Developmental Genetics2

7/30/2019 Developmental Genetics2

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Cytoplasmic Determinants

These maternal substances, cytoplasmic determinants,

regulate the expression of genes that affect thedevelopmental fate of the cell.

After fertilization,the cell nuclei

resulting from mitoticdivision of the zygoteare exposed todifferent cytoplasmic

environments.

This shows unequal sharing

of cellular material in case

you could not tell.


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Peer Pressure

The other important source of developmental

information is the environment around the

cell, especially signals impinging on an

embryonic cell from other nearby embryonic

cells.

The synthesis of these signals is controlled by the

embryos own genes.


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Pattern Formation

Cytoplasmic determinants, inductive signals, and their effects

contribute to pattern formation, the development of a spatialorganization in which the tissues and organs of an organismare all in their characteristic places.

The major axes of an animal are established very early as the

molecular cues that control pattern formation, positionalinformation, tell a cell its location relative to the body axesand to neighboring cells.

They also determine how the cells and its progeny willrespond to future molecule signals.


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The anterior-posterior polarity of the embryo, larva, and adult has its origin in the

anterior-posterior polarity of the egg

maternal effect genes expressed in the mothers ovaries produce messenger RNAs that

are placed in different regions of the egg.

Bicoid and Hunchback, regulate the production ofanterior structures,

Nanos and Caudal, regulates the formation of the

posterior parts of the embryo

Gap genes are the zygotic genes that are regulated

by these maternal genes and are the first ones to gettranscribed in the embryo.

Different Pair-rule genes get transcribed depending

on the concentration of the Gap protein in that part of

the embryo


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Gurken mRNA and protein is localized to the future dorsal side of embryo in the oocyte

localized activation of Torpedo receptor on follicle cells to the future dorsal side of

embryo in the oocyte

PIPE protein on the ventral side of the embryo leads to the activation of spatzle

in the extracellular space which now binds to Toll receptor which determines the

nuclear localization of dorsal protein

DORSO-VENTRAL PATTERN FORMATION


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ANTERIO-POSTERIOR POLARITY


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Segmentation Genes

The bicoid protein and other morphogens are

transcription factors that regulate the activity of some ofthe embryos own genes.

Gradients of these morphogens bring about regional

differences in the expression ofsegmentation genes, thegenes that direct the actual formation of segments after

the embryos major axes are defined.


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Segmentation

Sequential activation of

three sets of segmentationgenes provides thepositional information forincreasingly fine details of

the body plan. These are gap genes, pair-

rule genes, and segmentpolarity genes.


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3 types of segmentation genes

1. GAP GENES: mutation in these genes produce gaps in the segmentation pattern

of the larva. Eg: kruppel lacks 8 segments T1-A5. 6 GAP GENES ARE PRESENT

2. PAIRRULE GENES: 8 genes. Mutations result in loss of alternate segments. Eg:even-skipped (eve) odd numbered segments lost, fushi tarazu (ftz) lacks even

numbered

3. SEGMENT-POLARITY GENES: Mutations produce larvae with a normal number of

segments but with a part of each segment deleted and replaced by a mirror-

image duplicate


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PAIR-RULE GENE

SEGMENT-POLARITY GENE (gooseberry)

The body of Drosophila melanogaster is built from 14 segments:

3 segments make up the head with its antennae and mouth

parts.

3 segments make up the thorax. Each thoracic segment has a

pair of legs.,the middle thoracic segment carries a single pair of

wings; the hind segment a pair of halters.

8 abdominal segments.

Pair-rule genes divide syncytial balstoderm into 7

segments and after this cell membrane is formed around

each nuclei transforming into cellular blastoderm.


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Homeotic Genes

In a normal fly, structures such as antennae, legs, and

wings develop on the appropriate segments. The anatomical identity of the segments is controlled by

master regulatory genes, the homeotic genes.

Discovered by Edward Lewis, these genes specify thetypes of appendages and other structures that each

segment will form.


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The order of the genes on the chromosome reflects the order that they are

expressed along the anterior-posterior axis of the developing embryo.

Structures characteristic of a particular part of the animal arise in

the wrong place.

Like other developmental genes, the homeotic genes code for

transcription factors.

The Antennapedia group includes labial, antennapedia, sex combs reduced,

deformed, andproboscipedia.

Labial and Deformed proteins are expressed in head segments where they

activate the genes that define head features.

Sex-combs-reduced and Antennapedia specify the properties of thoracic

segments.

The bithorax group control the specializations of the third thoracic segment

and the abdominal segments.


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Mutations to homeotic genes produce flies with suchstrange traits as legs growing from the head in place of

antennae.

Mutation in antennapedia hox gene results in the formation of a leg from the

head of a fruit fly in stead of the expected antenna.

Homeotic transformation


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Apoptosis

Lineage analysis ofC. elegans highlights another outcome of cell

signaling, programmed cell death or apoptosis. The timely suicide of cells occurs exactly 131 times in the course of

C. eleganss normal development.

At precisely the same points in development, signals trigger theactivation of a cascade of suicide proteins in the cells destined to

die.


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Apoptosis Apoptosis is regulated not at the level of transcription or

translation, but through changes in the activityofproteins that are continually present in the cell.


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Apoptosis pathways in humans and other mammals are more complicated.

Research on mammals have revealed a prominent role for mitochondria inapoptosis.

Signals from apoptosis pathways or others somehow cause the outermitochondrial membrane to leak, releasing proteins that promoteapoptosis.

Still controversial is whether mitochondria play a central role in apoptosisor only a subsidiary role.

A cell must make a life-or-death decision by somehow integrating boththe death and life (growth factor) signals that it receives.

Apoptosis is essential to the development of animal morphogenesis

(prevents webbing between fingers and toes).

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Developmental Genetics2