DNA contains the base sequence, which codes for proteins, and almost all cells have DNA (which stores all our genetic information in its base sequence). It is a remarkable molecule with a double-helix structure, but we can't forget RNA ! RNA , another nucleic acid, is equally important as DNA, especially for protein synthesis. DNA and RNA go hand-in-hand.
There are two specific types of RNA that we're going to be examining in detail, each of which has specialized functions in protein synthesis:
- Messenger RNA (mRNA)
- Transfer RNA (tRNA)
Let's take a closer look at each type.
Functions of mRNA
mRNA is the end product of transcription, the first step of protein synthesis, which occurs in the nucleus. These molecules are shorter than DNA, and they store the base sequence of a specific gene as RNA nucleotides. mRNA can migrate out of the nucleus towards a ribosome, where it acts as the template for the next step of protein synthesis. Therefore, its primary function is to transfer the genetic information of proteins to ribosomes, where it is used to determine the amino acid sequence of a polypeptide.
You'll come across pre-mRNA, which is the type of RNA made in eukaryotic cells. Eukaryotes contain introns and exons; Therefore, this pre-mRNA needs to undergo splicing, which removes unwanted introns. Prokaryotic cells do not have introns.
Functions of tRNA
tRNA is involved in translation, which occurs at the ribosomes. These interestingly clover-shaped molecules function to deliver the correct amino acid to the ribosomes to build the polypeptide chain. Think of them as mailmen; tRNA's transport mail (which are amino acids) to homes (which are the ribosomes).
Remember that ribosomes are found in the cytoplasm of cells and on the rough endoplasmic reticulum (RER).
What is the structure of RNA?
Now we know why RNA is essential, but what makes RNA? All types of RNA contain three common components:
- A phosphate group
- A ribose pentose sugar
- An organic nitrogenous base (adenine, uracil, cytosine or guanine)
As we take a deeper look at the structures of mRNA and tRNA, you'll understand why and how they carry out their specific functions.
Structure of mRNA
mRNA is a short single-stranded helical polynucleotide that is small enough to diffuse through the nuclear pore. The nucleotides contain the same base sequence, or codons, as the template strand of DNA (the only difference is that the thymine in DNA is uracil in RNA).
Polynucleotides are chains made of monomeric nucleotides, in the case of mRNA. The monomers made of RNA nucleotides are composed of a phosphate group, a ribose sugar and a nitrogenous base.
Fig. 1 - The structure of mRNA
Structure of tRNA
Like mRNA, tRNA is made of one polynucleotide strand, folded into an incredible cloverleaf-shaped structure. These vary in size, ranging from 60 to 95 nucleotides. You'll need to know the two functionally essential regions of tRNA:
- An anticodon loop
- An amino acid attachment site
The anticodon loop is composed of three RNA nucleotides complementary to the codons found on the mRNA. For example, the start codon for mRNA, AUG, will bind to the tRNA with an anticodon loop of UAC.
The amino acid attachment site is, as its name suggests, where the corresponding amino acid temporarily binds. If we come back to our 'tRNA mailman' analogy, the amino acid is the mail delivered to the ribosome!
How does the correct amino acid know to which tRNA it should bind? Enzymes, called aminoacyl-tRNA synthetases, are specific to an amino acid, and its tRNA helps attach the amino acid. But don't worry; you won't need to know each enzyme involved in this process. Just appreciate that it's enzymes that do all the work!
Comparing RNA and DNA structure
First, look at previous articles about the structure and function of DNA if you need to refresh you knowledge. We're going to compare the two types of nucleic acids, DNA and RNA .
Similarities between RNA and DNA structure
Structurally, there is only one main similarity between these nucleic acids, and this is that both are made of at least one polynucleotide strand.
Fig. 2 - The cloverleaf-shaped tRNA structure contains an anticodon and an amino acid attachment site
Differences between RNA and DNA structure
As you can imagine, there are more differences between DNA and RNA, which are listed below:
- RNA is composed of one polynucleotide strand, while DNA is made of two polynucleotide strands.
- RNA is relatively shorter than DNA as it corresponds with one gene; DNA is much longer as it contains the whole genome.
- RNA contains the ribose pentose sugar, while DNA contains the deoxyribose pentose sugar.
- RNA contains the uracil base; in contrast, DNA contains the thymine base.
Comparing mRNA and tRNA structure
We've looked at the details of the structure of each type of RNA, so it's clear that they are structurally different. But let's note them down into points that are easier to understand:
- mRNA is a single helical polynucleotide, while tRNA is folded into a cloverleaf shape.
- mRNA contains codons, while tRNA contains complementary anticodons.
- mRNA is linear; However, tRNA has an anticodon loop and amino acid attachment site.
The Structure of RNA - Key takeaways
- RNA , specifically mRNA and tRNA, are the chief polynucleotides in protein synthesis.
- mRNA functions to carry the codons of a gene to the ribosomes. tRNA functions to deliver the correct amino acid to the ribosomes.
- mRNA is a single-chained helical polynucleotide, shorter than DNA. It contains the codons derived from the template strand of DNA.
- tRNA is a single-chained polynucleotide folded into a cloverleaf shape. This structure contains an anticodon loop, complementary to the mRNA codons, and an amino acid attachment site.
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