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How does a fertilized egg become

ananimal?

Clam egg and sperm

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Developmental Biology is the study of a PROCESS whereby a single cell (the fertilized egg) divides and selectively activates expression of genes to produce a complex organism composed of many cell types.

Ex ovo omnia!

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To form an embryo, the following (and more!) must occur:

• Gametes form and fuse (Reproduction)• Cells multiply (Growth)• Generation of Asymmetry• Axis Determination (Positional information)

- Anterior/Posterior (Head-Tail)- Dorsal/Ventral (Back-Front)

- Left/Right • Cells differentiate• Structures are built from cells (Morphogenesis)

-Animal cells organize into sheets and move-Plant cells form structures without moving

What kinds of PROCESSES are required?

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Differentiation is a central idea of development:

All cells have the same DNA, but DIFFERENT CELLS express DIFFERENT GENES

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Nature supports an incredible diversity of plant

and animal body plans

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Yet all of these organisms share conserved developmental mechanisms

that are evidence of their evolution from a common ancestor.

Our challenge is to understand both this diversity and this unity.

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Developmental Biology is studied using the following TOOLS

1. Cell Biology2. Genetics3. Molecular Biology

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Let’s Review the Basics

1. The body is made of millions to billions of cells.2. Cellular machinery is largely made up of proteins3. Because of their different tasks, different cells

contain different proteins4. Proteins are made up of chains of amino acids, and

these amino acids are "encoded" in the cell's DNA1. Information flows from DNA to RNA to Protein2. When one gene is mutated, one protein is affected

(usually disabled).5. All cells have the same DNA but different cells

express different genes

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Development Occurs at an Unfamiliar Scale

If a cell was the size of a basketball (8 inches)

• a mouse would be the size of Chapel Hill (10 miles) • a gene would be about an inch long.

 

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Development Occurs at an Unfamiliar Scale

If a protein was the size of a Volvo (10 feet)

• a cell would be the size of Chapel Hill (10 miles)• a gene would be about 1.5 miles long but the strand of DNA would only be a few feet wide.

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TESTS

• Exam #3 March 31 (covers March 3-29)

• FINAL May 5 (covers April 5-26)

• NO make-up exams

• Regrade requests must be submitted to your TA within one week of exam

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Two Extreme Models for Differentiationfrom the late 1800’s (neither is correct)

1. Mosaic development2. Regulative development

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Roux’s landmark experiments with frog embryos:

do cells have fixed identities that they can maintain without influence from their neighbors?

The Mosaic Development model proposesthat cells become progressively committed to specific cell fates

Only half an embryo develops

“YES”!

Differential segregation of genetic potential?

4-cell stage

Kill 2 cells with a hot needle and allow the remaining

2 cells to develop

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Roux’s landmark experiments

Figure 3.16. Destroying (but not removing) one cell of a 2-cell frog embryo results in the development of only half the embryo.

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Driesch’s experiments with sea urchin embryos:

do cells have fixed identities that they can maintain without influence from their neighbors?

The Regulative Development model proposes that cells retain the ability to adjust their fates in response to their cellular environment

(No differential segregation of genetic potential)

“NO”!Each cell regulated its development

to produce an entire embryo

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C. elegans

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How do cells know which genes to activate as they go through development?

Most organisms use 2 sources of info1. parents2. neighbors

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Information from parents:The Cell lineage

But what makes “red” different from “blue” in the first place?

Mother cell

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Mother cell

Unequal localization of "determinants"

Cell division transfers determinants to a single daughter cell

Cells are now different.

Segregation of determinants

Cell type A Cell type B

• mechanism to generate asymmetry and subsequent cellular differentiation

• determinants are usually proteins or mRNA. • information (proteins/RNA) can be passed on uniformly, or can be segregated to one of the progeny cells.

Information from parents:

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Mother cell

Cell interactions

Cell type A

• an alternative mechanism to generate asymmetry and subsequent cellular differentiation

• cell division places daughter cells in different environments

• different environments lead to different cell fates

Cell type B

Cell division

Information from neighbors:

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Cells don’t have to be inside an animal to communicate with each other

Examples

1. Yeast2. Slime mold (Dictyostelium)

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"A" cell

"alpha" cell

• 2 yeast cell types: "A" and "alpha"

• only cells of different mating types can mate

HOW?

each cell type makes a specific signal (factor) and has receptors

only for the opposite signal

"A" cell

"alpha" cell

"A" factor

"A" factor receptor

"alpha" factor receptor

"alpha" factor

Single yeast cells talk to each other when they want sex!

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These cell-cell signalslead the yeast cells that receive them tomove together, change shapeand ultimately fuse,producing a diploid cell

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The slime mold develops into an animal only when it (they?) gets hungry!

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The remarkable life cycle of a slime mold

cAMP signal

Slug/Grex

Figure 2.10

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Dictyostelium discoideum (slime mold) slug stage

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The Cells of the Grex Differentiate

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Conclusion: Even cells of the most simple eukaryotic

organisms sense their environment, migrate, adhere to each other,

differentiate, and interact

Now, on to more complicated ones!

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Breakthroughs in Modern Biology

1. All organisms share similar cellular machinery

2. All animals use this machinery in similar ways to direct embryonic development

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Model Organisms in Developmental Biology

Plants Invertebrates Vertebrates

Why use model organisms?What features do they have in common?