Lecture 7: Cytogenetics 3 Meiosis and recombination Flashcards
Meiosis and recombination: 5.
- Genes are on chromosomes
- Four types of gametes can be
produced in a dihybrid - The gametes are 1:1:1:1
- The combination of them will give the phenotype of 9:3:3:1
- This is the cytogenetic the foundation of Mendelian Genetics
EXPLAINING EACH IN DETAIL
- Genes on Chromosomes:
- Genes, the units of inheritance, are located on chromosomes within cells.
- Chromosomes contain the genetic information that determines an organism’s traits. - Four Types of Gametes in a Dihybrid:
- During dihybrid crosses, where two traits are considered, four types of gametes can be produced.
- These gametes are the result of independent assortment of the alleles for each trait during meiosis. - Gametes in a 1:1:1:1 Ratio:
- In a dihybrid cross, each of the four possible types of gametes is produced in equal proportions.
- The ratio of these gametes is 1:1:1:1, meaning each gamete has an equal chance of being formed. - Phenotypic Ratio of 9:3:3:1:
- When the gametes from the dihybrid cross combine, the resulting offspring show a phenotypic ratio of 9:3:3:1.
- This ratio represents the different combinations of traits that can occur in the offspring. - Cytogenetic Foundation of Mendelian Genetics:
- The observation of the 9:3:3:1 phenotypic ratio resulting from the combination of dihybrid gametes laid the cytogenetic foundation of Mendelian Genetics.
- It provided evidence for the independent assortment of genes on chromosomes and the concept of gene segregation during meiosis.
Explain Meiois I: cross over happens at 4 chromatid stages: PROPHASE I.
- Prophase I:
- Prophase I is the longest phase of meiosis I and can be further divided into several sub-stages.
- During the pachytene stage of prophase I, homologous chromosomes pair up to form bivalents or tetrads.
- Each bivalent consists of two pairs of sister chromatids, with a total of four chromatids present. - Crossing Over:
- Crossing over, also known as recombination, occurs during the 4-chromatid stage of prophase I.
- At this stage, non-sister chromatids of homologous chromosomes exchange genetic material through a process called genetic recombination.
- The crossover event involves the breakage and exchange of corresponding sections of DNA between the chromatids.
- This genetic exchange results in the mixing of genetic information between homologous chromosomes, promoting genetic diversity. - Importance of Crossing Over:
- Crossing over plays a vital role in increasing genetic variation within a population.
- It shuffles the alleles (variations of genes) between homologous chromosomes, creating new combinations of genetic information.
- This process helps generate unique combinations of traits in offspring, contributing to the diversity of individuals within a species. - Subsequent Meiosis I Events:
- After crossing over, the homologous chromosomes remain physically connected at points called chiasmata.
- The chiasmata ensure proper alignment and segregation of homologous chromosomes during subsequent phases of meiosis I.
- Meiosis I then proceeds with the separation of homologous chromosomes into two separate daughter cells.
Explain Meiois and prophase I:
- Single crossover, double, triple or multiple.
- Homologous chromosomes crossing over.
Recombination takes place through the breakage, alignment and repair of DNA strand: 5
- Breakage of DNA Strands:
- During meiosis, specifically in prophase I, recombination occurs through the breakage of DNA strands.
- The breakage is initiated by the formation of protein complexes that cause DNA double-strand breaks. - Alignment of DNA Strands:
- After the breakage, the homologous chromosomes pair up and align with each other.
- This pairing process is facilitated by proteins and structures, such as the synaptonemal complex. - Exchange of Genetic Material:
- The broken ends of the DNA strands from one chromosome are then repaired using the intact DNA strands from the homologous chromosome.
- This repair process involves the exchange of genetic material between the non-sister chromatids of the homologous chromosomes. - Repair of DNA Strands:
- The repair mechanism involves the formation of Holliday junctions, which are temporary joint structures between the DNA strands.
- The DNA strands are then enzymatically cleaved and rejoined, resulting in the exchange of genetic material between the chromatids. - Genetic Recombination:
- The exchange of genetic material between the DNA strands leads to genetic recombination.
- This recombination creates new combinations of alleles and promotes genetic diversity within a population.
Explain/The steps of The Holiday model of homologous recombination (5)
- Single DNA strand breakage
- Strand migration
- Holiday junction formation
- Strand displacement
- Branch migration
- Cleavage and rejoining
The Holliday model is a widely accepted model that explains the process of homologous recombination. It involves several steps that occur during the exchange of genetic material between homologous DNA strands. Here’s an explanation of each step in the Holliday model:
- Single DNA Strand Breakage:
- The process begins with the introduction of a single-strand break in one of the DNA molecules involved in recombination.
- This break allows the DNA strands to separate and initiate the recombination process. - Strand Migration:
- Once the break occurs, the separated DNA strands can migrate or move along the DNA molecule.
- This migration enables the DNA strands to search for homologous regions on another DNA molecule. - Holliday Junction Formation:
- When complementary or homologous regions are encountered between the DNA strands, they align and form a structure known as a Holliday junction.
- The Holliday junction consists of two crossover points, where the DNA strands are intertwined. - Strand Displacement:
- During strand displacement, one of the DNA strands involved in the Holliday junction displaces a corresponding strand from the other DNA molecule.
- This displacement allows for the exchange of genetic information between the DNA strands. - Branch Migration:
- Branch migration is the movement of the Holliday junction along the DNA molecule.
- It involves the migration of the crossover points, which leads to the extension or expansion of the exchanged DNA region. - Cleavage and Rejoining:
- After branch migration, the Holliday junction can be resolved by the cleavage and rejoining of the DNA strands.
- The cleavage and rejoining events determine the final outcome of the recombination process, either resulting in the crossover or non-crossover products.
Explain The double-strand -break model of recombination (5)
The double-strand-break model involves:
* Double-strand breaks
* Single-strandformation
* Single-strand invasion
* Tow holiday junctions
* Cleavage and reunion
The double-strand-break model is another model that explains the process of homologous recombination. It involves several steps that occur during the exchange of genetic material between double-stranded DNA molecules. Here’s an explanation of each step in the double-strand-break model:
- Double-Strand Breaks:
- The process begins with the introduction of double-strand breaks (DSBs) in the DNA molecules involved in recombination.
- DSBs are breaks in both DNA strands within a DNA molecule, creating free ends. - Single-Strand Formation:
- After the DSBs occur, the free ends of the DNA strands are resected or processed to generate single-stranded DNA (ssDNA) tails.
- Resection can occur in one or both DNA strands to expose the ssDNA regions. - Single-Strand Invasion:
- The ssDNA tails then invade the intact, complementary DNA molecule.
- The invasion is guided by the homology between the ssDNA and the complementary sequence on the intact DNA molecule. - Two Holliday Junctions:
- Once the ssDNA invades the complementary DNA molecule, two Holliday junctions are formed.
- Holliday junctions are structures that contain crossover points between the invading ssDNA and the intact DNA. - Cleavage and Reunion:
- The Holliday junctions can be cleaved or resolved by enzymatic activities.
- Cleavage involves the breaking of DNA strands at specific positions within the Holliday junction.
- Reunion refers to the rejoining of DNA strands after cleavage, resulting in the formation of new DNA molecules.
In Metaphase I there is one pair of homologous chromosomes.
Explain Independent Assortment in Metaphase I
- Homologous chromosomes align independently along the metaphase plate
- Number of possible combinations = 2n, where n = haploid chromosome #
– e.g. 22 = 4 possible chr combinations
– 22^3 = 8,388,608 combinations
– Note this is when WITHOUT crossovers - WITH crossovers, it will be 2n where n is the number of genes/loci which can be separated by crossovers.
———————————————————————Metaphase I - Independent Assortment:
- During metaphase, I, the homologous chromosome pairs, known as bivalents or tetrads, align at the equatorial plane of the cell.
- Independent assortment occurs as the homologous pairs randomly align along the metaphase plate.
- The alignment of homologous pairs is independent of other pairs, meaning that the arrangement is not influenced by the orientation of other bivalents.
- This random alignment results in different combinations of maternal and paternal chromosomes facing each pole of the cell.
- The number of possible combinations is 2^n, where n is the number of homologous pairs.
Explain Independent Assortment in Metaphase II: (5)
Independent assortment is a key principle of genetics that describes how homologous chromosomes segregate and align during metaphase I and metaphase II of meiosis.
Metaphase II - Independent Assortment:
1. In metaphase II, the replicated chromosomes (sister chromatids) align at the equatorial plane of the cell.
- Independent assortment occurs as the sister chromatids randomly align along the metaphase plate.
- Similar to metaphase I, the arrangement of sister chromatids is independent of other chromatids.
- As a result, different combinations of sister chromatids, which carry identical genetic information, are segregated to each pole of the cell.
- The number of possible combinations is 2^n, where n is the number of sister chromatids.
Overall, independent assortment in metaphase I and metaphase II ensures the random distribution of homologous chromosomes and sister chromatids to the resulting cells. This process contributes to genetic diversity by creating unique combinations of alleles in the gametes and ultimately in the offspring.
Explain Cohesin and shugoshin at mitosis and meiosis
Sure! Here’s an explanation of cohesin and shugoshin in both mitosis and meiosis, providing additional detail while keeping the structure of the flashcards:
Mitosis:
1. Sister kinetochores orient toward different poles…
- During the METAPHASE of mitosis, sister chromatids align along the equatorial plane of the cell.
- The sister kinetochores, located at the centromeres of sister chromatids, face opposite poles of the cell.
- …and cohesin keeps sister chromatids together.
- Cohesin is a protein complex that holds the sister chromatids together along their entire length.
- It ensures the cohesion of sister chromatids until they are ready to separate during ANAPHASE. - The breakdown of cohesin allows sister chromatids to separate.
- In the ANAPHASE of mitosis, cohesin is selectively degraded by proteases.
- This breakdown of cohesin at the centromeres allows the sister chromatids to separate and move toward opposite poles of the cell.
Meiosis:
4. Cohesin along chromosome arms holds homologs together at the chiasmata.
- During prophase I of meiosis, homologous chromosomes pair up and form structures called bivalents or tetrads.
- Cohesin along the chromosome arms keeps the homologous chromosomes together at specific regions called chiasmata.
METAPHASE I
- Cohesin along chromosome arms breaks down, allowing homologs to separate,…
- At the onset of anaphase I in meiosis, cohesin along the chromosome arms undergoes cleavage.
- This breakdown of cohesin permits the separation of homologous chromosomes, ensuring their distribution to different daughter cells.
ANAPHASE 1 - …but cohesin at the centromere is protected by shugoshin.
- Shugoshin is a protein that protects cohesin at the centromeres during meiosis I.
- It ensures that cohesin remains intact at the centromeres, preventing premature separation of sister chromatids.
ANAPHASE 1 - Shugoshin is degraded. Cohesin at the centromere breaks down, allowing chromatids to separate.
- At anaphase II of meiosis, shugoshin is degraded, leading to the breakdown of cohesin at the centromeres.
- This breakdown enables the separation of sister chromatids, ensuring their distribution to the final gametes.
ANAPHASE 2
Cohesin and shugoshin play crucial roles in maintaining chromosome cohesion and regulating the separation of chromosomes in both mitosis and meiosis. While cohesin ensures the proper alignment and separation of sister chromatids, shugoshin protects cohesin at the centromeres during meiosis I to allow proper segregation of homologous chromosomes.
Meiosis: What is it all for? (2) AND 3 WAYS OF NEW COMBOS
- Halving the chromosome number
- Creating new combinations of alleles
- Three ways it does this:
1. crossing over during prophase I
2. random alignments at metaphase I & II 3. random fertilization
Recombination through meiosis is the main source of genetic diversity = how?
- Life would be very dull without meiosis
- Geneticists would have to find something else to do for a living
- It is closely related to:
- Mendelian genetics
- Population genetics
- Quantitative genetics
- Conservation genetics
- Human disease genetics
What do geneticists do with diversity? (2) and an example
- Breeders can select new cultivars
- Conservation geneticists need to conserve genetic diversity
- Example: from the pasture legume tedera:
- Genetic fingerprints taken from 27 wild plants from around the Mediterranean and Canary Islands
- How are they related?
What is genetic fingerprinting? How does it work?
Genetic fingerprinting, also known as DNA fingerprinting or DNA profiling, is a technique used to identify and compare individuals based on their unique genetic makeup. It relies on the analysis of specific regions of an individual’s DNA, which exhibit variations between individuals.
from previous plant example:
* They are different individuals.
* Each peak is an allele.
* This is only showing one locus.
Here’s how genetic fingerprinting works:
- Sample Collection: A biological sample containing DNA is collected from the individual, typically through non-invasive methods like swabbing the inside of the cheek or collecting a small blood or saliva sample.
- DNA Extraction: The DNA is extracted from the collected sample using specialized laboratory techniques. This process separates the DNA from other cellular components.
- Polymerase Chain Reaction (PCR): Specific regions of the DNA, known as short tandem repeats (STRs), are amplified using a technique called PCR. STRs are regions where a sequence of nucleotides is repeated multiple times.
- STR Analysis: The amplified DNA fragments containing the STRs are separated and analyzed using a technique called gel electrophoresis. The DNA fragments are loaded onto a gel matrix and subjected to an electric field, causing them to migrate through the gel. The fragments separate based on their size, with smaller fragments moving faster than larger ones.
- DNA Banding Pattern: After electrophoresis, the DNA fragments form distinct banding patterns on the gel. These banding patterns represent the different alleles present at each STR locus. Each individual has a unique combination of alleles, which creates a specific pattern of bands.
- Comparison: The DNA banding patterns obtained from the individual’s sample can be compared with those from other samples, such as crime scene evidence or samples from other individuals. The aim is to determine if there are matching patterns or similarities that indicate a potential relationship or identity.
- Interpretation: The banding patterns are analyzed and interpreted by experts who look for similarities and differences between samples. The more matches in the banding patterns, the higher the probability of a match between the individuals’ DNA.
- Statistical Analysis: To quantify the likelihood of a match, statistical analysis is performed using population databases that provide the frequency of different alleles in a given population. This helps determine the probability of finding the same DNA profile by chance.
Genetic fingerprinting has various applications, including forensic investigations, paternity testing, identification of human remains, and establishing genetic relationships. The technique has proven to be highly accurate and reliable due to the uniqueness and stability of an individual’s DNA profile.
Summary of the lecture:Meiosis makes it possible for the genetic variation to occur: (3)
Meiosis makes it possible for genetic variation to occur:
– Crossing over
– Random assortment of chromosomes
– Random combination of gametes to produce individuals
