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Biology 240 – General Zoology
Lecture 12 Outline - Development II
II. Regulation of Development
A. Differentiation of Cells, Tissues and Organs
- differentiation of cells and body structures results from differential gene expression.
- protein transcription factors control patterns of gene expression in cells
- expression of different genes in different cell types produces tissue-specific proteins
1. Nuclear equivalence – all cells in an organism have the same DNA
- information to guide development comes from factors outside the nucleus
Ex: Briggs and King (1952) - Nuclear transplantation experiments
- nucleus from donor frog blastula transplanted into recipient egg
→ developed into a complete embryo
- demonstrated that a single cell contains all the genetic information of the organism
totipotent cells of very early embryos can form any cell type
pluripotent cells can form multiple cell types (e.g., embryonic stem cells, bone marrow cells)
As development proceeds, cells become increasingly determined - can only develop into one cell type;
determination is usually irreversible, but can be reversed in some cells under certain conditions
(e.g., sheep cloning experiments - mammary gland cells were treated to yield a totipotent nucleus)
2. Cytoplasmic determinants (maternal factors) – regulatory molecules (mRNA or protein) in the
egg that influence early development of the embryo
- coded for by maternal effect genes in the mother
- concentration gradients of morphogens are distributed non-randomly in the egg → embryo
- have a role in establishing body axes of the embryo (A/P, D/V)
Ex: Drosophila – bicoid factor determines the A/P axis
bicoid mRNA → bicoid protein concentrated at one pole of egg → future head end of embryo
bicoid mutant mother produces an embryo with two tails, no head
3. Induction – one group of embryonic cells influences the development of neighboring
embryonic cells
- acts via diffusion of signaling molecules (morphogens) or cell-cell surface interactions
Ex: Spemann and Mangold (1924) – frog gastrula cell transplantation experiments
- cells from the dorsal lip area were removed and transplanted to the opposite side of a gastrula
- transplanted cells induced formation of a second notochord, neural tube, somites, etc.
Dorsal lip area functions as an organizer that induces development of other embryonic structures.
Ex: vertebrate eye formation - developing retina layer induces lens formation from overlying cells
Ex: chick limb development - zone of polarizing activity (ZPA) induces differentiation of digits;
transplantation of ZPA to opposite side of developing limb bud results in mirror-image digits
B. Control of Gene Expression in Development
Development involves sequential activation of regulatory genes that control differentiation
1. Master genes – regulatory genes that control the expression of other, subordinate genes that
affect development
- code for transcription factors that bind to regulatory regions (enhancers) upstream of target genes
- act as developmental “switches” that turn on/off subordinate genes in a specific sequence
- leads to expression of genes that code for tissue-specific proteins in differentiated cells
Biology 240, Lecture 12
Ex: myoD - muscle cell master gene, activated in myoblasts
myoD gene → transcription(Tx), translation (Tl) → MyoD protein = transcription factor
…which activates the expression of…
subordinate genes → Tx,Tl → subordinate gene products = more transcription factors
…which activate the expression of…
structural protein genes → Tx,Tl → actin, myosin → differentiated muscle cells
C. Pattern Formation and Body Plan Development in Drosophila
fertilized egg → → → segmented embryo → larva → → → pupa → adult
1. Cytoplasmic determinants establish the A/P and D/V axes of the embryo.
a. maternal effect genes
2. Sequential activation of regulatory genes forms the general body plan.
a. “gap” genes - divide the embryo into major regions
b. “pair-rule” genes are expressed in even- and odd-numbered segments
c. “segment polarity” genes - identify individual segments
3. Homeotic/homeobox (Hox) genes – master genes that specify the identity of body segments
- activate subordinate genes that control development of segment-specific body structures
Homeotic mutants in Drosophila (“bithorax” and “antennapedia” ) have duplicated body regions
or misplaced structures in the wrong body segments
Hox genes code for segment-specific transcription factors
- “homeobox” region of Hox gene consists of 180 base pairs which codes for a 60 amino acid
DNA binding domain (homeodomain) of a transcription factor protein;
- variable region of Hox gene codes for a segment-specific activation domain of the transcription
factor that interacts with the enhancer DNA and other transcription factor to control patterns of
gene expression in the body segment
- Hox gene sequences are highly conserved through evolution
Hox genes are arranged on the chromosome in the same order as expressed along the A/P axis
- vertebrates have four Hox gene clusters located on four different chromosomes
- close sequence homology in Hox genes between distantly related taxa (insect vs. mammal)
Evolutionary Implications of Hox Genes
1. Molecular homology - Hox genes and their chromosomal organization are highly conserved in
the animal kingdom, even among distantly-related phyla.
2. “Hopeful monsters” - mutations in Hox genes can cause major changes in body organization;
most mutations will be detrimental but occasionally an advantageous, large change could occur.
3. Origins of body plan diversity – evolution of Hox genes may have provided a genetic basis for the
Cambrian Explosion.
Study Questions
1. Understand the concept of differential gene expression and its importance in development.
What are transcription factors and what role do they play in the regulation of development?
2.
3.
4.
5.
What is a cytoplasmic determinant? What is a morphogen?
Explain the process of induction in animal development and provide examples.
What are master genes and how (in general) do they function during development?
What is a homeotic (Hox) gene and what is a primary function of Hox genes in animal
development? Explain the possible importance of Hox genes in animal evolution.