Mitosis Somatic and Germline Cells • Multicellular organisms are made up of somatic cells and germ line cells – Somatic cells: Make up the body of an organism. • Have twice as much DNA as a germ line cell • Chromosomes arranged in homologous pairs • Diploid 2n: – Germ line: Reproductive cells (gametes). • Haploid1n: Chromosomes are unpaired; • Fusion of male (sperm) and female (ova) 1n cells forms a 2n zygote. Chromosomes in a human Chromosome Pairs 1-22 are called autosomes Homologous pair of chromosomes 23rd pair contain the sex chromosomes. Homologous Chromosomes Homologous pair of chromosomes (replicated and joined at centromere Cell Cycle G1 phase S phase (DNA synthesis) G2 phase M (mitosis) Cell Cycle: Interphase • G1 phase: Cell is growing and/or performing normal metabolic functions. • DNA is in the form of chromatin • Long thread-like DNA • S phase: Cell replicates its DNA. Sister chromatids are formed and joined at the centromere. • G2 phase: Cell continues growth and metabolism and prepares for mitosis. Cell Cycle: Mitosis (activity 1) • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis Cytokinesis Cell Cycle Time Estimation (activity 2) • In a population of cells only a few are in the M phase at a given time. • You will determine mitotic index and duration of each stage of mitosis in Allium root tips Meristems: regions in plants where new tissue is formed (mitosis) Exercise 13: Meiosis Gametes carry 22 autosomal chromosomes and either an X or Y sex chromosome. Human somatic cells: 23 pairs = 46 chromosomes (diploid, 2n) Gamete 23 unpaired chromosomes (haploid 1n) Meiosis Meiosis Activities • 1) Meiosis simulation • 2)Human cytogenetics – Compare patient karyotypes with normal karyotype • 3) Solve various genetic problems using punnett square analysis Exercise 12: Introduction to Genetics and Cell Division The purpose of this part of the exercise is to get some hands on time learning the steps in mitosis. This is a process best learned by working with actual chromosome models. Once you are familiar with the various phases, you will then examine some real cells and make some estimates of the amount of time that is spent in each phase of the cell cycle. Part 1: Mitosis Simulation Worksheet In this part of the lab, you will be doing a simulation of the steps in the mitosis process. Normally we use the pipe cleaners in the lab and the paper sheets with each of the stages of mitosis to describe the various phases. In this case, there is a Mitosis worksheet in Canvas that you can use to walk through the process steps. Draw two homologous pairs of chromosomes, then. start with phase G1 in interphase, and then describe what happens in S, G2 and each of the phases of mitosis. You can use your textbook to help you work through the stages. Be sure to include the following information in your description of the process: Interphase Prophase Metaphase Anaphase Telophase Cytokinesis G1: Cell grows, performs normal functions S: DNA is replicated; sister chromatids remain joined at the centromere G2: Cell stockpiles molecules required to complete the division process DNA in the form of chromatin begins to condense into visible chromosomes Nuclear membrane begins to disintegrate Spindle fibers begin to form, originating at poles of the cell and joined to each sister chromatid at the centromere By the end of prophase, DNA is fully condensed and spindle fibers are formed Chromosomes are pulled by the spindle fibers and aligned across the equator of the cell (metaphase plate) sister chromatids are separated at the centromere and pulled to opposite poles of the cell by spindle fibers Chromosomes are enclosed in a new nuclear membrane; spindle fibers break down, DNA in chromosomes returns to chromatin form The cytoplasm of the cell divides in half in plant cells, a cell plate form, dividing the cell in two in animal cells, a cleavage furrow forms and pinches the cell into two End result is two, genetically identical daughter cells Complete the worksheet and submit it to your instructor. Part 2: Cell Cycle Time Estimation In a population of cells that are actively growing and dividing, only some of the cells are actually in mitosis at any one time. The percentage of dividing cells is known as the mitotic index. The approximate duration of the various stages of mitosis can be obtained simply by multiplying the mitotic index by the total duration of the cell cycle. You will determine both the mitotic index and duration of mitosis in Allium root tips. In plants, the growth and formation of new tissues and organs occurs in specialized regions called meristems. A plant root has a meristem located just above the tip of the root, behind the root cap. Growth in meristems occurs largely by mitosis. 1. Obtain the Onion root tip photo from Canvas. 2. Randomly select 100 cells from the photo, and determine whether each cell is in interphase, or one of the steps of mitosis. 3. Observe a minimum of 100 cells in the root tip region. Identify each of the cells as to their stage of the cell cycle: Interphase, Prophase, Metaphase, Anaphase, Telophase. Record the number of cells that are in each of the phases in Table 1. 4. Calculate the mitotic index for each category on the chart including the total cells in mitosis and each of the separate phases. The mitotic index is equal to the number of cells dividing in each category divided by the total number of cells observed. a. Divide the number of cells in interphase by100 and record the percentage in the table. b. Repeat the previous step for the remaining 4 stages of the cell cycle. Record each percentage in Table 1. Onion root tip mitosis Anaphase cell Table 1: Mitosis Data and Calculations Interphase Number of cells in stage Mitotic Index = # Cells Dividing / Total # cells Observed Prophase Metaphase Anaphase Telophase Questions: Which phase of mitosis has the longest duration in this organism? How do you know? Which of the phases of mitosis has the shortest duration in this organism? How do you know? Which phase is longer overall, mitosis or interphase? How do you know? If you were to assume that the total cell cycle time for these cells was 16 hours, use the information in your table to determine how much time would be spent in each of the 5 phases (interphase, prophase, metaphase, anaphase & telophase). Pre-Lab Exercise 12: Introduction to Genetics and Cell Division The process of cell division is central to all life. All cells come from other cells, whether as part of the process of reproduction, growing a multi-cellular organism, or for replacing worn out or damaged cells. In this exercise you will be learning how cells replicate, and looking at how the cell’s chromosomes are replicated and sorted among cells. Introduction Multi-cellular organisms are composed of different types of cells that are designed to perform a wide variety of functions. The cells can be divided into two different groups, somatic cells and germline or reproductive cells. Somatic cells are generally described as cells that form the body of an organism (the word somatic is derived from a Greek word soma, meaning “body”). Somatic cells cannot divide to produce a new generation of offspring. In humans, there are about 210 different types of somatic cells that serve many purposes, from carrying oxygen gas that we breathe, to producing proteins that fight off foreign invaders. Somatic cells include those that make up our bones, blood, connective tissue, skin and other internal organs. These somatic cells all reproduce by a process known as mitosis. Each of the offspring cells of the division process are called daughter cells; they are produced by the process of mitosis, and are genetically identical. Germline cell types (also known as gametes) are much more limited. In mammals, the sperm cells produced by males, and ova produced by females are created through a process known as meiosis. These cells are produced from other germline cells called gametocytes that are also part of the germline. It is these germline cells that are capable of fusing together to form a new individual. Each of the germline cells produced is genetically unique. Genetics 101 Every living organism contains a set of genetic blueprints composed of molecules of deoxyribonucleic acid (DNA). A complete copy of this set of blueprints is found in almost every cell type in humans This DNA is typically organized into individual pieces of DNA called chromosomes. Humans, for example, have a genome that is made of 46 pieces of DNA which can be organized into 23 pairs of chromosomes. An alternate way to define somatic and germ line cells is by the amount of DNA they contain. In mammals, the amount of DNA in a somatic cell is twice as much as in a germ line cell. Somatic cells with chromosomes arranged in pairs are known as diploid cells, while the germ line cells that contain only single, unpaired chromosomes are known as haploid cells. Diploid cells are often referred to as 2n, while haploid cells are described as 1n. Haploid cells fuse during fertilization to produce the first cell of a new diploid organism, known as a zygote. The mammalian embryo develops from the zygote. This means that half of the genetic material Maternal Paternal in the new diploid organism came from the Chromosome Chromosome mother and half came from the father. Each of the parents contributes one set of chromosomes, which when combined, form Page 1 Pre-Lab:: Intro Genetics & Cell Division the complete set of 23 pairs. Twenty-two of the 23 pairs of chromosomes will contain information that is of the same type, and these pairs of chromosomes are known as homologous chromosomes or homologous pairs. These 22 pairs of chromosomes are collectively known as autosomes. The 23rd pair of chromosomes is the sex chromosomes. In females, there is a pair of X chromosomes, while in males there is one X chromosome, and one Y chromosome. Each of the homologous chromosomes has a particular size and shape that allows them to be paired up in a visual manner called a karyotype. The pairs are numbered from 1-23, with pair number one being the physically largest pair, and decreasing in size to pairs 21 and 22 which are the smallest. Each of these pairs of chromosomes contains certain sets of genes, with one copy of each gene coming from each of the two parent organisms. A Human Male Karyotype A pair of homologous chromosomes will have the same types of genes on each of the chromosomes. For example, if one member of the pair has a gene for freckles in a particular position on the chromosome, the other member of the homologous pair will also have a gene that codes for freckles in the same position. The two genes may be identical in function, or they may be different variants of the same gene, for example, one gene coding for freckles and one that codes for no freckles. These alternative Two similar, but non-identical alleles on homologous chromosomes Two identical alleles on homologous chromosomes Page 2 Pre-Lab:: Intro Genetics & Cell Division forms of a gene that may be present on chromosomes are known as alleles. Depending upon the gene, there may be two or more possible alleles that could be present. The Cell Cycle Despite the great diversity of cells in multi-cellular organisms, the processes that are utilized to replicate cells are remarkably similar. During its lifetime, each eukaryotic cell progresses through an ordered series of events known as the cell cycle. The cell cycle is typically divided into a number of different phases including G1, S, G2 and M (mitosis). The Cell Cycle The G1 phase of the cell cycle is the time when the cell is growing and performing its normal metabolic functions in addition to preparing to replicate its DNA. This may include increasing the amount of cytoplasm and the number of organelles as the cell grows. During this phase the DNA is in the form of chromatin, and there is one copy of each of the 46 chromosomes in a human somatic cell. During S phase, the cell duplicates its DNA. Each of the 46 homologous chromosomes is replicated, but the new replicates remain joined together at the centromere. The two replicated chromatids are called sister chromatids, and will remain joined together until anaphase of mitosis. Sister chromatids are identical duplicates of each other. By the end of S phase, each of the homologous chromosomes will be a pair of sister chromatids. A pair of sister chromatids joined at the centromere During G2 phase, the cell continues with growth and metabolism and is preparing for the actual division steps that are defined as mitosis. The combination of G1, S and G2 phases is described as Interphase. The cell spends the greatest proportion of its life in interphase, as much as 90% for some types of cells. Page 3 Pre-Lab:: Intro Genetics & Cell Division In M phase (mitosis), the cell segregates its chromosomes into two new daughter cells so that each of the new cells is diploid (2N). There are four phases of mitosis—prophase, metaphase, anaphase and telophase. Telophase is also accompanied by cytokinesis, the division of the cytoplasm of the cell to produce two complete, diploid cells. In a plant cell, this process includes the formation of a cell plate, which lays down a new cell wall structure. In animal cells, the cell is “pinched” in two by a ring of proteins called actin. Detailed pictures of the stages of mitosis can be found in your textbook. Page 4 Pre-Lab:: Intro Genetics & Cell Division Mitosis Worksheet For each stage of mitosis, label the stage with the name, draw the configuration of two sets of chromosomes and spindle fibers, then below your drawing list the things that happen for the stage. You can work with other students in the class, but you must do your own drawing. When you are done, upload the drawing for your instructor to check. Interphase Prophase Metaphase Anaphase Telophase Table 1: Mitosis Data and Calculations Interphase Prophase Metaphase Anaphase Telophase Number of cells in stage Mitotic Index = # Cells Dividing / Total # cells Observed Questions: Which phase of mitosis has the longest duration in this organism? How do you know? Which of the phases of mitosis has the shortest duration in this organism? How do you know? Which phase is longer overall, mitosis or interphase? How do you know? If you were to assume that the total cell cycle time for these cells was 16 hours, use the information in your table to determine how much time would be spent in each of the 5 phases (interphase, prophase, metaphase, anaphase & telophase).
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