THE CELL CYCLE, MITOSIS AND MEIOSIS

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1 Biology 160 Name and Lab Section THE CELL CYCLE, MITOSIS AND MEIOSIS INTRODUCTION In single celled organisms cell reproduction gives rise to the next generation. In multicellular organisms cell division occurs not just to produce a whole new organism but for growth and replacement of worn out cells within the organisms. Cell division is always highly regulated and follows a highly orchestrated series of steps. The term cytokinesis refers to dividing a cell in half, while mitosis and meiosis refer to two different forms of nuclear division. Mitosis results in two nuclei that are identical to the original nucleus. Meiosis on the other hand results in four nuclei that each have ½ the chromosomes of the original cell. In animals meiosis only occurs in the cells that give rise to the sex cells (gametes), i.e., the egg and the sperm. This handout will be used for two laboratory sessions. In the first lab section you will use pipe cleaners to demonstrate the chromosomes in the stages of the cell cycle and mitosis. You should also know the difference between mitosis and cytokinesis. In next weeks lab you will continue with investigating cell and nuclear division but we will focus on meiosis and why meiosis and sexual reproduction results in variability. Objectives: 1. Be able to demonstrate knowledge of the steps involved in the cell cycle, mitosis and meiosis 2. Understand the difference in cytokinesis in plants and animals 3. Be able to identify stages of mitosis in onion root tips 4. Understand how meiosis leads to genetic diversity 5. Know which cells undergo mitosis and which undergo meiosis in humans The cell cycle: The cell cycle is a cell s life cycle. In eukaryotes the cell cycle is divided into two phases, Interphase and the M-phase. Each of these phases is subdivided. Interphase accounts for the majority of a cell s life. During Interphase the cell is doing its job, i.e., all its various functions while also replicating everything it needs to make two cells. Interphase is further divided into Gap1 (G 1 ), DNA synthesis (S) and Gap2 (G 2 ). G 1 is when the cell performs the majority of its functions, increases its proteins, organelles and size in preparation for replication. Cells that never replicate such as many nerve or muscle cells, stay in G 1 for their entire existence. This is often referred to as G 0 If the cell is going to replicate it must double its DNA so that is has enough chromosomal material for each new cell to get a complete set of chromosomes. This occurs in the S phase. After the S phase is the G 2 when the cell finishes replicating any proteins needed for nuclear and cell division. The M-phase consists of mitosis or meiosis and cytokinesis. Mitosis and meiosis refer to division of the nucleus and cytokinesis refers to division of the cytoplasm into two cells. Mitosis results in nuclei that are identical to the original nuclei. 1

2 The Cell Cycle Chromosomes Organisms that are diploid, such as humans, have two complete sets of chromosomes in most of the cells of their body. One set comes from the mother and one from the father. The matched chromosomes are called homologous chromosomes. Humans have two sets of 23 chromosomes for a total of 46 chromosomes. Each of the 23 chromosomes in one set are different lengths and are labeled 1 through 22 with chromosome 1 being the longest and chromosomes 22 being the shortest. There is no chromosome 23 because those are the sex chromosomes and are called the X or Y chromosome. Chromosomes hold the genetic information that is passed generation to generation i.e., the genes. Each gene, which is composed of DNA, codes for a specific protein or RNA. Homologous chromosomes carry the same gene at the same location but that gene may code for different versions of the protein. The different versions of the gene are called alleles. A good example of an allele would be alleles of eye color. The gene codes for the eye color, but it is possible to get a different version of that gene and therefore have blue or brown eyes. During G 1 each chromosome (which is made of DNA and protein) consists of one strand called a chromatid. During the S-phase DNA is duplicated and each chromosome now consists of two identical sister chromatids. This form of a chromosome looks somewhat like an X and is the shape that most people associate with a chromosome. Chromosome duplication must occur before mitosis or meiosis can occur. Mitosis Mitosis followed by cytokinesis results in two daughter cells that are identical to the original cell. Mitosis is a continuum of changes but biologists like break down the stages of mitosis into four main stages, prophase, metaphase, anaphase and telophase. The focus in this class is on an understanding of the process and not a memorization of phases. The only phase name you will need to remember is metaphase. In mitosis the nuclear membrane is broken down, spindle fibers (microtubules) attach to the chromatids at the centromere and pull apart the chromatids. When the chromatids reach separate ends of the cells the spindle fibers disintegrate and a nuclear membrane rebuilds around the chromosomes making two nuclei. Each nucleus is identical to the original nucleus as it was in G 1. 2

3 Mitosis Cytokinesis Because of the cell wall, cytokines is quite different in plant and animal cells. In animal cells the cell is divided into two by microfilaments (actin working with myosin) to pinch the cell into two. Plant cells cannot be pinched in two because of the wall. In plant cells vesicles with materials to form the cell wall collect in the middle of the dividing cell. The vesicles fuse together to form a plasma membrane and the material builds the cell wall. Meiosis Meiosis is the form of nuclear cell division that results in daughter cells that have one half the chromosome numbers as the original cell. In organisms that are diploid, the end result is cells that are haploid. Each daughter cell gets one complete set of chromosomes, i.e., one of each homologous pair of chromosomes. In humans this means the chromosome number is reduced from 46 to 23. The only cells that undergo meiosis will become sperm or eggs. The joining together of a sperm and egg during fertilization returns the chromosomes number to 46. Cells that undergo meiosis go through the cell cycle including the S phase so begin the process with chromosomes that consist of two chromatids just as in mitosis. Meiosis consists of meiosis I and meiosis II. In meiosis I homologous chromosomes are separated into different nuclei. This is the reduction division; chromosome number is cut in half. Meiosis II is very similar to mitosis, chromatids are separated into separate nuclei. As in mitosis, it is spindle fibers that pull the chromosomes and chromatids apart. The end result of meiosis is four cells, each with one complete set of chromosomes instead of two sets of chromosomes. 3

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5 Procedure for Day 1: Mitosis: Making Identical Daughter Cells During this lab you will use pipe cleaners as model representations of chromosomes to demonstrate what happens during the cell cycle. You will make cells that have 4 chromosomes, two homologous pairs. To represent homologous chromosomes you will use different colors of pipe cleaners. One color represents the set that came from the mother and the other color the set that came from the father. To represent the different sized chromosomes you will use different sized pieces of pipe-cleaners. To represent chromosomes in G 1 of the cell cycle please use one pipe cleaner. To represent chromosomes that have duplicated in the S-phase use another pipe cleaner and form an X. Sister chromatids are identical so the two strands must be the same color. Your instructor will explain this further. 1) You will be asked to demonstrate a knowledge of the cell cycle, showing when chromosomes replicate (synthesis their DNA) and when mitosis and cytokinesis occurs. 2) You will make highly simplified representations of cell(s) in G 1, metaphase and after telophase and cytokinesis has ended in the circles provided on page 6. Your representations you should only include the chromosomes, nuclear membrane and spindle fibers (i.e., do not add in the other organelles). The plasma membrane is represented by the circles. Please use different colors to represent the homologous chromosomes and lengths in your drawings to represent the different chromosomes. 3) Please examine the prepared onion root tip slides. You should focus towards the bottom of the root but not at the very bottom. You should be able to see cells in various stages of the cell cycle including interphase and M phase. In interphase the chromosomes will not be distinct since they have not condensed. In the M phase it is possible to see distinct chromosomes. The easiest stage to see distinct chromosomes is during anaphase of the M phase when the chromosomes are being pulled apart. Please answer questions 1-3 on the last page before you put away the onion root tip slide. Procedure for Day 2: Meiosis: Reduction Division You will make highly simplified representations of a cell(s) in metaphase I of meiosis I, metaphase II of meiosis II, and after meiosis is complete and four cells been made. In this exercise we will once again use pipe cleaners to represent chromosomes and but we will also use beads to represent genes. Please use different colors to represent the homologous chromosomes and lengths in your drawings to represent the different chromosomes. Different genes will be represented with different colored beads, while alleles (which are different versions of the same gene) will be represented by different shapes of beads. Your instructor will demonstrate this before you begin. You will need to repeat this procedure three different times, on pages 7, 8 and 9. The first two representations will be different from each other because during metaphase I of meiosis I the chromosomes can line up in two different ways and you will represent this. The third representation will show crossing over. 5

6 Phases of the Cell Cycle and Mitosis: G1 Metaphase End of Telophase and Cytokinesis 6

7 Meiosis (Possibility 1) Metaphase of Meiosis I Metaphase of Meiosis II End of Meiosis II (four cells) 7

8 Meiosis (Possibility 2) Metaphase of Meiosis I Metaphase of Meiosis II End of Meiosis II (four cells) 8

9 Meiosis (Crossing over) Metaphase of Meiosis I Metaphase of Meiosis II End of Meiosis II (four cells) 9

10 Questions and drawings: 1) In the prepared onion root tip slides in what phase were the majority of the cells? 2) Please sketch an onion root tip cell in anaphase. 3) Please sketch an onion root tip cell in interphase. 4) Which form of nuclear division results in cells that are identical to the original cell? 5) If a cell underwent mitosis but not cytokinesis how would the cell appear? 6) Which stage of meiosis, meiosis I or meiosis II, is more similar to mitosis? 7) Why is the process of cytokinesis different in plant and animal cells? 8) If a diploid organism has 20 chromosomes in its lung cells, how many chromosomes will it have in its sperm? 9) In mitosis, on any one chromosome the sister chromatids are identical. In meiosis, chromatids that are attached to each other are NOT always identical. What process results in sister chromatids results in chromatids not being identical? 10