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UNC-Chapel Hill BIOL 205 - Lecture #1 INTRODUCTION TO DEVELOPMENTAL BIOLOGY

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1BIOLOGY 205/SECTION 7 DEVELOPMENT- LILJEGRENLecture #1INTRODUCTION TO DEVELOPMENTAL BIOLOGYPpts will be posted by 5 pm the day before class. My lecture notes will beposted after the class period. Not all of the material covered in the lectures willbe included in the ppts or lecture notes, so be sure to come to class!1. What is the study of development?• The process by which a fertilized egg transforms itself into a complex organism is one ofthe most exciting and complex mysteries in the field of biology! How does it happen??Each of us sitting in this room developed from a single egg.• Developmental Biology is the study of a PROCESS whereby a single cell divides andselectively activates expression of genes to produce a complex organism composed ofmany cell types.• Ex ovo omnia=all from the egg. William Harvey (1651) was the first to propose that allanimals originate from eggs.• What are the types of PROCESSES required for development?2. A central idea of development that we will talk about many times the rest of thesemester is DIFFERENTIATION: ALL CELLS HAVE THE SAME DNA, BUT DIFFERENTCELLS EXPRESS DIFFERENT GENES.Since different cell types express different genes, different cell types possess different cellularmachinery. Different cells specialize to do different jobs. We will learn how this happens, howcells learn to activate different sets of instructions that lead to production of different proteins.DIVERSITY: Earth's myriad animals & plants display an incredible diversity of body plans, and yetshare many conserved developmental mechanisms, evidence of their evolution from a commonancestor. Our challenge is to understand both this diversity and this unity.3. This PROCESS is studied using TOOLS, many of which you have already heard aboutin Bio202 and the first half of Bio205:a) Cell Biology - how cells are made, move, and talk to each otherb) Genetics - what is the role of specific proteins? study the effect of mutations ondevelopmental processesc) Molecular Biology - how cells selectively activate a subset of genes that will produce aunique set of cellular traitsWhat does this mean? You will now take your box of biological tools and use them tounderstand how development occurs.Before we get into developmental mechanisms, let’s REVIEW THE BASICS1. The body is made of millions to billions of cells.a) Each cell has a specialized task, e.g. muscle cell, nerve cell, etc.b) To carry out these specialized tasks, each cell has special machineryc) Nerve cell has machinery for transmitting electrical and chemical signalsd) blood cell has machinery to transport oxygen21. Cellular machinery is largely made up of proteinsProteins are long chains of amino acids. Each protein molecule has evolved to carry out aparticular task. For example: i. hemoglobin in the blood binds to and carries oxygen ii. actin forms filaments that make up the cytoskeleton iii. ribosomal proteins help make up the ribosome, the cell's protein factory iv. insulin is secreted by cells of the pancreas and serves as a signal to other cells.2. Because of their different tasks, different cells contain different proteinsWhile some proteins are found in all cells (actin), others are made only in specialized cells i. e.g. muscles make myoglobin to store oxygen for work ii. lymphocytes make antibodies to neutralize foreign invaders iii. skin cells make cytokeratin which serves as the structural element of skin andhair3. Proteins are made up of chains or sequences of amino acids, and these amino acidsare "encoded" in the cell's DNAa. DNA is organized in very large segments known as chromosomes, but eachchromosome is a package of thousands of genesb. 1 gene encodes 1 proteinc. Thus there is a myoglobin gene, an actin gene, an insulin gene etc. Genes are theinstructions to make individual cellular machines.d. Mutations in single genes thus result in failure to produce single proteins, and mutantcells are thus lacking a particular protein machine4. All cells have the same DNA but different cells express different genesa. Different sets of cellular instructions are activated in different cells, leading to theproduction of different proteinsb. hemoglobin genes are only active in red blood cellsc. the insulin gene only in the ß-cells of the pancreasd. the pepsin gene only in stomach cells5. Development occurs at an unfamiliar scale.• A mouse (3 inches = 7.5 cm = 750 mm) is about 100,000 times bigger than a cell-10-30 µm = 0.01-0.03 mm• A gene is 10,000 times bigger than a protein, which are generally 2-10 nm long =0.001 µm = 0.000001 mm. A gene averages 1-10µm in length when unwound, but isonly about 2nm wide when wound up with histones etc (it is extremely folded up to fitin cell since there are 15-30,000 genes per cell). Another way to think of thisIn other words if a mouse were the size of Chapel Hill (10 miles)• a cell would be about the size of a basketball (8 inches),• and a gene would be on average about an inch long. If a cell were the size of Chapel Hill (10 miles):• an average protein would be the size of a Volvo (10 feet)• an average gene would be about 1.5 miles long but the strand of DNA would onlybe a few feet wide.3DEVELOPMENTAL MECHANISMS AND THE EVOLUTION OF MULTICELLULARITY1. Differentiation mechanisms- Two extreme models (REALITY INVOLVES BOTH!)a) MOSAIC DEVELOPMENT/autonomous specification: cells become progressivelycommitted to particular cell fates: there’s no looking back! In other words, cellsacquire fixed identities that they then maintain without influence from neighbors; whenisolated their descendants only develop into particular parts of the body.Wilhelm Roux's experiment (1888) illustrates this.He killed two of the first four cells in a frog embryo with a hot needle, and allowed theremaining cells to develop.WHAT HAPPENS? The remaining two cells adopt the fates they would have in anintact embryo.b) "REGULATIVE" DEVELOPMENT/conditional specification: cells are flexible and able toadjust to alterations in their neighbors and environment; when isolated theirdescendants can develop into an entire organism.Hans Driesch's experiment (1892) illustrates this. Separate the first fourcells in a sea urchin embryo by vigorous shaking.WHAT HAPPENS? To his surprise, rather than finding the same results as Roux.Instead of each cell developing into a specific part, each cell senses its neighborsabsence and regulates its fate to make an


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UNC-Chapel Hill BIOL 205 - Lecture #1 INTRODUCTION TO DEVELOPMENTAL BIOLOGY

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