Meiosis SKill Check and Karyotype Analysis
Meiosis and Reproduction Skill Check Worksheet NAME:_______________________ LAB SECTION_________________ Activity 2: Karyotype Analysis Case 1: What is the name of the disorder? What is the underlying cause of the syndrome? Describe the symptoms that will show in the individual with the syndrome. Is the syndrome treatable? What is the survival rate the child who has this disorder? As a genetic counselor, what would you tell the parents of the child who has this disorder? Case 3: What is the name of the disorder? What is the underlying cause of the syndrome? Describe the symptoms that will show in the individual with the syndrome. Is the syndrome treatable? What is the survival rate? As a genetic counselor, what would you tell the parents of the child who has this disorder? Case3: What is the name of the disorder? What is the underlying cause of the syndrome? Describe the symptoms that will show in the individual with the syndrome. Is the syndrome treatable? What is the survival rate? As a genetic counselor, what would you tell the parents of the child who has this disorder? Case4: What is the name of the disorder? What is the underlying cause of the syndrome? Describe the symptoms that will show in the individual with the syndrome. Is the syndrome treatable? What is the survival rate? As a genetic counselor, what would you tell the parents of the child who has this disorder? Case5: What is the name of the disorder? What is the underlying cause of the syndrome? Describe the symptoms that will show in the individual with the syndrome. Is the syndrome treatable? What is the survival rate? As a genetic counselor, what would you tell the parents of the child who has this disorder? Case6 What is the name of the disorder? What is the underlying cause of the syndrome? Describe the symptoms that will show in the individual with the syndrome. Is the syndrome treatable? What is the survival rate? As a genetic counselor, what would you tell the parents of the child who has this disorder? 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 Meiosis and Reproduction What will my children look like? We all wonder about the answer to this question, and when the time comes, we look at our children’s characteristics, often puzzled by the outcome. In this exercise you will look at the basis of genes and genetics, and will learn how to predict the likelihood that certain combinations of genes will occur in an organism. You will also see that in some cases, mistakes in forming reproductive cells can occur, resulting in offspring that have genetic diseases. INTRODUCTION In the previous exercise you learned that somatic cells contain 46 chromosomes within the nucleus. These chromosomes can be organized into homologous pairs, where half of the chromosomes (23, one of each type) were inherited from the mother, and the other half of the chromosomes (the other 23 chromosomes) were inherited from the father. The cells that carry the chromosomes from the mother or the father are called gametes (sperm from the male, and ova (or eggs) from the female). How does the number of chromosomes change from the normal diploid number of 46 in somatic cells, to the haploid number of 23 that is found in the sperm or eggs? The type of cell division that reduces the number of chromosomes from diploid to haploid is called meiosis. The specific steps of meiosis are in many ways similar to those of mitosis, but the outcome of the process is very different. Meiosis produces 4 cells (either egg or sperm), each of which carries only one copy of each type of chromosome. A human sperm cell, for example will carry one of each of the 22 types of autosomes, and either an X or a Y chromosome. Each chromosome contains a variety of genes that code for different characteristics or traits. Each gene may come in a few alternate forms called alleles. Approximately one half of the gametes produced by an organism will carry alleles that came from the original maternal chromosome, and the other half will carry alleles that came from the original paternal chromosome. When an egg and sperm cell unite to form a new zygote, the first cell of the new organism, the chromosome carried in the sperm cell, and the chromosome in the egg cell produce a new homologous pair of chromosomes that result in a new combination of alleles in this organism. The question that is asked most often is how can we predict what the new organism or offspring will look like? The truth is, we can’t predict with 100% accuracy, but for some genes, we can determine the probability that a certain characteristic or trait will appear. In order to make these predictions, we need to understand what types of gametes will be formed, and we can do that by understanding the meiosis process. In genetics, we look at the possible gametes produced by each individual, and then the possible combinations of offspring those gametes can produce by mating specific male and female parents. For simple gamete combinations, where only one or two traits are being examined, it is possible to determine possible gamete combinations using a Punnett square. For more complex situations, when more than two traits are being followed, statistical analysis is used. For example, in a monohybrid cross (a cross where one trait with two alternative alleles is followed), a 2 x 2 square is used, placing one set of gametes on each side of the square, and looking at the offspring combinations in the four center squares. Male Gamete 1 Male Gamete 2 Female Gamete 1 Male Gamete Male Gamete 2 & Female 1 & Female Gamete 2 Gamete 1 Female Gamete 2 Male Gamete 1 & Female Gamete 2 Male Gamete 2 & Female Gamete 2 Punnett Square Analysis The Punnett square allows us to predict possible combinations of alleles that will come together on pairs of homologous chromosomes in a new organism. In genetics, alleles are represented by letters that represent the organism’s genotype. The physical description of a trait is called the phenotype (for example, tall, type A blood, blonde hair). If the two alleles that are inherited represent different forms of the gene, they will be represented by different letters, and the individual will be considered heterozygous for that trait. The allele that is expressed or visible in heterozygous individuals is referred to as the dominant allele, and is represented by a capital letter such as “A”. The allele that is not expressed or visible in these individuals is referred to as recessive, and is represented by a lower case letter such as “a”. Individuals that inherit two identical alleles for a trait (AA or aa) are homozygous for that trait. In the next lab exercise you will learn to use Punnett squares to predict the probabilities that certain genotypes and phenotypes occur in an organism. In this exercise, you will learn how the process of meiosis leads to the formation of gametes. Activity 1: Meiosis Simulation In the first part of this exercise, you will simulate the process of meiosis in humans in a drawing, in a manner like you did last week with mitosis. You will find that the steps in the process are very similar to those in mitosis, and so many of the terms should be familiar. You should go through the process with one homologous pair of chromosomes first, and then repeat the process with two homologous pairs. Start with phase G1, and then describe what happens in S, G2 and each of the phases of Meiosis I and Meiosis II. Use your textbook to help you walk through the stages. Be sure to include the following information in your description of the process: Meiosis Interphase Prophase 1 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 Homologous chromosomes are drawn together via synapsis and form structures called tetrads; recombination occurs between homologous chromatids Metaphase 1 Anaphase 1 Telophase 1 Cytokinesis Interphase 2 Tetrads are pulled by the spindle fibers and aligned across the equator of the cell (metaphase plate) Tetrads are separated 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 Cells recharge for a second round of division DNA in the form of chromatin begins to condense into visible chromosomes Nuclear membrane begins to disintegrate Prophase 2 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 Metaphase 2 Chromosomes are pulled by the spindle fibers and aligned across the equator of the cell (metaphase plate) Anaphase 2 Sister chromatids are separated and pulled to opposite poles of the cell by spindle fibers Telophase 2 Chromosomes are enclosed in a new nuclear membrane; spindle fibers break down, DNA in chromosomes returns to chromatin form Cytokinesis The cytoplasm of the cell divides in half (same as previously described); end result is a total of four, haploid reproductive cells When you think you understand the steps in the process and can describe them completely, fill out the meiosis worksheet and turn it in to your instructor to check. Activity 2: Human Cytogenetics You have graduated at the top of your class and are now a trained cytogeneticist. It is your job to construct karyotypes of patients in order to look for possible chromosomal abnormalities. You will look at 4 of the karyotypes that you have constructed and determine what if any abnormalities exist, and what the prognosis is for the patient. The vast majority of people in the world have only minor variations in their DNA which leads to the diversity that we see in the world. On occasion, however, major errors can occur during the meiosis process. These errors generally result in alterations in chromosome number or alterations in chromosome structure that can lead to significant genetic syndromes or, in many cases, death. The following section describes some of the ways in which these types of genetic defects occur. Alterations in Chromosome Number Alterations in chromosome number occur when homologous chromosome fail to separate during anaphase I of meiosis, or when sister chromatids fail to separate during anaphase II. This is known as nondisjunction. The result is that one gamete that has 2 copies of one chromosome, and the other has no copies of that chromosome. All of the other chromosomes would be distributed normally to the gamete cells. Nondisjunction can affect any of the 22 pairs of autosomes, or it may occur with the sex chromosomes (X or Y). Nondisjunction actually occurs at a fairly high frequency in humans, but the results are usually so devastating (especially with autosomal nondisjunction) to the growing zygote that a miscarriage occurs very early in the pregnancy. Nondisjunction of the sex chromosomes can also be fatal, but many people can live with these genomes. In cases where the zygote continues development into a fetus, and ultimately survives to birth, the child will have a set of symptoms known as a syndrome that is caused by the abnormal number of chromosomes. If either of these gametes (the one with the extra chromosome, or the one with a chromosome missing) unites with another gamete during fertilization, the result is an abnormal chromosome number in the new zygote. This is known as aneuploidy. In humans, aneuploidy will result in one of two conditions, either trisomy or monosomy. • Trisomy: the developing zygote has one extra chromosome in its genome (2n + 1) for a total of 47 chromosomes • Monosomy: the developing zygote has one missing chromosome (2n-1) for a total of 45 chromosomes; there is only one monosomy syndrome in humans where the child is able to survive. The following are brief descriptions of some of the most common human syndromes due to alterations in chromosome number. Down Syndrome. Also known as trisomy 21, this syndrome is the result of an extra copy of chromosome 21. People with Down syndrome have a total of 47 chromosomes, with 3 copies of the number 21 chromosome. Down syndrome is correlated with the age of the mother; the older the mother, the greater the probability of Down syndrome. But the syndrome also can be the result of non-disjunction of the father’s number 21 chromosome. Down syndrome occurs in approximately 1:700 live births. Patau Syndrome. Also known as trisomy 13, this syndrome is the result of an extra copy of chromosome 13. People with this syndrome have a total of 47 chromosomes, with three copies of the number 13 chromosome. Patau syndrome occurs in approximately 1:5000 live births. Edward’s Syndrome. Also known as trisomy 18, this syndrome is the result of an extra copy of chromosome 18. People with this syndrome have a total of 47 chromosomes, with three copies of the number 18 chromosome. Edward’s syndrome occurs in approximately 1:3000 live births. It affects girls more than three times as often as boys. Klinefelter Syndrome: Men with Klinefelter syndrome have a total of 47 chromosomes, with 2 X chromosomes, and also a Y chromosome (XXY). This syndrome occurs in approximately 1:500 to 1:1000 live male births Trisomy X: Females with this syndrome have three X chromosomes instead of two, for a total of 47 chromosomes. This syndrome occurs in approximately 1:1000 live births. XYY Syndrome: In this case the male has a total of 47 chromosomes, including 2 copies of the Y chromosome instead of the usual one. The syndrome occurs in approximately 1:1000 live male births. Turner’s syndrome: is the only monosomy syndrome in humans where the fetus is able to survive and live through birth. Other monosomies are so devastating that the growing fetus does not survive. It occurs in approximately 1:5000 live births, and women with Turner’s syndrome have only 45 chromosomes. Alterations in Chromosome Structure It is possible for chromosomes to break during the meiosis process, leading to physical changes in chromosome structure. This generally results in one of three different types of changes. • Deletions: a portion of a chromosome is lost during meiosis. That chromosome is now missing the genes that are usually found on that part of the chromosome. If this chromosome is passed on to the offspring, the result is often lethal due to the missing genes. • Duplication: in some cases a fragment may break off one chromosome, and then join onto the other homologous chromosome of the pair. If that occurs, then that region of the chromosome is duplicated. • Translocation: a fragment of a chromosome is moved or “trans-located” from one chromosome to another, joining a non-homologous chromosome. The balance of genes is normal because nothing has been gained or lost, but the change can result in changes in phenotype because the genes have been placed in a different environment. The following are brief descriptions of some of the most common human syndromes due to alterations in chromosome number. Cri du Chat: A small piece of the short arm of chromosome number 5 is deleted. This syndrome is also called 5p minus syndrome because part of the p arm is deleted. It is a relatively rare condition, with an estimated incidence between 1:25,000 and 1:50,000 live births. Approximately 80% of cases are caused by a spontaneous deletion in one of the child’s number 5 chromosomes. Fragile X: the X chromosome of some people is unusually fragile at one tip, and appears to be hanging by a thread” when viewed under a microscope. This seems to be due to an unusually high number of sequence repeats at this end of their chromosome (the normal number is 29; Fragile X people have over 700 repeats due to duplications). Activity 2: Karyotype Analysis On your Skill Check worksheet you will find the series of karyotypes that you, as a cytogeneticist, were responsible for creating for a series of patients. For each of the karyotypes, answer the questions that are listed below the karyotype. You may use textbooks that are in the lab, or the Internet to find information about the syndrome. A normal female karyotype and a normal male karyotype are found below. Normal Female Karyotype Normal Male Karyotype Activity 3: Genetics Practice Problems 1) Bartter syndrome results from damaged NaCl transport in the loop of Henle of the kidney. After birth, children have poor growth rates and appear malnourished. Most patients have low or low-normal blood pressure, and the inability to retain K, Ca or Mg. This can lead to muscle weakness, spasms, and in some cases, mental retardation. The syndrome is inherited in an autosomal recessive manner. A man who had a sister with Bartter syndrome marries a woman who is homozygous dominant for the normal allele. a) What are the possible genotypes for the man? b) What is the genotype of the woman? c) Depending upon the genotype of the man, what are the possibilities for genotypes and phenotypes of the children? 2) Fanconi anemia is an inherited anemia that leads to bone marrow failure (aplastic anemia). It is an autosomal recessive disorder. It occurs equally in males and females, and found in all ethnic groups; it can affect all systems of the body, and many patients eventually develop acute myelogenous leukemia at an early age; they also develop a variety of other head, neck, gynecological and or gastrointestinal cancers; the patient can be cured of the FA blood problem by having a successful bon…
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