Chapter 21

The Genetic Basis of Development

Zygote and Cell Division

When the zygote divides, it undergoes 3 major changes:1. Cell division2. Cell differentiation3. Morphogenesis

Cell Signaling

Cell signaling is largely responsible for the developmental processes.

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1. Cell Division Cell division,

mitosis gives rise to numerous cells.

2. Cell Differentiation Cell differentiation

is the process by which cells become specialized in form and function.

These cells undergo changes that organize them into tissues and organs.

3. Morphogenesis As the dividing cells begin to take form, they are

undergoing morphogenesis which means the “creation of form.”

Morphogenetic events lay out the development very early on in development.

These morphogenetic events “tell” the organism where the head and tail are, which is the front and back, and what is left and right.

As time progresses, later morphogenetic events will give instructions as to where certain appendages will be located.

Morphogenetic events, as well as cell division and differentiation, take place in all multicellular organisms.

Morphogenetic Events Morphogenesis differs in 2 major ways

in plants and animals:1. In animals, movements of cells and tissues

are required for the transformation of the early embryo into the characteristic 3D form of the organism.

2. In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods, they occur throughout the life of the plant.

Apical Meristems

Remember, apical meristems of plants are responsible for a plant’s continued growth and development and the formation of new organs throughout the plant’s life.

These are perpetually embryonic regions in the tips of shoots and roots.

The Experiments of F.C. Steward

In the 1950’s, Steward was working with carrot plants.

He showed that cells taken from the root of the plant would grow into an adult carrot when cultured in growth medium. These plants were clones of the original.

It demonstrated that differentiation doesn’t involve irreversible changes in DNA; that cells can dedifferentiate.

In plants, cells can remain totipotent: they retain the potential to make all parts of the plant. (can become any kind of cell) while other cells are pluripotent (can become many cell types).

Animals: Stem Cells Stem Cells: relatively unspecialized cells that

have two important properties: They continually reproduce themselves. They can differentiate into specialized cells of one or

more types. The adult body has various kinds of stem cells

which serve to replace nonreproducing specialized cells.

Example stem cells in bone marrow give rise to all the different kinds of blood cells.

Stem cells that can become multiple cell types are called pluripotent.

Tutorial 19.2 (E) Early Asymmetry in the Embryo

Multicellular Organisms The cells of multicellular organisms

come almost entirely from differences in gene expression.

Regulatory mechanisms turn certain genes on and off during development.

These regulatory mechanisms are what makes cells different because nearly all cells have the same genetic complement.

Cloning Using the somatic (body) cells of a

multicellular organism to generate a new organism is called cloning.

Each clone is genetically identical to the parent plant.

Differentiated cells don’t usually divide in culture, so researchers had to take a different approach to decide if animal cells were totipotent.

What Researchers Did…

They removed the nucleus of an unfertilized egg and replaced it with one from a differentiated cell.

The process is called nuclear transplantation.

If the transplanted cell retains all of its genetic information, the recipient cell should develop with all of the necessary tissues and organs.

Nuclear Transplantation As these experiments

were conducted on frogs, it was determined that something in the DNA does change.

In tadpoles, normal development proceeded, but as the age of the donor nucleus increased, the percentage of organisms that developed correctly decreased.

Nuclear Transplantation Continued research showed that the DNA

remains the same for the most part, but the chromatin changes in a way that problems arise.

Often times, the histones get modified or DNA is methylated and these changes in the chromatin prevent dedifferentiation.

Sometimes the process is reversible, but usually it isn’t.

One thing is certain, most scientists agree that all cells contain the necessary genetic information to make an entire organism.

However, the different cell types exist because of the variations in gene expression.

Nuclear Transplanting and Cloning

In 1997, Scottish researchers cloned a sheep named Dolly.

They used cells from mammary tissue in an adult sheep, implanted the nucleus from the cell into egg cells from which the nucleus had been removed and implanted into the uterus of a lamb.

Nuclear Transplanting and Cloning

Analysis of the DNA from Dolly showed it was identical to that of the original sheep, and its mitochondria matched that of the mother lamb.

However, Dolly’s cells appeared older than her age would indicate.

Dolly’s Problems

She suffered from a lung disease seen in older sheep.

She had arthritis. These results indicate that not all

of the DNA had been reprogrammed.

Problems With Animal Cloning In General:

Many of the animals exhibit a variety of defects such as obesity and premature death.

Only a small percentage of the embryos created develop correctly resulting in live birth.

Possible reasons for these results include: Epigenetic changes in chromatin (acetylation

of histones and/or methylation of DNA) result in only a small number of genes being turned on while the others remain suppressed.

Stem Cells The use of stem cells, especially

embryonic stem cells, has many obvious medical applications.

There are obvious ethical dilemmas that arise from the research.

There are moral issues on both sides: One is that it is immoral to tamper with

human embryos for medical purposes. The other is that it is immoral not to because

the benefits outweigh the cost of doing nothing.

Tutorial 19.1 Embryonic Stem Cells

Cell Differentiation There are 2 major things telling a cell when

and how to differentiate:1. The “stuff” found within the egg at the time of

conception. The egg cell’s cytoplasm contains RNA and protein

molecules encoded by the mother’s DNA. What has been received in the cytoplasm will determine

the developmental fate of each of the cells.2. The environment in which the embryo develops.

an embryo’s genes signal the expression of proteins that cause changes in nearby target cells.

These signals send a cell down a specific developmental pathway--inducing further differentiation of the many specialized cells within the new organism.

Pattern Formation Pattern formation is the development of

spatial organization in which the tissues and organs of an organism are all in their characteristic places. In plants, pattern formation occurs through

the life of the plant. In animals, it is restricted to the embryonic

or juvenile stage. Pattern formation in animals begins in the embryo

when the major axes are determined.

Pattern Formation, An Example

Here is an example of pattern formation and cell signaling as seen in the fruit fly. Movie

Tutorial 19.3 Pattern Formation in the Drosophila Embryo

Apoptosis

Apoptosis is the programmed cell death that occurs through the normal course of development.

It is usually triggered by signals that activate a cascade of signal proteins in cells that are to die.

During the process, the cell shrinks, the nucleus breaks down and the nearby cells quickly engulf and break down the contents of the cell.

The process helps in the growth and development of the major structures and systems of an organism.

It controls cell division helping to slow or stop division in certain cells.

Homeotic Genes Looking across species, there are many

similarities in the genes controlling development.

A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of development (morphogenesis). The homeobox-encoded region is part of the protein that functions as a transcription regulator.

In this way, the homeobox genes work to switch certain developmental genes on and off.

The most studied the Hox genes, which control segmental patterning during development.

Many distantly related eukaryotes such as plants and yeasts also have these Hox genes (regulatory sequences).

Homeotic Genes Genes that have a

homeobox are called homeobox genes and form the homeobox gene family.