Chromosome And Karyotype Review Answer Key

Chromosome and Karyotype: A Comprehensive Review with Answer Key

Understanding chromosomes and karyotypes is fundamental to genetics and medicine. This comprehensive review delves into the intricacies of chromosome structure, function, and analysis, culminating in a detailed answer key to common practice questions. We'll explore the essential concepts, providing a robust foundation for students and professionals alike.

What are Chromosomes?

Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. They are made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

Chromosome Structure: A Closer Look

Each chromosome consists of:

• Chromatin: This is the complex of DNA and proteins (histones) that make up chromosomes. The histones help organize and compact the DNA, preventing it from becoming tangled.
• Centromere: This is a constricted region of the chromosome that plays a crucial role in cell division. It's the point where the two sister chromatids are joined.
• Telomeres: These are repetitive DNA sequences located at the ends of chromosomes. They protect the chromosome ends from degradation and fusion with other chromosomes.
• Sister Chromatids: These are identical copies of a chromosome created during DNA replication. They are joined together at the centromere until they separate during cell division.

Types of Chromosomes

Chromosomes are categorized based on the position of their centromeres:

• Metacentric: The centromere is located in the middle, resulting in two arms of approximately equal length.
• Submetacentric: The centromere is slightly off-center, leading to one long arm and one short arm (p arm and q arm respectively).
• Acrocentric: The centromere is located near one end, resulting in one very short arm and one very long arm.
• Telocentric: The centromere is located at the very end of the chromosome. These are rare in humans.

Karyotypes: Visualizing Chromosomes

A karyotype is a visual representation of an individual's chromosomes. It's a complete set of chromosomes arranged in pairs according to size, shape, and banding pattern. Karyotyping is a crucial cytogenetic technique used for:

• Detecting chromosomal abnormalities: This includes aneuploidy (abnormal number of chromosomes), translocations (chromosome rearrangements), and deletions.
• Diagnosing genetic disorders: Many genetic disorders are associated with specific chromosomal abnormalities.
• Prenatal diagnosis: Karyotyping can be performed on fetal cells obtained through amniocentesis or chorionic villus sampling to detect chromosomal abnormalities before birth.
• Cancer diagnosis: Karyotyping can help identify chromosomal abnormalities associated with cancer, aiding in diagnosis and prognosis.

Obtaining a Karyotype: The Process

Creating a karyotype involves several steps:

1. Cell Collection: Cells are collected from a blood sample, amniotic fluid, or other tissue.
2. Cell Culture: The cells are grown in a laboratory to increase their number.
3. Chromosome Harvesting: The cells are treated to arrest cell division at metaphase, when the chromosomes are most condensed and easily visible.
4. Chromosome Staining: Chromosomes are stained with dyes (like Giemsa) to create distinct banding patterns that help identify individual chromosomes.
5. Microscopy and Image Capture: The chromosomes are viewed under a microscope, and images are captured.
6. Chromosome Arrangement: The images of the chromosomes are arranged in pairs according to size and banding pattern to create the karyotype.

Common Chromosomal Abnormalities

Many genetic disorders result from chromosomal abnormalities. Some of the most common include:

• Down Syndrome (Trisomy 21): Characterized by the presence of an extra copy of chromosome 21.
• Turner Syndrome (Monosomy X): Affects females and involves the absence of one X chromosome.
• Klinefelter Syndrome (XXY): Affects males and involves an extra X chromosome.
• Cri-du-chat Syndrome: Caused by a deletion of part of chromosome 5.
• Philadelphia Chromosome: A specific translocation involving chromosomes 9 and 22, often associated with chronic myeloid leukemia.

Interpreting Karyotypes: A Step-by-Step Guide

Interpreting a karyotype requires careful observation and understanding of chromosome nomenclature. The standard format is as follows:

• Number of chromosomes: The total number of chromosomes is listed first.
• Sex chromosomes: The sex chromosomes (XX for females, XY for males) are then specified.
• Abnormalities: Any chromosomal abnormalities are described, including the specific chromosome involved and the type of abnormality (e.g., trisomy, deletion, translocation).

Example: A karyotype written as 47,XX,+21 indicates a female with Down syndrome (47 total chromosomes, two X chromosomes, and an extra chromosome 21).

Practice Questions and Answer Key

Here are some practice questions to test your understanding, followed by a detailed answer key:

Question 1: What is the function of the centromere?

Answer 1: The centromere is the constricted region of a chromosome that joins the two sister chromatids. It plays a vital role in chromosome segregation during cell division.

Question 2: Describe the difference between a metacentric and an acrocentric chromosome.

Answer 2: A metacentric chromosome has a centromere located in the middle, resulting in two arms of approximately equal length. An acrocentric chromosome has a centromere located near one end, resulting in one very short arm and one very long arm.

Question 3: What is a karyotype, and what is its clinical significance?

Answer 3: A karyotype is a visual representation of an individual's chromosomes, arranged in pairs according to size, shape, and banding pattern. It's clinically significant because it allows for the detection of chromosomal abnormalities, aiding in the diagnosis of genetic disorders and cancers, and providing valuable information for prenatal diagnosis.

Question 4: Explain the significance of telomeres.

Answer 4: Telomeres are repetitive DNA sequences at the ends of chromosomes. They protect chromosome ends from degradation and fusion with other chromosomes, maintaining genomic stability.

Question 5: What is Down syndrome, and how is it detected using karyotyping?

Answer 5: Down syndrome (Trisomy 21) is a genetic disorder characterized by the presence of an extra copy of chromosome 21. Karyotyping reveals this extra chromosome, providing a definitive diagnosis. The karyotype would typically be written as 47,XX,+21 (for females) or 47,XY,+21 (for males).

Question 6: What are some limitations of karyotyping?

Answer 6: While karyotyping is a powerful technique, it has limitations. It might not detect all small-scale chromosomal abnormalities (like microdeletions or microduplications) and requires living cells for analysis. Furthermore, it cannot identify single-gene mutations.

Question 7: A karyotype shows 45,X. What condition does this represent?

Answer 7: This karyotype represents Turner Syndrome, a condition affecting females characterized by the absence of one X chromosome.

Question 8: A karyotype reveals a translocation between chromosomes 9 and 22. What is the likely diagnosis?

Answer 8: This translocation, known as the Philadelphia chromosome, is strongly associated with chronic myeloid leukemia (CML).

Question 9: Describe the process of preparing a karyotype from a blood sample.

Answer 9: The process involves collecting a blood sample, culturing the cells to increase their number, arresting cell division at metaphase using colchicine, staining the chromosomes with Giemsa dye, viewing them under a microscope, capturing images, and arranging the chromosomes in pairs to create the karyotype.

Question 10: What is the significance of chromosome banding in karyotype analysis?

Answer 10: Chromosome banding, produced by staining techniques like Giemsa banding, creates unique banding patterns on each chromosome. These patterns help identify individual chromosomes and detect subtle structural abnormalities such as deletions, duplications, and inversions that might not be visible without banding. This greatly enhances the accuracy and detail of karyotype analysis.

Conclusion

Chromosomes and karyotypes are essential concepts in genetics and medicine. Understanding their structure, function, and analysis is crucial for diagnosing and managing a wide range of genetic disorders and cancers. This review provides a solid foundation for further exploration into this fascinating field. Remember, continued learning and practice are key to mastering this complex but rewarding subject. This comprehensive review, along with the detailed answer key, should equip you to confidently tackle the challenges of understanding chromosome structure and karyotype interpretation.