Development and Genes

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Development is the process of timed genetic controlled changes that occurs in an organism’s life cycle.

• Mitosis

• Cell differentiation

• Pattern formation

• Morphogenesis

All four processes are anchored by differentiation with regard to gene expression. Most cells in a multicellular organism have the same genome or DNA. Genes must be turned on and turned off during development.

Characteristics of Development for Multicellular Organisms

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CleavageCleavage is the time of rapid mitosis without significant growth of daughter cells. Cells become increasing smaller. Each cell is called a blastomere. G1

and G2 phases of cell cycle is shortened or eliminated.

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Nematode Development

The embryonic development and fate of adult cells has been mapped with the nematode.

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Gastrulation

After cleavage, development in animals is often accompanied by mass movement of cells called gastrulation.

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Animal Gastulation

The exact mechanism for gastrulation can vary from animal species to animal species.

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Plant Development

Plants do not have mass movement of cells during development due to the cell walls. Certain tissues set aside for cell division and this tissue is called the meristem or meristematic tissue.

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Cells have two general classes of genes.

• Housekeeping genes which are necessary to go about the “business” of life. For example genes that code for the enzymes for cellular respiration are housekeeping genes. Most cells have all of these activated

• Specialized genes that produce unique gene product important to the cells differentiation. For example the activation of the crystallin gene that produces product necessary for the development of the lens of the eye.

Cell, Genes and Development

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Genes and Development

It would be wasteful for lens cells to produce albumin, and in the same way it would be wasteful for the liver cells to produce crystallin. These specialized genes must be regulated so that they are only activated when they are needed and timing is critical.

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Determination comes before Differentiation.

Determination are those things or processes necessary to commit a cell to a particular type of cell or fate. Most often when a cell is committed to a particular fate it is usually irreversible.

Differentiation is those changes that occur in a cell to make it a certain cell type.

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The graph shows the percentage of nuclear transplants embryos that develop normally in relationship to the age of the donor.

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Determination – Events that lead to the observable differentiation of a cell. Once determination has occurred then the final fate of the cell is sealed.

If a determined cell is placed in another location in the organism, it will still differentiate into the cell that was its normal fate.

Determination

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Determination

If cells are placed in another location in the organism, and the cells take on the identity of the surrounding tissue, then the cells have not been determined. If the cells retain their original identity then the cells have been determined

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Determination

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How Determination Occurs

There are two sources responsible for determining the fate or development of cells.

• Cytoplasmic Determinants

• Induction via signals secreted by neighboring cells

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Initial Development Governed by Cytoplasmic

Determinants

Development can begin with fertilization of an egg and subsequent division of cytoplasmic determinants during cytokinesis. There is unequal distribution of the cytoplasmic determinants to the daughter cells as illustrated.

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Initial Development Governed by Inductive Signals

molecules that can interact with receptor sites and receiving cells. This causes the activation of a signal transduction pathway for the receiving cell. This can send the cell down a specific developmental pathway.

Once there are a multitude of cells, neighboring cells may produce signal

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Initial Development Governed by Cytoplasmic Determinants

Cell Differentiation

• Differentiation of activated genes and inactive genes

• Appearance of mRNA for cell specific proteins

• Changes in cellular structure

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Using Drosophila as a Model Organism for Development

Drosophila and human development are homologous processes. They utilize closely related genes working in highly conserved regulatory networks. Unlike humans, Drosophila is subject to easy genetic manipulation. As a result, most of what we know about the molecular basis of animal development has come from studies of model systems such as Drosophila.

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Using Drosophila as a Model Organism for Development and Determination of Axes

The Drosophila life cycle consists of a number of stages: embryogenesis, three larval stages, a pupal stage, and (finally) the adult stage!

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Pattern Formation or Setting Up the Body Plan

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Maternal effect genes are genes that when mutant in the mother results in a mutant phenotype in the offspring, regardless of the offspring’s own genotype.

In the fruit fly, mRNA or proteins of the maternal effect genes are synthesized in the egg while it is still in the mother’s ovary. A mutation in the maternal effect gene can cause fertilized eggs to fail to develop normally.

Maternal effect genes control the polarity of the egg and ultimately the fly and are also called egg-polarity genes. One set of genes controls the anterior-posterior axis and another set controls the ventral-dorsal axis. Mutations in these genes are generally lethal.

Importance of Material in the Cytoplasm of an Egg

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Bicoid and nanos genes are responsible for the patterning of the anterior and posterior ends respectively.

Nurse cells secrete maternally produced bicoid and nanos mRNA into a maturing oocyte. They are differentially transported along microtubules to opposite poles of the oocyte due to the use of different motor proteins to transport the two different mRNA.

Two maternal effect genes are called bicoid and a nanos.

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One the mRNA arrive at their respective ends, the mRNAs become anchored in the cytoplasm where the mRNA are translated.

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After fertilization, the mRNAs are translated, creating opposing gradients of bicoid and nanos proteins.

These proteins control the translation of two other maternal genes, hunchback (needed for anterior structures) and caudal (needed for posterior structures)

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The mRNA is translated into bicoid protein at the anterior end. The bicoid protein then diffuses toward the posterior end forming a gradient. Substances that form a gradient in the zygote or embryo and affects development or morphogenesis are called morphogens. The bicoid protein is classified as a morphogen.

Example Mutations in Bicoid

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These bicoid and nanos proteins control the trans-lation of two other maternal genes, hunchback (needed for anterior structures) and caudal (needed for posterior structures), however the mRNAs are evenly distributed in the oocyte.

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The hunchback and caudal proteins will form a gradient because of interaction with the bicoid and nanos protein.

The bicoid protein binds to and inhibits the translation of the mRNA for caudal, and the nanos protein binds to and inhibits the translation of hunchback. This interaction causes a gradient for both.