follow a particular differentiation pathway. 

Genes like “master regulatory genes” regulate other genes by induction, promote differentiation and morphogenesis, differential gene expression, and programmed cell death/apoptosis. [Know what each of those things are/do.]

induction: process that influences the fate of another group of nearby cells through signaling mechanism

promote differentiation: process by which cells become specialized in structure and function 

morphogenesis: process by which cells, tissues and organs develop their shape and structure during embryonic development. 

differential gene expression: activation or deactivation of certain genes

programmed cell death/ apoptosis: highly regulated and essential part of development in both plants and animals.
apoptosis: most common type of cell programmed death. 

Pattern formation in fruit Fly; positional information; know the major axes.

pattern formation: development of spatial organization of tissues and organs.

major axes

1. anterior (front)

2. posterior (back)

3. central 

4. dorsal 

5. left

6. right

positional information: molecular cues that control pattern formation

• tells the cell its location relative to major axes and neighbor cells. 

Nusselhein-Volhard/Weischaus mutant screen identified genes controlling development patterning; know what maternal effect genes do, where they come from, and what happens to the embryo if maternal effect genes are mutated; problems with deposition of maternal effect mRNA in an egg will affect all offspring equally; know what bicoid does.

maternal effect genes: moms effect on developing offspring. mom gives plan to structure your body with mRNA’s. (also called egg polarities)

what happens if maternal effect genes are mutated: it can lead to severe developmental defects in the embryo even tho the embryos genome is intact. 

1. lack of body axis formation

2. misplaced cell differentiation 

3. sever patterning defects

bicoid: affects front half of the body (head). 

morphogen concentration gradient

• higher concentration was on the anterior end (front/head)

• bicoid mRNA is made from mom cells 

Know what morphogens are and how researchers figured out that bicoid was expressed as a morphogen, where it was found in the embryo, and what type of protein it is.

morphogens: signaling molecules that play a key role in development of an organism/ typically secrete by specific cells and form concentration gradients within a developing embryo. 

how researchers figured out that bicoid was expressed as a morphogen: genetic experiments showing that bicoid mutants lack anterior structures, and using techniques like in situ hybridization to show that mRNA is localized in the anterior of the gg and that is protein forms a gradient. 

where is it found: in the anterior forn pole of the oocyte during oogenesis, prior to fertilization. 

what type of protein is it: it is the homeodomain transcriptor factor that activates the expression of games required for anterior development in the embryo. 

Understand what homeotic genes are. recognize hox gene organization, evolutionary conservation (which organisms have Hox genes) and the consequences of altering expression of homeobox genes (phenotypes like Antennapedia and bithorax in the fruit fly).

homeosis: transformation of one organ into another via mutation. 

homeotic genes: genes that determine organ formation

homeobox genes are examples of homeotic genes

organisms with hox genes: 

1. drosophila 

2. humans

3. zebrafish 

4. mice

hox gene organization: highly conserved and specific manner within the genome.

• crucial for proper function during development

• order of genes is the same as the order in which they are expressed

• all animals contain a related set of hox genes 

  • spatial arrangement is related to their expression patterns along the anterior and posterior axis. 

diverse groups: 

1. prokaryotes

2. yeast

3. plants

4. animals

EVOLUTION: 

shrimp to mosquito

consequences of altering hox box expression genes: 

• lead to transformation in the identity of entire body parts

  • antennapedia: grow legs in the head instead of antenna

  • bithorax: development of a third thoracic segment and the posterior abdominal segments 

Be able to explain how gaining or losing developmental patterning genes can lead to gain or loss of certain structures (example: loss of Hox6/8 overlapping expression results in loss of forelimbs, while loss of Sonic hedgehog could result in loss of hindlimbs)

developmental patterning genes are essential for determining the formation, identity and positioning of various structures during embryonic development.

• when genes are gained or loss or misregulated that can lead to gain or loss of certain body structures. 

• typically work by establishing spatial and temporal expression gradients which control the development of various tissues and organs

LOSS OF HOX 6/8

crucial for forelimbs and hindlimbs in vertebrates

• If there is a loss of Hox6/8 overlapping expression. for example, through mutations or the disruption of regulatory elements, this can disrupt the proper signaling that would normally promote forelimb development.

• Result: Loss of forelimbs (or severe limb malformations), because the lack of proper Hox6/8 expression means the forelimb patterning is not properly initiated in the embryo. The forelimb structures will not form, and instead, the embryo may only develop hindlimb structures.

Loss of Sonic Hedgehog (Shh) Leading to Loss of Hindlimbs

 critical signaling molecule that plays a fundamental role in regulating the growth and patterning of limbs, particularly during early limb development 

• When Sonic hedgehog is lost or misregulated, the ZPA does not secrete Shh properly, which leads to disruption of the signaling required for proper limb development.

• The posterior structures of the limb, such as the hindlimb (in vertebrates), fail to develop properly. This can also affect the proper formation of the distal structures of the limb (such as fingers or toes).

• Loss of Sonic hedgehog (Shh) can result in the complete absence of hindlimbs (or other structures like digits), depending on the extent of the loss of Shh signaling

Understand how gene expression control and the types of genes controlling development reveal an evolutionary connectedness among eukaryotes.

gene expression control: mechanisms that regulate the timing, location and amount of gene activity in cells. 

types: 

1. homeobox genes (including hox genes)

2. sonic hedgehog

3. wnt signaling pathway 

4. pax genes

evolutionary connectedness: showing that many fundamental developmental processes have been conserved across species over evolutionary time. These conserved genes and pathways, such as Hox genes, Shh, Wnt, and Notch, serve as key examples of how evolutionary pressures have shaped the development of diverse body plans while maintaining core genetic mechanisms.