The Cell Cycle & Mitosis Learning Targets • Explain the main events of the cell cycle. • Explain what occurs in the four phases of mitosis. Copying Genetic Material • Prior to cell division, all cells must copy or replicate their DNA. • Each daughter cell then gets a complete copy of the DNA. Prokaryotic Cell Division • In most prokaryotes, after the DNA is copied, the contents of the cell are simply separated into two parts. Eukaryotic Cell Division • Eukaryotic cell division is more complex and occurs in two main stages. • The first stage, division of the cell nucleus, is called mitosis. • The second stage, division of the cytoplasm, is called cytokinesis. • Mitosis divides the _______, while cytokinesis divides the _______. • Nucleus, cytoplasm Chromosomes • In eukaryotic cells, the genetic information that is passed on from one generation of cells to the next is carried by chromosomes. • Chromosomes are condensed chromatin, DNA tightly coiled around proteins called histones. • At the beginning of mitosis, chromatin condenses to form chromosomes. Chromatids • Before cell division, each chromosome (DNA) is replicated, or copied. • Each chromosome consists of two identical “sister” chromatids. • When the cell divides, the “sister” chromatids separate from each other with one chromatid going to each of the two new cells. Chromatids • Each pair of chromatids is attached at an area called the centromere. • Centromeres are usually located near the middle of the chromatids. • If a human cell normally has 46 chromosomes, how many chromatids would it have just prior to cell division, after replication? • 92 • What is the difference between chromatin and chromosomes? • Chromosomes are condensed chromatin. The Cell Cycle • The cell cycle is the series of events that cells go through as they grow and divide. • During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cell cycle again. • The cell cycle consists of interphase, mitosis and cytokinesis. The Cell Cycle Interphase • During interphase, cells perform normal functions, grow, replicate their DNA and produce organelles and molecules required for cell division. Mitosis • After interphase, mitosis begins. • Mitosis is divided into four main phases: prophase, metaphase, anaphase and telophase (PMAT). Prophase • Longest phase of mitosis, 50-60% of the total time required. • Chromatin condenses into visible chromosomes. • Centrioles, two tiny structures located in the cytoplasm near the nuclear envelope, separate and move to opposite sides of the cell. • The mitotic spindle forms between the pair of centrioles. • The spindle consists of microtubules and other proteins and helps separate chromatids and organize cell division. Prophase • Chromosomes coil more tightly. • The nucleolus disappears. • The nuclear envelope (or membrane) breaks down. Metaphase • Often only lasts a few minutes. • Chromosomes line up across the center of the cell. • Microtubules connect the centromere of each chromosome to the two poles of the spindle. • “M” for middle Anaphase • The centromeres that join the sister chromatids split, allowing the sister chromatids to separate and become individual chromosomes. • The chromosomes continue to move until they have separated into two groups near the poles of the spindle. • “A” for away. Telophase • Chromosomes begin to uncondense. • The nuclear envelope re-forms around each cluster of chromosomes. • The spindle begins to break apart, and the nucleolus becomes visible in each daughter nucleus. • “T” for two • Mitosis is complete! Cytokinesis • Mitosis results in two nuclei being produced, each with its own set of chromosomes. • Cytokinesis splits the cytoplasm and organelles in half completing the cell cycle. Mitosis Animation Purpose of Mitosis • The purpose of mitosis is to create new cells that have identical genetic information. • It is a form of asexual reproduction that does not result in genetic variation. • In sexual reproduction, the gametes (sex cells) of two different organisms are combined increasing genetic variation. Interphase Prophase Metaphase Anaphase Telophase Cytokinesis • Centrioles separate • Cytoplasm divides in half • Sister chromatids separate • Nuclear envelope breaks down • Spindle appears • Chromosomes connected to • Chromosomes line up across spindle fiber at centromere center of cell • Chromatin condenses into • Chromosomes gather at chromosomes opposite ends of cell and lose • Cell grows and copies DNA distinct shapes. • Two nuclear envelopes re-form.