Chromosomes, Cell division, meiosis and chromosome abnormalities Flashcards
Describe the basic structure and packaging of chromosomes.
Basic Structure and Packaging of Chromosomes
Chromosomes are highly organized structures of DNA and proteins that carry genetic information. The structure and packaging of chromosomes are essential for ensuring that DNA fits within the cell nucleus while still being accessible for processes like replication, transcription, and repair. The process of chromosome packaging allows the long DNA molecule to fit into the relatively small volume of a cell’s nucleus while maintaining its structural integrity.
Basic Structure of Chromosomes
DNA Molecule:
At its core, a chromosome is made of DNA (deoxyribonucleic acid), a long polymer composed of repeating units called nucleotides. Each nucleotide consists of a phosphate group, a sugar molecule (deoxyribose in DNA), and a nitrogenous base (adenine [A], thymine [T], cytosine [C], or guanine [G]).
The DNA sequence determines the genetic information carried by the chromosome.
The DNA in chromosomes is double-stranded and forms a double helix structure, which was first described by James Watson and Francis Crick in 1953.
Histones:
DNA in chromosomes is wrapped around proteins called histones. These proteins help in organizing and compacting the DNA. Histones are positively charged, which facilitates the binding of negatively charged DNA.
There are five major types of histones: H1, H2A, H2B, H3, and H4.
Nucleosome:
The fundamental unit of chromatin packaging is the nucleosome. A nucleosome consists of about 146 base pairs of DNA wrapped around a core of eight histone proteins (two of each: H2A, H2B, H3, and H4).
The DNA wraps around the histone core like thread around a spool, forming the “beads on a string” appearance under the microscope.
The H1 histone binds the DNA where it enters and exits the nucleosome, helping to stabilize the structure.
Chromatin:
Chromatin is the complex of DNA and histone proteins that makes up chromosomes. Chromatin exists in two forms:
Euchromatin: Loosely packed chromatin that is transcriptionally active, meaning the DNA in euchromatin is accessible for transcription and gene expression.
Heterochromatin: Tightly packed chromatin, which is usually transcriptionally inactive and often contains repetitive sequences or silenced genes.
Higher Levels of Chromatin Packaging
30 nm Fiber:
The next level of organization involves the coiling of nucleosomes into a thicker structure, often referred to as the 30 nm fiber. This structure is formed by the linker DNA (the DNA between nucleosomes) folding in a zig-zag pattern, resulting in a more compact form of chromatin.
Looping and Further Compaction:
The 30 nm fiber is further organized into loops anchored to a protein scaffold or matrix inside the nucleus. These loops form large domains of chromatin.
This looping creates higher-order structures that help further compact the DNA into a more organized form, especially during cell division.
Chromosome Condensation:
As the cell prepares to divide (during mitosis or meiosis), the chromatin further condenses into distinct chromosomes. This is necessary for ensuring the proper segregation of genetic material during cell division.
The most condensed form of a chromosome during cell division is called a metaphase chromosome, which consists of two sister chromatids joined at the centromere.
Centromere:
The centromere is a constricted region of the chromosome that holds the two sister chromatids together. During cell division, it plays a crucial role in the proper segregation of chromosomes to daughter cells.
The kinetochore, a protein complex, assembles on the centromere and attaches to spindle fibers, facilitating chromosome movement during cell division.
Telomeres:
Telomeres are specialized structures at the ends of chromosomes that protect them from degradation and prevent the loss of important genetic information during DNA replication.
Telomeres consist of repetitive nucleotide sequences (e.g., TTAGGG in humans) that shorten with each cell division. This progressive shortening is associated with cellular aging.
Chromosome Packaging Summary:
DNA is wrapped around histone proteins to form nucleosomes (the basic packaging unit).
Chromatin is the combination of DNA and histones, and it exists in euchromatin (active) and heterochromatin (inactive) forms.
The chromatin is further compacted into a 30 nm fiber, and these fibers are looped and anchored to form higher-order structures.
During cell division, chromatin condenses into distinct chromosomes, which consist of two sister chromatids connected by a centromere.
Telomeres protect the ends of chromosomes and prevent the loss of genetic information during DNA replication.
This intricate organization allows the DNA, which can be up to several meters long in a human cell, to fit into the tiny nucleus while still being accessible for the processes of transcription, replication, and repair.
Describe the laboratory diagnosis of genetic disease due to abnormalities in chromosome number.
Chromosomal abnormalities, such as aneuploidy (the presence of an abnormal number of chromosomes), can lead to various genetic disorders. These abnormalities can be diagnosed through cytogenetic testing, which includes methods that visualize chromosomes directly or analyze genetic material for numerical or structural changes. Below are the key methods used for diagnosing genetic diseases related to abnormalities in chromosome number.
- Karyotyping
What is it?
Karyotyping is a laboratory technique used to examine the number, size, and shape of chromosomes in a cell. It involves arranging and visualizing chromosomes from a cell sample under a microscope.
It is commonly used to diagnose chromosomal aneuploidies (abnormal number of chromosomes) and structural chromosomal abnormalities (such as translocations, deletions, duplications).
How It Works:
A sample of cells (usually from blood, amniotic fluid, or bone marrow) is collected.
The cells are cultured and treated with chemicals that arrest them in the metaphase stage of mitosis, where chromosomes are most visible.
The chromosomes are stained with a dye (such as Giemsa) to create a banding pattern and are then photographed.
The chromosomes are arranged into pairs based on size, banding pattern, and centromere position to create a karyogram, where abnormalities can be identified.
What It Detects:
Aneuploidies: For example, Down syndrome (Trisomy 21), where there is an extra copy of chromosome 21, or Turner syndrome (Monosomy X), where there is only one X chromosome instead of two.
Structural Abnormalities: These include deletions, duplications, inversions, and translocations.
Advantages:
Direct visualization of chromosome number and structure.
Effective for detecting large chromosomal abnormalities.
Limitations:
Resolution is limited (it may not detect small chromosomal changes, such as microdeletions or duplications).
It requires live cells and can take several days for results. - Fluorescence In Situ Hybridization (FISH)
What is it?
Fluorescence in situ hybridization (FISH) is a molecular cytogenetic technique used to detect and localize the presence or absence of specific DNA sequences on chromosomes.
It uses fluorescently labeled DNA probes that bind to complementary sequences on chromosomes. This allows for the detection of specific chromosomal regions and aneuploidies.
How It Works:
Cells are fixed to slides, and specific DNA probes labeled with fluorescent dyes are applied.
The probes hybridize to specific chromosome regions (for example, a probe for chromosome 21 for Down syndrome).
Under a fluorescent microscope, the presence of the probe indicates the location of the target chromosome or chromosomal region.
What It Detects:
Aneuploidies: For example, Trisomy 13, Trisomy 21, or Klinefelter syndrome (XXY) by targeting specific chromosomes.
Microdeletions and microduplications: Smaller chromosomal imbalances that are too small to be detected by karyotyping.
Chromosomal translocations: Specific rearrangements of chromosomes, such as the Philadelphia chromosome in Chronic Myelogenous Leukemia (CML).
Advantages:
High sensitivity for detecting specific chromosomal abnormalities.
Faster than karyotyping and can be performed on interphase nuclei, making it useful for non-dividing cells (like in prenatal samples).
Higher resolution than karyotyping for detecting smaller chromosomal changes.
Limitations:
Limited to detecting predefined abnormalities (requires prior knowledge of what is being tested for).
Cannot detect unknown abnormalities or those without a specific probe. - Chromosomal Microarray (CMA)
What is it?
Chromosomal Microarray Analysis (CMA), also known as array comparative genomic hybridization (aCGH), is a high-resolution method used to detect copy number variations (CNVs), including both deletions and duplications of chromosomal regions.
CMA detects submicroscopic chromosomal imbalances that may not be visible with traditional karyotyping.
How It Works:
DNA from the patient and a reference sample are labeled with different fluorescent dyes and hybridized to an array of probes that correspond to specific regions of the genome.
The differences in fluorescence intensity indicate gains (duplications) or losses (deletions) in the chromosomal material.
What It Detects:
Copy number variations (such as microdeletions and microduplications) that are associated with genetic syndromes like Williams syndrome, DiGeorge syndrome, and Prader-Willi syndrome.
Larger chromosomal imbalances (like those detected in karyotyping, but with higher resolution).
Advantages:
High resolution—detects smaller chromosomal abnormalities that karyotyping might miss.
No need for live cells—can be done on DNA extracted from blood or other tissues.
Comprehensive analysis of the genome for known and unknown chromosomal abnormalities.
Limitations:
Does not detect balanced translocations or mosaicism (where two or more cell populations with different chromosomal compositions exist).
Limited to detecting imbalances, not structural rearrangements like inversions. - Non-Invasive Prenatal Testing (NIPT)
What is it?
Non-invasive prenatal testing (NIPT) uses cell-free fetal DNA circulating in a pregnant woman’s blood to detect chromosomal abnormalities in the fetus. This test is commonly used to screen for Down syndrome (Trisomy 21), Trisomy 18, and Trisomy 13.
How It Works:
A blood sample is taken from the mother, and the fetal DNA is extracted from the plasma.
The fetal DNA is analyzed to determine the relative number of chromosomes, and any abnormalities in chromosome number can be detected, such as an extra chromosome in Trisomy disorders.
What It Detects:
Aneuploidies: Specifically, Trisomy 21, Trisomy 18, and Trisomy 13.
It may also detect sex chromosome abnormalities like Turner syndrome (monosomy X) or Klinefelter syndrome (XXY).
Advantages:
Non-invasive—does not carry the risk of miscarriage that invasive tests like amniocentesis or chorionic villus sampling (CVS) do.
Early detection—can be performed as early as the 10th week of pregnancy.
High accuracy for detecting common aneuploidies.
Limitations:
Screening test rather than a diagnostic test (false positives and false negatives are possible).
Limited ability to detect other chromosomal abnormalities (such as balanced translocations). - Quantitative PCR (qPCR) and Next-Generation Sequencing (NGS)
What is it?
Quantitative PCR (qPCR) and Next-Generation Sequencing (NGS) are advanced molecular techniques used to detect chromosomal abnormalities by analyzing specific genetic sequences.
qPCR quantifies DNA by amplifying target sequences, and NGS sequences the entire genome to identify small variations, including chromosomal abnormalities.
How It Works:
qPCR amplifies specific regions of DNA, and the quantity of amplification is used to detect duplications or deletions in the genome.
NGS sequences the entire genome, allowing for detailed analysis of single-nucleotide polymorphisms (SNPs), structural variants, and copy number variations.
What It Detects:
Copy number variations, including smaller deletions and duplications.
Aneuploidy in specific regions or whole chromosomes.
Point mutations and structural variations.
Advantages:
High resolution and ability to detect a wide range of chromosomal abnormalities, including those missed by other methods.
Can provide comprehensive genetic analysis in a single test.
Limitations:
Complexity and cost of testing.
May require bioinformatics expertise for interpreting large amounts of data.
Describe the types of structural chromosomal abnormalities.
- Deletions
Definition:
A deletion occurs when a segment of a chromosome is lost or deleted. This can result in the loss of one or more genes, which may lead to various genetic disorders depending on which genes are missing.
Types of Deletions:
Terminal Deletion: Involves the loss of genetic material from the end of a chromosome.
Interstitial Deletion: Involves the loss of genetic material from the middle of a chromosome, leaving the ends intact.
Examples:
Cri-du-chat syndrome: Caused by a deletion on chromosome 5 (specifically 5p), leading to developmental delays, a characteristic high-pitched cry, and intellectual disabilities.
Williams syndrome: Caused by a deletion on chromosome 7 (7q11.23), leading to cardiovascular problems, intellectual disability, and a distinctive personality.
Consequences:
Loss of essential genes can cause developmental and health problems depending on the genes involved.
2. Duplications
Definition:
A duplication occurs when a segment of a chromosome is repeated. This can result in an increased dosage of the genes in the duplicated region, which can lead to overexpression of certain genes and cause disease.
Types of Duplications:
Tandem Duplication: The duplicated segment is adjacent to the original segment, in the same orientation.
Dispersed Duplication: The duplicated segment is scattered or located in different parts of the chromosome.
Examples:
Charcot-Marie-Tooth disease type 1A: Caused by a duplication of a region on chromosome 17, which leads to peripheral neuropathy and muscle weakness.
Duchenne muscular dystrophy: In some cases, duplications of regions in the DMD gene on X chromosome lead to this condition, causing muscle wasting and weakness.
Consequences:
Gene dosage imbalance can lead to diseases depending on the functions of the duplicated genes.
It may result in developmental issues, neurological problems, or other phenotypic changes.
3. Inversions
Definition:
An inversion occurs when a chromosomal segment breaks in two places, and the segment is reversed (inverted) and reinserted into its original location. While the total amount of genetic material remains unchanged, the rearrangement may disrupt gene function or cause problems during meiosis.
Types of Inversions:
Pericentric Inversion: The inversion includes the centromere.
Paracentric Inversion: The inversion does not include the centromere.
Examples:
Inv(9)(p11q13): A common inversion found in humans that typically does not cause any apparent health problems but may affect the outcome of future pregnancies due to potential complications in meiosis.
Hemophilia A (in some cases): Can result from inversions in the F8 gene on the X chromosome, leading to clotting factor deficiencies.
Consequences:
Inversions can cause problems in chromosome pairing during meiosis and result in miscarriages or infertility.
Inversions may also disrupt important genes if the breakpoints occur within functional gene regions.
4. Translocations
Definition:
A translocation occurs when a segment of one chromosome breaks off and attaches to a different chromosome. This can result in an exchange of material between two chromosomes and can lead to gene disruptions or altered gene expression.
Types of Translocations:
Reciprocal Translocation: The exchange of material between two chromosomes. Both chromosomes involved lose and gain material, but the total genetic content remains the same (no loss of genetic material).
Robertsonian Translocation: A type of translocation that typically involves two acrocentric chromosomes (chromosomes with a very small short arm) that fuse near the centromere, resulting in the loss of the short arms and the creation of a single chromosome.
Examples:
Philadelphia Chromosome: A reciprocal translocation between chromosomes 9 and 22 (t(9;22)) that leads to Chronic Myelogenous Leukemia (CML) due to the creation of an abnormal fusion gene (BCR-ABL).
Down syndrome: A form of Down syndrome can be caused by a Robertsonian translocation between chromosome 14 and chromosome 21, leading to trisomy 21 in the offspring without a true extra chromosome 21.
Consequences:
Translocations can result in genetic imbalance, disruption of genes, or creation of abnormal fusion genes (like the BCR-ABL fusion gene in CML).
Translocations may also lead to miscarriages, infertility, or birth defects if they disrupt critical genes.
5. Ring Chromosomes
Definition:
A ring chromosome forms when the ends of a chromosome are lost and the remaining ends of the chromosome rejoin to form a ring structure. This can result in the loss of important genetic material.
Examples:
Ring chromosome 14: This can lead to developmental delays, seizures, and other neurological issues due to the loss of genetic material from chromosome 14.
Ring chromosome 22: Associated with cat-eye syndrome, a disorder that causes eye abnormalities, heart defects, and other physical and developmental issues.
Consequences:
Loss of genetic material can cause developmental problems, neurological issues, and growth retardation depending on which genes are missing.
Ring chromosomes may also lead to difficulties in chromosome segregation during cell division, which can cause mosaicism (where different cells have different chromosomal compositions).
6. Isochromosomes
Definition:
An isochromosome is a type of chromosomal abnormality where a chromosome loses one arm and the remaining arm is duplicated. The result is a chromosome with two identical arms rather than the usual two different arms.
Examples:
Isochromosome Xq: Seen in some cases of Turner syndrome (monosomy X), where the long arm (q) of the X chromosome is duplicated, leading to developmental abnormalities.
Isochromosome 12p: Involved in some cases of Pallister-Killian syndrome, causing developmental and intellectual disabilities.
Consequences:
Isochromosomes often result in imbalanced genetic content, leading to developmental and cognitive impairments.
The extra genetic material from the duplicated arm can lead to overexpression of certain genes and disrupt normal development.
