Development of a complex multicellular organism is more than just mitosis- we certainly do not look like gigantic fertilized eggs.

Zygote -> many specialized cell types (differ in size, shape, longevity, biochemistry)

Plus they are arranged spatially in tissues (cells that share a certain function)

What does it take to go from a single celled zygote to a multicellular individual??

1. Mitosis…..2. Differentiation

Wormbook.org

How do cells grow to look a certain way and occur in a particular place? C. elegans

How do cells grow to look a certain way and occur in a particular place?

Different genes turned on and off due to

• Transcription factors

• Epigenetics

Differential transcription and translation of genes drives differentiation!

Figure 14.9

Transcription factors

TATA box

Promoter Nontemplate strand

Start point

Transcriptioninitiationcomplex forms.

Transcription initiation complex

DNA

RNA transcript

A eukaryoticpromoter

Several transcriptionfactors bind to DNA.

35

5 3 35

35

35

3

2

1Template

strand

Transcription factors

RNA polymerase II

35

35 T A T A A A A

A T A T T T T

…. each cell is “told” by transcription factors to transcribe different genes to make mRNA and then to make different proteins

When different proteins are made the cell differentiates

So lets go back to our zygote that is waiting to go through mitosis to grow into an adult with many different cell types….

Transcription factors initiate cell differentiation.

So genes are being transcribed, mRNA is being made and in turn proteins are made.

NOW…… “The expression of one gene can influence the expression of several other genes.”

What does that mean???

The protein being made may be a transcription factor for another gene!

Figure 16.4-3

Other muscle-specific genesNucleus Master regulatory

gene myoD

DNAOFF OFF

OFF

mRNA mRNAmRNA mRNA

MyoD

Embryonicprecursor cell

MyoD protein(transcriptionfactor)

Anothertranscriptionfactor

Myosin, othermuscle proteins,and cell cycle–blocking proteins

Myoblast(determined)

Part of a muscle fiber(fully differentiated cell)

mRNA

A specific example!!!!

In turn, the altered expression patterns of these genes can then influence the expression of an even larger number of genes.

By this process, called a cascade, a change in one or a few genes can alter the expression patterns of numerous genes.

Transcription factors bind to DNA sequences located near the coding region of the gene in question.

When they bind they alter the “transcriptional machinery” and, thus, the level of transcription can change.

Can cause transcription to increase (up-regulation) or decrease (down-regulation).

Figure 16.3

(b) Induction by nearby cells

Unfertilized egg Early embryo(32 cells)Sperm

Fertilization

Two-celledembryo

Mitoticcell division

Zygote(fertilizedegg) Signal

transductionpathway

Signalingmolecule(inducer)

Signalreceptor

NUCLEUS

Who puts those little green triangles and brown circles in the cell?

Cytoplasmic determinants can serve as transcription factors and mRNA can make proteins that then serve as transcription factors.

Both regulate expression of the cell’s genes as cells differentiate.

Back to the ZYGOTE There are transcription factors and mRNA already INSIDE zygotes!

Figure 16.3

(a) Cytoplasmic determinants in the eggInduction by nearby cells

Unfertilized egg Early embryo(32 cells)

Sperm

Fertilization

Nucleus

Two-celledembryo

Mitoticcell division

Zygote(fertilizedegg)

Molecules oftwo differentcytoplasmicdeterminants Signal

transductionpathway

Signalingmolecule(inducer)

Signalreceptor

NUCLEUS

Sometimes transcription is also initiated from OUTSIDE cell during development.

Revisit one way this process starts…

Egg establishes a molecular coordinate system-provides a way of telling "which end is up”

A specific cytoplasmic determinant (an mRNA) establishes the anterior-posterior polarity in Drosophila.

This mRNA is put into the egg by “nurse cells”

This one mRNA tends to aggregate in anterior end because that is where nurse cells are.

If there is a problem with this mRNA then it does not aggregate in the anterior region and you end up with a mutant fly.

If it is put there by mom…who is mutated?

If mom homozygous for “Bicoid” (remember fly mutants named by mutation) will have offspring with two tails-lack front half of body!

Figure 16.7b-5

Larval stage

(b) Development from egg to larva

Segmentedembryo

Bodysegments

Fertilized egg

Unfertilized egg

Eggdeveloping withinovarian follicle

Hatching0.1 mm

Embryonicdevelopment

EggshellDepleted

nurse cells FertilizationLaying of egg

Egg

Nurse cell

Follicle cellNucleus1

2

3

4

5

Figure 16.7a

0.5 mm

Head Thorax Abdomen

Dorsal

Ventral

PosteriorAnteriorRight

Left

(a) Adult

BODYAXES

Body plans! Ultimately positional information comes from cytoplasmic determinants but inductive signals also come into play…

These cues determine how the cell and the cell’s progeny will react to future molecular signals

ApoptosisDuring development but also when cells are damaged or infected..

What happens? Cells parts are packaged up and scavenged by other cells(saves neighbors from reactive chemicals that might leak out)

Details were worked out in C. elegans (as adults they have 1,000 cells but on the way cell death happens regularly)

Are triggered to die thru signal transduction pathway by external signals with relay molecules passing message to nucleus-then proteins are produced that start process!

Figure 16.6

1 mm Interdigital tissue Cells undergoing apoptosis

Space between digits

http://www.youtube.com/watch?v=LFG-aLidT8s

Questions for-Cells in the early embryo battle each other….

• What did the researchers study and what did they find??

• If all cells in a body come from a fertilized zygote and are thus genotypically identical why would there be differences between cells?? Where would these differences come from??

• Why might this process be an important force in longer lived animals?

• Is this just like the process of apoptosis??