Answers to Chapter 13 (Meiosis)

1/29/2019 Ch 13 Ch 13 Activity: Meiosis Part A What is the ploidy of organisms that have two sets of chromosomes? Hint 1. How is ploidy designated? ANSWER: 4n 3n n 2n Correct An organism that has two sets of chromosomes has a ploidy of 2n. Part B What information can not be obtained from an individual's karyotype? Hint 1. What structures can be examined in a karyotype? ANSWER: The number of autosomes in the individual The types of chromosomes present in the individual The sequence of bases of a particular gene The ploidy of the individual 1/30 1/29/2019 Ch 13 Correct Karyotypes only give information about the chromosomes in an individual. Part C True or false? A haploid organism has one pair of homologous chromosomes. Hint 1. How many sets of chromosomes are in a haploid organism? ANSWER: True False Correct A haploid organism only has one set of chromosomes, so it has no homologous chromosomes. Part D If a cell has 24 chromosomes, how many chromosomes would each of its four daughter cells have after meiosis? Hint 1. Think about what happens to the chromosomes during the two divisions that make up meiosis. ANSWER: six 36 12 24 Correct Meiosis is a reduction division that reduces the number of chromosomes passed on to daughter cells by one-half. Part E Which of the following statements about meiosis and mitosis is true? Hint 1. Consider how mitosis and meiosis differ. ANSWER: . 2/30 1/29/2019 Ch 13 In mitosis and meiosis I, homologous chromosomes move independently of each other. Each daughter cell produced during meiosis and mitosis is genetically identical to the parent cell. In meiosis II and mitosis, a diploid cell divides. Meiosis II is similar to mitosis in that sister chromatids of each chromosome separate. Correct Both meiosis II and mitosis are cell divisions involving the separation of sister chromatids. Part F Which structure is directly correlated with the production of genetic variability in the daughter cells produced during meiosis? Hint 1. Genetic variability is generated when homologous chromosomes exchange segments. ANSWER: Synapsis Tetrad Chiasma Centromere Correct The chiasma is an X-shaped structure formed from the crossing over of homologous chromosomes, which can then exchange segments at the crossover sites. Part G What is the composition of a tetrad at the beginning of prophase I? Hint 1. Tetrads are formed by synapsis during meiosis I. ANSWER: One pair of homologous chromosomes Two sister chromatids Four different chromosomes Two pairs of homologous chromosomes Correct One chromosome in a tetrad is paternally inherited, while the other is maternally inherited. Each chromosome consists of two chromatids. Meiosis (1 of 3): Genes, Chromosomes, and Sexual Reproduction (BioFlix tutorial) . 3/30 1/29/2019 Ch 13 Meiosis ensures the transmission of traits from one generation to the next. At the same time, it is a key process that introduces genetic variation into the traits that offspring inherit from their parents. In this tutorial, you will explore the genetic context of meiosis. Before beginning the tutorial, watch the Meiosis animation. You can review relevant parts of the animation at any point in the tutorial. Part A - Meiosis terminology Drag the labels from the left to their correct locations in the concept map on the right. Hint 1. The structure of DNA, chromatin, and chromosomes DNA (deoxyribonucleic acid) is a double helix. Most of the time, nuclear DNA exists in various states of packing. DNA is associated with proteins called histones. This DNA-protein complex is called chromatin, and chromosomes are made up of chromatin. When the cell is not dividing and during DNA replication, each chromosome is in the form of a long, thin chromatin fiber. As chromosomes condense during cell division, each chromatin fiber becomes densely packed and folded, making the chromosomes much shorter and so thick that you can see them with a light microscope. Hint 2. Can you rank the size of three DNA-containing structures? Rank these DNA-containing structures from largest to smallest. Hint 1. Genes and genomes Genes are the fundamental units of inheritance. A gene is a segment of DNA with a well-defined sequence, whose information determines some characteristic or trait of an organism. The term genome typically refers to all of the genetic material within some unit such as an organelle (e.g., the nuclear genome), a cell (e.g., a bacterial genome), or an organism (e.g., the human genome). ANSWER: . 4/30 1/29/2019 Ch 13 Reset genome chromosome Help gene Hint 3. What are gametes? Select the statement that accurately describes gametes. ANSWER: Gametes, such as eggs and sperm, are diploid reproductive cells that are involved in both sexual and asexual life cycles. Gametes, such as eggs and sperm, are haploid reproductive cells that fuse during fertilization in sexual life cycles. Gametes, such as eggs or sperm, are diploid reproductive cells that are produced in asexual life cycles. ANSWER: . 5/30 1/29/2019 Ch 13 Reset Help chromatin genes traits chromosomes locus genome gametes Correct Knowing the terms and relationships shown in this concept map will help you understand the role that meiosis plays in heredity, sexual reproduction, and genetic variability. Part B - Interactions among chromosomes This diagram shows a diploid nucleus (2n=8) in which chromosome replication has occurred in preparation for mitosis (top) and meiosis (bottom). The nucleus at top right is now in prophase of mitosis; the nucleus at bottom right is now in prophase I of meiosis. Drag the labels to their appropriate targets to correctly identify the various chromosome structures. Labels can be used more than once. Hint 1. Can you match the terms to their definitions? Drag the terms on the left to the appropriate blanks in the sentences on the right. ANSWER: Reset Help 1. Chromosome pairs of the same length and centromere location are called homologous chromosomes . 2. Replicated forms of a chromosome are called sister chromatids . 3. Crossing over occurs between the nonsister chromatids of homologous chromosomes early in meiosis I. 4. Chromosomes that differ in length and centromere location are called non-homologous chromosomes . Hint 2. What chromosome structure is produced by DNA replication? . 6/30 1/29/2019 Ch 13 DNA replication occurs before the beginning of mitosis or meiosis. During DNA replication, an exact copy of each chromosome is made. What is the term for the original chromosome and its exact copy? ANSWER: sister chromatids homologous chromosomes nonsister chromatids centromeres Hint 3. How does the alignment of chromosomes after DNA replication differ between mitosis and meiosis? Before either mitosis or meiosis can begin, the chromosomes must replicate. However, the replicated chromosomes behave differently in cells entering mitosis and cells entering meiosis. Which statement correctly describes a key difference between cells entering prophase of mitosis versus prophase I of meiosis? ANSWER: Cells entering meiosis contain twice as much DNA as cells entering mitosis, which is essential because meiosis involves two cell divisions. For cells entering mitosis, homologous chromosomes function independently; for cells entering meiosis, homologous chromosomes pair up. DNA replication before mitosis produces sister chromatids, whereas DNA replication before meiosis produces homologous chromosomes. Hint 4. The centromere The centromere is a region of the chromosome where sister chromatids attach to each other. The location of the centromere along the length of the chromosome is identical for homologous chromosomes but differs for non-homologous chromosomes. ANSWER: Reset non-homologous chromosomes nonsister chromatids homologous chromosomes non-homologous chromosomes Help sister chromatids homologous chromosomes centromere sister chromatids homologous chromosomes centromere nonsister chromatids . sister chromatids 7/30 1/29/2019 Ch 13 Correct To understand the process of meiosis, it is essential that you can differentiate between sister chromatids, nonsister chromatids, homologous chromosomes, and non-homologous chromosomes. Part C - Animal life cycles In the life cycle of an organism, meiosis is paired with the process of fertilization. Understanding the life cycle of an organism is the key to understanding how sexual reproduction ensures the inheritance of traits from both parents and also introduces genetic variation. Complete the diagram to show the life cycle of a typical animal. Follow these steps: 1. First, drag blue labels onto blue targets only to identify each stage of the life cycle. 2. Next, drag pink labels onto pink targets only to identify the process by which each stage occurs. 3. Then, drag white labels onto white targets only to identify the ploidy level at each stage. Labels can be used once, more than once, or not at all. Hint 1. Ploidy level defined A cell produced by fertilization (called a zygote) contains two sets of chromosomes, one from the maternal parent and one from the paternal parent. Such a cell is described as diploid. The two gametes (egg and sperm) that fuse in the process of fertilization each contain only a single set of chromosomes and are described as haploid. If n represents the number of chromosomes in a single set, haploid cells have n chromosomes whereas diploid cells have 2n chromosomes. Hint 2. How does mitosis affect ploidy level? If a parent cell is diploid, the daughter cells produced by mitosis are _____. ANSWER: diploid haploid Hint 3. How do meiosis and fertilization affect ploidy levels? As a result of meiosis and fertilization, chromosomes from the maternal and paternal parents are mixed in the next generation. What effect do meiosis and fertilization have on the ploidy level of cells produced by each process? ANSWER: Meiosis decreases the ploidy level from 2n to n; fertilization does not change the ploidy level. Meiosis decreases the ploidy level from 2n to n; fertilization increases the ploidy level from n to 2n. Meiosis does not change the ploidy level; fertilization increases the ploidy level from n to 2n. Meiosis increases the ploidy level from n to 2n; fertilization decreases the ploidy level from 2n to n. Neither meiosis nor fertilization changes the ploidy level. ANSWER: . 8/30 1/29/2019 Ch 13 Reset 2n n 2n 2n meiosis meiosis egg n sperm Help n egg fertilization zygote sperm 2n zygote mitosis meiosis fertilization 2n mitosis meiosis n egg Correct Meiosis creates gametes (eggs and sperm) with only a single chromosome set (haploid or n) from parental cells with two chromosome sets (diploid or 2n). During fertilization, the haploid sperm (n) and egg (n) fuse, producing a diploid zygote (2n). The cells of the zygote then divide by mitosis (which does not change the ploidy level) to produce an adult organism (still 2n) of the next generation. In sexual life cycles, meiosis and fertilization keep the number of chromosomes constant from generation to generation. Meiosis (2 of 3): The Mechanism (BioFlix tutorial) Meiosis is mechanistically similar to mitosis in many ways, although it involves two sequential nuclear and cellular divisions rather than one. The two stages of meiosis are Meiosis I, which consists of prophase I, metaphase I, anaphase I, and telophase I (followed by cytokinesis) Meiosis II, which consists of prophase II, metaphase II, anaphase II, and telophase II (followed by cytokinesis) Watch the Meiosis animation to learn about each stage in the process. You can review relevant parts of the animation at any point in the tutorial. Part A - The stages of meiosis Can you recognize the eight stages of meiosis based on the location and behavior of the chromosomes? Drag the diagrams of the stages of meiosis onto the targets so that the four stages of meiosis I and the four stages of meiosis II are in the proper sequence from left to right. (Note that only one of the two daughter cells is shown for meiosis II.) Hint 1. What are the key events of the stages of meiosis? . 9/30 1/29/2019 Ch 13 Drag the words on the left to the appropriate blanks to complete the sentences on the right ANSWER: Reset Help 1. During metaphase I , pairs of homologous chromosomes align in the center of the cell. 2. During telophase II , the separated chromatids elongate and (usually) cytokinesis occurs, forming four genetically distinct haploid daughter cells. 3. During anaphase I , homologous chromosomes separate by moving with the spindle microtubules toward the poles. 4. During prophase II , a spindle apparatus forms and individual chromosomes (each composed of sister chromatids) begin to move toward the center of the cell. 5. During prophase I , chiasmata form and crossing-over occurs. 6. During anaphase II , sister chromatids separate and move toward the poles. 7. During telophase I , the separated chromosomes cluster at the poles and cytokinesis occurs, forming two daughter cells, each with a haploid set of replicated chromosomes. 8. During metaphase II , individual chromosomes (each composed of sister chromatids) align in the center of the cell. Hint 2. What distinguishes meiosis I from meiosis II? In meiosis, there are two sequential divisions of the parent cell, producing four daughter cells. In terms of the behavior of the chromosomes, how does meiosis I compare to meiosis II? Hint 1. Sister chromatids and homologous chromosomes Sister chromatids are the two copies of a duplicated chromosome that form during the replication of DNA. While joined, the two sister chromatids make up one chromosome, though they eventually separate and form two separate chromosomes. Homologous chromosomes are a pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the paternal parent and the other from the maternal parent. Early in meiosis, homologous chromosomes (each consisting of two sister chromatids) pair loosely along their lengths. . 10/30 1/29/2019 Ch 13 ANSWER: Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II. Sister chromatids separate in both meiosis I and meiosis II. Sister chromatids separate in meiosis I; homologous chromosomes separate in meiosis II. Homologous chromosomes separate in both meiosis I and meiosis II. ANSWER: Reset Help Correct Meiosis involves two sequential cellular divisions. In meiosis I, homologous chromosomes pair and then separate. Thus, although the parent cell is diploid (containing two chromosome sets, one maternal and one paternal), each of the two daughter cells is haploid (containing only a single chromosome set). In meiosis II, the sister chromatids separate. The four daughter cells that result are haploid. Part B - Crossing over . 11/30 1/29/2019 Ch 13 Crossing over plays a critical role in increasing the genetic variation among offspring of sexual reproduction. It is important to understand how crossing over occurs and its consequences in meiosis. Look carefully at the diagrams depicting different stages in meiosis in a cell where 2n = 6. Assume that the red chromosomes are of maternal origin and the blue chromosomes are of paternal origin. Drag the labels to fill in the targets beneath each diagram of a cell. Note that the diagrams are in no particular order. Drag the blue labels to the blue targets to identify the stage of meiosis depicted in each diagram. Drag the pink labels to the pink targets to identify whether the configuration of the chromosomes related to crossing over is possible or not. Hint 1. What are the key events of the stages of meiosis? Drag the words on the left to the appropriate blanks to complete the sentences on the right. ANSWER: Reset Help 1. During metaphase I , pairs of homologous chromosomes align in the center of the cell. 2. During telophase II , the separated chromatids elongate and (usually) cytokinesis occurs, forming four genetically distinct haploid daughter cells 3. During anaphase I , homologous chromosomes separate by moving with the spindle microtubules toward the poles. 4. During prophase II , a spindle apparatus forms and individual chromosomes (each composed of sister chromatids) begin to move toward the center of the cell. 5. During prophase I , chiasmata form and crossing-over occurs. 6. During anaphase II , sister chromatids separate and move toward the poles. 7. During telophase I , the separated chromosomes cluster at the poles and cytokinesis occurs, forming two daughter cells, each with a haploid set of replicated chromosomes. 8. During metaphase II , individual chromosomes (each composed of sister chromatids) align in the center of the cell. Hint 2. Review the animation showing crossing over The association of homologous chromosomes and the effects of crossing over are usually shown in two-dimensional diagrams. This short animation shows the process in three dimensions. . 12/30 1/29/2019 Ch 13 Hint 3. What are the requirements and consequences of crossing over? Crossing over is unique to meiosis. Its occurrence depends on specific events early in prophase I and it has important consequences for the rest of meiosis and beyond. Consider these statements about crossing over: 1. For crossing over to occur, homologous chromosomes must align precisely early in prophase I so that nonsister chromatids can exchange corresponding segments of DNA. 2. Crossing over occurs at the ends of chromosomes, rather than near the centromeres, because segments of DNA near the centromeres cannot break and rejoin easily. 3. As a result of crossing over, sister chromatids are no longer identical to each other. 4. Crossing over prevents homologous chromosomes from separating during meiosis I. Which of the above statements are correct? ANSWER: 1 and 2 only 1 and 3 only 2 and 3 only 1, 2, and 3 1, 2, 3, and 4 ANSWER: Reset Help anaphase I telophase I prophase II anaphase II metaphase II prophase I metaphase I yes no no yes telophase II yes no Correct Crossing over occurs during prophase I when homologous chromosomes loosely pair up along their lengths. Crossing over occurs only between nonsister chromatids within a homologous pair of chromosomes, not between the sister chromatids of a replicated chromosome. Only segments near the ends of the chromatids, not segments nearest the centromeres, can exchange DNA. Part C - Changes in ploidy and DNA content during meiosis The parent cell that enters meiosis is diploid, whereas the four daughter cells that result are haploid. Which statement correctly describes how cellular DNA content and ploidy levels change during meiosis I and meiosis II? . 13/30 1/29/2019 Ch 13 Hint 1. Changes in cellular DNA content Meiosis, like mitosis, is always preceded by the replication of a cell’s chromosomes, which doubles the DNA content of the cell (an exact copy of each chromosome is made). Note that DNA replication does not alter a cell’s ploidy level because the number of chromosome sets in the cell remains unchanged. During meiosis, the DNA content of a cell is reduced in half twice—once during anaphase I and once during anaphase II. At each of these stages, the DNA is divided evenly between two daughter cells. Hint 2. Which processes change cellular ploidy level? Ploidy refers to whether a cell is haploid or diploid. Diploid cells (2n) have two sets of chromosomes – one inherited from the maternal parent and the other from the paternal parent. Haploid cells (n) have a single set of chromosomes, none of which has a homologous match. Sort these processes into the appropriate bin depending on whether the process increases a cell’s ploidy level from n to 2n, decreases the cell’s ploidy level from 2n to n, or does not change the cell’s ploidy level. ANSWER: Reset fertilization meiosis I DNA replication mitosis Help meiosis II ANSWER: DNA content is halved in both meiosis I and meiosis II. Ploidy level changes from diploid to haploid only in meiosis II. DNA content is halved only in meiosis I. Ploidy level changes from diploid to haploid only in meiosis II. DNA content is halved only in meiosis I. Ploidy level changes from diploid to haploid in meiosis I, and remains haploid in meiosis II. DNA content is halved only in meiosis II. Ploidy level changes from diploid to haploid in meiosis I, and remains haploid in meiosis II. DNA content is halved in both meiosis I and meiosis II. Ploidy level changes from diploid to haploid in meiosis I, and remains haploid in meiosis II. Correct During anaphase of both meiosis I and meiosis II, the DNA content (number of copies of chromosomes) in a cell is halved. However, the ploidy level changes only when the number of unique chromosome sets in the cell changes. This occurs only in meiosis I (where separation of homologous chromosomes decreases the ploidy level from 2n to n and produces daughter cells with a single chromosome set). Activity: Asexual and Sexual Life Cycles . 14/30 1/29/2019 Ch 13 Click here to complete this activity. Then answer the questions. Part A Human gametes are produced by _____. ANSWER: meiosis the cell cycle mitosis fertilization asexual reproduction Correct Meiosis produces haploid gametes from a diploid parental cell. Part B Normal human gametes carry _____ chromosomes. ANSWER: 46 46 pairs of 5 23 23 pairs of Correct This is the number of chromosomes in a single set of human chromosomes. Part C Which of these cells is (are) haploid? . 15/30