Cells from Cells Study Guide

8 Cellular Reproduction: Cells from Cells Why Cellular Reproduction Matters ◀ Having too few or too many chromosomes is almost always fatal. ▲ In certain species of sea star, a severed arm may be able to regrow a whole new body. If stretched out, the DNA in any one of your cells would be taller than you. ▶ ▲ Every tumor is the result of a malfunction in cell division. CHAPTER CONTENTS What Cell Reproduction Accomplishes 156 The Cell Cycle and Mitosis 157 Meiosis, the Basis of Sexual Reproduction 164 CHAPTER THREAD Life with and without Sex BIOLOGY AND SOCIETY Virgin Birth of a Dragon 155 THE PROCESS OF SCIENCE Do All Animals Have Sex? 171 EVOLUTION CONNECTION The Advantages of Sex Life with and without Sex BIOLOGY AND SOCIETY Virgin Birth of a Dragon Zookeepers at the Chester Zoo in England were startled to discover that Flora, a female Komodo dragon— Varanus komodoensis, the largest living species of lizard, capable of growing up to 10 feet long—had laid a clutch of 25 eggs. It wasn’t surprising that a captive Komodo dragon would breed. In fact, Flora was at the zoo for that very reason: She was one of two female Komodo dragons that were part of a captive breeding program intended to help repopulate the species. What made Flora’s clutch of eggs so remarkable is that she had not yet been in the company of a male, let alone mated with one. As far as anyone knew, Komodo dragons, like the vast majority of animal species, create offspring only through sexual reproduction involving the union of a male’s sperm and a female’s egg. But despite Flora’s virginity, eight of her eggs developed normally and hatched into live, healthy Komodo dragons. DNA analysis confirmed that Flora’s offspring derived their genes solely from her. The new dragons must have resulted from parthenogenesis, the production of offspring by a female without involvement of a male. Parthenogenesis is one form of asexual reproduction, the creation of a new generation without participation of sperm and egg. Parthenogenesis is rare among vertebrates (animals with backbones), although it has been documented in species as diverse as sharks (including the hammerhead), domesticated birds, and now Komodo dragons. Soon, zoologists identified a second Komodo at a difThe Komodo dragon. The Komodo is the world’s ferent zoo who had also borne young by parthenogenesis. This largest lizard and is found in the wild only on three same ­Komodo dragon later had additional offspring via sexual ­reproduction—indicating that this species is capable of switch- islands in Indonesia. ing between two reproductive modes. Biologists are investigating the evolutionary basis of this phenomenon and considering what implications it may have on efforts to repopulate this rare species. The ability of organisms to procreate is the one characteristic that best distinguishes living things from nonliving matter. All organisms—from bacteria to lizards to you—are the result of repeated cell divisions. The perpetuation of life therefore depends on cell division, the production of new cells. In this chapter, we’ll look at how individual cells are copied and then see how cell reproduction underlies the process of sexual ­reproduction. Throughout our discussion, we’ll consider examples of asexual and sexual reproduction among both plants and animals. 174 Ordinary cell division produces two daughter cells that are genetically identical. Name three functions of this type of cell division. Which of these functions occur in your body? Answer: cell replacement, growth of an organism, asexual reproduction of an organism; only the first two occur in your body ▶ Figure 8.1 Three f­ unctions of cell division by mitosis. reproduce by dividing in half, and the offspring are When you hear the word reproduction, you probably genetic replicas of the parent. Because it does not inthink of the birth of new organisms. But reproduction volve fertilization of an egg by a sperm, this type of actually occurs much more often at the cellular level. reproduction is called asexual reproduction. Consider the skin on your arm. Skin cells are Offspring produced by asexual reproduction constantly reproducing themselves and movIn certain inherit all of their chromosomes from a sining outward toward the surface, replacing species of sea gle parent and are thus genetic duplicates. dead cells that have rubbed off. This renewal star, a severed arm Many multicellular organisms can reproof your skin goes on throughout your life. may be able to duce asexually as well. For example, some And when your skin is injured, additional regrow a whole sea star species have the ability to grow new cell reproduction helps heal the wound. new body. individuals from fragmented pieces. And if When a cell undergoes reproduction, or you’ve ever grown a houseplant from a clipping, cell division, the two “daughter” cells that reyou’ve observed asexual reproduction in plants. In sult are genetically identical to each other and to the asexual reproduction, there is one simple principle of original “parent” cell. (Biologists traditionally use the inheritance: The lone parent and each of its offspring word daughter in this context to refer to offspring cells, have identical genes. The type of cell division responsible but of course cells lack gender.) Before the parent cell for asexual reproduction and for the growth and maintesplits into two, it duplicates its chromosomes, the strucnance of multicellular organisms is called mitosis. tures that contain most of the cell’s DNA. Then, during Sexual reproduction is different; it requires fertilizacell division, each daughter cell receives one identical set tion of an egg by a sperm. The production of gametes— of chromosomes from the original parent cell. egg and sperm—involves a special type of cell division As summarized in Figure 8.1, cell division plays sevcalled meiosis, which occurs only in reproductive eral important roles in the lives of organisms. For examorgans. As we’ll discuss later, a gamete has only half as ple, within your body, millions of cells must divide every many chromosomes as the parent cell that gave rise to it. second to replace damaged or lost cells. Another function In summary, two kinds of cell division are involved in of cell division is growth. All of the trillions of cells in the lives of sexually reproducing organisms: mitosis for your body are the result of repeated cell divisions that begrowth and maintenance and meiosis for reproduction. gan in your mother’s body with a single fertilized egg cell. The remainder of the chapter is divided into two main Another vital function of cell division is reproducsections, one devoted to each type of cell division. tion. Many single-celled organisms, such as amoebas, FUNCTIONS OF CELL DIVISION BY MITOSIS Cell Replacement Division of a human kidney cell into two Growth via Cell Division The cells of an early human embryo Colorized SEM 810× CHECKPOINT What Cell Reproduction Accomplishes LM 590× Chapter 8 Cellular Reproduction: Cells from Cells The Cell Cycle and Mitosis The Cell Cycle and Mitosis Most of the time, the chromosomes exAlmost all of the genes of a eukaryotic cell— ist as thin fibers that are much longer than around 21,000 in humans—are located on the nucleus they are stored in. In fact, if chromosomes in the cell nucleus. (The If stretched out, fully extended, the DNA in just one of main exceptions are genes on small DNA the DNA in any one of your cells would be your cells would be more than 6 feet long! ­molecules found in mitochondria and taller than you. Chromatin in this state is too thin to be chloroplasts.) Because chromosomes are seen using a light microscope. As a cell the lead players in cell division, we’ll focus on them before turning our attention to the cell as a whole. Species Eukaryotic Chromosomes Each eukaryotic chromosome contains one very long DNA molecule, typically bearing thousands of genes. The number of chromosomes in a eukaryotic cell depends on the species (Figure 8.2). For example, human body cells have 46 chromosomes, while the body cells of a dog have 78 and those of a koala bear have 16. Chromosomes are made up of a material called chromatin, fibers composed of roughly equal amounts of DNA and protein molecules. The protein molecules help organize the chromatin and help control the activity of its genes. Indian muntjac deer Koala Opossum Number of chromosomes in body cells 6 16 22 Giraffe Mouse Human Duck-billed platypus Bison Figure 8.2 The number of chromosomes in the cells of selected mammals. Notice that humans have 46 chromosomes and that the number of chromosomes does not correspond to the size or complexity of an organism. ▶ Dog 30 40 46 54 60 78 Red viscacha rat 102 LM 250× Asexual Reproduction Reproduction of an amoeba Fragmentation and regeneration of a sea star. The sea star on the right lost and replaced an arm. The severed arm grew into the new sea star on the left. Reproduction of an African violet from a clipping (large leaf)