H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003), страница 8

The Teton Mountains inWyoming, now about 14,000 feet high and still growing, didnot exist a mere 10 million years ago. Yet horseshoe crabs,with a life span of about 19 years, have faithfully reproducedtheir ancient selves more than half a million times during thatperiod. The common impression that biological structure istransient and geological structure is stable is the exact opposite of the truth. Despite the limited duration of our individual lives, reproduction gives us a potential for immortalitythat a mountain or a rock does not have.The simplest type of reproduction entails the division ofa “parent” cell into two “daughter” cells.

This occurs as partof the cell cycle, a series of events that prepares a cell to dividefollowed by the actual division process, called mitosis. Theeukaryotic cell cycle commonly is represented as four stages(Figure 1-17). The chromosomes and the DNA they carry arecopied during the S (synthesis) phase. The replicated chromosomes separate during the M (mitotic) phase, with eachdaughter cell getting a copy of each chromosome during celldivision. The M and S phases are separated by two gap stages,the G1 phase and G2 phase, during which mRNAs and proteins are made. In single-celled organisms, both daughter cellsOverview Animation: Life Cycle of a CellTranscriptional control of gene expression was first decisively demonstrated in the response of the gut bacteriumE.

coli to different sugar sources. E. coli cells prefer glucoseas a sugar source, but they can survive on lactose in a pinch.These bacteria use both a DNA-binding repressor proteinand a DNA-binding activator protein to change the rate oftranscription of three genes needed to metabolize lactose depending on the relative amounts of glucose and lactose present (Chapter 4). Such dual positive/negative control of geneexpression fine tunes the bacterial cell’s enzymatic equipmentfor the job at hand.Like bacterial cells, unicellular eukaryotes may be subjected to widely varying environmental conditions that require extensive changes in cellular structures and function.For instance, in starvation conditions yeast cells stop growing and form dormant spores (see Figure 1-4).

In multicellular organisms, however, the environment around most cells isrelatively constant. The major purpose of gene control in usand in other complex organisms is to tailor the properties ofvarious cell types to the benefit of the entire animal or plant.Control of gene activity in eukaryotic cells usually involves a balance between the actions of transcriptional activators and repressors. Binding of activators to specific DNAregulatory sequences called enhancers turns on transcription,and binding of repressors to other regulatory sequencescalled silencers turns off transcription.

In Chapters 11 and12, we take a close look at transcriptional activators and repressors and how they operate, as well as other mechanismsfor controlling gene expression. In an extreme case, expression of a particular gene could occur only in part of thebrain, only during evening hours, only during a certain stageof development, only after a large meal, and so forth.Many external signals modify the activity of transcriptional activators and repressors that control specific genes.For example, lipid-soluble steroid hormones, such as estrogen and testosterone, can diffuse across the plasma membrane and bind to their specific receptors located in thecytoplasm or nucleus (Figure 1-16b). Hormone bindingchanges the shape of the receptor so that it can bind to specific enhancer sequences in the DNA, thus turning the receptor into a transcriptional activator.

By this rather simplesignal-transduction pathway, steroid hormones cause cells tochange which genes they transcribe (Chapter 11). Sincesteroid hormones can circulate in the bloodstream, they canaffect the properties of many or all cells in a temporally coordinated manner. Binding of many other hormones and ofgrowth factors to receptors on the cell surface triggers different signal-transduction pathways that also lead to changesin the transcription of specific genes (Chapters 13–15). Although these pathways involve multiple components and aremore complicated than those transducing steroid hormonesignals, the general idea is the same.1718CHAPTER 1 • Life Begins with Cellsoften (though not always) resemble the parent cell. In multicellular organisms, stem cells can give rise to two differentcells, one that resembles the parent cell and one that does not.Such asymmetric cell division is critical to the generation ofdifferent cell types in the body (Chapter 22).During growth the cell cycle operates continuously, withnewly formed daughter cells immediately embarking on theirown path to mitosis.

Under optimal conditions bacteria can divide to form two daughter cells once every 30 minutes. At thisrate, in an hour one cell becomes four; in a day one cell becomes more than 1014, which if dried would weigh about 25grams. Under normal circumstances, however, growth cannotcontinue at this rate because the food supply becomes limiting.Most eukaryotic cells take considerably longer than bacterial cells to grow and divide. Moreover, the cell cycle in adultplants and animals normally is highly regulated (Chapter 21).This tight control prevents imbalanced, excessive growth oftissues while assuring that worn-out or damaged cells are replaced and that additional cells are formed in response to newcircumstances or developmental needs. For instance, the proliferation of red blood cells increases substantially when a person ascends to a higher altitude and needs more capacity tocapture oxygen.

Some highly specialized cells in adult animals,such as nerve cells and striated muscle cells, rarely divide, ifat all. The fundamental defect in cancer is loss of the abilityto control the growth and division of cells. In Chapter 23, weexamine the molecular and cellular events that lead to inappropriate, uncontrolled proliferation of cells.Mitosis is an asexual process since the daughter cellscarry the exact same genetic information as the parental cell.In sexual reproduction, fusion of two cells produces a thirdcell that contains genetic information from each parentalcell.

Since such fusions would cause an ever-increasing number of chromosomes, sexual reproductive cycles employ aspecial type of cell division, called meiosis, that reduces thenumber of chromosomes in preparation for fusion (see Figure 9-3). Cells with a full set of chromosomes are calleddiploid cells. During meiosis, a diploid cell replicates its chromosomes as usual for mitosis but then divides twice withoutcopying the chromosomes in-between.

Each of the resultingfour daughter cells, which has only half the full number ofchromosomes, is said to be haploid.Sexual reproduction occurs in animals and plants, andeven in unicellular organisms such as yeasts (see Figure 1-5).Animals spend considerable time and energy generating eggsand sperm, the haploid cells, called gametes, that are used forsexual reproduction.

A human female will produce about halfa million eggs in a lifetime, all these cells form before she isborn; a young human male, about 100 million sperm each day.Gametes are formed from diploid precursor germ-line cells,which in humans contain 46 chromosomes. In humans the Xand Y chromosomes are called sex chromosomes because theydetermine whether an individual is male or female.

In humandiploid cells, the 44 remaining chromosomes, called autosomes, occur as pairs of 22 different kinds. Through meiosis, aman produces sperm that have 22 chromosomes plus either anX or a Y, and a woman produces ova (unfertilized eggs) withFEMALEMALE44 AXX44 AXYDiploid (2n)MeiosisHaploid (1n)22 AX22 AX22 AX22 AYOne typeof femalegameteDiploid (2n)Fertilization44 AXX44 AXYFemalezygoteMalezygoteTwo typesof malegamete▲ FIGURE 1-18 Dad made you a boy or girl. In animals,meiosis of diploid precursor cells forms eggs and sperm(gametes).

The male parent produces two types of sperm anddetermines the sex of the zygote. In humans, as shown here,X and Y are the sex chromosomes; the zygote must receive aY chromosome from the male parent to develop into a male.Aautosomes (non-sex chromosomes).22 chromosomes plus an X. Fusion of an egg and sperm (fertilization) yields a fertilized egg, the zygote, with 46 chromosomes, one pair of each of the 22 kinds and a pair of X’s infemales or an X and a Y in males (Figure 1-18).

Errors duringmeiosis can lead to disorders resulting from an abnormal number of chromosomes. These include Down’s syndrome, causedby an extra chromosome 21, and Klinefelter’s syndrome,caused by an extra X chromosome.Cells Die from Aggravated Assault or anInternal ProgramWhen cells in multicellular organisms are badly damaged orinfected with a virus, they die. Cell death resulting from sucha traumatic event is messy and often releases potentiallytoxic cell constituents that can damage surrounding cells.Cells also may die when they fail to receive a life-maintainingsignal or when they receive a death signal. In this type of programmed cell death, called apoptosis, a dying cell actuallyproduces proteins necessary for self-destruction. Death byapoptosis avoids the release of potentially toxic cell constituents (Figure 1-19).Programmed cell death is critical to the proper development and functioning of our bodies (Chapter 22).

Duringfetal life, for instance, our hands initially develop with “webbing” between the fingers; the cells in the webbing subsequently die in an orderly and precise pattern that leaves the1.4 • Investigating Cells and Their Parts19fingers and thumb free to play the piano. Nerve cells in thebrain soon die if they do not make proper or useful electrical connections with other cells. Some developing lymphocytes, the immune-system cells intended to recognize foreignproteins and polysaccharides, have the ability to reactagainst our own tissues. Such self-reactive lymphocytes become programmed to die before they fully mature. If thesecells are not weeded out before reaching maturity, they cancause autoimmune diseases in which our immune system destroys the very tissues it is meant to protect.1.4 Investigating Cells and Their Parts▲ FIGURE 1-19 Apoptotic cells break apart withoutspewing forth cell constituents that might harm neighboringcells.