17 Developmental Genetics

Lecture Outline

I. Cell differentiation usually does not involve changes in DNA

A. All of the 200 differentiated cells in a human body descended from a single fertilized egg cell (zygote)

1. With a few exceptions, cells do not lose genes that are not needed

2. Nuclear equivalence is a concept that describes the genetic similarity of all cells in a multicellular organism

a) The somatic cells in an adult animal have the same genes

b) Totipotent cells provide the evidence for nuclear equivalence

B. Differential gene expression determines which genes are turned on in which cells to make certain proteins

1. Differentiation of cells requires more limiting of the products produced through transcription and translation

C. A totipotent nucleus contains of all the instructions required to direct development

1. Plant tissue culture is based on totipotent plant cells, first developed by Steward and colleagues in the 1950s

a) Tissue culture is of importance in biotechnology

b) Cells are induced to divide, then are transferred to agar where they grow into an entire plant

c) Plantlets can be cloned in this way from genetically engineered plants

2. Totipotency in animal cells has not been as successful as in plant experimentation

a) Nuclei from amphibian cells have been successfully transplanted into egg cells

D. The first successful cloning of a mammal occurred in 1996 and resulted in the sheep Dolly

a) Dolly was the result of a cultured mammary gland cell from an adult sheep that was fused with an enucleated sheep egg

b) There has been low overall success rate to date, partly due to the fact that the cell cycles of the donor cell and the recipient cell are difficult to be in synchrony

2. Transgenic organisms, in which foreign genes are introduced, have been the subject of most cloning research

a) Research is attempting to improve the efficiency of this technique and widen its horizons

b) In human therapeutic cloning, no newborn human is formed

c) The primary purpose of human therapeutic cloning is to produce stem cells

E. Stem cells divide and give rise to differentiated cells

a) Totipotent cells can give rise to all tissues of the body

b) Pleuripotent stem cells are more specialized and give rise to cells of the line to which they belong; e.g., pleuripotent neural stem cells give rise to various types of brain cells

 

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c) Stem cells have the potential for transplantation into humans with degenerative diseases such as Parkinson= s

d) Most of these studies are funded by private companies because of governmental restrictions on stem cell research

e) The source of totipotent stem cells is early human embryos left over from in vitro fertilization, a newborn human= s umbilical cord, and some sources from mice as well as human tissues

F. There are many ethical issues surrounding human cloning

a) Human reproductive cloning means producing an exact copy of an adult by producing a genetically identical newborn

b) Human therapeutic cloning involves producing cells that could be used to treat disease

c) Controversies stem from the benefit vs. moral/religious beliefs, especially if embryos are produced for research and then destroyed

II. The genetic control of development

A. Certain organisms are particularly well suited for studies on the genetic control of development

1. The fruit fly, the nematode, yeast, and the lab mouse have been intensely studied

B. Drosophila melanogaster provides researchers with a wealth of developmental mutants

1. The maternal genome controls early development in Drosophila melanogaster

2. Polytene chromosomes in the salivary glands of the larvae consist of more than 1000 DNA strands side by side; they have replicated many times without mitosis and cytokinesis

a) Banding patterns are useful in mapping mutations

b) Chromosome puffs appear in locations of rapid transcription

3. Foreign DNA can be injected into fly eggs and incorporated into their DNA by transformation

C. The Drosophila life cycle includes egg, larval, pupal, and adult stages

1. Larvae hatch from the egg, molt several times, then pupate

a) Precursor cells of adult structures in the larvae are known as imaginal discs (the adult is called the imago)

2. During pupation, the insect undergoes complete metamorphosis

3. Over 50 genes have been identified that control the imaginal discs and their development

D. Many Drosophila developmental mutants affect the body plan

1. Maternal effect genes organize the egg cytoplasm

a) mRNA stores in the egg are derived entirely from the mother fly

b) Many of these genes are involved in establishing the polarity of the egg

c) Protein gradients within the egg may provide positional information

2. Zygotic segmentation genes continue and extend the developmental program

a) Immediately after fertilization, the zygote nucleus divides 13x without cytokinesis

b) Some of the resultant nuclei migrate to the periphery and nuclear membranes begin to form, and some of the genes of these nuclei begin to be expressed

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c) Zygotic segmentation genes result in a repeating pattern of segments in the embryo

(1) Gap genes are the first to act and begin the anterior-posterior orientation

(2) Pair-rule genes delete alternating segments

(3) Segment polarity genes produce segments that are mirror images of each other

(4) Action of these genes and maternal segmentation genes is under the influence of morphogens

(5) These genes code for transcription factors of various kinds

3. Homeotic genes specify the identity of each segment

a) Homeotic genes are regulated by zygotic segmentation genes

b) Mutations result in substitution of body parts

(1) Antennapedia mutants have legs instead of antennae on their heads

c) Homeoboxes are relatively short nucleotide sequences characteristic of many homeotic genes

d) Homeoboxes have been shown to be similar in many organisms, such as sea urchins, yeast, and humans

e) A homeobox codes for a portion of a protein called the homeodomain, which allows the protein to act as a transcription factor

(1) Homeodomains consist of 60 amino acids that form 4 alpha-helices

(2) Homeodomains may serve as a recognition helix that binds to a specific DNA sequence and therefore affect transcription

f) Hox genes are clusters of homeobox-containing genes

g) Hox genes contain some basic directional developmental genes and are very similar in all animals studied except sponges and cnidarians

h) The same developmental controls are seen in insects, nematodes, and humans, showing it developed early and was evolutionarily conserved

E. Caenorhabditis elegans has a very rigid early developmental pattern and has given much information about apoptosis

1. Early studies by Sydney Brenner in the 1960s on C. elegans showed its promise as a model organism for developmental biology

2. This nematode is only 1.5 mm long at maturity and contains about 1000 somatic cells and about 2000 germ-line cells; cell-constant animals

3. C. elegans is either hermaphroditic or male

4. Because it is transparent, the cell lineages can be followed relatively easily

5. Stem or founder cells have a fixed fate and are formed early in development

a) The embryo is said to be highly mosaic, as the cells have fixed fates

b) Each organ is believed to have been derived from a single stem cell

6. Destruction of a single cell in most instances leads to the absence of the structure or structures derived from it but to normal development of adjacent cells

7. However, differentiation of some cells may be influenced by adjacent cells (induction)

8. Apoptosis, or programmed cell death, is also under genetic control, and C. elegans also has capases

9. Chronogenes are involved in timing of development

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10. Genes that resemble homeoboxes have been found but are rather dissimilar to those found in Drosophila

11. Other well known C. elegans researchers include Horvitz, Brenner, and Sulston, who received the Nobel Prize in 2002 for their research on development and apoptosis

F. The mouse is a model for mammalian development

1. Cells of very early mouse embryos are totipotent

a) A chimera can be formed by fusing 2 early mouse embryos

b) Mouse development is highly regulative; it can accommodate missing or extra cells

2. Transgenic mice are used in studies on developmental regulation

a) A transgenic mouse results from a fertilized egg in which a transgene (foreign gene) has been introduced

b) Mouse genes can be inactivated (A knocked out@ ) by gene targeting

c) Transgenic mice express transgenes in appropriate tissues

G. Homeotic-like mutations occur in plants

1. Economically important plants like corn are under much study

2. A member of the mustard family, Arabidopsis is an important plant in research as it is small in size and some homeotic-like mutations have been isolated

3. Arabidopsis is the first plant to have its genome sequenced (2000)

a) One example of a mutation involves variation in development of floral parts

(1) The ABC model states that the A gene codes for the sepals, and the B and C code for the remainder of the parts of the flower

(2) Another class of genes are SEPALLATA

(3) All of these increase the number of molecular probes available from plants

III. Cancer and development

A. Cancerous cells lack normal biological inhibition

1. A cancerous group of cells may produce a tumor

2. A tumor that escapes from local controls has metastasized

B. Cancers are caused by altered expression of genes required for cell division

1. An oncogene, or oncogenes, cause cancerous changes in a cell

2. Proto-oncogenes are most likely the progenitors for oncogenes

3. Oncogenes were first discovered in viruses that transform mammalian cells into cancerous cells

4. The change to cancer involves growth factors, growth factor receptors, protein kinases, and transcription factors

C. About half of all cancers are caused by a mutation in a tumor suppressor gene (called an anti-ncogene)

1. These normally interact with growth inhibiting factors and block cell division

2. Mutations in tumor suppressor genes result in uncontrolled cell division

D. To date, more than 100 oncogenes and 15 tumor suppressor genes have been identified

1. Great Britain= s Cancer Genome Project is examining human genes for cancer-related mutations

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