Human Chromosomes lab 5

Transcription

1 Human Chromosomes lab 5 Objectives Upon completion of this activity, you should be able to: describe the structure of human chromosomes with reference to size, centromere position, and presence or absence of satellites. prepare a metaphase spread using a human tumor cell line known as HeLa. describe representative chromosomal abnormalities using standard cytogenetic designations. prepare a human karyotype and determine the number of chromosomes present, sex of the individual, and the presence or absence of chromosome abnormalities. understand the importance of cytogenetics in the study of cancer and chromosomal aberrations. Each somatic cell in the human body contains 23 pairs of chromosomes. During interphase of the cell cycle each of these chromosomes is duplicated and consists of two chromatids joined by a common centromere (spindle attachment region). During mitosis the chromatids separate and become independent chromosomes which move to opposite ends of the cell. The subsequent division of the cytoplasm results in the formation of two new daughter cells each containing the same diploid number of chromosomes as the parent cell. Colchicine, a plant alkaloid, has the unique property of arresting cells in metaphase of the mitotic cycle by interfering with the formation of spindle fibers needed for the separation of chromosomes during anaphase. Since the chromosomes are most coiled and condensed during metaphase this stage of mitosis is the most convenient to observe. Once treated with colchicine the cells are then exposed to a hypotonic solution that causes the cell to swell and become more fragile. The cells are then fixed with an acetic acid-methanol solution which preserves the existing cell components. The cells are then ready to be splattered onto slides and stained. This is followed by a search for ideal chromosome spreads for the study of chromosome number and structure. When these techniques were applied by Tijo and Levan (in 1956) to cultured cells of human lung embryonic tissue, the human chromosome number was established at 2n = 46. This observation was soon confirmed by other cytogeneticist. The significance of the number of chromosomes present in humans became apparent in 1959 when Lejeune and his coworkers attributed the disorder Down Syndrome to an abnormal chromosome number (47, +21). All normal human cells contain identical numbers and types of chromosomes.

2 Each chromosome pair contains unique physical attributes which distinguish them from all the others. The three main criteria used to distinguish and identify individual chromosomes are: 1) length of the chromosome, 2) position of the centromere, and 3) staining/banding pattern of a chromosome when exposed to specific chemicals. Using these criteria, cytogeneticist have set up a classification system for chromosomes which labels each pair of autosomes with a number, or for the sex chromosomes an X or Y. This system of standardization allows for accurate communication among scientists and medical professionals. Many genetics conditions have been associated with a specific change or abnormality. The abnormalities include: polyploidy - an increase in sets of chromosomes, e.g., 3n or 4n aneuploidy - extra of missing chromosomes, e.g., 2n + 1 or 2n - 1 translocations - attachment of a chromosome or piece of a chromosome to another, e.g., t(14/21) structural alterations - deletions, duplications, inversions, fragile sites, ring chromosomes, e.g., 5p- Most alterations in chromosome number are lethal especially numerical changes involving extra sets of chromosomes or any of the larger autosomes. Unbalanced chromosome counts involve thousands of genes and are not compatible with normal growth and development of the fetus. Sex chromosomal abnormalities are less severe than autosomal abnormalities. Irregular chromosome counts are associated with a constellation of abnormalities that may include morphological, physiological, and psychological deviations from normal. Such a complex of symptoms is called a syndrome. Some examples of genetic conditions and their respective chromosome aberrations are: Down Syndrome - typically due to an extra chromosome #21 (trisomy 21) Cri du chat - characterized by a deletion of the short arm of chromosome #5. Fragile X Syndrome - characterized by a fragile site or break point on the long arm of the X sex chromosome One practical application of karyotype analysis is the early detection of chromosomal defects through amniocentesis or chorionic villus sampling (CVS). In amniocentesis some of the amniotic fluid surrounding the fetus is removed by a physician. The fluid contains fetal cells which will propagate under special laboratory conditions. Once the cells have increased in number a karyotype can be performed on these fetal cells for prenatal diagnosis of sex and chromosome abnormalities. In karyotyping not involving fetuses the cell type most often used for analysis are lymphocytes or WBC s. As with fetal cells these cells are grown in culture, treated with hypotonic solution, and fixed prior to performing a karyotype. Several kinds of cancers have been associated with chromosomal abnormalities and many malignant cells tend to have abnormal chromosome numbers. For example, chronic myelogenous leukemia (CML) is associated with the so-called Philadelphia chromosome (Ph). The defective chromosomes in CML occur only in white blood cells and involves a reciprocal translocation (exchange of segments) between the long arms of chromosomes 9 and 22. The proper designation for this condition is 46, (XX or XY), [t(9/22)].

3 Cells grown in tissue culture are used for most human chromosome studies. In this exercise, the human tumor cell line, HeLa, is used for karyotyping. The HeLa cell line originated in the early 1950 s from the cancerous cervical cells of a women named Henrietta Lacks. Because the cells are of tumor origin they are immortal and have continued to divide and multiply in tissue culture for over 40 years. Cancer cells result from uncontrolled mitosis when mutant cells lose cell cycle checkpoints which normally trigger apoptosis in chromosomally abnormal cells. Many cancer cells are aneuploids containing chromosome counts greater than 46 with one, two or even three extra copies of a particular chromosome. Try to make a chromosome count of your spreads to determine if the cells have more than 46 chromosomes. PART I. Preparing A Metaphase Spread Of HeLa Cells A log phase culture of HeLa cells were incubated with colchicine, treated with trypsin, fixed, and placed in a hypotonic solution prior to shipping. The cell suspension will be stable for a few days but should be refrigerated when not in use. 1. Place a new, clean microscope slide at a 45 0 angle. 2. With a pipette, gently resuspend the cells in the tube provided. Remove a small sample of cell suspension with the pipette and hold the pipette 2 feet above the slide. Allow one drop of cell suspension to "splat onto the slide about 3/4 inch from the upper end so that it tumbles down the slide. 3. Carefully apply 6-8 more drops from various heights, one drop at a time, onto the same region of the slide. It is important to release the cell suspension one-drop-at-atime. Do not expel all of your cell suspension in one squirt, or you will obtain poor results. 4. Gently blow across the slide for 2-3 seconds. The drying will help "spread the chromosomes. 5. Allow the cells to air dry completely. 6. Dip the slide into the tube containing Stain #1 for one second only. Remove the slide and dip into Stain #1 again for one second only. Remove the slide and dip into Stain #1 for one second only a third time. 7. Caution should be taken to avoid carry over of stains. Wipe the bottom of the slide with a paper towel before transferring. 8. Drain off the stain and dip the slide into the tube containing Stain #2 for one second only. Remove the slide and dip again into Stain #2 for one second, dip a third time into Stain #2 for one second. 9. Remove the slide from the stain and thoroughly rinse with distilled water. 10. Allow the slide to air dry completely. A stream of warm air or blowing may help speed up the drying process. Incomplete drying will result in poor resolution when the mounting medium (Permount) is added. While your waiting for your slides to dry work on Part II.

4 11. Place two drops of Permount as demonstrated by your instructor on the stained area of your slide and place a coverslip over the glue. Apply gently pressure to the coverslip to spread the Permount evenly under the coverslip. You may wish to place 2 coverslips side by side so as to allow viewing of the entire microscope slide. 12. Once the Permount has dried the slide is ready for viewing. 13. Under low power (100 x) scan your spread for cells which appear to have ruptured and released their chromosomes. Shift to high power (450x) to examine each spread more carefully. Low and high dry observations can be made immediately. Observations with the oil immersion objective should not be done until the Permount has dried completely (48-72 hours) or should be done very carefully so as not to transfer Permount to the objective. Caution: Only use the oil immersion lens on those areas of the slide containing a coverslip. If Permount does get onto an objective notify your instructor and try to remove it immediately using lens paper and xylene. 14. An ideal chromosome spread will contain chromosomes which appear distinct and do not overlap with adjacent chromosomes. Chromatids should be visible. The hunt requires careful observation so take your time when scanning the slide. Once you have found what appears to be a clear and distinct set of chromosomes place a small drop of immersion oil (if available) on the coverslip over the area and switch to 1000x. Note: When using the oil immersion lens you will have to increase the amount of light passing through the specimen. This can be done by increasing the light intensity and adjusting the iris diaphragm. 15. Try to identify and locate the three different types of chromosomes: metacentric, submetacentric and acrocentric. 16. Try to count the number of chromosomes present in 5 different cells on the slide. Remember that this cell line is aneuploid and each cell will probably contain a different number of chromosomes, each greater than the diploid number (46). Cell #1 #2 #3 #4 #5 # of chromosomes

5 Procedure for Preparing a Metaphase Spread

6 PART II. Morphology Of Chromosomes On the basis of chromosome length and position of the centromere, normal human chromosomes have been arranged in seven groups of autosomes, A to G, and one pair of sex chromosomes, XX or XY. Chromosome bands are observed with Q- G- and R-staining methods (Quinacrine, Giemsa and reversed Giemsa bands or R bands). Autosomes are numbered in order from 1 to 22, and Y and X are listed separately. Short arms of chromosomes are designated p and long arms q. Regions are numbered from 1 (next to centromere) to the distal end of each arm, and bands are numbered within the regions from the centromere end to the distal end of each arm according to the 1971 Paris Conference on standardization in human cytogenetics. Observe the relative length and centromere location of the chromosomes shown in the diagram. Draw brackets around and assign the appropriate letter to each of the eight groups (A-G and sex chromosomes). Note that, in terms of size and centromere position, the X chromosome is similar to the chromosomes in the C group and the Y chromosome is similar to the G autosomes. How to Identify Human Chromosomes

7 2. The firs PART III. Preparing Human Karyotypes In medical cytogenetics laboratories chromosome spreads for each patient are examined through the microscope. The best appearing chromosome spreads are photographed and enlarged. Individual chromosomes are then cut out, arranged in matched pairs, and glued in proper order on a standardized form. Each karyotype includes a complete set of chromosomes for the person or fetus under investigation. Evaluations for structural and numerical chromosome abnormalities are made form the karyotype and verified when necessary from direct microscopic observations of the slides. A karyotype provides a tangible record for comparison, display, and filing for future use. In recent years most cytogenetics laboratories have begun using an automated, computerdriven karyotyping system. Although expensive, such a system can be accurate and efficient. 1. Before you begin, place the number found in the upper left hand corner of your metaphase spread on a blank standard karyotype form. This number identifies the karyotype to your instructor. 3. Carefully cut out each chromosome and attempt to arrange them from large to small. 4. To identify chromosomes more accurately you must examine the banding patterns. Use the normal karyotype provided as a guide to chromosome identification. 5. Use the standard karyotype form to arrange your chromosomes. Be sure to double check your arrangement before permanently pasting or taping any of the chromosomes to the form. 6. Using standard nomenclature, give the descriptive chromosome symbols, sex, and phenotype or syndrome for each of the karyotypes you prepare.

8 Name Human Chromosome Post-Lab Questions Answer as completely as possible. Turn in with your kayotypes at the beginning of next weeks lab. 1. Which of the human chromosomes are metacentric? Answer by giving individual chromosome numbers. 2. Which human chromosomes are submetacentric? 3. Acrocentric chromosomes have centromeres located near one end and satellite regions that represent the site of ribosomal RNA genes (nucleolar organizing region; NOR). Which human chromosomes are acrocentric? 4. What is a syndrome? 5. Give the complete chromosomal description for the following individuals as illustrated in "a. a. An infant male with Patau syndrome 47, XY, +13 b. A newborn female with Cri du Chat syndrome c. A man with Klinefelter syndrome 6. Why is it necessary to expose cells to a hypotonic solution when preparing them for karyotyping? 7. During what stage of mitosis are chromosomes in their most condensed state? 8. Explain the difference between polyploidy and aneuploidy.