Chapter 8 The Continuity of Life: How Cells Reproduce Lectures by Gregory Ahearn University of North Florida Ammended by John Crocker Copyright 2009 Pearson Education, Inc.
Review Questions for Chapters 8-11 (as always answers must be full and complete) 1. Describe the structure of DNA. Be sure to include what forms the skeleton and how are the strands held together? 2. Compare and contrast chromosomes, chromatids, genes, and alleles. 3. Compare and contrast prokaryotic and eukaryotic cell division. 4. Describe the process of asexual reproduction in eukaryotic cells. 5. Compare and contrast animal and plant cell asexual reproduction. 6. Compare and contrast mitosis and meiosis. 7. Without genetic testing how could you determine if an organism is homozygous or heterozygous for a specific trait (ie hair color)? 8. Describe three ways that genetic variability is increased. 9. Two fruitflies are bred. One is true breeding for red eyes and one is true breeding for white eyes. Red eyes are dominant. What will the genotype and phenotype of the offspring be? 10.If two of the offspring of the above match are crossbred what will the genotype and phenotype of their offspring be? 11.In the above examples how would the genotypes and phenotypes be different if red eye color was partially dominant producing pink eyes when heterozygous? 12.In the example above (#9) the red-eyed fruitfly has straight wings and the white-eyed fruitfly has wrinkled wings. Straight wings are dominant. What would the genotype and phenotype of the offspring be? 13.What characteristics can make genetic disorders more likely to be passed from one generation to the next? (at least 3) 14.Describe the process of DNA replication. What is meant by semiconservative replication? How are continuous synthesis and discontinuous synthesis involved in the process? 15.How common are mistakes in replication? What safeguards are in place to prevent mistakes? What types of mistakes are relatively common? 16.Compare and cont rast DNA and RNA. 17.Describe the proce ss of transcription. 18.Describe the proce ss of translation. 19.What are codons and how do they function in protein synthesis? 20.Describe the w ays by which gene expression may be regulated.
8.1 Why Do Cells Divide? Cells reproduce by cell division. One cell gives rise to two or more cells, called daughter cells. Each daughter cell receives: a complete set of heredity information identical to that in the parent cell and about half of the cytoplasm.
8.1 Why Do Cells Divide? Cell division transmits hereditary information to each daughter cell. DNA contains the hereditary information in each cell. DNA is contained in chromosomes. Nucleotides are the monomers of DNA.
8.1 Why Do Cells Divide? A nucleotide consists of a phosphate, a sugar (deoxyribose), and one of four bases. The four bases are adenine (A) thymine (T) guanine (G) cytosine (C). The nucleotides are held together by hydrogen bonding between the bases in the two strands, called a double helix.
8.1 Why Do Cells Divide? The structure of DNA phosphate nucleotide base sugar A C T G A G C C C G A T A C G A T T A T (a) A single strand of DNA (b) The double helix Flash Fig. 8-1
8.1 Why Do Cells Divide? Genes are the units of inheritance made up of segments of different lengths along a DNA molecule. Each gene spells out the instructions for making a proteins or regulating the expression of another gene. When a cell divides, it first replicates its DNA, and each copy is transferred into each daughter cell. Flash
8.1 Why Do Cells Divide? Cell division is required for growth and development. Cell division in which the daughter cells are genetically identical to the parent cell is called mitotic cell division. After cell division, the daughter cells may grow and divide again, or may differentiate, becoming specialized for specific functions. The repeating pattern of division, growth, and differentiation followed again by division is called the cell cycle.
8.1 Why Do Cells Divide? Most multicellular organisms have three categories of cells. Stem cells: retain the ability to divide and can differentiate into a variety of cell types Other cells capable of dividing: typically differentiate only into one or two different cell types (progenitor cells) Permanently differentiated cells: differentiated cells that can never divide again
8.1 Why Do Cells Divide? Cell division is required for sexual and asexual reproduction. Sexual reproduction in eukaryotic organisms occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults. Gametes are produced by meiotic cell division, which results in daughter cells with exactly half of the genetic information of their parent cells. Fertilization of an egg by a sperm results in the restoration of the full complement of hereditary information in the offspring.
8.1 Why Do Cells Divide? Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg, is called asexual reproduction. Asexual reproduction produces offspring that are genetically identical to the parent. Examples of asexual reproduction occur in bacteria, single-celled eukaryotic organisms, multicellular organisms such as Hydra, and many trees, plants, and fungi.
The trees in this grove have already lost their leaves (a) Dividing bacteria (b) Cell division in Paramecium bud The trees in this grove are still green The trees in this grove have begun to change color (c) Hydra reproduces asexually by budding (d) A grove of aspens often consists of genetically identical trees produced by asexual reproduction Fig. 8-2
8.2 What Occurs During The Prokaryotic Cell Cycle? The prokaryotic cell cycle consists of a long period of growth, during which the cell duplicates its DNA. cell division by binary fission cell growth and DNA replication (a) The prokaryotic cell cycle Fig. 8-3a
8.2 What Occurs During the Prokaryotic Cell Cycle? Cell division in prokaryotes occurs by binary fission, which means splitting in two. The prokaryotic chromosome is attached at one point to the plasma membrane of the cell.
8.2 What Occurs During The Prokaryotic Cell Cycle? The prokaryotic cell cycle cell wall plasma membrane attachment site The circular DNA double helix is attached to the plasma membrane at one point. circular DNA Fig. 8-3b(1)
8.2 What Occurs During The Prokaryotic Cell Cycle? During the growth phase of the cell cycle, the DNA is replicated, producing two identical chromosomes that become attached to the plasma membrane at two separate points. As the cell grows, new plasma membrane is added between the attachment points of the chromosomes, pushing them apart.
8.2 What Occurs During The Prokaryotic Cell Cycle? The prokaryotic cell cycle (continued) The DNA replicates and the two DNA double helices attach to the plasma membrane at nearby points. New plasma membrane is added between the attachment points, pushing them further apart. Fig. 8-3b(2)(3)
8.2 What Occurs During The Prokaryotic Cell Cycle? Once the cell has doubled in size, the plasma membrane in the middle of the cell grows inward between the two DNA attachment sites.
8.2 What Occurs During The Prokaryotic Cell Cycle? The prokaryotic cell cycle (continued) The plasma membrane grows inward at the middle of the cell. Fig. 8-3b(4)
8.2 What Occurs During The Prokaryotic Cell Cycle? Fusion of the plasma membrane along the equator of the cell completes binary fission, producing two daughter cells, each with its own chromosomes The two daughter cells are genetically identical to each other and to the parent cell
8.2 What Occurs During The Prokaryotic Cell Cycle? The prokaryotic cell cycle (continued) The parent cell divides into two daughter cells. Fig. 8-3b(5)
8.3 How Is The DNA In Eukaryotic Cells Organized? Unlike prokaryotic chromosomes, eukaryotic chromosomes are separated from the cytoplasm by a membrane-bound nucleus. Eukaryotic cells always have multiple chromosomes. Eukaryotic chromosomes contain more DNA than prokaryotic chromosomes.
8.3 How Is The DNA In Eukaryotic Cells Organized? The eukaryotic chromosome consists of DNA bound to protein. Human chromosomes contain a single DNA double helix that is 50 to 250 million nucleotides long, which would be about 3 inches long if the DNA were completely relaxed.
8.3 How Is The DNA In Eukaryotic Cells Organized? During cell division, proteins fold up the DNA into compact structures that are 10 times shorter than during the rest of the cell cycle. Fig. 8-4
8.3 How Is The DNA In Eukaryotic Cells Organized? Duplicated chromosomes separate during cell division. Prior to cell division, the DNA within each chromosome is replicated. The duplicated chromosomes then consist of two DNA double helixes and associated proteins that are attached to each other at the centromere.
8.3 How Is The DNA In Eukaryotic Cells Organized? Duplicated chromosomes separate during cell division (continued). Each of the duplicated chromosomes attached at the centromere is called a sister chromatid. During mitotic cell division, the sister chromatids separate and each becomes a separate chromosome that is delivered to one of the two resulting daughter cells.
8.3 How Is the DNA In Eukaryotic Cells Organized? Eukaryotic chromosomes during cell division centromere genes duplicated sister chromosome chromatids (2 DNA double helices) (a) A replicated chromosome consists of two sister chromatids independent daughter chromosomes, each with one identical DNA double helix (b) Sister chromatids separate during cell division Fig. 8-5
8.3 How Is The DNA In Eukaryotic Cells Organized? Eukaryotic chromosomes usually occur in pairs. An entire set of stained chromosomes from a single cell is called a karyotype. sex chromosomes Fig. 8-6
8.3 How Is The DNA In Eukaryotic Cells Organized? Eukaryotic chromosomes usually occur in pairs (continued). The nonreproductive cells of many organisms have chromosomes in pairs, with both members of the pair being the same length. The chromosomes are the same length and have the same staining properties because they have the same genes arranged in the same order.
8.3 How Is The DNA In Eukaryotic Cells Organized? Chromosomes with the same genes are called homologous chromosomes, or homologues. Cells with pairs of homologous chromosomes are called diploid.
8.3 How Is The DNA In Eukaryotic Cells Organized? Homologous chromosomes are usually not identical. The same genes on homologous chromosomes may be different from each other due to changes in the sequence of nucleotides in the DNA, called mutations. A given mutation may have occurred recently, or may have occurred generations ago and has been inherited ever since.
8.3 How Is The DNA In Eukaryotic Cells Organized? A typical human cell has 23 pairs of chromosomes. 22 of these pairs have a similar appearance and are called autosomes. Human cells also have a pair of sex chromosomes, which differ from each other in appearance and in genetic composition. Females have two X chromosomes. Males have one X and one Y chromosome.
8.3 How Is The DNA In Eukaryotic Cells Organized? Not all cells have paired chromosomes. The ovaries and testes undergo a special kind of cell division, called meiotic cell division, to produce gametes (eggs and sperm). Gametes contain only one member of each pair of autosomes, plus one of the two sex chromosomes. Cells with half the number of each type of chromosome are called haploid cells. Fusion of two haploid cells at fertilization produces a diploid cell with the full complement of chromosomes.
8.3 How Is The DNA In Eukaryotic Cells Organized? The number of different types of chromosomes in a species is called the haploid number and is designated n. In humans, n = 23. Diploid cells contain 2n chromosomes. Humans body cells contain 2n = 46 (2 x 23) chromosomes.
8.4 What Occurs During The Eukaryotic Cell Cycle? The eukaryotic cell cycle is divided into two major phases: interphase and cell division. During interphase, the cell acquires nutrients from its environment, grows, and duplicates its chromosomes. During cell division, one copy of each chromosome and half of the cytoplasm are parceled out into each of two daughter cells.
8.4 What Occurs During The Eukaryotic Cell Cycle? The eukaryotic cell cycle prophase metaphase anaphase mitotic cell division cytokinesis telophase and cell growth and differentiation cell growth interphase synthesis of DNA; chromosomes are duplicated Fig. 8-7
8.4 What Occurs During The Eukaryotic Cell Cycle? There are two types of division in eukarytic cells: mitotic cell division and meiotic cell division. Mitotic cell division may be thought of as ordinary cell division, such as occurs during development from a fertilized egg, during asexual reproduction, and in skin, liver, and the digestive tract every day. Meiotic cell division is a specialized type of cell division required for sexual reproduction.
8.4 What Occurs During The Eukaryotic Cell Cycle? Mitotic cell division Mitotic cell division consists of nuclear division (called mitosis) followed by cytoplasmic division (called cytokinesis) and the formation of two daughter cells.
8.4 What Occurs During The Eukaryotic Cell Cycle? Meiotic cell division Is a prerequisite for sexual reproduction in all eukaryotic organisms. Meiotic cell division involves a specialized nuclear division called meiosis. It involves two rounds of cytokinesis, producing four daughter cells that can become gametes.
8.4 What Occurs During The Eukaryotic Cell Cycle? The life cycle of eukaryotic organisms include both mitotic and meiotic cell division. A new generation begins with the fusion of two gametes. Through mitosis and differentiation, the fertilized egg grows and develops a multicellular body. Meiotic cell division generates new gametes that may unite with other gametes to produce the next generation.
8.4 What Occurs During The Eukaryotic Cell Cycle? Mitotic and meiotic cell division in the human life cycle mitotic cell division, differentiation, and growth mitotic cell division, differentiation, and growth baby adults mitotic cell division, differentiation, and growth haploid diploid embryo fertilized egg egg fusion of gametes meiotic cell division in ovaries sperm meiotic cell division in testes Flash Fig. 8-8
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Mitosis is divided into four phases. Prophase Metaphase Anaphase Telophase Flash
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Interphase, prophase, and metaphase nuclear envelope chromatin nucleolus condensing chromosomes spindle pole spindle microtubules centriole pairs beginning of spindle formation spindle pole kinetochore (a) Late Interphase (b) Early Prophase (c) Late Prophase The (d) Metaphase Duplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered. Chromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs. nucleolus disappears; the nuclear envelope breaks down; spindle microtubules attach to the kinetochore of each sister chromatid. Kinetochores interact; spindle microtubules line up the chromosomes at the cell s equator. Fig. 8-9a d
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Anaphase, telophase, cytokinesis, and interphase unattached spindle microtubules chromosomes extending nuclear envelope re-forming (e) Anaphase Sister (f) Telophase One set of (g) Cytokinesis (h) Interphase of chromatids separate chromosomes reaches The cell divides in daughter cells Spindles and move to opposite each pole and relaxes two; each daughter disappear, intact nuclear poles of the cell; spindle into the extended state; cell receives one envelopes form, microtubules that are nuclear envelopes start nucleus and about chromosomes extend not attached to the to form around each set; half of the cytoplasm. completely, and the chromosomes push the spindle microtubles nucleolus reappears. poles apart. begin to disappear. Fig. 8-9e h
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During prophase, the chromosomes condense and are captured by the spindle microtubules. Three major events happen in prophase: The duplicated chromosomes condense. The spindle microtubules form. The chromosomes are captured by the spindle.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? The centriole pairs migrate with the spindle poles to opposite sides of the nucleus. When the cell divides, each daughter cell receives a centriole. Every sister chromatid has a structure called a kinetochore located at the centromere, which attaches to a spindle apparatus.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Prophase Fig. 8-9b c
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During metaphase, the chromosomes line up along the equator of the cell. At this phase, the spindle apparatus lines up the sister chromatids at the equator, with one kinetochore facing each cell pole.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Metaphase Fig. 8-9d
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During anaphase, sister chromatids separate and move to opposite poles of the cell. Sister chromatids separate, becoming independent daughter chromosomes. The kinetochores pull the chromosomes poleward along the spindle microtubules.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Anaphase Fig. 8-9e
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During telophase, nuclear envelopes form around both groups of chromosomes. Telophase begins when the chromosomes reach the poles. The spindle microtubules disintegrate and the nuclear envelop forms around each group of chromosomes.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Telophase Fig. 8-9f
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis occurs during telophase, separating each daughter nucleus into a separate cell that then begins interphase.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis Fig. 8-9g
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During cytokinesis, the cytoplasm is divided between two daughter cells. Microfilaments attached to the plasma membrane form a ring around the equator of the cell. During cytokinesis, the ring contracts and constricts the cell s equator. Eventually, the constriction divides the cytoplasm into two new daughter cells.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During cytokinesis, the cytoplasm is divided between two daughter cells. Microfilaments form a ring around the cell s equator. The microfilament ring contracts, pinching in the cell s waist. The waist completely pinches off, forming two daughter cells (a) Microfilaments contract, pinching the cell in two (b) Scanning electron micrograph of cytokinesis. Fig. 8-10
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis in plant cells is different than in animal cells. In plants, carbohydrate-filled vesicles bud off the Golgi apparatus and line up along the cell s equator between the two nuclei. The vesicles fuse, forming a cell plate. The carbohydrate in the vesicles become the cell wall between the two daughter cells.
8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis in a plant cell Fig. 8-11
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Meiosis is the production of haploid cells with unpaired chromosomes derived from diploid parent cells with paired chromosomes. Meiosis includes two nuclear divisions, known as meiosis I and meiosis II. In meiosis I, homologous chromosomes pair up, but sister chromatids remain connected to each other. In meiosis II, chromosomes behave as they do in mitosis sister chromatids separate and are pulled to opposite poles of the cell. Flash
8.6 How Does Meiotic Cell Division Produce Haploid Cells? paired homologous chromosomes recombined chromatids chiasma spindle microtubule kinetochores (a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I Duplicated chromosomes condense. Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts by crossing over. The nuclear envelope disintegrates, and spindle microtubules form. Paired homologous chromosomes line up along the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to the spindle microtubules via the kinetochore (blue). Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. Spindle microtubules disappear. Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II. Fig. 8-12a d
8.6 How Does Meiotic Cell Division Produce Haploid Cells? (e) Prophase II (f) Metaphase II (g) Anaphase II (h) Telophase II (i) If the chromosomes have relaxed after telophase I, they recondense. Spindle microtubules re-form and attach to the sister chromatids. The chromosomes line up along the equator, with sister chromatids of each chromosome attached to spindle microtubules that lead to opposite poles. The chromatids separate into independent daughter chromosomes, one former chromatid moving toward each pole. The chromosomes finish moving to opposite poles. Nuclear envelopes re-form, and the chromosomes become extended again (not shown here). Four haploid cells Cytokinesis results in four haploid cells, each containing one member of each pair of homologous chromosomes (shown here in the condensed state). Fig. 8-12e i
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Meiosis I separates homologous chromosomes into two haploid daughter nuclei. During prophase I, homologues pair up. The two homologues in a pair intertwine, forming chiasmata (singular, chiasma). At some chiasmata, the homologues exchange parts in a process known as crossing over.
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Prophase I Fig. 8-12a
8.6 How Does Meiotic Cell Division Produce Haploid Cells? During metaphase I, paired homologues line up at the equator of the cell. Interactions between the kinetochores and the spindle microtubules move the paired homologues to the equator of the cell.
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Metaphase I Fig. 8-12b
8.6 How Does Meiotic Cell Division Produce Haploid Cells? During anaphase I, homologous chromosomes separate. One duplicated chromosome (consisting of two sister chromatids) from each homologous pair moves to each pole of the dividing cell. At the end of anaphase I, the cluster of chromosomes at each pole contains one member of each pair of homologous chromosomes.
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Anaphase I Fig. 8-12c
8.6 How Does Meiotic Cell Division Produce Haploid Cells? After telophase I and cytokinesis, there are two haploid daughter cells. The spindle microtubules disappear and the nuclear envelope may reappear. Cytokinesis takes place and divides the cell into two daugher cells; each cell has only one of each pair of homologous chromosomes and is haploid. Each chromosome still has two sister chromatids.
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Telophase I Fig. 8-12d
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Meiosis II separates sister chromatids into four haploid daughter cells. Meiosis II is virtually identical to mitosis, although it occurs in haploid cells.
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Prophase II: the spindle microtubules reform Fig. 8-12e
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Metaphase II: duplicated chromosomes line up at the cell s equator Fig. 8-12f
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Anaphase II: sister chromatids move to opposite poles Fig. 8-12g
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Telophase II and cytokinesis: four haploid cells are formed Fig. 8-12h i
8.6 How Does Meiotic Cell Division Produce Haploid Cells? Flash
8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Ways to produce genetic variability from meiotic cell division and sexual reproduction Shuffling of homologues Crossing over Fusion of gametes
8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Shuffling of homologues creates novel combinations of chromosomes. There is a random assortment of homologues to daughter cells at meiosis I. At metaphase I, paired homologues line up at the cell s equator. Which chromosome faces which pole is random, so it is random as to which daughter cell will receive each chromosome.
8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Random separation of homologues during meiosis produces genetic variability. (a) The four possible chromosome arrangements at metaphase of meiosis I (b) The eight possible sets of chromosomes after meiosis I Fig. 8-13
8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Crossing over creates chromosomes with novel combinations of genetic material. Exchange of genetic material during prophase I, through crossing over, is a unique event each time. Genetic recombination through crossing over results in the formation of new combinations of genes on a given chromosome. As a result of genetic recombination, each sperm and each egg is genetically unique.
8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Crossing over sister chromatids of one duplicated homologue pair of homologous duplicated chromosomes chiasmata (sites of crossing over) parts of chromosomes that have been exchanged between homologues Fig. 8-14
8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Fusion of gametes creates genetically variable offspring. Because every egg and sperm are genetically unique, and it is random as to which sperm fertilizes which egg, every fertilized egg is also genetically unique.