14.4
Estructura del ARN
Descripción general
La estructura básica del ARN está formada por un azúcar de cinco carbonos y una de cuatro bases nitrogenadas. Aunque la mayor part…
- [Narrador] Cada molécula de ácido ribonucleico,
o ARN, está compuesta de una cadena de nucleótidos.
Cada nucleótido consiste de ribosa,
azúcar de cinco carbones,
unida a un grupo de fosfato por un lado
y a uno de cuatro bases nitrogenosas del otro,
adenina, guanina, citosina o uracilo.
Estas bases se unen a sus
bases complementarias en el ADN durante la transcripción,
cuando se sintetiza el ARN mensajero, ARNm.
La adenina se une a la tiamina, la guanina a la citosina,
y el uracilo, que se usa en el ARN
en vez de la tiamina, se une a la adenina.
Los nucleótidos luego se entrelazan
a través de enlaces fosfodiéster
entre el grupo de fosfato de un nucleótido
y el grupo de hidroxilo OH en la ribosa
del nucléotido adyacente,
lo que crea un esqueleto de azúcar-fosfato.
Las moléculas del RNA tienen un extremo cinco primo
con un grupo de fosfato en
el cinco primo de la ribosa,
y un extremo tres primo,
con un grupo de hidroxil en el carbono tres primo.
El RNA se ensambla en dirección
del cinco primo al tres primo,
y usualmente permanece en un una hebra.
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Q1: What are the basic components of an RNA nucleotide?
Each RNA nucleotide consists of three components: ribose, a five-carbon sugar; a phosphate group; and one of four nitrogenous bases—adenine, guanine, cytosine, or uracil. The phosphate group attaches to the five-carbon of ribose, while the nitrogenous base attaches to the one-carbon. These components link together to form the RNA chain.
Q2: How do RNA bases pair with DNA during transcription?
During transcription, RNA bases bind to complementary DNA bases through specific pairing rules: adenine binds to thymine, guanine binds to cytosine, and uracil binds to adenine. Uracil replaces thymine in RNA, making it the distinguishing base between RNA and DNA. This complementary binding ensures accurate mRNA synthesis from the DNA template.
Q3: What is the sugar-phosphate backbone and how is it formed?
The sugar-phosphate backbone forms through phosphodiester bonds linking adjacent nucleotides. These bonds connect the phosphate group of one nucleotide to the hydroxyl group on the ribose of the next nucleotide. This backbone creates the structural framework of RNA and gives the molecule its directionality and stability.
Q4: Why are the 5' and 3' ends of RNA important?
The 5' end has an unbound phosphate group on the five-carbon of ribose, while the 3' end has a free hydroxyl group on the three-carbon. RNA is always assembled in the 5' to 3' direction, with new nucleotides adding to the 3' end. This directionality is essential for proper RNA synthesis and function during gene expression transcription splicing and translation.
Q5: How do secondary structures form in RNA and what is their function?
Secondary structures form when distant nucleotides on the same single-stranded RNA bind through complementary base pairing. Hairpin loops form between bases 5-10 nucleotides apart, while stem-loops involve bases separated by 50 to hundreds of nucleotides. In prokaryotes, these structures regulate transcription; in eukaryotes, they stabilize mRNA by preventing degradation.
Q6: What is the three-dimensional structure of tRNA and how does it function?
tRNA has an L-shaped three-dimensional structure that folds into a cloverleaf pattern of 70-80 nucleotides. The amino acid binding site is at one end, and the anticodon—a three-nucleotide sequence complementary to mRNA codons—is at the other. This unique shape enables tRNA to bind to ribosomes and serve as an adaptor molecule during protein synthesis.
Q7: What are pseudoknots and why are they significant in RNA structure?
Pseudoknots are tertiary structures formed when bases in the loop regions of secondary structures interact with complementary bases outside the loop. These complex three-dimensional structures play essential roles in RNA function and regulation. They represent a higher level of RNA organization beyond simple secondary structure formation.