8.15
Conversión génica
Además de mantener la estabilidad del genoma mediante la reparación del ADN, la recombinación homóloga desempeña un papel importante en la diversifica…
A diferencia de la mitosis, en la que las roturas de doble cadena son accidentales, en la meiosis, son creadas por una enzima llamada Spo11, que escinde la columna vertebral del fosfodiéster. Los extremos helicoidales rotos son recortados por un complejo de proteínas llamado MRX y el daño se repara mediante un proceso llamado conversión de genes.
Aquí, la hebra de ADN "aceptora" dañada invade un dúplex de ADN "donante" homólogo para formar un bucle de desplazamiento. Esto crea regiones de ADN heterodúplex donde una hebra del ADN del donante se empareja con una hebra complementaria del ADN aceptor.
La ADN polimerasa extiende la hebra invasora, y el bucle D extendido luego se empareja con la cola 3' libre. La síntesis de ADN en la hebra recién capturada da como resultado la formación de un intermediario con dos estructuras de cuatro hebras llamadas uniones de Holliday.
Este intermedio de doble unión de Holliday se resuelve mediante enzimas reparadoras del ADN llamadas resolvasas y hay dos orientaciones en las que se pueden escindir las uniones. En el primero, la resolvasa corta cada unión horizontalmente para que las hebras parentales aún estén intactas.
Esto da como resultado un producto no cruzado, llamado así porque las hebras después de la rotura permanecen con su compañero original, y no hay un cruce importante de las hebras donante y aceptora.
Alternativamente, si la escisión ocurre verticalmente, las regiones que flanquean el daño se intercambian, lo que conduce a un producto cruzado, donde la hebra donante después de la ruptura se recombina con la hebra aceptora.
La conversión génica tiene un impacto significativo en la diversidad genómica. En los organismos que se reproducen sexualmente, la descendencia hereda un conjunto de genes del padre y otro de la madre.
Los conjuntos de ADN parental se recombinan y las cromátidas hermanas se someten a una conversión génica. Esto da como resultado que la descendencia tenga cromosomas nuevos en comparación con los de los padres.
Q1: What is gene conversion and how does it differ from other recombination mechanisms?
Gene conversion is a process where one DNA sequence is replaced by a highly similar sequence from a homologous chromosome, resulting in non-reciprocal exchange of genetic material. Unlike homologous recombination and strand invasion, which typically involve reciprocal exchange between two DNA molecules, gene conversion produces asymmetrical outcomes where one allele is converted to match another without equivalent reciprocal change.
Q2: How does gene conversion contribute to genetic variation in populations?
Gene conversion increases genetic diversity by creating new allelic combinations without requiring crossing over. This mechanism allows cells to generate variation through localized sequence replacement, contributing to genomic stability while maintaining genetic flexibility. The process is particularly important in regions with repetitive sequences where multiple similar templates exist.
Q3: What role does gene conversion play in DNA repair?
Gene conversion facilitates DNA repair by using homologous sequences as templates to restore damaged regions. When double-strand breaks or other lesions occur, the cell can employ gene conversion to replace corrupted sequences with intact copies from sister chromatids or homologous chromosomes, ensuring accurate restoration of genetic information.
Q4: Which proteins and enzymes are essential for gene conversion?
Gene conversion requires recombination proteins including RAD51, which catalyzes strand invasion and homology search, along with nucleases and helicases that process DNA ends. These proteins work together to identify homologous sequences, facilitate strand exchange, and resolve recombination intermediates, ensuring accurate conversion of one DNA sequence to match its homolog.
Q5: How does gene conversion maintain genomic stability during meiosis?
Gene conversion corrects mismatches and ensures proper chromosome pairing during meiosis by converting one allele to match its homolog. This process reduces genetic conflicts between homologous chromosomes and prevents the propagation of mutations, thereby maintaining genomic stability across generations while allowing controlled genetic recombination.
Q6: What is the relationship between gene conversion and mismatch repair?
Gene conversion and mismatch repair both address sequence discrepancies but operate differently. Mismatch repair corrects errors introduced during DNA replication by identifying and removing incorrect bases. Gene conversion, by contrast, replaces entire sequence segments with homologous templates, serving as a broader mechanism for maintaining sequence fidelity and genetic consistency.
Q7: Can gene conversion occur outside of meiosis, and what are the consequences?
Gene conversion occurs in both meiotic and mitotic cells, though with different frequencies and consequences. In somatic cells, mitotic gene conversion can generate genetic diversity within tissues and may contribute to immune system diversity. Uncontrolled gene conversion in somatic cells can occasionally lead to loss of heterozygosity or altered gene expression patterns.