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Questions and Answers
What is the chromosomal composition of Turner syndrome?
Which syndrome is characterized by a male genotype of 47,XY,+21?
Which of the following is a common feature of XXX syndrome?
What is the primary characteristic of XYY syndrome?
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Which chromosomal abnormality leads to Cri-du-chat syndrome?
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What is the total number of chromosomes in human somatic cells?
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What is a karyotype?
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What are chromosomal abnormalities (CAs) a common cause of?
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Which of the following describes aneuploidy?
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What is trisomy?
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What year did the Human Genome Project officially begin?
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What percentage of the human genome did the Human Genome Project account for?
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Which of the following organisms' genomes were sequenced as part of the Human Genome Project?
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What is the estimated number of genes in the human genome?
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Which of the following bases predominantly compose gene-rich regions of the genome?
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What is the largest known human gene?
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What percentage of the genome codes for proteins?
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How much of the DNA sequence in any two individuals is identical?
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What term describes the process of filling gaps in DNA sequences?
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What is a characteristic of noncoding DNA regions in the human genome?
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Study Notes
Chromosome Basics
- Human somatic cells contain 23 pairs of chromosomes, totaling 46.
- Composed of 22 pairs of autosomes and 2 sex chromosomes (either XX or XY).
- Gametes (sperm and egg cells) have 23 chromosomes.
Karyotype
- A karyotype is the complete set of chromosomes in an individual, typically displayed in a standardized format.
Chromosome Banding
- Chromosome banding involves alternating light and dark regions along the chromosome, revealed through staining.
Chromosomal Abnormalities
- Chromosomal abnormalities (CAs) are found in 50% of spontaneous fetal losses in the first trimester and are a common cause of congenital malformations.
- Types of CAs include:
- Numerical abnormalities
- Structural abnormalities
Numerical Chromosomal Abnormalities
- Changes in chromosome number can occur:
- Euploidy: Multiple complete sets of chromosomes (e.g., 3n = 69 chromosomes, 4n = 92 chromosomes).
- Aneuploidy: Deviation from the normal diploid number, such as 2n + 1 (trisomy) or 2n - 1 (monosomy).
Types of Aneuploidy
- Trisomy: Presence of three copies of a specific chromosome.
- Monosomy: Presence of one copy of a specific chromosome.
- Both types can affect autosomes or sex chromosomes.
- Non-disjunction leads to the formation of gametes with abnormal chromosome numbers.
Aneuploidy in Sex Chromosomes
- Turner syndrome: Characterized by a 45,XO karyotype; typically involves various phenotypic features.
- Klinefelter syndrome: Male with XXY karyotype; often undiagnosed until puberty, associated with infertility.
- Triple X syndrome (XXX): Generally associated with learning disabilities, tall stature, hypotonia, and sometimes infertility.
- Jacob’s syndrome (XYY): Males typically have tall stature, mild learning difficulties, normal sexual development, and fertility.
Autosomal Aneuploidies
- Generally result in early spontaneous abortion.
- Trisomy 13 (Patau Syndrome): Karyotype 47,XX,+13 (female) or 47,XY,+13 (male).
- Trisomy 21 (Down Syndrome): Karyotype 47,XX,+21 (female) or 47,XY,+21 (male).
- Trisomy 18 (Edward Syndrome): Karyotype 47,XX,+18 or 47,XY,+18.
Structural Chromosomal Abnormalities
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Deletions:
- Cri-du-chat syndrome: Microdeletion of chromosome 5; presents with a distinctive cat-like cry and severe mental retardation.
- Di-George syndrome: Microdeletion of chromosome 22.
- Angelman syndrome & Prader-Willi syndrome: Microdeletions of chromosome 15.
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Duplications:
- Example includes Fragile X syndrome, linked to duplications on chromosome Xq27.
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Translocation:
- In Down syndrome, an extra part or whole chromosome 21 is attached to another chromosome.
Overview of the Human Genome Project
- Genomics encompasses the complete DNA sequence study of organisms; initiated in the mid-1980s.
- The Human Genome Project (HGP) began in 1990, utilizing gene mapping and DNA sequencing technologies.
- Organisms such as E. coli, C. elegans, and D. melanogaster were sequenced as part of the project.
Assembling and Finishing Genome Sequences
- DNA sequences are assembled using computer algorithms based on overlapping sequences.
- Remaining gaps are filled through additional sequencing, a process called finishing.
- HGP lasted from 1990 to 2003, yielding a high-quality human genome sequence covering 92% of the genetic material.
- In 2022, a complete ‘telomere-to-telomere’ human genome sequence was achieved.
Draft Sequence and Gene Information
- Estimated number of human genes is between 20,000 and 25,000.
- Average gene length is about 40,000 bases; the largest known gene, dystrophin, is approximately 2.4 million bases long.
- DNA sequences among individuals are 99.9% identical.
Genome Composition
- Over 50% of discovered genes have unknown functions.
- Less than 2% of the genome is responsible for protein coding.
- Repeated non-coding sequences constitute at least 50% of the human genome.
- Chromosome sizes vary from 50 million to 250 million bases.
Gene Distribution
- Gene-rich regions predominantly consist of guanine and cytosine bases, while gene-poor regions are rich in adenine and thymine.
- Genes are randomly distributed throughout the genome, often surrounded by extensive areas of noncoding DNA.
Studying That Suits You
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Description
Explore the fundamentals of chromosomes with this quiz, focusing on human somatic cells, their structure, and karyotype analysis. Understand how chromosome banding is utilized to visualize genetic material in a standardized format. Test your knowledge on key concepts and terms related to chromosomes.
