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GENES AND DEVELOPMENT
Chapter 16
Development is the process by which the descendants of a single cell specialize and organize into a
complex organism.
An organism contains many cells that are specialized both in structure and function.
During development, cells become gradually committed to specific patterns of gene activity through a
process called determination.
Differentiation is the process by which a cell becomes specialized with characteristic appearance and
activities.
Determination leads to differentiation.
Morphogenesis is a multistep process by which cells in different locations become progressively
organized into recognizable structures.
There is no evidence that genes are lost during development.
Nuclei of differentiated somatic cells of an organism are identical to each other. This is called nuclear
equivalency.
There are some exceptions to nuclear equivalency.
Because somatic cells contain the entire genome of the organism, they are said to be totipotent.
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They can potentially form a complete new organism.
A totipotent nucleus contains all the information required to direct normal development.
Most differences among cells are due to differential gene expression.
The expression of genes can be regulated in many different ways.
The quantities of gene products vary from one tissue to another.
EXPERIMENTAL ORGANISMS
Drosophila melanogaster.
The fruit fly has many types of developmental mutants.
Genetic analysis of Drosophila has been facilitated by the unusual large chromosomes found in certain
tissues.
These chromosomes are called polytenes.
Polytenes form when the DNA replicate many times without mitosis or cytokinesis. It may consist of
1000 or more DNA double helices with its associated histones.
Bands are formed when the region associated with an active gene uncoils and forms a “puff”, the region
of active RNA synthesis.
The life cycle involves complete metamorphosis:
Egg  first instar larva  second instar larva  third instar larva  pupa  adult fly
Early in the larval stage, precursor cells of many of the adult structures are organized as undifferentiated
paired structures called imaginal discs.
Each imaginal disc occupies a specific position in the larva and will produce a specific structure.
The very early stages of development are controlled by maternal genes that pass mRNA into the eggs
when they are in the ovary of the mother.
The genes that produce these mRNA molecules are called maternal effect genes.
These maternal effect genes are active prior to fertilization and produce gradients of morphogens.
A morphogen is a chemical that affects the differentiation of cells and the development of form.
Zygotic segmentation genes become active when the embryo is no longer a zygote!
These genes generate the repeating pattern of segments.
There are three types of zygotic segmentation genes:
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Gap genes interpret the anterior-posterior information in the egg and begin segmentation.
Pair-rule genes delete every other segment when mutations in the gene occur.
Segment polarity genes produce segments with one part missing and the remaining part is
duplicated as a mirror image.
Most of the maternal effect genes and zygotic segmentation genes code for transcription factors that are
involved in regulating DNA transcription of m RNA.
Homeotic genes are responsible for producing the structures formed by each of the imaginal discs.
Mutation of the homeotic genes will cause the formation of parts normally formed in other segments
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Legs on the head instead of antennae.
Homeotic genes contain short sequences of approximately 160 base pairs called homeobox.
The homeobox sequence codes for a protein that bind to the DNA called homeodomain.
Homeodomains consist of 60 amino acids that form alpha helices that affect transcription when bound to
DNA. They are transcription factors.
Caenorhabditis elegans.
Caenorhabditis is a roundworm or nematode.
The worm contains about 1000 somatic cells and about 2000 germ-line cells.
The lineage of every somatic cell in the adult is known, and each can be traced to a single founder cell in
the early embryo.
If a somatic cell is destroyed the structure that is derived from that cell does not form.
The fate of the cells are largely predetermined. This pattern of development is called mosaic
development.
The worms can be either hermaphroditic or males.
Some cells interact with surrounding cells and induce them to differentiate.
Several mutations have been identified that affect induction, apoptosis (programmed cell death) and
timing of development (chronogenes).
Mus musculus, the mouse.
Mammalian embryos develop differently from those of Drosophila and Caenorhabditis.
The mouse show regulative development.
Regulative development means that the early embryo is self –regulating and can produce an entire
individual even if some cells are missing.
Cells of the early embryo are totipotent.
Early embryos have been fused together to produce a normal mouse, a chimera.
Later embryonic stages do not have totipotency.
Transgenic mice have been used in studies to determine how genes are regulated and activated during
development.
Plants.
Homeotic-like genes code for transcription factors.
These genes are controlled by regulatory genes that are very similar to genes that regulate expression of
parts in Drosophila.
EXCEPTIONS
There are exceptions to the rule of nuclear equivalency.
Genomic rearrangement is the physical rearrangement of the DNA
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Part of a gene is rearranged to make a different coding sequence.
Gene replacement: the active gene near the expression site is replaced by a silent gene, which
becomes active.
Gene amplification increases the number of copies of specific genes.
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It provides more mRNA copies.