Lecture 9:Cytogenetics 5 Variations in Chromosome Number and Structure 2 Flashcards
4 Variations in Chromosome Structure
Variation in Chromosome Structure
1. Deletions
2. Duplications
3. Inversions
4. Translocations
Define Deletion:
- Deletions:
- Deletions refer to the loss of a segment of a chromosome. It occurs when a portion of the chromosome breaks off and fails to rejoin during DNA replication or due to structural abnormalities.
- Deletions can vary in size, ranging from small segments to large portions of a chromosome.
- Deletions can lead to gene loss, and disruption of gene function, and can cause genetic disorders depending on the genes affected.
Explain deletions (4)
Deletions:
1. - When a CH breaks and a portion of it is lost, the missing piece is called a deletion
- Deletion may occur in the middle (intercalary) or @ the end (terminal) of a CH
- The synapses between a normal CH & a CH with intercalary deletion will result in a deletion or compensation loop of unpaired region
4.- Often lethal except for the deletion of tiny CH piece (Cri-du-chat-Syndrome)
explain deletions: origin of terminal deletion, origin of intercalary deletion, formation of deficiency loop
Deletions refer to the loss of a segment of a chromosome. They can occur in different ways and have varying effects on genetic material. Let’s explore the different aspects:
- Origin of Terminal Deletion:
- Terminal deletions occur when a segment is lost from the end of a chromosome.
- This type of deletion can result from a breakage event near the end of a chromosome followed by loss of the broken segment.
- Terminal deletions can lead to the loss of specific genes or regulatory regions located at the end of the chromosome, which can have significant effects on gene function and phenotype. - Origin of Intercalary Deletion:
- Intercalary deletions, also known as interstitial deletions, occur when an internal segment of a chromosome is lost.
- This type of deletion can result from breakage events within the chromosome followed by loss of the intervening segment.
- Intercalary deletions can cause the loss of one or more genes within the affected segment, leading to disruptions in gene dosage and gene interactions. - Formation of Deficiency Loop:
- A deficiency loop, also called a deletion loop, is a structural abnormality that forms during meiosis in individuals with a deletion.
- In the case of a deletion, the affected chromosome is missing a segment, resulting in unpaired regions during pairing of homologous chromosomes.
- To facilitate proper pairing, the intact homologous chromosome forms a loop to compensate for the missing segment, creating a deficiency loop.
- Deficiency loops can affect chromosome alignment during meiosis and may lead to abnormal segregation of genetic material, resulting in gametes with missing or duplicated genes.
It’s important to note that the effects of deletions can vary depending on the specific genes and regulatory regions involved. Deletions can disrupt normal gene function, leading to haploinsufficiency (where one functional copy of a gene is not sufficient for normal function), or cause genetic disorders depending on the genes affected. Understanding the nature and consequences of deletions is crucial in studying genetic disorders and their underlying mechanisms.
Impact of Deletions: 4
- Large deletions are generally lethal
- In a very general way, the larger the deletion, the larger the impact
- Deletion of genes essential for survival is always lethal
- Pseudodominance, in which typically recessive alleles behave in a dominant fashion may result from deletions due to short stretches of chromosomes which become hemizygous due to a deletion in one homolog
Deletion: CRI-DU-CHAT SYNDROME: THE CRY OF THE CAT (4)
- Cri-du-chat syndrome results from deletion of approximately 1/2 the human chromosome 5
*It is one of the very few human deletions known to survive to live birth
- The larger the deletion the more severe the symptoms which include:
– Gastrointestinal and cardiac malformation
– Mental retardation
– Abnormal development of the glottis and larynx resulting in a cry resembling that of a mewing cat - Incidence of about 1/50,000 live births
Define duplication:
Duplications:
1. Duplications refer to the presence of an extra copy of a segment of a chromosome. It occurs when a segment of a chromosome is copied, resulting in two identical or similar segments in the chromosome.
- Duplications can occur through errors during DNA replication or due to structural abnormalities.
- Duplications can have varying effects on the phenotype, depending on the genes involved. They can lead to gene dosage imbalance, altered gene expression levels, or the emergence of new genetic functions.
Production of Duplications
Duplications are thought to be the result
of unequal crossing over
Explain Duplication Also Produces Compensation Loops
Note: This is during chromosome pairing at meiosis in a duplication heterozygous individual.
When a duplication occurs, which involves the presence of an extra copy of a segment of a chromosome, compensation loops can form to facilitate proper pairing and alignment of chromosomes during meiosis. Here’s an explanation of compensation loops in the context of duplications:
- Origin of Duplication:
- Duplication events can happen due to various mechanisms, such as errors during DNA replication, unequal crossing over, or transposable element-mediated events.
- These mechanisms can result in the generation of additional copies of a segment within a chromosome. - Compensation Loops:
- Compensation loops, also known as duplication loops, are structural abnormalities that occur during meiosis in individuals with duplications.
- In the case of a duplication, the affected chromosome carries an extra copy of a specific segment, resulting in unpaired regions during the pairing of homologous chromosomes.
- To facilitate proper pairing and alignment, the homologous chromosome with the duplicated segment forms a loop to compensate for the presence of the additional DNA.
- This loop allows for matching of the duplicated segment with its counterpart on the other homologous chromosome. - Role of Compensation Loops:
- Compensation loops help ensure correct chromosome pairing and alignment during meiosis, allowing for proper segregation of genetic material.
- These loops play a crucial role in maintaining the fidelity of chromosome segregation by facilitating the pairing of homologous chromosomes and preventing misalignments or irregular chromosomal associations.
- Compensation loops also help in preserving the integrity of the duplicated segment and preventing recombination events that could lead to further genetic alterations.
It’s important to note that compensation loops are specific to duplications and their presence aids in maintaining the stability of the genome during meiosis. The formation of compensation loops helps to ensure accurate chromosome segregation and maintain genetic information within the duplicated segment.
Impact of Duplications: 3.
- More copies of a gene results in more of
the gene product of the gene - This can result in dosage problems
- Position effects may also have an influence on the way gene duplication impacts organisms
Examples of duplications: 2
- Bar mutation in Drosophila: having narrow, slit-like eyes compared to normal oval eyes
- Bar eyes are caused by duplication(s) of region 16A of the X chromosome; one copy 16A = normal eyes, two copies = Bar flies, three copies = doubled Bar flies…
Define Inversion
Inversions:
- Inversions involve the rearrangement of a chromosome segment in the reverse orientation.
- Inversions occur when a segment of chromosome breaks, flips in orientation and rejoins with the same chromosome.
- Inversions can be either pericentric, involving the centromere, or paracentric, occurring outside the centromere.
- Inversions can affect gene expression, disrupt gene function, or have no apparent effect depending on the location and size of the inversion and the genes involved.
Explain Inversions: 5
- A segment of the CH is turned around for 1800
- Paracertric (centromere is not part of the inversion) vs pericentric (centromere included) inversion
- Inversion heterozygotes form an inversion loop during meiosis
- Crossing over may produce dicentric and acentric chromatids
- The viability of these gametes decreased dramatically
Explain: The origin of inversion; 5
The origin of an inversion, a structural rearrangement of a chromosome, typically involves breakage and subsequent rejoining of chromosome segments in an inverted orientation. Here’s an explanation of the origin of inversions:
- Breakage Points:
- Inversions usually arise from DNA breakage events that occur within a chromosome.
- These breaks can happen at two different points along the chromosome, referred to as breakpoints. - Segment Inversion:
- After the initial breakage, the segment between the breakpoints becomes detached from the chromosome. - Rejoining in Inverted Orientation:
- The detached segment reattaches to the chromosome but in an inverted orientation.
- In other words, the segment is flipped around or reversed before joining back to the chromosome. - Forms of Inversions:
- There are two main types of inversions: paracentric and pericentric inversions.
- Paracentric inversions occur when the breakpoints are located on the same arm of the chromosome.
- Pericentric inversions involve breakpoints on both arms of the chromosome, including the centromere. - Consequences of Inversions:
- Inversions can disrupt the normal gene order and arrangement on a chromosome.
- They may also lead to changes in gene expression and function, as the inverted segment may affect the regulation of nearby genes.
- Inversions can also impact meiotic recombination, as crossovers within the inverted region can result in chromosomal abnormalities or non-viable gametes.
It’s important to note that inversions can occur spontaneously due to DNA breakage and repair mechanisms, or they can be induced by external factors such as radiation or chemicals. Inversions are structural variations that can have significant effects on gene function and can be associated with genetic disorders or phenotypic variations in organisms.
Explain paracentric - inversion heterozygote
A paracentric inversion heterozygote refers to an individual who carries two different versions of a specific chromosome due to the presence of a paracentric inversion. Here’s an explanation of a paracentric inversion heterozygote:
- Paracentric Inversion:
- A paracentric inversion is a type of chromosomal inversion where the inverted segment does not include the centromere.
- It involves the rearrangement of a chromosome segment within a single arm of the chromosome. - Heterozygote:
- In a paracentric inversion heterozygote, an individual possesses two copies of the same chromosome, but one of the copies carries a paracentric inversion, while the other copy is normal (non-inverted).
- Heterozygote indicates that the individual has two different versions of the chromosome at that specific locus. - Chromosome Structure:
- In a paracentric inversion heterozygote, the two chromosomes have different structures due to the presence of the inversion.
- The non-inverted chromosome has a normal linear structure.
- The inverted chromosome carries an inverted segment, which results in a rearrangement of genes within that segment. - Meiotic Consequences:
- During meiosis, when the paracentric inversion heterozygote undergoes gamete formation, the two chromosomes can form different types of recombinant chromosomes.
- Crossovers within the inverted region can lead to abnormal chromosome structures or non-viable gametes.
- The occurrence of crossing over within the inverted segment may result in genetic imbalances or rearrangements in the offspring.
It’s important to note that paracentric inversion heterozygotes can have implications for fertility, as meiotic recombination events within the inverted region can lead to issues in chromosome segregation and the production of abnormal gametes. Additionally, the presence of a paracentric inversion heterozygote can result in phenotypic variations or genetic disorders, depending on the specific genes affected by the inversion.
