Download Cell Cycle HW - Brookwood School

Name ____________________________________________________________ Date ___________________ Tab ______________________ The Cell Cycle – The Life Cycle of Eukaryotic Cells In life, we often go through phases that are defined by our interests and activities. For many years when we are young we go to school. Early adulthood holds first time jobs and college for some people and as we grow older, we focus more on our career and families. Most Eukaryotic cells do this too. They may spend a long period of time on activities relating only to the growth of the cell and then pause those activities as they move into a period devoted exclusively to reproducing themselves. This cycle between processes related to cell division and processes related to cell growth is called the cell cycle – it is the life cycle of the cell. Before we go any further in discussing the cell cycle, we need to note an important difference among the cells of the human body. All of the cells of a multicellular eukaryotic organism can be divided into two types: somatic cells are the cells forming the body of the organism (skin cells, muscle cells, blood cells, etc.); reproductive cells are the sex cells or gametes (sperm and egg cells). The processes of cell division by which somatic cells and reproductive cells are produced are different from each other. In this reading, we will focus on somatic cells and save the production of gametes for our Human Sexuality unit. When cells reproduce, they divide in order to create new life. One parent cell splits into two daughter cells. So, when we refer to cell division, we are referring to cell reproduction. The average cell cycle in human cells lasts 24 hours, but some cells can take more or less time to complete the cycle. The cell cycle describes the series of phases that lead to somatic cell division (reproduction). There are three main stages in the cell cycle (check out the graphic above). The first stage is Interphase, during this time the cell grows and prepares to divide. The second stage is Mitosis, during this time the nucleus and DNA divides. The final stage is Cytokinesis; this is when the whole cell and organelles divide into two new cells. CJ Bell Brookwood School We will start with a detailed look at Interphase, which is further divided into three distinct phases (Gap 1, DNA Synthesis, and Gap 2). A Eukaryotic cell moves through the phases and stages of the cell cycle in precise order and, at any given moment, all somatic cells in the body are at some point in this cycle. The phases and stages can be summarized as follows. (Phelan (2009) What is Life?) Interphase Gap 1 (G1) During this period, a cell grows and performs all cellular functions (making proteins, getting rid of waste, creating energy, etc.). Cells that divide infrequently, such as neurons and heart muscles, spend most of their time in Gap 1 phase. DNA Synthesis (S) During this phase, the cell begins to prepare for cell division. Every piece of DNA (chromatin) creates an exact duplicate of itself by replication. The cell now has two times the normal amount of DNA. Before replication, each piece of DNA is a long linear strand of genetic material. After replication, each piece of DNA has become a pair of identical long linear strands held together near the center by the centromere. Gap 2 (G2) In this phase, the cell finishes growing and developing. This phase differs from Gap 1, because the genetic material has now been duplicated. Gap 2 is usually much shorter than Gap 1. CJ Bell Brookwood School Mitosis After the cell completes Interphase, it is ready to begin dividing by first dividing its nucleus and genetic material and then the rest of the organelles and cytoplasm. During Mitosis, the nucleus of the parent cell divides. This process pulls apart the two identical copies of DNA in the cell and moves them to opposite sides of the cell. Mitosis is broken down into four phases – Prophase, Metaphase, Anaphase, and Telophase (pictured below). More on this later! Cytokinesis During this stage the cytoplasm, organelles, and the rest of the materials in the parent cell duplicate and the cell separates into two daughter cells. Each of these two new daughter cells has an EXACT copy of the parent cell’s DNA and other cellular structures. The daughter cells then proceed into a new cell cycle, starting with interphase. The daughter cells become parent cells in the next cycle. The transition from one phase to the next is trigged by specific events. For example, a cell moves from Gap 1 (G1) to DNA Synthesis (S) only if environmental conditions are good or favorable for cell division. Until the right conditions are present, the cell will remain in G1. Similarly, the cell only begins mitosis after the genetic material has been replicated or duplicated. Take Home Message! Eukaryotic somatic cells move through a specific cycle during their lives, which has different stages. The cell spends most of its life in Interphase, which consists of three phases: G1, S phase, and G2. During these phases the cell carries out life functions, copies its DNA, and grows to the full size. The nucleus and DNA divide during the stage of Mitosis and the actual cellular division takes place during the stage of Cytokinesis. Cell Cycle Homework Questions 1. What is the “cell cycle?” _________________________________________________________________________________________________________________________ 2. What is the difference between somatic cells and gametes? _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 3. How is cell division the same thing as cell reproduction? _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 4. Briefly describe what happens during each stage of the Cell Cycle below: Interphase: __________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ -­Gap 1: ________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ -­DNA Synthesis: _____________________________________________________________________________________________ _________________________________________________________________________________________________________________________ -­Gap 2: ________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ Mitosis: _______________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ Cytokinesis: _________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 5. Describe the characteristics of the daughter cells. _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 6. What triggers the transition between stages and phases of the cell cycle? _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 7. Summarize the “take-­‐home message.” _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________