CHAPTER 8 The Cellular Basis of Reproduction and Inheritance

CHAPTER 8The Cellular Basis of Reproduction and Inheritance Modules 8.1 – 8.3

How to Make a Sea Star — With and Without Sex • The life cycle of a multicellular organism includes • development • reproduction • This sea star embryo (morula) shows one stage in the development of a fertilized egg • The cluster of cells will continue to divide as development proceeds

This sea star is regenerating a lost arm • Regeneration results from repeated cell divisions • Some organisms can also reproduce asexually

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION • Cell division is at the heart of the reproduction of cells and organisms • Organisms can reproduce sexually or asexually

8.1 Like begets like, more or less • Some organisms make exact copies of themselves, asexual reproduction Figure 8.1A

Other organisms make similar copies of themselves in a more complex process, sexual reproduction Figure 8.1B

8.2 Cells arise only from preexisting cells • All cells come from cells • Cellular reproduction is called cell division • Cell division allows an embryo to develop into an adult • It also ensures the continuity of life from one generation to the next

8.3 Prokaryotes reproduce by binary fission • Prokaryotic cells divide asexually • These cells possess a single chromosome, containing genes • The chromosome is replicated • The cell then divides into two cells, a process called binary fission Prokaryotic chromosomes Figure 8.3B

Plasmamembrane Prokaryoticchromosome Cell wall • Binary fission of a prokaryotic cell Duplication of chromosomeand separation of copies Continued growth of the cell and movement of copies Division intotwo cells Figure 8.3A

THE EUKARYOTIC CELL CYCLE AND MITOSIS 8.4 The large, complex chromosomes of eukaryotes duplicate with each cell division • A eukaryotic cell has many more genes than a prokaryotic cell • The genes are grouped into multiple chromosomes, found in the nucleus • The chromosomes of this plant cell are stained dark purple Figure 8.4A

Individual chromosomes are only visibleduring cell division • They are packaged as chromatin • Chromosomes contain a very long DNA molecule with thousands of genes

Sister chromatids • Before a cell starts dividing, the chromosomes are duplicated • This process produces sister chromatids Centromere Figure 8.4B

Chromosomeduplication • Two daughter cells are produced • Each has a complete and identical set of chromosomes • When the cell divides, the sister chromatids separate Sister chromatids Centromere Chromosomedistributiontodaughtercells Figure 8.4C

8.5 The cell cycle multiplies cells • The cell cycle consists of two major phases: • Interphase, where chromosomes duplicate and cell parts are made • The mitotic phase, when cell division occurs Figure 8.5

8.6 Cell division is a continuum of dynamic changes • Eukaryotic cell division consists of two stages: • Mitosis • Cytokinesis

After the chromosomes coil up, a mitotic spindle moves them to the middle of the cell • In mitosis, the duplicated chromosomes are distributed into two daughter nuclei

INTERPHASE PROPHASE Centrosomes(with centriole pairs) Early mitoticspindle Centrosome Fragmentsof nuclearenvelope Kinetochore Chromatin Centrosome Spindlemicrotubules Nucleolus Nuclearenvelope Plasmamembrane Chromosome,consisting of twosister chromatids Figure 8.6

The process of cytokinesis divides the cell into two genetically identical cells • The sister chromatids then separate and move to opposite poles of the cell

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Cleavagefurrow Nucleolusforming Metaphaseplate Nuclearenvelopeforming Spindle Daughterchromosomes Figure 8.6 (continued)

8.7 Cytokinesis differs for plant and animal cells • In animals, cytokinesis occurs by cleavage • This process pinches the cell apart Cleavagefurrow Cleavagefurrow Contracting ring ofmicrofilaments Figure 8.7A Daughter cells

Cell plateforming Wall ofparent cell Daughternucleus • In plants, a membranous cell plate splits the cell in two Cell wall New cell wall Vesicles containingcell wall material Cell plate Daughtercells Figure 8.7B

8.8 Anchorage, cell density, and chemical growth factors affect cell division • Most animal cells divide only when stimulated, and others not at all • In laboratory cultures, most normal cells divide only when attached to a surface • They are anchorage dependent

This is called density-dependent inhibition • Cells continue dividing until they touch one another Cells anchor to dish surface and divide. When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the dish with a single layer and then stop (density-dependent inhibition). Figure 8.8A

Growth factors are proteins secreted by cells that stimulate other cells to divide After forming a single layer, cells have stopped dividing. Providing an additional supply of growth factors stimulates further cell division. Figure 8.8B

8.9 Growth factors signal the cell cycle control system • Proteins within the cell control the cell cycle • Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide G1 checkpoint Controlsystem M checkpoint Figure 8.9A G2 checkpoint

The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division Growth factor Plasma membrane Relayproteins G1 checkpoint Receptor protein Signal transduction pathway Cell cyclecontrolsystem Figure 8.8B

8.10 Connection: Growing out of control, cancer cells produce malignant tumors • Cancer cells have abnormal cell cycles • They divide excessively and can form abnormal masses called tumors • Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division

Malignant tumors can invade other tissues and may kill the organism Lymphvessels Tumor Glandulartissue Metastasis 1 A tumor grows from a single cancer cell. 2 Cancer cells invade neighboring tissue. 3 Cancer cells spread through lymph and blood vessels to other parts of the body. Figure 8.10

8.11 Review of the functions of mitosis: Growth, cell replacement, and asexual reproduction • When the cell cycle operates normally, mitotic cell division functions in: • Growth (seen here in an onion root) Figure 8.11A

Deadcells • Cell replacement (seen here in skin) Epidermis, the outer layer of the skin Dividingcells Dermis Figure 8.11B

Asexual reproduction (seen here in a hydra) Figure 8.11C

MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs • Somatic cells of each species contain a specific number of chromosomes • Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Centromere Sister chromatids Figure 8.12

8.13 Gametes have a single set of chromosomes • Cells with two sets of chromosomes are said to be diploid • Gametes are haploid, with only one set of chromosomes

Repeated mitotic divisions lead to the development of a mature adult • The adult makes haploid gametes by meiosis • All of these processes make up the sexual life cycle of organisms • At fertilization, a sperm fuses with an egg, forming a diploid zygote

Haploid gametes (n = 23) Egg cell • The human life cycle Sperm cell MEIOSIS FERTILIZATION Diploidzygote (2n = 46) Multicellulardiploid adults (2n = 46) Mitosis anddevelopment Figure 8.13

8.14 Meiosis reduces the chromosome number from diploid to haploid • Meiosis, like mitosis, is preceded by chromosome duplication • However, in meiosis the cell divides twice to form four daughter cells

While they are paired, they cross over and exchange genetic information • The homologous pairs are then separated, and two daughter cells are produced • In the first division, meiosis I, homologous chromosomes are paired

MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes(withcentriolepairs) Microtubules attached tokinetochore Metaphaseplate Sister chromatidsremain attached Sites of crossing over Spindle Nuclearenvelope Sisterchromatids Tetrad Centromere(with kinetochore) Homologouschromosomes separate Chromatin Figure 8.14, part 1

The sister chromatids of each chromosome separate • The result is four haploid daughter cells • Meiosis II is essentially the same as mitosis

MEIOSIS II: Sister chromatids separate TELOPHASE IAND CYTOKINESIS TELOPHASE IIAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavagefurrow Sister chromatidsseparate Haploiddaughter cellsforming Figure 8.14, part 2

8.15 Review: A comparison of mitosis and meiosis • For both processes, chromosomes replicate only once, during interphase

MITOSIS MEIOSIS PARENT CELL(before chromosome replication) Site ofcrossing over MEIOSIS I PROPHASE I Tetrad formedby synapsis of homologous chromosomes PROPHASE Chromosomereplication Chromosomereplication Duplicatedchromosome(two sister chromatids) 2n = 4 Chromosomes align at the metaphase plate Tetradsalign at themetaphase plate METAPHASE I METAPHASE ANAPHASE I TELOPHASE I ANAPHASETELOPHASE Sister chromatidsseparate duringanaphase Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together Haploidn = 2 Daughtercells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cellsof mitosis n n n n Daughter cells of meiosis II Figure 8.15

8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring • Each chromosome of a homologous pair comes from a different parent • Each chromosome thus differs at many points from the other member of the pair

The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes • Random fertilization also increases variation in offspring

POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

8.17 Homologous chromosomes carry different versions of genes • The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

Coat-color genes Eye-color genes C E Brown Black C E C E c e c e c e White Pink Tetrad in parent cell(homologous pair ofduplicated chromosomes) Chromosomes ofthe four gametes Figure 8.17A, B

8.18 Crossing over further increases genetic variability • Crossing over is the exchange of corresponding segments between two homologous chromosomes • Genetic recombination results from crossing over during prophase I of meiosis • This increases variation further

CHAPTER 8 The Cellular Basis of Reproduction and Inheritance