Principles of Development Biology

3.1.1 The Making of a Living Organism

The structure and composition of living organisms varies greatly, from single-celled bacteria to complex multi-cellular organisms with differentiated cell types and interconnected organ systems. Regardless of the complexity, every living entity contains a blueprint for its construction in the form of a double-helical chain of molecules called deoxyribonucleic acid or DNA (Figure 1-14). DNA is an amazingly simple chemical structure, yet it contains an entire library of information on how to make, maintain, and reproduce an organism, and also keeps a record of clues to the organism's evolutionary history. The entire sequence of DNA in an organism is called its genome. A genome can be as small as the 9,750 bases of the human immunodeficiency virus (HIV; the cause of AIDS) or, as large as the +3 billion bases in mouse, human, and frogs.

DNA encodes the unique nature of different organisms by specifying the precise structure of each protein in a cell. In analogy to DNA, proteins are made from a linear sequence of amino acids, and the exact sequence of amino acids is what determines the function of the protein. A DNA sequence is translated into the protein sequence by a code, where a triplet of bases (a codon) specifies a single amino acid; some codons specify the end of the protein. Humans are built from an estimated 20,000-35,000 proteins, yet, only a small percentage of the 3 billion bases in the human genome codes for these proteins. The discrete sections of DNA that encode proteins are referred to as genes.

Protein production is a highly regulated process. For example, a cell does not want to waste energy making the proteins needed for cell division if it is busy with other functions, such as secreting a hormone. This process of turning a gene on and off depending on the cell's need for a particular set of proteins is referred to as regulation of gene expression. Certain proteins, and even other regions of DNA, physically bind to the DNA sequence surrounding a gene to affect its expression. This interaction occurs at regions of the DNA with apt names, such as promoters or enhancers of gene expression.

Gene regulation is an essential part of life. Since every cell in an organism contains the same genetic blueprint, turning on different genes at different times during development creates different cell types. In fact, it is differential gene expression that allows stem cells to become unique cell types. Gene regulation is also critical for cellular response to metabolic needs.

The entire genome is not read all in one piece. Instead, cells make copies of selected genes at selected times via the process of transcription. These copies are transported out of the nucleus to a cellular factory, or organelle, called the ribosome, where translating the original DNA code makes proteins.

3.1.2 Reproduction

Fertilization activates the egg and brings together the nuclei of the sperm and egg. Fertilization forms the diploid zygote and triggers the onset of embryonic development. The sperm nucleus swells and merges with the egg nucleus to form the zygote and DNA replication begins with the first division occurring in about 90 minutes.

Duplication of a cell's DNA is required for both cellular replication to replenish dying cells, and for sexual reproduction. In unicellular organisms, these two processes are the same. DNA is duplicated before the cell divides to produce two separate organisms, each with the original amount of DNA. This asexual method of reproduction is known as binary fission. In multi-cellular organisms, a similar process called mitosis is used to replenish lost cells (Figure 1-15). However, reproduction is more complex and begins with specialized cells called gametes (eggs and sperm). Through the process of meiosis, these cells have only half of the DNA of other cells; that is, only one copy of each of the 23 chromosomes.

Maintaining genetic variability in successive generations can be achieved through recombination of genetic material. The order of the genes on the chromosome remains the same, but the specific versions of the genes become shuffled. Recombination is one of the primary reasons that offspring from the same parents do not look alike, as new combinations of alleles are formed in every gamete.

Viruses are simply a strand of genetic material, either DNA or RNA, encapsulated in an outer protein shell, and sometimes a membrane. They cannot reproduce on their own or perform basic cellular tasks like protein synthesis, so most people do not consider them to be living organisms. Viruses reproduce by hijacking the replication machinery of a host cell. They transfer their genome into the cell of another organism, integrate their DNA sequence into the host DNA, and let the host cell replicate, transcribe and translate their genes. Viral genomes contain genes for directing the replication and packaging of complete copies of the virus, so that eventually, the host cell bursts open and releases new viruses to infect other cells.

Figure 1-15. This image depicts a ceil in mid-prometaphase. In this cell, the spindle is forming between the well-separated centrosomes. Some of the chromosomes have established connections to both poles and are aligned at the spindle equator, while others are still connected only to one pole. Photo courtesy of NASA.

Cell division, the process by which our cells grow and multiply, is normally tightly controlled. In embryos and young children, cell division is required primarily for growth. However, its main role in adults is to repair and replace old cells. Cell division is a very complex process, and it involves a very ordered sequence of events. For example, cancer occurs when a cell

Figure 1-15. This image depicts a ceil in mid-prometaphase. In this cell, the spindle is forming between the well-separated centrosomes. Some of the chromosomes have established connections to both poles and are aligned at the spindle equator, while others are still connected only to one pole. Photo courtesy of NASA.

breaks free from normal constraints and starts multiplying uncontrollably. Tens, if not hundreds, of molecules are involved in cell division, and many of these have been implicated in cancer.

Cleavage is a succession of rapid mitotic cell divisions following fertilization and produces a multi-cellular embryo, the bias tula. A definite polarity results is shown in the egg caused by the concentration of cellular components as mRNA4, proteins, and yolk. The yolk is a key factor in determining polarity and influencing cleavage in frogs and other animals. The vegetal pole of the egg has the highest concentration of yolk. The animal pole has the lowest concentration and is the area where polar bodies bud off of the cell. The animal pole marks where the most anterior part of the animal will form. The animal hemisphere is gray due to the presence of the pigment melanin. The vegetal hemisphere is slightly yellow due to the yellow yolk. Cleavage in the animal hemisphere is more rapid than in the vegetal hemisphere. If there is little yolk in the vegetal hemisphere cleavage will proceed equally. The first two cleavage divisions are vertical and divide the embryo into four cells. The third cleavage plane is horizontal and produces an eight-cell embryo with two levels. Continual cleavage produces a solid ball of cells called the morula. A fluid-filled cavity, called the blastocoel, develops within the morula forming a hollow ball of cells called the bias tula (Figure 116).

Gastrulation then rearranges the blastula to form a three-layered embryo with a primitive gut. The three layers produced by gastrulation are embryonic tissues called embryonic germ layers. These three germ layers will eventually develop into all parts of the adult animal.

Figure 1-16. Fertilization of a Xenopus egg is followed by cleavage (a succession of cell divisions that partition the large fertilized egg cell in smaller cells), different-tiation, and organogenesis. After hatching from the egg, the tadpole wilt exist in an aquatic stage with gills and a tail, until complex hormonal changes transform it into an adult frog. Source NASA.

4 mRNA stands for messenger RNA. The DNA of a gene is transcribed into mRNA molecules, which then serve as a template for the synthesis of proteins.

3.1.3 Differentiation and Embryogenesis

Multi-cellular organisms develop from a single cell into a complex entity replete with a variety of cell types, such as skin, muscle, nerve, and bone. Different cell types require different sets of enzymes, structural proteins, and regulatory proteins to drive their specific chemical processes and support their unique needs. Embryonic cells differentiate into new cell types by regulating gene expression turning on and off the transcription and translation of individual genes.

Biologists have found that the organism's development is mostly determined by the genome and the organization of the egg's cytoplasm. As the zygote undergoes cleavage, the cytoplasm is compartmentalized causing the nuclei of the different cells to be exposed to different cytoplasmic environments. These different cytoplasmic environments result in the expression of different genes in different cells. Inherited traits then emerge, in an orderly fashion, in space and time by mechanisms controlling gene expression.

Cell differentiation occurs very early in embryogenesis. In mammals, after just a few divisions of the original fertilized egg, cells begin to migrate and form defined ends of the organism and a narrow grove that will become the spinal cord. Cells quickly begin to differentiate, expressing genes that are specific to their future cell type. Gene regulation not only guides differentiation, but also allows an organism to respond to a changing environment as well as to the needs of a developing fetus versus those of an adult.

Complex organisms have three general types of cells: somatic, stem, and germ cells. Somatic cells, from the Greek "soma" or body, make up most of the organism. When fully differentiated, somatic cells become damaged or worn out, many are replaced by simple mitosis. However, some are replenished from a pool of stem cells. Stem cells exist in an earlier, less differentiated state and have the ability to mature into a variety of cell types depending on the extracellular signals they receive. Stem cells identified in humans include brain, bone marrow (Figure 1-17), blood, blood vessel, skeletal muscle, skin, and liver. The term germ cell refers to the reproductive cells or gametes, sperm, and eggs.

Organogenesis forms the organs of the animal body from the three embryonic layers. The first evidence of organogenesis is morphogenetic changes (folds, splits, condensation of cells) that occur in the layered embryonic tissues. The neural tube and notochord are the first organs to develop in frogs and other chordates. The notochord stretches the embryo lengthwise and forms the core around witch the mesoderm cells will develop the muscles of the axial skeleton. As organogenesis continues, other organs and tissues develop from the embryonic germ layers. Morphogenesis includes the final physical processes that give shape to the animal's body and organs.

Figure 1-17. This graphic illustrates the variety of blood cells that are derived fi'om a single stem cell in bone marrow of mammals. The red blood cells contain hemoglobin for the transportation of oxygen. Platelets are not cells, but fragments of megakarocytes that form clumps to assist in clotting. Macrophages are white blood cell that ingests foreign substances in the body. The other white blood cells contain granules that are tiny sacs of enzymes for digesting microorganisms that invade the body. B-cells produce antibodies (proteins) directed against specific antigens (foreign bodies). Some mature into "memory cells " that recognize reinfection by the same foreign body and stimulates further production of antibodies. T-cells are responsible for the destruction of foreign bodies.

Figure 1-17. This graphic illustrates the variety of blood cells that are derived fi'om a single stem cell in bone marrow of mammals. The red blood cells contain hemoglobin for the transportation of oxygen. Platelets are not cells, but fragments of megakarocytes that form clumps to assist in clotting. Macrophages are white blood cell that ingests foreign substances in the body. The other white blood cells contain granules that are tiny sacs of enzymes for digesting microorganisms that invade the body. B-cells produce antibodies (proteins) directed against specific antigens (foreign bodies). Some mature into "memory cells " that recognize reinfection by the same foreign body and stimulates further production of antibodies. T-cells are responsible for the destruction of foreign bodies.

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