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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 Lecture 10 The Cellular Basis of Reproduction and Inheritance 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 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): 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 (Prokarytoes) 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 Eukaryotic Cell Division • Eukaryotic cell division is more complex and time consuming process than binary fission • Features of Eukaryotic 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 Found only during cell division Tightly packaged DNA DNA is not being used for macromolecule synthesis. Chromatin Found throughout cell cycle Unwound DNA DNA is being used for macromolecule synthesis. Eucaryotic Chromosomes Duplicate Before Each Cell Division Cell Cycle of Eucaryotic Cells • The Cell Cycle – is the sequence of events from the time a cell is formed, until the cell divides once again. • Before cell division, the cell must: a) precisely copy genetic material (DNA) b) roughly double its cytoplasm c) synthesize organelles, membranes, proteins, and other molecules. • The 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 Eukaryotic 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 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. Mitosis 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 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 . Malignant tumor: Cancer cells have ability to “detach” from tumor and spread to other organs or tissues 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 Eukaryotes 1. Growth: All 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 Chromosome -1 • • • • Chromosomes are structures that contain information 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 Chromosomes: Code for the Same Genetic Traits, but Have Different Alleles Chromosomes-2 • There are two types of chromosomes • Autosomes - Found in both males and females. In humans there are 22 pairs of autosomes. Autosomes are of the same size and are homologous. • Sex Chromosomes (X and Y)- Determine an individual’s gender. • 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. 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. E.g 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. Chromosomes-3 • Humans have two sets of chromosomes. • One inherited from each parent. • Diploid Cells: Cells whose nuclei contain two homologous sets of chromosomes (2n). Almost all cells in our body are diploid . In humans the diploid number (2n) is 46. • Haploid Cells: Cells whose nuclei contain a single set of chromosomes (n). Egg and sperm cells are haploid. In humans the haploid number (n) is 23. • Fertilization: Haploid egg fuses with a haploid sperm to form a diploid zygote (fertilized egg). Mitosis versus Meiosis Mitosis Meiosis One cell division Produces two (2) cells Produces diploid cells Daughter cells are genetically Two successive cell divisions Produces four (4) cells Produces haploid gametes Cells are genetically identical to mother cell different from mother cell and each other No crossing over Crossing over* Functions: Growth, Functions: Reproduction cell replacement *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. 2. Prophase I: (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. 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 Significance of Meiosis • Sexual Reproduction – if meiosis did not occur the fusion of gametes would lead to double the number if chromosomes, each generation. • Genetic Variation – Opportunity for new combination of genes in gametes.