15.5
ADN complementario
Descripción general
Sólo los genes que se transcriben en ARN mensajero (ARNm) están activos o se expresan. Por tanto, los científicos pueden extraer e…
- [Narrador] Casi todas las células contienen el mismo ADN
pero algunas, como las neuronas y las fibras musculares,
expresan genes diferentes
porque solo algunos genes son transcritos
en ARN mensajero, o ARNm, en cada célula.
En el laboratorio, los ARNm son utilizados como un modelo
para sintetizar ADN complementario, o ADNc,
para estudiar la expresión genética.
Un método común es extraer ARN de las células,
luego aislar el ARNm de los otros tipos de ARN,
como ARN ribosomal o ARN de transferencia,
dejando pasar la muestra por una columna de cuentas
con segmentos de nucleótidos de timina asociados.
Estos se unen al extremo poli-A,
una cadena de nucleótidos de adenina específicamente
presente en las terminaciones prima-3 de ARNm eucarionte.
Los otros ARN no se unen y son arrastrados con la muestra.
Después de aislar el ARNm,
un cebador poli-T se une al extremo poli-A,
siendo un origen para que la enzima transcriptasa inversa
transcriba una cadena sencilla de ADNc a partir del ARNm.
Químicos, como las enzimas ribonucleasas,
se añaden despúes para degradar el ARN.
Entonces se utilizan enzimas polimerasas de ADN
para sintetizar una cadena complementaria para el ADNc,
obteniendo una cadena doble de ADNc,
el cual puede ser insertado en un vector bacterial o viral
y utilizado en investigaciones de biología molecular.
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Q1: Why do scientists use complementary DNA instead of genomic DNA to study gene expression?
Complementary DNA (cDNA) lacks introns and regulatory sequences present in genomic DNA, allowing researchers to directly determine the amino acid sequence of encoded peptides. Since cDNA is synthesized from mRNA, it represents only the genes actively expressed in specific cells or tissues, making it ideal for studying gene expression patterns across different cell types.
Q2: How does the poly-A tail help isolate mRNA from other RNA types?
Mature eukaryotic mRNA has a poly-A tail—a string of adenine nucleotides—added to its 3' end, while transfer RNA and ribosomal RNA do not. Researchers attach thymine nucleotides (oligo-dTs) to a column or magnetic beads, which specifically base-pair with the poly-A tail. This allows mRNA to be captured while other RNA types are washed away.
Q3: What role does reverse transcriptase play in cDNA synthesis?
Reverse transcriptase is a DNA polymerase enzyme from retroviruses that synthesizes a single-stranded cDNA strand from mRNA. Since reverse transcriptase can only add nucleotides to the 3' end of a chain, a poly-T primer binds to the poly-A tail to provide a starting point for cDNA synthesis.
Q4: Why is the RNA degraded after the first cDNA strand is synthesized?
After reverse transcriptase creates single-stranded cDNA, the original mRNA template must be removed to allow DNA polymerase to synthesize the complementary second strand. RNA is degraded using alkali treatment or RNase enzymes, leaving the single-stranded cDNA intact for the next synthesis step.
Q5: How is double-stranded cDNA created from single-stranded cDNA?
DNA polymerase synthesizes a second DNA strand complementary to the first cDNA strand, often using the hairpin loop at the end of the first strand or a nicked piece of the mRNA as a primer. This produces double-stranded cDNA, which can be inserted into bacterial or viral vectors for molecular biology research.
Q6: What is a cDNA library and why would researchers construct one?
A cDNA library is a collection representing all the mRNAs expressed in specific cells or tissues of interest. Researchers construct cDNA libraries to comprehensively study gene expression patterns, identify active genes in particular cell types, and preserve genetic information for additional research and analysis.
Q7: How does different gene expression in neurons versus muscle cells relate to cDNA analysis?
Although neurons and muscle cells contain the same DNA, they express different genes because only certain genes are transcribed into mRNA in each cell type. By extracting and analyzing mRNA from these different cell types, researchers can use cDNA synthesis to study which genes are active and understand cell-type-specific gene expression patterns.