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Chapter 8: The Cellular Basis of Reproduction and Inheritance Introduction Stages of an Organism’s Life Cycle: Development: All changes that occur from a fertilized egg or an initial cell to an adult organism. Reproduction: Production of offspring that carry genetic information in the form of DNA, from their parents. Two types of reproduction: 1. Sexual Reproduction 2. Asexual Reproduction Types of reproduction: 1. Sexual Reproduction: Most common type of animal reproduction. Male and female gametes or sex cells (sperm and egg cell) join together to create a fertilized egg or zygote. The offspring has genetic information from both parents. Offspring are genetically different from each parents and their siblings. Advantages: Ensures genetic diversity of offspring. • Population more likely to survive changing environment. Disadvantages: Cannot reproduce without a partner of opposite sex. Considerable time, energy, and resources spent to find a suitable mate. Parents only pass on 1/2 (50%) of their genetic information to each offspring. 2. Asexual Reproduction: Production of offspring by a single parent through: Splitting: Binary fission in bacteria. Budding: Yeasts, plants Fragmentation: Sea stars Parthenogenesis: “Virgin birth”. Several insect species. Offspring inherit DNA form one parent only. Offspring are genetically identical to parent and siblings, unless mutations occur. Advantages: Can reproduce without a partner of opposite sex. Don’t spend time, energy, and resources to find a suitable mate. Parents pass on 100% of their genetic information to each offspring. Disadvantage: No genetic diversity of offspring. • Population less likely to survive changing environment. Cells Only Arise from Preexisting Cells New cells are made through cell division: Unicellular organisms (Bacteria, protozoa): Division of one cell into two new organisms through binary fission or mitosis. Multicellular organisms (Plants, animals): If sexual reproduction: 1. Growth and development from zygote or fertilized egg. Original cell divides by mitosis to produce many cells, that are genetically identical to first cell. Cells later develop specific functions (differentiation). 2. Reproduction requires: Meiosis: Special type of cell division that will generate gametes or sex cells, with 50% of individual’s genetic material. Bacteria (Procarytoes) Reproduce Asexually by Binary Fission Features of Bacterial DNA Single, relatively small circular chromosome: About 3-5 million nucleotide base pairs Contains only about 5-10,000 genes Binary fission Single circular DNA is replicated Bacterium grows to twice normal size Cell divides into two daughter cells Each daughter cell with an identical copy of DNA Rapid process, as little as 20 minutes. Bacteria Reproduce Asexually by Binary Fission Eucaryotic cell division is a more complex and time consuming process than binary fission Features of Eucaryotic DNA 1. DNA is in multiple linear chromosomes. Unique number for each species: • Humans have 46 chromosomes. • Cabbage has 20, mosquito 6, and fern over 1000. 2. Large Genome: Up to 3 billion base pairs (humans) Contains up to 50,000-150,000 genes Human genome project is determining the sequence of entire human DNA. 3. DNA is enclosed by nuclear membrane. Correct distribution of multiple chromosomes in each daughter cell requires a much more elaborate process than binary fission. Human Body Cells Have 46 Chromosomes DNA: Found as Chromosomes or Chromatin Chromosomes Tightly packaged DNA Found only during cell division DNA is not being used for macromolecule synthesis. Chromatin Unwound DNA Found throughout cell cycle DNA is being used for macromolecule synthesis. Eucaryotic Chromosomes Duplicate Before Each Cell Division Cell Cycle of Eucaryotic Cells Sequence of events from the time a cell is formed, until the cell divides once again. Before cell division, the cell must: Precisely copy genetic material (DNA) Roughly double its cytoplasm Synthesize organelles, membranes, proteins, and other molecules. Cell cycle is divided into two main phases: Interphase: Stage between cell divisions Mitotic Phase: Stage when cell is dividing Eucaryotic Cell Cycle: Interphase + Mitotic Phase The Life Cycle of a Eucaryotic Cell: Interphase: Time between cell divisions. Most cells spend about 90% of their time in interphase. Cells actively synthesize materials they need to grow. Chromosomes are duplicated. Interphase can be divided into three stages: 1. G1 phase: Just after cell division. Cell grows in size, increases number of organelles, and makes proteins needed for DNA synthesis. 2. S phase: DNA replication. Single chromosomes are duplicated so they contain two sister chromatids. 3. G2 phase: Just before cell division. Protein synthesis increases in preparation for cell division. Duplication of Chromosomes During S stage of Interphase DNA replication during S stage of Interphase Single chromosome Two identical sister chromatids joined by a centromere ( ) The Life Cycle of a Eucaryotic Cell: Mitosis: The process of eucaryotic cell division. Most cells spend less than 10% of time in mitosis. Mitosis is divided into four stages: 1. Prophase: Cell prepares for division. 2. Metaphase: Chromosomes line up in “middle” of cell. 3. Anaphase: Sister chromatids split and migrate to opposite sides of the cell. 4. Telophase: DNA is equally divided into two new daughter cells. Cytokinesis usually occurs. Cytokinesis: Division of cytoplasm. Mitotic Phase: Mitosis + Cytokinesis Mitotic Phase: Mitosis + Cytokinesis Mitosis: The Stages of Cell Division 1. Prophase Chromatin condenses into chromosomes, which appear as two sister chromatids joined by a centromere. Nucleoli disappear. Nuclear envelope breaks apart. In animal cells, mitotic spindle begins to form as mictotubules grow out of two centrosomes or microtubule organizing centers (MTOCs). • Each centrosome is made up of a pair of centrioles. Microtubules attach to kinetochores on chromatids and begin to move chromosomes towards center of cell. Centrosomes begin migrating to opposite poles of cell. Interphase and Prophase of Mitosis in Animal Cell Mitosis: The Stages of Cell Division 2. Metaphase Short period in which chromosomes line up along equatorial plane of cell (metaphase plate). Chromosomes are completely condensed and easy to visualize. Mitotic spindle is fully formed. Kinetochores of sister chromatids face opposite sides and are attached to spindle microtubules at opposite ends of the cell. Metaphase, Anaphase, and Telophase of Mitosis in an Animal Cell Mitosis: The Stages of Cell Division 3.Anaphase Centromeres of sister chromatids begin to separate. Each chromatid is now an independent daughter chromosome. The separate chromosomes are pulled toward opposite ends by spindle microtubules, attached to the kinetochores. Cell elongates as poles move farther apart. Anaphase ends when a complete set of chromosomes reaches each pole. Mitosis: The Stages of Cell Division 4. Telophase Cell continues to elongate. Cell returns to interphase conditions: • A nuclear envelope forms around each set of chromosomes. • Chromosomes uncoil, becoming chromatin threads. • Nucleoli reappear. • Spindle microtubules disappear. Cytokinesis usually occurs at the end of this stage Mitotic Phase: Mitosis + Cytokinesis Cytokinesis The division of cytoplasm to produce two daughter cells. Usually begins during telophase. • In animal cells: Division is accomplished by a cleavage furrow that encircles the cell like a ring in the equator region. • In plant cells: Division is accomplished by the formation of a cell plate between the daughter cells. Each cell produces a plasma membrane and a cell wall on its side of the plate. Cytokinesis in Animal and Plant Cells Animal Cell Plant Cell External Factors Control Mitosis 1. Anchorage Most cells cannot divide unless they are attached to a solid surface. May prevent inappropriate growth of detached cells 2. Nutrients and growth factors Lack of nutrients can limit mitosis Growth factors: Proteins that stimulate cell division. 3. Cell density Density-dependent inhibition: Cultured cells will stop dividing after a single layer covers the petri dish. Mitosis is inhibited by high cell density. Cancer cells do not demonstrate density inhibition Density Dependent Inhibition of Mitosis Normal Cells Stop Dividing at High Cell Density Cancer Cells are Not Inhibited by High Cell Density Cell-Cycle Control System There are three critical points at which the cell cycle is controlled*: 1. G1 Checkpoint: Prevents cell from entering S phase and duplicating DNA. Most important checkpoint. Amitotic cells (muscle and nerve cells) are frozen here. 2. G2 Checkpoint: Prevents cell from entering mitosis. 3. M Checkpoint: Prevents cell from entering cytokinesis. *Cells must have proper growth factors to get through each checkpoint. Cell Division is Controlled at Three Key Stages Growth factors are required to pass each checkpoint Cancer is a Disease of the Cell Cycle Cancer kills 1 in 5 people in the United States. Cancer cells divide excessively and invade other body tissues. Tumor: Abnormal mass of cells that originates from uncontrolled mitosis of a single cell. Benign tumor: Cancer cells remain in original site. Can easily be removed or treated tumor: Cancer cells have ability to “detach” from tumor and spread to other organs or tissues Malignant Metastasis: Spread of cancer cells form site of origin to another organ or tissue. Tumor cells travel through blood vessels or lymph nodes. Metastasis: Cancer Cells Spread Throughout Body Functions of Mitosis in Eucaryotes: 1. Growth: All somatic cells that originate after a new individual is created are made by mitosis. 2. Cell replacement: Cells that are damaged or destroyed due to disease or injury are replaced through mitosis. 3. Asexual Reproduction: Mitosis is used by organisms that reproduce asexually to make offspring. Mitosis Replaces Dead Skin Cells Chromosomes are matched in homologous pairs Homologous Chromosomes: Eucaryotic chromosomes come in pairs. Normal humans have 46 chromosomes in 23 pairs. One chromosome of each pair comes from an individual’s mother, the other comes from the father. Homologous chromosomes carry genes that control the same characteristics. Examples: Eye color, blood type, flower color, or height Locus: Physical site on a chromosomes where a given gene is located. Allele: Different forms of the same gene. Example: Alleles for blood types A, B, or O. Homologous Pair of Chromosomes: One Comes From Each Parent Homologous Chromosomes: Code for the Same Genetic Traits, but Have Different Alleles There are two types of chromosomes: 1. Autosomes: Found in both males and females. In humans there are 22 pairs of autosomes. Autosomes are of the same size and are homologous. 2. Sex Chromosomes: Determine an individual’s gender. One pair of chromosomes (X and Y). The X and Y chromosomes are not homologous. The X chromosome is much larger than the Y chromosome and contains many genes. The Y chromosome has a small number of genes. In Humans and other mammals females are XX and males are XY. Chromosomes of Normal Human Male: 44 (22 Pairs) Autosomes + XY Normal Genetic Complement of Humans: Females: 44 autosomes (22 pairs) + XX Males: 44 autosomes (22 pairs) + XY Note: In most cases, having additional or missing chromosomes is usually fatal or causes serious defects. Down’s syndrome: Trisomy 21. Individual’s with an extra chromosome 21. Most common chromosomal defect (1 in 700 births in U.S.). Mental retardation, mongoloid facial features, heart defects, etc. Gametes have a single set of chromosomes Humans have two sets of chromosomes, one inherited from each parent. Diploid Cells: Cells whose nuclei contain two homologous sets of chromosomes (2n). Somatic cells are diploid (almost all cells in our body). In humans the diploid number (2n) is 46. Haploid Cells: Cells whose nuclei contain a single set of chromosomes (n). Gametes are haploid (egg and sperm cells). In humans the haploid number (n) is 23. Fertilization: Haploid egg fuses with a haploid sperm to form a diploid zygote (fertilized egg). Meiosis Produces Haploid Gametes From Diploid Parents Fertilization Produces Diploid Offspring from Haploid Gametes Mitosis versus Meiosis Mitosis Meiosis One cell division Two successive cell divisions Produces two (2) cells Produces four (4) cells Produces diploid cells Produces haploid gametes Daughter cells are genetically Cells are genetically different from identical to mother cell mother cell and each other No crossing over Crossing over* Functions: Growth, Functions: Sexual reproduction cell replacement, and asexual reproduction *Crossing over: Exchange of DNA between homologous chromosomes. Meiosis: Generates haploid gametes Reduces the number of chromosomes by half, producing haploid cells from diploid cells. Also produces genetic variability, each gamete is different, ensuring that two offspring from the same parents are never identical. Two divisions: Meiosis I and meiosis II. Chromosomes are duplicated in interphase prior to Meiosis I. Meiosis I: Separates the members of each homologous pair of chromosomes. Reductive division. Meiosis II: Separates chromatids into individual chromosomes. STAGES OF MEIOSIS Interphase: Chromosomes replicate Meiosis I: Reductive division. Homologous chromosomes separate Meiosis II: Sister chromatids separate Meiosis I: Separation of Homologous Chromosomes 1. Prophase I: Most complex phase of meiosis (90% of time) Chromatin condenses into chromosomes. Nuclear membrane and nucleoli disappear. Centrosomes move to opposite poles of cell and microtubules attach to chromatids. Synapsis: Homologous chromosomes pair up and form a tetrad of 4 sister chromatids. Crossing over: DNA is exchanged between homologous chromosomes, resulting in genetic recombination. Unique to meiosis. Chiasmata: Sites of DNA exchange. Prophase I: Crossing Over Between Homologous Chromosomes Meiosis I: Separation of Homologous Chromosomes 2. Metaphase I: Chromosome tetrads (homologous chromosomes) line up in the middle of the cell. Each homologous chromosome faces opposite poles of the cell. Meiosis I: Homologous Chromosomes Separate Stages of Meiosis: Meiosis I 3. Anaphase I: Chromosome tetrads split up. Homologous chromosomes of each pair separate, moving towards opposite poles. Random assortment: One chromosome from each homologous pair is shuffled into the two daughter cells, randomly and independently of the other pairs. Random assortment increases genetic diversity of offspring. Possible combinations: 2n. One human cell can generate 223 or over 8.3 million different gametes by random assortment alone. Random Assortment of Homologous Chromosomes During Meiosis I Generates Many Possible Gametes Meiosis I: Separation of Homologous Chromosomes 4. Telophase I and Cytokinesis: Chromosomes reach opposite poles of the cell. Nucleoli reorganize, chromosomes uncoil, and cytokinesis occurs. New cells are haploid. Meiosis II: Separation of Sister Chromatids During interphase that follows meiosis I, no DNA replication occurs. Interphase may be very brief or absent. Meiosis II is very similar to mitosis. 1. Prophase II: Very brief, chromosomes reform. No crossing over or synapsis. Spindle forms and starts to move chromosomes towards center of the cell. Meiosis II: Separation of Sister Chromatids 2. Metaphase II: Very brief, individual chromosomes line up in the middle of the cell. Kinetochores of chromatids face opposite poles. 3. Anaphase II: Chromatids separate and move towards opposite ends of the cell. Meiosis II: Separation of Sister Chromatids Meiosis II: Separation of Sister Chromatids 4. Telophase II: Nuclei form at opposite ends of the cell. Cytokinesis occurs. Product of meiosis: Four (4) haploid gametes, each genetically different from the other. Meiosis Produces Four Genetically Different Gametes Meiosis in Males and Females Spermatogenesis: Four sperm cells are made. Starts in puberty and occurs continuously. Males produce millions of sperm cells a month. Oogenesis: Only one large egg is produced. The other three cells are small polar bodies. Oogenesis starts before birth in females, stops at Prophase I, and resumes during puberty. Meiosis is completed only after fertilization. Females make one mature egg/month. Mitosis versus Meiosis (Review) Mitosis Meiosis One cell division Two successive cell divisions Produces two (2) cells Produces four (4) cells Produces diploid cells Produces haploid gametes Daughter cells are genetically Cells are genetically different from identical to mother cell mother cell and each other No crossing over Crossing over* Functions: Growth, Functions: Sexual reproduction cell replacement, and asexual reproduction *Crossing over: Exchange of DNA between homologous chromosomes. Crossing Over in Meiosis Increases Genetic Diversity Sources of Genetic Variability in Sexual Reproduction 1. Crossing Over: After crossing over and synapsis, sister chromatids are no longer identical. 2. Independent Assortment: Each human can produce over 8.3 million different gametes by random shuffling of chromosomes in meiosis I. 3. Fertilization: A couple can produce over 64 trillion (8.3 million x 8.3 million) different zygotes during fertilization. This figure does not take into account diversity created by crossing over. Accidents During Meiosis Can Cause Chromosomal Abnormalities Nondisjunction: Chromosomes fail to separate. Members of a pair of homologous chromosomes fail to separate during meiosis I or: Sister chromatids fail to separate during meiosis II. Nondisjunction increases with age. Gametes (and zygotes) will have an extra chromosome, others will be missing a chromosome. Trisomy: Individuals with one extra chromosome, three instead of pair. Have 47 chromosomes in cells. Monosomy: Missing a chromosome, one instead of pair. Have 45 chromosomes in cells. Nondisjunction of Chromosomes During Meiosis Produces Abnormal Gametes Accidents During Meiosis Can Result in a Trisomy or Monosomy Most abnormalities in numbers of autosomes are very serious or fatal. Down’s syndrome: Caused by a trisomy of chromosome number 21 (1 in 700 births). Mental retardation, mongoloid features, and heart defects. Most abnormalities of sex chromosomes do not affect survival. Klinefelter Syndrome: Males with an extra sex chromosome (XXY) (1 in 1000 male births). Turner Syndrome: Females missing one sex chromosome (XO) (1 in 2500 female births). Down’s Syndrome is More Common in Children Born to Older Mothers Abnormal Numbers of Sex Chromosomes Usually Do Not Affect Survival Klinefelter Syndrome (XXY) Incidence: 1:1000 male births Turner Syndrome (XO) Incidence: 1 in 2500 female births