 Molecular Basis of Heredity What an organism looks like and how it functions is determined largely by its genetic material. The basic principles of heredity were developed by Gregor Mendel, who experimented with pea plants in the 19th century. He mathematically analyzed the inherited traits (such as color and size) of a large number of plants over many generations. The units of heredity are genes carried on chromosomes. Genetics can explain why children look like their parents, and why they are, at the same time, not identical to the parents. Phenotype and Genotype The collection of physical and behavioral characteristics of an organism is called a phenotype. For example, your eye color, foot size, and ear shape are components of your phenotype. The genetic makeup of a cell or organ- ism is called the genotype. The genotype is like a cook- book for protein synthesis and use. Phenotype (what an organism looks like or how it acts) is determined by the genotype (its genes) and its environment. By environ- ment, we don’t mean the Earth, but the environment surrounding the cell or organism. For example, hor- mones in the mother’s body can influence the gene expression. Reproduction Asexual reproduction on the cellular level is called mito- sis. It requires only one parent cell, which, after exactly multiplying its genetic material, splits in two. The result- ing cells are genetically identical to each other and are clones of the original cell before it split. Sexual reproduction requires two parents. Most cells in an organism that reproduces sexually have two copies of each chromosome, called homologous pairs—one from each parent. These cells reproduce through mitosis. Gamete cells (sperm and egg cells) are exceptions. They carry only one copy of each chromosome, so that there are only half as many chromosomes as in the other cells. For example, human cells normally contain 46 chromo- somes, but human sperm and egg cells have 23 chro- mosomes. At fertilization, male and female gametes (sperm and egg) come together to form a zygote, and the number of chromosomes is restored by this union. The genetic information of a zygote is a mixture of genetic information from both parents. Gamete cells are manu- factured through a process called meiosis, whereby a cell multiples its genetic material once, but divides twice, producing four new cells, each contains half the number of chromosomes present in the original cell before divi- sion. In humans, gametes are produced in testes and ovaries. Meiosis causes genetic diversity within a species by generating combinations of genes different from those present in the parents. – LIFE SCIENCE– 235 2 2 2 2 2 2 2 2 2 2 2 Cytoplasm Endoplasmic reticulum Plasma membrane Nucleolus Nucleus Vacuole Cell wall Ribosomes Mitochondria Centriole Chloroplast Lysosome Animal Cell Plant Cell Golgi complex Alleles Alleles are alternative versions of the same gene. An organism with two copies of the same allele is homozy- gous, and one with two different alleles is heterozygous. For example, a human with one gene for blue eyes and one gene for brown eyes is heterozygous, while a human with two genes for blue eyes or two genes for brown eyes is homozygous. Which of the two genes is expressed is determined by the dominance of the gene. An allele is dominant if it alone determines the phe- notype of a heterozygote. In other words, if a plant has a gene for making yellow flowers and a gene for making red flowers, the color of the flower will be determined by the dominant gene. So if the gene for red flowers is dom- inant, a plant that has both the gene for red and the gene for yellow will look red. The gene for yellow flowers in this case is called recessive, as it doesn’t contribute to the phenotype (appearance) of a heterozygote (a plant con- taining two different alleles). The only way this plant would make yellow flowers is if it had two recessive genes—two genes both coding for yellow flowers. For some genes, dominance is only partial and two different alleles can be expressed. In the case of partial dominance, a plant that has a gene that codes for red flowers and a gene that codes for white flowers would produce pink flowers. A Punnett square can be used to represent the possi- ble phenotypes that offspring of parents with known genotypes could have. Take the example with the yellow and red flower. Let’s label the gene for the dominant red gene as R and the gene for yellow flowers as r. Cross a plant with yellow flowers (genotype must be rr) with a plant with red flowers and genotype Rr. What possible genotypes and phenotypes can the offspring have? In a Punnett square, the genes of one parent are listed on one side of the square and the genes of the other parent on the other side of the square. They are then combined in the offspring as illustrated here: The possible genotypes of the offspring are listed inside the square. Their genotype will be either Rr or rr, causing them to be either red or yellow, respectively. Sex Determination In many organisms, one of the sexes can have a pair of unmatched chromosomes. In humans, the male has an X chromosome and a much smaller Y chromosome, while the female has two X chromosomes. The combination XX (female) or XY (male) determines the sex of humans. In birds, the males have a matched pair of sex chromosomes (WW), while females have an unmatched pair (WZ). In humans, the sex chromosome supplied by the male determines the sex of the offspring. In birds, the female sex chromosome determines the sex. Plants, as well as many animals, lack sex chromo- somes. The sex in these organisms is determined by other factors, such as plant hormones or temperature. Identical twins result when a fertilized egg splits in two. Identical twins have identical chromosomes and can be either two girls or two boys. Two children of different sex born at the same time can’t possibly be identical twins. Such twins are fraternal. Fraternal twins can also be of the same sex. They are genetically not any more alike than siblings born at different times. Fraternal twins result when two different eggs are fertilized by two dif- ferent sperm cells. When meiosis goes wrong, the usual number of chro- mosomes can be altered. An example of this is Down’s syndrome, a genetic disease caused by the presence of an extra chromosome. Changes in DNA (mutations) occur randomly and spontaneously at low rates. Mutations occur more fre- quently when DNA is exposed to mutagens, including ultraviolet light, X-rays, and certain chemicals. Most mutations are either harmful to or don’t affect the organ- ism. In rare cases, however, a mutation can be beneficial to an organism and can help it survive or reproduce. Ultimately, genetic diversity depends on mutations, as mutations are the only source of completely new genetic material. Only mutations in germ cells can create the variation that changes an organism’s offspring. Plant rr RRr Rr rrr rr Plant – LIFE SCIENCE– 236  Biological Evolution Mutations cause change over time. The result of a series of such changes is evolution, or as Darwin put it, “descent with modification.” The great diversity on our planet is the result of more than 3.5 billion years of evo- lution. The theory of evolution argues that all species on Earth originated from common ancestors. Evidence for Evolution Several factors have led scientists to accept the theory of evolution. The main factors are described here. ■ Fossil record. One of the most convincing forms of evidence is the fossil record. Fossils are the remains of past life. Fossils are often located in sedimentary rocks, which form during compres- sion of settling mud, debris, and sand. The order of layers of sedimentary rock is consistent with the proposed sequence in which life on Earth evolved. The simplest organisms are located at the bottom layer, while top layers contain increas- ingly complex and modern organisms, a pattern that suggests evolution. ■ Biogeography. Another form of evidence comes from the fact that species tend to resemble neigh- boring species in different habitats more than they resemble species in similar, but far away, habitats. ■ Comparative anatomy. Comparative anatomy provides us with another line of evidence. It refers to the fact that the limb bones of different species, for example, are similar. Species that closely resemble one another are considered more closely related than species that do not resemble one another. For example, a horse and a donkey are considered more closely related than a horse and a frog. Biological classifications (kingdom, phylum, class, order, family, genus, and species) are based on how organisms are related. Organ- isms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships. ■ Embryology. Embryology provides another form of evidence for evolution. Embryos go through the developmental stages of their ancestors to some degree. The early embryos of fish, amphib- ians, reptiles, birds, and mammals all have com- mon features, such as tails. ■ Comparative molecular biology. Comparative molecular biology confirms the lines of descent suggested by comparative anatomy and fossil record. Darwin also proposed that evolution occurs gradually, through mutations and natural selection. He argued that some genes or combinations of genes give an individual a survival or reproductive advantage, increasing the chance that these useful combinations of genes will make it to future generations. Whether a given trait is advantageous depends on the environment of the organism. Natural selection is only one of several mechanisms by which gene frequency in a population changes. Other factors include mating patterns and breeding between popula- tions.  Interdependence of Organisms The species in communities interact in many ways. They compete for space and resources, and they can be related as predator and prey, or as host and parasite. Plants and other photosynthetic organisms harness and convert solar energy and supply the rest of the food chain. Herbivores (plant eaters) obtain energy directly from plants. Carnivores are meat eaters and obtain energy by eating other animals. Decomposers feed on dead organisms. The flow of energy can then be repre- sented as follows: Sun → Photosynthetic organisms → Herbivores → Carnivores → Decomposers The food chain is not the only example of the inter- dependence of organisms. Species often have to compete for food and space, so that the increase in population of one can cause the decrease in population of the other. Organisms also may have a symbiotic relationship (live in close association), which could be classified as parasitism, mutualism, or commensalism. In a parasitic relationship, one organism benefits at the expense of the other. Commensalism is symbiosis in which one organ- ism benefits and the other is neither harmed nor rewarded. In mutualism, both organisms benefit. Under ideal conditions, with ample food and space and no predators, all living organisms have the capacity to reproduce to infinite number. However, resources are limited, limiting the population of a species. – LIFE SCIENCE– 237 Humans probably come closest to being a species with seemingly infinite reproductive capacity. Our population keeps increasing. Our only danger seems to come from viruses and bacteria, which at this point, we more or less have under control. When we need more food, we grow more, and when we need more space, we clear some by killing off other biomes. By doing this, humans modify ecosystems and destroy habitats through direct harvest- ing, pollution, atmospheric changes, and other factors. This attitude is threatening current global stability and has the potential to cause irreparable damage.  Behavior of Organisms Even the most primitive unicellular organisms can act to maintain homeostasis. More complex organisms have nervous systems. The simplest organism found to have learning capability is a worm, suggesting a more complex nervous system. The function of the nervous system is collection and interpretation of sensory signals as trans- mission of messages from the center of the nervous sys- tem (brain in humans) to other parts of the body. The nervous system is made of nerve cells, or neurons, which conduct signals in the form of electrical impulses. Nerve cells communicate by secreting excitatory or inhibitory molecules called neurotransmitters. Many legal and ille- gal drugs act on the brain by disrupting the secretion or absorption of neurotransmitters. Many animals have sense organs that enable them to detect light, sound, and specific chemicals. These organs provide the animals with information about the outside world. Animals engage in innate and learned social behavior. These behaviors include hunting or searching for food, nesting, migrating, playing, caring for their young, fighting for mates, and fighting for territory. Plants also respond to stimuli. They turn toward the sun and let their roots run deeper when they need water. – LIFE SCIENCE– 238 E ARTH AND SPACE science are concerned with the formation of the Earth, the solar system and the universe, the history of Earth (its mountains, continents and ocean floors), the weather and seasons on Earth, the energy in the Earth system, and the chemical cycles on Earth.  Energy in the Earth Systems Energy and matter can’t be created or destroyed. But energy can change form and travel great distances. Solar Energy The sun’s energy reaches our planet in the form of light radiation. Plants use this light to synthesize sugar mol- ecules, which we consume when we eat the plants. We obtain energy from the sugar molecules and our bodies use it. Ultimately, our energy comes from the sun. The sun also drives the Earth’s geochemical cycles, which will be discussed in the next section. The sun heats the Earth’s surface and drives convection within the atmosphere and oceans, producing winds and ocean currents. The winds cause waves on the surface of oceans and lakes. The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Strong winds cause large CHAPTER Earth and Space Science HUMANS HAVE always wondered about the origin of the Earth and the universe that surrounds it. What kinds of matter and energy are in the universe? How did the universe begin? How has the Earth evolved? This chapter will answer these fundamental questions and review the key concepts of Earth and space science. 25 239 waves. Tsunamis, or tidal waves, are different. They result from underwater earthquakes, volcanic eruptions, or landslides, not wind. Energy from the Core Another source of Earth’s energy comes from Earth’s core. We distinguish four main layers of Earth: the inner core, the outer core, the rocky mantle, and the crust. The inner core is a solid mass of iron with a temperature of about 7,000° F. Most likely, the high temperature is caused by radioactive decay of uranium and other radioactive elements. The inner core is approximately 1,500 miles in diameter. The outer core is a mass of molten iron that surrounds the solid inner core. Electri- cal currents generated from this area produce the earth’s magnetic field. The rocky mantle is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium and is about 1,750 miles thick. This mantle accounts for most of the Earth’s mass. When parts of this layer become hot enough, they turn to slow moving molten rock, or magma. The Earth’s crust is a layer from four to 25 miles thick, consisting of sand and rock. The upper mantle is rigid and is part of the litho- sphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The crust is divided into plates that drift slowly (only a few cen- timeters each year) on the less rigid mantle. Oceanic crust is thinner than continental crust. This motion of the plates is caused by convection (heat) currents, which carry heat from the hot inner mantle to the cooler outer mantle. The motion results in earthquakes and volcanic eruptions. This process is called plate tectonics. Tectonics Evidence suggests that about 200 million years ago, all continents were a part of one landmass, named Pangaea. Over the years, the continents slowly separated through the movement of plates in a process called continental drift. The movement of the plates is attributed to con- vection currents in the mantle. The theory of plate tec- tonics says that there are now twelve large plates that slowly move on the mantle. According to this theory, earthquakes and volcanic eruptions occur along the lines where plates collide. Dramatic changes on Earth’s land- scape and ocean floor are caused by collision of plates. These changes include the formation of mountains and valleys.  Geochemical Cycles Water, carbon, and nitrogen are recycled in the bios- phere. A water molecule in the cell of your eye could have been, at some point, in the ocean, in the atmosphere, in a leaf of a tree, or in the cell of a bear’s foot. The circula- tion of elements in the biosphere is called a geochemical cycle. Water Oceans cover 70% of the Earth’s surface and contain more than 97% of all water on Earth. Sunlight evapo- rates the water from the oceans, rivers, and lakes. Living beings need water for both the outside and the inside of their cells. In fact, vertebrates (you included) are about 70% water. Plants contain even more water. Most of the water passes through a plant unaltered. Plants draw on water from the soil and release it as vapor through pores in their leaves, through a process called transpiration. Our atmosphere can’t hold a lot of water. Evaporated water condenses to form clouds that produce rain or snow on to the Earth’s surface. Overall, water moves from the oceans to the land because more rainfall reaches the land than is evaporated from the land. (See the figure on the next page.) Carbon Carbon is found in the oceans in the form of bicarbon- ate ions (HCO 3 − ), in the atmosphere, in the form of car- bon dioxide, in living organisms, and in fossil fuels (such as coal, oil, and natural gas). Plants remove carbon diox- ide from the atmosphere and convert it to sugars through photosynthesis. The sugar in plants enters the food chain, first reaching herbivores, then carnivores, and finally scavengers and decomposers. All these organ- isms release carbon dioxide back into the atmosphere when they breathe. The oceans contain 500 times more carbon than the atmosphere. Bicarbonate ions (HCO 3 – ) settle to the bottoms of oceans and form sedimentary rocks. Fossil fuels represent the largest reserve of carbon on Earth. Fossil fuels come from the carbon of organisms that had lived millions of years ago. Burning fossil fuels releases energy, which is why these fuels are used to power human contraptions. When fossil fuels burn, car- bon dioxide is released into the atmosphere. Since the Industrial Revolution, people have increased the concentration of carbon dioxide in the atmosphere – EARTH AND SPACE SCIENCE– 240