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Chromosomes and Cellular Reproduction Chapter 6 Prokaryotic Cell Reproduction •Prokaryotes reproduce via Binary Fission, occurs in two stages: 1. DNA is replicated and prokaryote adds a new cell membrane between the two copies (FtsZ ring). • "Filamenting temperature-sensitive mutant Z” Prokaryotic Cell Reproduction •Prokaryotes reproduce via Binary Fission, occurs in two stages: 1. DNA is replicated and prokaryote adds a new cell membrane between the two copies (FtsZ ring). • 2. "Filamenting temperature-sensitive mutant Z” FtsZ ring pinches cell into two independent identical cells. Eukaryotic Cell Reproduction • Large amounts of information are encoded in DNA, and they are organized into individual units called genes. • A gene is a segment of DNA that codes for a protein or RNA molecule (Traits). • Eukaryotic Cell Reproduction HeLa cells stained for DNA with the Blue Hoechst dye. The central and rightmost cell are in interphase, thus their entire nuclei are labeled. On the left a cell is going through mitosis and its DNA has condensed ready for division. • As the eukaryotic cell prepares to divide, the chromosomes-(the DNA and its associated proteins)-become visible, a process called DNA condensation • Before this happens, however, the DNA has already been copied. Eukaryotic Cell Reproduction • DNA condensation – The linear DNA strand “super coils” around histone proteins. – Exact copies are called chromatids, which are attached at the centromere. 1. First Growth Phase (G₁-Gap Phase) • A cell grows rapidly, and carries out routine functions. Cells that are not dividing or in the process of remain in this phase 2. Synthesis Phase (S phase) • The cells DNA is Copied, at the end of this phase, each chromosome consists of two sister chromatids joined together at the centromere 3. Second Growth Phase (G₂ phase) • Preparations are made for the nucleus to divide. Microtubules are rearranged during this phase in preparation for mitosis 4. Mitosis • The Nucleus of the cell is divided into two Nuclei, each nucleus end up with the same number and kinds of chromosomes as the original cell. 5. Cytokinesis • The cytoplasm divides and two daughter cells arise from a single parent cell. Control of the Cell Cycle • Three Major Checkpoints: – 1. Cell Growth (G₁) checkpoint • This checkpoint is to determine if conditions are favorable for a cell to divide (size, health, etc…) – 2. DNA Synthesis (G₂) checkpoint • DNA replication is checked by DNA repair enzymes, if this checkpoint is passed proteins trigger mitosis – 3. Mitosis Checkpoint • Triggers the exit from mitosis, signals beginning of the G₁ phase. What happens when Control of the Cell Cycle is Lost? The Cell Cycle • A cell spends 90% of its time in the first three phases, collectively called interphase. • Total, there are 5 phases of the cell cycle Mitosis and Cytokinesis Chapter 6 Section 3 1. First Growth Phase (G₁-Gap Phase) • A cell grows rapidly, and carries out routine functions. Cells that are not dividing or in the process of remain in this phase 2. Synthesis Phase (S phase) • The cells DNA is Copied, at the end of this phase, each chromosome consists of two sister chromatids joined together at the centromere 3. Second Growth Phase (G₂ phase) • Preparations are made for the nucleus to divide. Microtubules are rearranged during this phase in preparation for mitosis 4. Mitosis • The Nucleus of the cell is divided into two Nuclei, each nucleus end up with the same number and kinds of chromosomes as the original cell. 5. Cytokinesis • The cytoplasm divides and two daughter cells arise from a single parent cell. 4. Mitosis • The Nucleus of the cell is divided into two Nuclei, each nucleus end up with the same number and kinds of chromosomes as the original cell. Forming the Spindle • Spindles are structures made up of both centrioles and microtubule fibers. – Involved in moving chromosomes during cell division – The Centrosome is a organelle that organizes the assembly of the spindle. • Two centrioles in each centrosome. The Centriole • Each spindle fiber is made of an individual microtubule • Each centriole is made up of nine triplets of microtubules arranged in a circle. • Unlike animal cells, plant cells do not have centrioles, but they do form a spindle that is almost identical to that of animal cells. Separation of Chromatids by Attaching Spindle Fibers • Two sets of microtubules attach on each side of the centromere and extend towards opposite poles. • Once attached to the centrosome apparatus, the chromatids will move along the fibers to opposite ends of the cell. • As soon as the chromatids separate from one another they are called chromosomes. Phases of Mitosis 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Prophase • Chromosomes coil up and become visible (condensation) • Nuclear envelope dissolves • Spindle fibers form Metaphase • Chromosomes move to center of the cell. • Line up along the equator. • Spindle fibers link the chromatids to opposite poles. Anaphase • Centromeres divide during anaphase. • The two chromosomes move toward opposite poles as the spindle fibers attached shorten Telophase • A nuclear envelope forms around the chromosomes at each pole. • Chromosomes now at opposite poles uncoil and the spindle dissolves. • Mitosis is complete 5. Cytokinesis • The cytoplasm divides and two daughter cells arise from a single parent cell. Cytokinesis Animal Cells • Cytoplasm is divided in half, cell membrane grows to enclose each cell. • Cell is pinched in half by belts of proteins (microfilaments) called a cleavage furrow. • Forms two identical cells. Plant Cells • Vesicles formed by the Golgi Apparatus fuse at the midline and form a cell plate. – Cell plate is a membrane bound cell wall that forms across middle of plant cell. • A new cell wall then forms on each side of the cell plate, forming two new cells. Cytokinesis