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Chapter 9

Chapter 9. The Continuity of Life: Cellular Reproduction. Why Do Cells Divide?. Cells reproduce by cell division , in which a parent cell normally gives rise to two daughter cells

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Chapter 9

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  1. Chapter 9 • The Continuity of Life: Cellular Reproduction

  2. Why Do Cells Divide? • Cells reproduce by cell division, in which a parent cell normally gives rise to two daughter cells • Each daughter cell receives a complete set of hereditary information from the parent cell and about half its cytoplasm • The hereditary information is usually identical with that of the parent cell

  3. Why Do Cells Divide? • Cell division transmits hereditary information to each daughter cell • deoxyribonucleic acid (DNA) - hereditary information • Polymer of nucleotides • Phosphate • a sugar (deoxyribose) • one of four bases • adenine (A), thymine (T), guanine (G), cytosine (C) • Chromosome - DNA in a double helix

  4. The Structure of DNA phosphate nucleotide T A base A sugar C G G C C G C A T A C G A T T A T Fig. 9-1 (a) A single strand of DNA (b) The double helix

  5. Why Do Cells Divide? • Cell division is required for growth and development • Mitotic cell division - The cell division of eukaryotic cells by which organisms grow or increase in number • After cell division, the daughter cells may differentiate, becoming specialized for specific functions • Cell cycle - The repeating pattern of divide, grow, and differentiate, then divide again • Most multicellular organisms have three categories of cells • Stem cells • Other cells capable of dividing • Permanently differentiated cells

  6. Why Do Cells Divide? • Stem cells • Two important characteristics: • self-renewal • retain the ability to divide • one daughter remains a stem cell, thus continuing the line; the other daughter undergoes several divisions • the ability to differentiate into a variety of cell types • Stem cells include most of the daughter cells formed by the first few cell divisions of a fertilized egg, as well as a few adult cells

  7. Why Do Cells Divide? • Other cells capable of dividing • typically differentiate into only one or two different cell types • Dividing liver cells, for example, can only become more liver cells

  8. Why Do Cells Divide? • Cell division is required for sexual and asexual reproduction • Sexual reproduction- eukaryotic organisms occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults • Asexual reproduction - Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg

  9. Cell Division Enables Asexual Reproduction Fig. 9-2

  10. The Prokaryotic Cell Cycle • The DNA is contained in a single, circular chromosome about a millimeter or two in circumference • Contained in a nucleoid

  11. The Prokaryotic Cell Cycle cell division by prokaryotic fission cell growth and DNA replication (a) The prokaryotic cell cycle New plasma membrane is added between the attachment points, pushing them farther apart. 3 attachment site cell wall circular DNA plasma membrane 1 The circular DNA double helix is attached to the plasma membrane at one point. 4 The plasma membrane grows inward at the middle of the cell. 2 The DNA replicates and the two DNA double helices attach to the plasma membrane at nearby but separate points. 5 The parent cell divides into two daughter cells. Fig. 9-3 (b) Prokaryotic fission

  12. DNA in Eukaryotic Chromosomes • Differ from prokaryotic chromosomes: • membrane-bound nucleus • always have multiple chromosomes (2-1200) • longer and have more DNA (human chromosomes are 10 to 80 times longer and have 10 to 50 times more DNA)

  13. DNA in Eukaryotic Chromosomes • A linear DNA double helix bound to proteins • Each human chromosome contains a single DNA double helix, about 50 million to 250 million nucleotides long • Most of the time, the DNA in each chromosome is wound around proteins called histones 1 DNA double helix histone proteins 2 DNA wound around histone proteins 3 Coiled DNA/histone beads 4 Loops attached to a protein scaffold; this stage of partial condensation typically occurs in a nondividing cell protein scaffold 5 Folded chromosome, fully condensed in a dividing cell

  14. DNA in Eukaryotic Chromosomes • Genes are segments of the DNA of a chromosome • Units of inheritance • Sequences of DNA from hundreds to thousands of nucleotides long • Each gene occupies a specific place, or locus (plural, loci) on a chromosome

  15. DNA in Eukaryotic Chromosomes • In addition to genes, every chromosome has specialized regions that are crucial to its structure and function: • Two telomeres • Protective caps on each end of a chromosome • Essential for chromosome stability • One centromere • Temporarily holds two daughter DNA double helices together after DNA replication • Is the attachment site for microtubules that move the chromosomes during cell division

  16. The Principal Features of a Eukaryotic Chromosome During Cell Division gene loci centromere telomeres (a) A eukaryotic chromosome before DNA replication duplicated chromosome (two DNA double helices) sister chromatids centromere (b) A eukaryotic chromosome after DNA replication independent daughter chromosomes, each with one identical DNA double helix (c) Separated sister chromatids become independent chromosomes Fig. 9-5

  17. DNA in Eukaryotic Chromosomes • Eukaryotic chromosomes usually occur in pairs with similar genetic information • Karyotype - an entire set of stained chromosomes from a single cell sex chromosomes

  18. DNA in Eukaryotic Chromosomes • These similarities occur because each chromosome in a pair carries the same genes arranged in the same order • Chromosomes that contain the same genes are called homologous chromosomes, or homologues • Cells with pairs of homologous chromosomes are called diploid, which means “double” 2n • Egg and sperm cells don’t have both pairs of chromosomes: called haploid n

  19. DNA in Eukaryotic Chromosomes • Homologous chromosomes are usually not identical • Diploid = 23 pairs of chromosomes, for a total of 46 • Autosomes - 22 pairs of chromosomes • Sex chromosomes - 1 pair and are different in the male and the female • Female - two X chromosomes that usually look similar • Male - an X and a Y chromosome that appear very different • However, in a male, the X and Y chromosomes behave as a pair during meiotic cell division

  20. The Eukaryotic Cell Cycle • Interphase, a cell grows in size, replicates its DNA, and often differentiates • Three phases: • G1 (growth phase 1) • Growth • Differentiates • Decides to divide (growth factors) • S(synthesis phase) • G2(growth phase 2) cytokinesis anaphase telophase and metaphase prophase G1: cell growth and differentiation mitotic cell division G2: cell growth and preparation for cell division interphase S: synthesis of DNA; chromosomes are duplicated

  21. The Eukaryotic Cell Cycle • During interphase, a cell grows in size, replicates its DNA, and often differentiates • Permanently differentiated cells are stuck in interphase. • Without enough cell divisions at the right time and in the right organs, development falters or body parts fail to replace worn-out or damaged cells • With too many cell divisions, cancers may form

  22. The Eukaryotic Cell Cycle • There are two types of cell division in eukaryotic cells • Mitotic cell division (mitosis) • Meiotic cell division (meiosis)

  23. The Eukaryotic Cell Cycle • Mitotic cell division • During mitosis (nuclear division), the nucleus of the cell and the chromosomes divide • Each daughter nucleus receives one copy of each of the replicated chromosomes of the parent cell • During cytokinesis (cytoplasmic division), the cytoplasm is divided roughly equally between the two daughter cells, and one daughter nucleus enters each of the daughter cells

  24. Mitotic Cell Division • Mitotic cell division takes place in all types of eukaryotic organisms • It is the mechanism of asexual reproduction • Mitotic cell division followed by differentiation of the daughter cells allows a fertilized egg to grow into an adult with perhaps trillions of specialized cells • It allows organisms to maintain, repair, and even regenerate body parts • It is the mechanism whereby stem cells reproduce

  25. Mitotic Cell Division • Four phases followed by cytokinesis • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis duplicated chromosome (two DNA double helices) sister chromatids centromere (b) A eukaryotic chromosome after DNA replication

  26. Mitotic Cell Division • Prophase • Spindlemicrotubules form • Pairs of centrioles, which serve as loci from which spindle microtubules form, begin to migrate to opposite sides of the cell, to regions called spindle poles • The spindle microtubules radiate from the poles, both toward the nucleus, forming a basket around it and outward toward the plasma membrane • The nuclear envelop disintegrates, releasing the duplicated chromosomes

  27. Mitotic Cell Division in an Animal Cell INTERPHASE MITOSIS nuclear envelope chromatin spindle pole condensing chromosomes nucleolus spindle microtubules kinetochore centriole pairs beginning of spindle formation kinetochore microtubules spindle pole (a) Late Interphase Duplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered. (b) Early Prophase Chromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs. (c) Late Prophase The nucleolus disappears; nuclear envelope breaks down; some spindle microtubules attach to the kinetochore (blue) of each sister chromatid. (d) Metaphase Kinetochore microtubules line up the chromosomes at the cell's equator.

  28. Mitotic Cell Division in an Animal Cell INTERPHASE polar microtubules chromosomes extending nuclear envelope re-forming nucleolus reappearing (f) Telophase One set of chromosomes reaches each pole & begins to decondense; nuclear envelopes start to form; nucleoli begin to reappear; spindle microtubules begin to disappear; microfilaments form rings around the equator. (g) Cytokinesis The ring of microfilaments contracts, dividing the cell in two; each daughter cell receives one nucleus and about half of the cytoplasm. (e) Anaphase Sister chromatids separate & move to opposite poles of the cell; polar microtubules push the poles apart. (h) Interphase of daughter cells Spindles disappear, intact nuclear envelopes form, and the chromosomes extend completely.

  29. Cytokinesis in a Plant Cell Fig. 9-10

  30. How Is the Cell Cycle Controlled? • The cells of some tissues, such as skin and intestines, divide frequently throughout the lifespan of an organism • Cell division occurs rarely or not at all in other tissues, such as brain, heart, and skeletal muscles • Cell division in eukaryotes is driven by enzymes and controlled at specific checkpoints

  31. Why Do So Many Organisms Reproduce Sexually? • Sexual reproduction is the prevalent form of reproduction • Asexual reproduction by mitosis produces genetically identical offspring • Sexual reproduction by meiosis shuffles the genes to produce genetically unique offspring • Two parents, each with a different advantageous trait (allele), can combine those traits in one individual (their offspring) through sexual reproduction

  32. Why Do So Many Organisms Reproduce Sexually? • Genetic variability among organisms is essential for survival in a changing environment • Mutations produce new variation but are relatively rare occurrences • The genetic variability that occurs from one generation to the next results almost entirely from meiosis and sexual reproduction • Gametes from two humans could produce about 64 trillion different combinations

  33. The Eukaryotic Cell Cycle • Meiotic cell division • occurs in animal ovaries and testes • prerequisite for sexual reproduction in all eukaryotic organisms • Meiotic cell division involves a specialized nuclear division called meiosis, and two rounds of cytokinesis • Two divisional steps produce four daughter cells that can become haploid gametes • Each gamete receives one homologue of each pair of chromosomes

  34. Meiosis Is a Reduction Division That Halves the Number of Chromosomes sister chromatids homologous chromosomes (a) Replicated homologues prior to meiosis (b) After meiosis I (c) After meiosis II Fig. 9-13

  35. Meiotic Cell Division • Fusion of gametes keeps the chromosome number constant between generations n 2n meiotic cell division 2n n 2n fertilization diploid parental cells haploid gametes diploid fertilized egg

  36. Meiotic Cell Division • Prophase I, homologous chromosomes pair up and exchange DNA • Crossing over • If the exchanged segments carry different traits, genetic recombination has occurred

  37. Meiotic Cell Division in an Animal Cell MEIOSIS I Fig. 9-15a, b, c, d paired homologous chromosomes recombined chromatids chiasma spindle microtubule kinetochores (a) Prophase I Duplicated chromosomes condense. Homologous chromosomes pair up and chromatids of homologues exchange parts by crossing over. The nuclear envelope disintegrates, and spindle microtubules form. (b) Metaphase I 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. (c) Anaphase I Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. (d) Telophase I Spindle microtubules disappear. Two clusters of chromosomes have formed. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II.

  38. Meiotic Cell Division in an Animal Cell Fig. 9-15e, f, g, h, i MEIOSIS II (f) Metaphase II The chromosomes line up along the equator. (e) Prophase II Spindle microtubules re-form and attach to the sister chromatids. (i) Four haploid cells result from Cytokinesis each containing one member of each pair of homologous chromosomes. (g) Anaphase II The chromatids separate into independent daughter chromosomes, one chromatid moving toward each pole. (h) Telophase II Nuclear envelopes re-form, and the chromosomes decondense.

  39. The Three Types of Eukaryotic Life Cycles mitotic cell division and growth or asexual reproduction mitotic cell division, differentiation, and growth meiotic cell division multicellular diploid adult spore n 2n n n n mitotic cell division, differentiation, and growth n n mitotic cell division, differentiation, and growth multicellular diploid adults meiotic cell division multicellular haploid adult 2n n 2n 2n 2n 2n n meiotic cell division n zygote zygote zygote n fusion of gametes fusion of gametes fusion of n n n gametes gametes gametes gametes (a) Haploid life cycle (protists, algae, fungi) (b) Diploid life cycle (animals) (c) Alternation of generations (plants) haploid (n) stages diploid (2n) stages Fig. 9-17

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