Download Chapter 21 Review – Genetic Basis of Development

Chapter 21 Review – Genetic Basis of Development
Here are the main concepts that you should be familiar with. Let me know if you have
any questions.
Keywords to know
Differentiation – cells become specialized in structure and function
Morphogenesis – physical process of giving an organism its shape
Stem cells – undifferentiated cells, they can become any kind of cell in the organism
Induction – signal molecules that induce changes in gene expression in nearby cells
Apoptosis – programmed cell death
Egg-polarity genes – genes that set up the anterior-posterior axis of the embryo
Segmentation genes – genes that direct formation of body segments
Homeotic genes – master regulatory genes in animals, specify types of appendages
attached to each segment (such as antennae, arms, legs, fins, etc.)
Organ identity genes – determine the type of structure that will grow from a meristem in
plants (analogous to homeotic genes in animals)
Basically, all organisms start out as one cell, but as that cell divides, not all cells end up
doing the same thing. Obviously, not all genes are expressed in every cell. Early in
development, all of the cells are stem cells – they can be programmed to turn into any
kind of cell. After the cell differentiates, it cannot go back to a stem cell. It will be a
heart cell or a bone cell or whatever. Every time it divides after that, it will make more of
that type of cell. For example, when skin cells divide, they make more skin cells.
Early in development, certain genes are turned on by chemical signals and they begin the
organization of the organism. The anterior and posterior regions are determined so that
eventually there is a head and a tail (at least in animals). Segmentation genes determine
where segments go. Homeotic genes determine what appendages will form at which
segments. Basically, each cell is doing its specific job and responding to chemical
stimuli, which turn on and off certain genes. When you put all of this together, an
organism develops from one cell to many specialized cells, tissues and organs working
together. At the genetic level, all of this happens because genes are regulated and some
genes are turned on and some genes are turned off. The genes that are turned on and off
are different in each type of cell. Also, genes are turned on and off at different times.
Another process that happens during normal development is apoptosis – cells are
programmed to die at certain times.
The result of all of this is that the organisms ends up to be the correct size and shape with
all of the right body parts in the right places.