1 Chap 21: The Genetic Basis of Development Points of Emphasis Know: 1.Most of the bold-faced terms. Areas of lowest priority are: 2.The basis of this

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Chap 21: The Genetic Basis of Development

Points of Emphasis

Know:

1. Most of the bold-faced terms. Areas of lowest priority are:

2. The basis of this chapter is to describe the molecular biology behind how cells become the tissues that form the organism that you are or that you see (plants or animals). There are certain control factors or proteins that cause the formation of structures at specific times so the organism can develop properly and it is the relationship between all these control factors that produce the proper form and structure.

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Figure 21.0 Electron micrograph of the head of an abnormal fly with small extra eyes on its antennae

• Both of the orange areas are extra eyes on the antennae of a fly.

• The gene for eye development was placed in the wrong spot.

• A similar gene for eye development exists in mammals.

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Figure 21.1 From early embryo to tadpole: what a difference a week makes

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Figure 21.2 Some key stages of development in animals and plants

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From Single Cell to Multicellular Organism

I. Introduction

• A fertilized egg gives rise to many different structures in a multicellular organism. We have liver, lung, heart and bones that must all be made or differentiated from this single fertilized egg. Plants have different cells (xylem and phloem) in their stems than in their leaves (mesophyll cells for PS and guard cells to open the stomata) as well as flowers of different shapes and colors.

• The point is. . . How does a single cell become a group of cells performing a similar function (tissue); how do these tissues forms organs; how these organs form organ systems and then how do these different systems coordinate with each other to form an organism.

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II. Embryonic development involves cell division, cell differentiation and morphogenesis

A. Three Basic Processes

1. Cell Division: but this would just produce a bunch of clones

2. Cellular Differentiation

3. Cellular Morphogenesis: processes that give an organism its shape.

a) A body plan is laid down so certain body parts end up in the correct location.

i. Animals: head and tail

ii. Plants: location of roots

From Single Cell to Multicellular OrganismFrom Single Cell to Multicellular Organism

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iii. Two major differences between how plants and animals obtain their shape.

- in order to obtain the 3D shape, cells and tissues move around during embryonic development of animals (but not plants)

- in plants, growth and morphogenesis occurs throughout the life of the plant where with animals there is a cessation of growth.


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III. Researchers study development in model organisms to identify general principles.

A. Amphibians were used in early studies because they have large eggs, fertilization and development occurred outside the body. Frogs were used to study cell movement in morphogenesis.

B. For genetic analysis you need small genomes, short generation times, and some knowledge about its genes. Drosophila, C. elegans, mouse, zebrafish and Arabidopsis are used.

1. Drosophila: good but first divisions produce large numbers of nuclei with a lot of cytoplasm which differs from us.

2. Caenorhabditis elegans: nematode, lives in soil, has a transparent body; its genome has been sequenced; most are hermaphroditic; recessive mutations are easily seen when they self fertilize. Has been used to follow the cell lineage or fate map of every cell of the embryo.

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Figure 21.3 Model organisms

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3. Mouse: the basic mammalian model; we can “knock-out” some its genes to observe its effect. Cannot watch embryonic development.

4. Zebrafish: another vertebrate model; transparent embryos that develop outside the mother’s body. May have been sequenced by now.

5. Arabidopsis: member of mustard family just like the Brassica rapa; can be grown in culture (in test tubes); has a small genome; each flower has egg and sperm; cells take up foreign DNA quickly.

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Differential Gene Expression

I. Different types of cells in an organism have the same DNA

A. Introduction

1. If you learn nothing else from this chapter you must remember that all cells of an organism have the same genome it is just the expression of those genes that makes the cells different.

2. So what happens to these genes as a cell begins to “obtain an identity of its own” or differentiate.

B. Totipotency in Plants

1. Plants can be cloned from root cells. In plants, whole new plants can be produced from differentiated somatic cells, such as root cells.

2. Since a differentiated cell, such as root cell, can then produce a whole new plant of stems, leaves, roots, etc. then the root cell’s DNA does not go through irreversible changes once the cell has differentiated.

3. Totipotency: the retention to produce a whole new organism.

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Figure 21.5 Test-tube cloning of carrots

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Differential Gene ExpressionDifferential Gene Expression

C. Nuclear Transplantation in Animals

1. Definition: to replace the nucleus of an unfertilized egg or even a zygote with the nucleus from a differentiated cell.

a) So what you are doing is taking DNA from a differentiated cell which means it has certain genes turned on and off and putting that DNA into a brand new environment. You are trying to see if the differentiated cell’s DNA can direct the formation of a totally new organism.

b) Experiments showed that although the DNA remains the same, gene activation and inactivation through methylation restricts the totipotency of animal cells. The totipotency decreases the older the differentiated cell is.

c) Sheep and mice have been cloned.

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Figure 21.6 Nuclear transplantation

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Figure 21.7 Cloning a mammal

When the cells were in culture they were exposed to low levels of nutrients that seem to cause them to dedifferentiate.

Identical to the mammary cell donor because it was her nucleus.

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D. Stem Cells of Animals

1. Properties of Stem Cells

a) They continually reproduce themselves

b) They can differentiate into one or more types of cells if provided with the proper environment.

c) Pluripotent: being able to give rise to various cell types

2. Where are stem cells located in you (adult’s body)?

a) In bone marrow which produce all of our blood cells

b) Adult brain that produce nerve cells

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3. Where are stem cells located in an embryo?

a) Most of early embryos are stem cells

4. The goal of working with stem cells is to produce cell types that are malfunctioning in a person’s body. Replace damaged pancreatic cells so insulin can be produced; replace brain cells to prevent Parkinson’s or Huntington’s.

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Figure 21.8 Working with stem cells

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II. Different cell types make different proteins, usually as a result of transcriptional regulation

A. The Concept of Determination

1. Definition: molecular changes that will lead to differentiation. When a cell is “determined” it has its fate decided whether to become a lung cell, liver cell, etc.

2. Molecular Changes

a) Tissue-specific proteins: specific genes are transcribed producing specific types of mRNA that will lead to changes in cellular structure, appearance and therefore function.

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3. Example: Molecular Changes in Skeletal Muscle Cells

a) Hypothesis: specific regulatory genes are activated in myoblasts (young muscle cells)

b) Isolated mRNA from cultured myoblasts since this mRNA was being transcribed as the myoblasts were becoming mature muscle cells.

c) Used reverse transcriptase to make cDNA from the various mRNAs.

d) Connected the cDNA with a promoter and inserted them into separate embryonic cells and observed differentiation.

e) In those cells that differentiated into myoblasts, certain key regulatory genes were identified.

f) Ultimately the myoblasts will make lots of myosin and actin.

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III. Transcriptional regulation is directed by maternal molecules in the cytoplasm and signals from other cells.

A. So if there are all these signals that must turn on genes, where is the “mother of all signals?” What starts it all?

1. Cytoplasmic Determinants: Researchers know that the distribution of mRNA, proteins, organelles in the unfertilized egg affect cellular differentiation and hence the transcription of specific genes.

2. Cellular Environment and Induction: signal molecules from neighboring cells or cell-to-cell contact can induce changes in adjacent target cells that affect gene transcription / expression.

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Figure 21.10 Sources of developmental information for the early embryo

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Genetic and Cellular Mechanisms of Pattern Formation

I. Introduction

A. Pattern Formation: the organization of tissues and body parts in a 3D formation by the cytoplasmic determinants and inductive signals.

B. Positional Information: these are the molecular cues that control where the different body parts will go,such as the head, tail, etc., and therefore locate these body parts along an organized body axis.

II. Genetic Analysis of Drosophila reveals how genes control development

A. Life Cycle of Drosophila

1. See next slide

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Figure 21.11 Key developmental events in the life cycle of Drosophila

Step 1: first mitotic divisions involve no new cellular development just one big multinucleated cell; also the amount of cytoplasm does not change.

Step 2/3: Nuclei will migrate out to periphery and plasma membrane forms around these nuclei so you get separate cells.

Step 4: A segmented body forms

Step 5/6: Larva forms and becomes encased as a pupa

Step 7: Pupa becomes an adult within the casing and fly emerges.

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B. Genetic Analysis of Early Development in Drosophila

III. Gradients of maternal molecules in the early embryo control axis formation.

A. Remember again that proteins, organelles, mRNA are just a few of the substances that are unequally distributed within the unfertilized egg.

1. These substances were coded for by maternal effect genes. A mutant in one of these genes results in a mutant phenotype of the offspring.

2. Also known as egg-polarity genes because they determine the the anterior-posterior (head-tail) axis and the dorsal-ventral (back-belly) axis of the embryo.

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B. The Bicoid Gene and Bicoid Protein (a transcription factor)

1. The bicoid protein is responsible for organizing the anterior end of the fly and so accumulates there.

2. A defect in the bicoid gene lacks a front end.

3. Proof: bicoid mRNA accumulates at the extreme anterior end of the egg cell. After fertilization, the mRNA is translated into protein which then diffuses posteriorly or towards the back end of the embryo. This produces a gradient of bicoid protein.

a) Bicoid protein is a morphogen: a substance that exerts its effects due to its gradient.

b) If bicoid mRNA was injected at various sites in a developing embryo, anterior anatomy developed at those sites so researchers new it was the bicoid mRNA that caused anterior end development.

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Figure 21.12 The effect of the bicoid gene, a maternal effect (egg-polarity) gene in Drosophila

Bicoid Research:

1. Established that a protein was responsible for organismal differentiation.

2. It identified the protein

3. Established that the mother cell’s gradient of this protein affected differentiation.

4. Established that a gradient of a chemical was responsible for its effects.

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IV. A cascade of gene activations sets up the segmentation pattern in Drosophila

A. Segmentation Genes

1. Once the major front to back and belly to back axes are defined, segments are formed in the fruit fly. These segments are formed by different kinds of segmentation genes.

2. Types of Segmentation Genes

a) Gap Genes

i. these are the first segmentation genes to be activated by transcription factors.

ii. Gap genes make general divisions of the head to tail body plan.

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b) Pair-rule Genes

i. These genes are regulated (activate) by the gap genes

ii. These genes produce proteins that produces finer divisions within a segment

c) Segment Polarity Genes

i. Activated by the Pair-rule genes

ii. These genes set the anterior-posterior positioning of each segment.

d) And why is this called a “cascade?”

-because the gap genes activate the pair-rule genes which activate the segment polarity gene.

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Figure 21.13 Segmentation genes in Drosophila

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V. Homeotic genes direct the identity of body parts

A. Homeotic Genes

1. These genes will produce the transcription factors that tell the cells in the segments which appendages to produce i.e., antennae, limbs, wings, etc.

2. These transcription factors control the specific body part to be developed in a specific location.

3. These genes are not activated until the anterior-posterior axis is completely laid down.

a) A mutation in a homeotic gene produces appendages that are misplaced.

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Figure 21.14 Homeotic mutations and abnormal pattern formation in Drosophila

The mutant is here with the legs coming out of its head!!

Normal small antennae

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VI. Homeobox genes have been highly conserved in evolution

A. The Concept of a Homeobox

1. Definition: “a specific nucleotide sequence” within a homeotic gene. Each homeotic gene has a common homeobox which is a nucleotide sequence that codes for a protein. This protein also binds to the DNA as part of the transcription factor complex.

2. The transcription factor complex then activates a body part gene (antennae) to be developed.

3. Homeoboxes are very common in many organisms therefore are thought to have evolved very early in evolution and important in many types of organisms and their development.

4. In mammals, homeobox-containing genes are called Hox genes.

Documents

1 Chap 21: The Genetic Basis of Development Points of Emphasis Know: 1.Most of the bold-faced terms. Areas of lowest priority are: 2.The basis of this