Chapter 2.

Cellular Reproduction and Model Genetic Organisms 1. Cell and Chromosomes 2. Mitosis 3. Meiosis 4. Genetics in the Laboratory: An Introduction to Some Model Research Organisms

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No father but three mothers Her genes were identical to those of one of her mothers 2

1. Cells and Chromosomes Some organisms consist of just a single cell. Others consist of trillions of cells. The simplest life forms, viruses, are not composed of cells. However, viruses must enter cells in order to function. Thus, all life has a cellular basis.

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The Cellular Environment Living cells are made of many different kinds of molecules. The most abundant is water. The inside of a cell, called the cytoplasm, contains both hydrophilic and hydrophobic substances

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The molecules that make up cells Carbohydrates such as starch and glycogen store chemical energy for work within cells. These molecules are composed of glucose, a simple sugar. The glucose subunits are attached one to another to form long chains, or polymers. Cells obtain energy when glucose molecules released from these chains are chemically degraded into simpler compounds.

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The molecules that make up cells Lipid. These molecules are formed by chemical interactions between glycerol and fatty acids. important constituents of many structures within cells. energy sources.

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The molecules that make up cells protein consists of one or more polypeptides which are chains of amino acids. components of many different structures. catalyze chemical reactions. enzymes Cells also contain nucleic acids—DNA and RNA Cells are surrounded by a thin layer called a membrane. -primary constituents are lipids and proteins.

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The molecules that make up cells Membranes are also present inside cells. specialized structures called organelles. Plant cell walls are composed of cellulose, a complex carbohydrate. Bacterial cell walls are composed of a different kind of material called peptidoglycan or murein (a polymer consisting of sugars and amino acids). Walls and membranes selectively allow other materials to pass through them via channels and gates.

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Prokaryote, Archaea and Eukaryote Prokaryotic cells are usually less than a thousandth of a millimeter long (1~10 µm), and they typically lack a complicated system of internal membranes and membranous organelles. Archaea are single celled organisms lacking nuclei. Eukaryotic cells are usually at least 10 times bigger than prokaryote.

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Electron micrograph of the bacterium Escherischia coli, a prokaryote, dividing into two cells. The light material inside the cell is where DNA is located.

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eukaryotic cells typically contain one or more mitochondria Light colored material: chromosomes

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• Eukaryotic cells are much larger than bacteria and contain a true nucleus bounded by a nuclear membrane housing the genetic material -The plasma membrane regulates the uptake and outflow of cellular materials and contains transmembrane proteins complexed with sugars; they can function as receptors binding ligands that alter gene expression and cellular activities.

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-The largest organelle in an eukaryotic cell is the nucleus, which contains DNA, RNA, and protein with the DNA located in chromosomes In prokaryotic cells, the DNA is usually not housed within a well-defined nucleus. -A vast network of interconnected tubular membranes called the endoplasmic reticulum or ER fills the cytoplasm. Rough ER contains bound ribosomes synthesizing polypetides that cross the membrane and move to the Golgi complex. Smooth ER is ribosome-free.

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-The Golgi complex: modify proteins synthesized in the rough ER or to finish their maturation to functional proteins. -Lysosomes are small vesicles that pinch off from the Golgi complex and contain powerful degradative enzymes capable of digesting any molecule in the cell.

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-Peroxisomes contain different enzymes that can break down fatty acids. -Membrane-bound liquid-filled vesicles termed vacuoles can comprise 90% of a plant cell's volume. They function to control intracellular electrolytes and toxic wastes and also provide a high internal pressure to physically support plants. • Mitochondria and chloroplasts probably arose from small prokaryotic cells and contain small circular DNA molecules and synthesize many of their own proteins. Mitochondria are the cell's power plant and convert lipids and sugars into ATP, which powers nearly all cellular activities. Mitochondrial mutations also cause a number of inherited disorders.

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• Chloroplasts are found only in plant cells and are the sites of photosynthesis in which sunlight is used to drive the synthesis of complex molecules from carbon dioxide and water. • An intracellular cytoskeleton is found in all eukaryotic cells and is comprised of a network of protein filaments controlling cell shape and movement.

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Chromosomes: Where Genes are Located • The genetic material in all cells is organized in structures called chromosomes; Prokaryotic cells typically contain only one chromosome. -possess many smaller DNA molecules called plasmids. -circular human sperm cells have 23. typically larger and more complex than those of prokaryotic cells. -linear

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Many eukaryotic cells possess two copies of each chromosome. Diploid: An organism or cell with two sets of chromosomes (2n) or two genomes. Somatic tissues of higher plants and animals are ordinarily diploid in chromosome constitution in contrast with the haploid (monoploid) gametes. Somatic: A cell that is a component of the body, in contrast with a germ cell that is capable, when fertilized, of reproducing the organism. Gametes: A mature male or female reproductive cell (sperm or egg). Haploid: An organism or cell having only one complete set (n) of chromosomes or one genome.

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Germ line: the reproductive tissue of an organism. Centromere: Spindle-fiber attachment region of a chromosome.

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Electron micrograph showing a bacterial chromosome. 22

Light micrograph of human chromosomes during cell division. 23

• Chromosomes have two functions: transmission of genetic information from cell to cell and generation to generation, and the orderly expression of genetic information to control cellular function and development. • A replicated chromosome contains two sister chromatids joined by a centromere. The centromere contains a protein structure called the kinetochore required for movement to the poles during cell division. The centromere can lie anywhere along the length of a chromosome. The chromosome ends are called telomeres. 24

• A chromosome contains critical DNA sequences including origins of replication where DNA replication begins, telomeres allowing correct replication of the ends, and centromeres to bind the sister chromatids together. A cell can divide into two cells A cell that is about to divide is called a mother cell, and the products of division are called daughter cells. Clone: All the individuals derived by vegetative propagation from a single original individual.

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prokaryotic cells divide, the contents of the mother cell are more or less equally apportioned between the two daughter cells. This process is called fission. Escherichia coli divides every 20 to 30 minutes. a single E. coli cell can produce enough progeny in a single day to form a mass visible to the unaided eye (250). We call such a mass of cells a colony.

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The eukaryotic cell cycle. This cycle is 24 hours long. The duration of the cycle varies among different types of eukaryotic cells.

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2. Mitosis Each chromosome in a mother cell is duplicated prior to the onset of mitosis, specifically during the S phase. Chromatin: The complex of DNA and proteins in eukaryotic chromosomes; originally named because of the readiness with which it stains with certain dyes. Microtubules: are components of the cytoskeleton. These fibers, composed of proteins called tubulins, attach to the chromosomes and move them about within the dividing mother cell.

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microtubule organizing centers (MTOCs): A region in a eukaryotic cell that generates the microtubules used during cell division. Centrosomes: In animal cells, the MTOCs are differentiated into small organelles. these organelles are not present in plant cells. centrosome contains two barrel-shaped centrioles

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The mitotic spindle in a cultured animal cell, which has been stained to show the microtubules (green) emanating from the two asters. 30

Electron micrograph showing two pairs of centrioles. 31

• Before and after mitosis, or mitotic cell division, the parental and daughter cells are diploid and contain 2n number of chromosomes. mitosis: interphase, which consists of G1, S and G2, prophase, metaphase, anaphase, and telophase. -During interphase, the centrally located centrosome is duplicated with each acting as a microtubule organizing center and functioning as the spindle pole during mitosis. A pair of centrioles at the center replicate and eventually separate as microtubules radiate outwardly, forming an aster.

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• During early prophase, the two centrosomes move to opposite poles of the cells and replicated chromosomes begin condensing so that by late prophase they are highly condensed. Kinetochores in the centromeres attach to microtubules and guide sister chromatid separation. The nuclear membrane fragments and the nucleolus disperses. • At metaphase, chromosomes containing fully condensed sister chromatids line up on the central equatorial plates between the centrosome poles. • During anaphase, centromeres "split" and each carries a sister chromatid to opposite poles of the cell via microtubule shortening. Emphasize that each chromatid is also a chromosome so chromosome number does not change even though the number of DNA molecules (and alleles) has been halved per cell. 33

• During telophase, chromosome movement ceases, microtubules disassemble, the nuclear membrane reforms, the nucleolus reappears, and chromosomes decondense. The cell has now entered G1 and interphase. • The cell completes division by separating into two daughter cells via cytokinesis. This occurs via a gradual midline constriction in most cells, but in plant cells with rigid cell walls a cell plate forms between daughters.

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The animal cell is a fertilized egg, which is dividing for the first time. 42

plant cells 43

3. Meiosis Meiosis from a Greek word meaning “diminution”—is the process that reduces the diploid state to the haploid state—that is, it reduces the number of chromosomes in a cell by half. The resulting haploid cells either directly become gametes, or divide to produce cells that later become gametes. a key role in reproduction among eukaryotes. Without it, ?

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human cells have 23 pairs of chromosomes. Each pair is distinct. Different pairs of chromosomes carry different sets of genes. The members of a pair are called homologous chromosomes, or simply homologues. Chromosomes from different pairs are called heterologues. During meiosis, homologues associate intimately with each other.

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Thin threads

paired threads

thick threads

two threads

Movement through

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homologous chromosomes come together intimately. This process of pairing between homologues is called synapsis. In some species, synapsis begins at the ends of chromosomes and then spreads toward their middle regions. Synapsis is accompanied by the formation of a proteinaceous structure between the pairing chromosomes (synaptonemal complex)

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Electron micrograph (a) and diagram (b) showing the structure of the synaptonemal complex that forms between homologous chromosomes during the zygotene stage of prophase I of meiosis. 50

If we count homologues, the pair is referred to as a bivalent of chromosomes, whereas if we count strands, it is referred to as a tetrad of chromatids. Bivalent: A pair of synapsed or associated homologous chromosomes that have undergone the duplication process to form a group of four chromatids. During pachynema, the paired chromosomes may exchange material. These contact points are called chiasmata.

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During anaphase I, the paired chromosomes separate from each other definitively. This separation, called chromosome disjunction

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Meiosis II and the Outcomes of Meiosis chromosome disjunction: the paired chromosomes separate from each other definitively. chromatid disjunction: In mitosis or meiosis, one of the two identical strands resulting from self-duplication of a chromosome. Mechanistically, meiosis II is much like mitosis. However, its products are haploid, and unlike the products of mitosis, the cells that emerge from meiosis II are not genetically identical.

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Comparison between mitosis and meiosis; c is the haploid amount of DNA in the genome 58

One reason these cells differ is that homologous chromosomes pair and disjoin from each other during meiosis I. Within each pair of chromosomes, one homologue was inherited from the organism's mother, and the other was inherited from its father. If there are 23 pairs of chromosomes, as there are in humans, meiosis I can produce 223 chromosomally different daughter cells—that is, more than 8 million possibilities. -homologous chromosomes exchange material by crossing over. -create countless different combinations of genes.

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In yeasts and unicellular algae, they divide mitotically to produce populations of haploid cells. In simple fungi, such as the bread mold Neurospora crassa, they divide mitotically to produce filaments of cells called hyphae (singular, hypha), which form the body of the fungus. Under appropriate environmental conditions, all these haploid organisms can produce sexual cells. When two different sexual cells from these organisms unite, they produce a diploid zygote, which usually proceeds into meiosis without any intervening mitotic growth.

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No male and female sexes. -mating types. In the alga Chlamydomonas reinhardii, they are denoted plus and minus; and in the mold Neurospora crassa, they are denoted A and a.

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Life cycles of the unicellular alga Chlamydomonas reinhardtii (for Chlamydomonas, n = 17)

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In lower plants, such as the mosses, the haploid cells derived from meiosis divide mitotically to produce branched filaments that eventually differentiate into tissues such as stems and leaves. When mature, these haploid plants produce gametes—either eggs or sperm. Consequently, they are called gametophytes. Gametophytes: That phase of the plant life cycle that bears the gametes; the cells have n chromosomes. The gametes unite at fertilization to form diploid zygotes, which divide mitotically and develop into structures called sporophytes. Sporophytes: The diploid generation in the life cycle of a plant that produces haploid spores by meiosis. 63

In higher plants, the gametophyte is much reduced in size—it consists of just a few haploid cells. Thus, the sporophyte is the conspicuous part of the life cycle. Meiosis occurs in distinct male and female reproductive tissues in the sporophytes of higher plants. Only one of the four haploid cells produced by female meiosis develops into a gametophyte; this cell is called the megaspore. All four of the haploid cells from male meiosis—called microspores—develop into gametophytes. The existence of both haploid and diploid organisms in the life cycles of plants is referred to as the alternation of generations.

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Alternation of generations between haploid gametophytes and diploid sporophytes in plants. In the lower plants, the gametophyte is dominant, whereas in the higher plants, the sporophyte is dominant. 65

4. Genetics in the Laboratory: An Introduction to Some Model Research Organisms Today a select group of microorganisms, plants, and animals are favored in genetic research. These creatures, often called model organisms, lend themselves well to genetic analysis. For the most part, they are easily cultured in the laboratory, their life cycles are relatively short, and they are genetically variable. In addition, through work over many years, geneticists have established large collections of mutant strains for these organisms.

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• Escherichia coli is a bacterium that occurs in the intestines of animals including humans. It can be grown on a simple medium, and it is amenable to biochemical and genetic analyses. • The DNA of the E. coli genome consists of 4.6 x 106 nucleotide pairs that are arranged in a single chromosome. The genome has been sequenced and is thought to contain 4,288 protein-encoding genes. • E. coli can be used to study the biology and genetics of viruses (bacteriophages; from a Greek word meaning “to eat bacteria,”) that infect them.

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Bacteriophage attaching to the surface of an E. coli cell. 68

• Saccharomyces cerevisiae, baker’s yeast is typically a unicellular fungi. It has developed as a model organism because it can be grown on a simple medium, large numbers of cells can be rapidly obtained from a single mother cell, and mutant strains with different growth characteristics can be readily isolated. • The yeast genome consists of 12 x 106 nucleotide pairs distributed among its 16 chromosomes, and it is thought to contain 6,268 genes. It was the first eukaryotic genome to be analyzed completely. • Yeast reproduces both sexually [mating types (denoted a and alpha)] and asexually.

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The four haploid products of meiosis are created in a sac called the ascus (plural, asci), and each of the products is called an ascospore.

Baker’s yeast

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• Drosophila melanogaster, has a relatively short life cycle. The adult fly is about 2 mm long. It possesses a complex nervous system and many specialized tissues and organs. A short life cycle (10 days at 25C). Like all insects, an adult Drosophila has three pairs of legs. However, unlike most insects, it has only one pair of wings; the second pair of wings has been modified into small appendages called halteres, which help the animal to balance during flight. The surface of the adult body is covered with sensory hairs and bristles, which are connected to the nervous system. Other prominent sensory organs—the eyes and the antennae—are located on the head. The reproductive organs are located in the abdomen.

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A female Drosophila can produce hundreds of eggs. • Thousands of mutant strains have been obtained affecting characters including eye color and anatomy. • The genome of D. melanogaster has been sequenced and contains 170 x 106 nucleotide pairs. It is estimated to contain 13,792 genes. • Caenorhaditis elegens is a nematode that has played a major role in the genetic analysis of development. • Its genome has been sequenced and consists of 100 x 106 nucleotide pairs housing an estimated 20,516 genes.

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Arabidopsis thaliana, a fast-growing plant By capturing solar energy and storing it in simple sugars such as glucose, plants play an indispensable role in the web of life. As a by-product of this process, they also generate oxygen, which we and all other nonphotosynthetic organisms breathe. • A thaliana is a weedy member of the mustard family. It is small with a relatively short generation time. • The genome of A thaliana consists of 157 x 106 nucleotide pairs and is thought to contain 27,706 genes. Since many of these genes are present in agriculturally important crop species, genetic analysis of Arabidopsis is providing a framework to understand how genes function in important food crops. 73

Like Mendel's garden peas, Arabidopsis is a self-fertilizing species; however, different strains can be cross-fertilized in the laboratory to produce hybrids.

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• Homo sapiens, our own species • Since human beings cannot be subjected to genetic experimentation like Drosophila or yeast, Homo sapiens it is not a true model system. The desire to learn about the genetics of our own species is very strong, and great efforts have been expended to satisfy this desire in spite of many obstacles. • Techniques such as cell culture and DNA cloning have added new dimensions to the study of genetic material of humans. Culturing of human cells allows genetic phenomena to be studied outside the human organism.

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• Human cell lines have been extensively used to study cell structure, metabolism, and growth, to localize genes to chromosomes, and to analyze the expression of genes and the functions of their products. • The human genome contains 3.2 x 109 nucleotide pairs containing 20,000 to 25,000 genes. • Recent focus has been on ways to isolate and culture human stem cells. Cells, obtained from certain types of adult tissues and from embryos, can be induced to differentiate in culture into specialized cells types and, therefore, have much potential medicinal value.

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HeLa cells growing in culture. 79