28.4D: Types of RNA
RNA is the nucleic acid that makes proteins from the code provided by DNA through the processes of transcription and translation.
- Describe the structure and function of RNA
Key Points
- The nitrogen bases in RNA include adenine (A), guanine (G), cytosine (C), and uracil (U).
- Messenger RNA (mRNA) carries the code from the DNA to the ribosomes, while transfer RNA (tRNA) converts that code into a usable form.
- Ribosomes are the sites where tRNA and rRNA assemble proteins.
- RNA differs from DNA in that it is single stranded, has uracil instead of thymine, carries the code for making proteins instead of directing all of the cell ‘s functions, and has ribose as its five-carbon sugar instead of deoxyribose.
Key Terms
- codon : a sequence of three adjacent nucleotides, which encode for a specific amino acid during protein synthesis or translation
- transcription : the synthesis of RNA under the direction of DNA
RNA Structure and Function
The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in all living organisms and is found in the nucleus of eukaryotes and in the chloroplasts and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope.
The other type of nucleic acid, RNA, is mostly involved in protein synthesis. Just like in DNA, RNA is made of monomers called nucleotides. Each nucleotide is made up of three components: a nitrogenous base, a pentose (five-carbon) sugar called ribose, and a phosphate group. Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to one or more phosphate groups.
RNA Structure : A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Carbon residues in the pentose are numbered 1′ through 5′ (the prime distinguishes these residues from those in the base, which are numbered without using a prime notation). The base is attached to the 1′ position of the ribose, and the phosphate is attached to the 5′ position. When a polynucleotide is formed, the 5′ phosphate of the incoming nucleotide attaches to the 3′ hydroxyl group at the end of the growing chain. Two types of pentose are found in nucleotides, deoxyribose (found in DNA) and ribose (found in RNA). Deoxyribose is similar in structure to ribose, but it has an H instead of an OH at the 2′ position. Bases can be divided into two categories: purines and pyrimidines. Purines have a double ring structure, and pyrimidines have a single ring.
In RNA, the nitrogenous bases vary slightly from those of DNA. Adenine (A), guanine (G), and cytosine (C) are present, but instead of thymine (T), a pyrimidine called uracil (U) pairs with adenine. RNA is a single stranded molecule, compared to the double helix of DNA.
The DNA molecules never leave the nucleus but instead use an intermediary to communicate with the rest of the cell. This intermediary is the messenger RNA (mRNA). When proteins need to be made, the mRNA enters the nucleus and attaches itself to one of the DNA strands. Being complementary, the sequence of nitrogen bases of the RNA is opposite that of the DNA. This is called transcription. For example, if the DNA strand reads TCCAAGTC, then the mRNA strand would read AGGUUCAG. The mRNA then carries the code out of the nucleus to organelles called ribosomes for the assembly of proteins.
Once the mRNA has reached the ribosomes, they do not read the instructions directly. Instead, another type of RNA called transfer RNA (tRNA) needs to translate the information from the mRNA into a usable form. The tRNA attaches to the mRNA, but with the opposite base pairings. It then reads the sequence in sets of three bases called codons. Each possible three letter arrangement of A,C,U,G (e.g., AAA, AAU, GGC, etc) is a specific instruction, and the correspondence of these instructions and the amino acids is known as the “genetic code.” Though exceptions to or variations on the code exist, the standard genetic code holds true in most organisms.
The ribosome acts like a giant clamp, holding all of the players in position, and facilitating both the pairing of bases between the messenger and transfer RNAs, and the chemical bonding between the amino acids. The ribosome has special subunits known as ribosomal RNAs (rRNA) because they function in the ribosome. These subunits do not carry instructions for making a specific proteins (i.e., they are not messenger RNAs) but instead are an integral part of the ribosome machinery that is used to make proteins from mRNAs. The making of proteins by reading instructions in mRNA is generally known as “translation.”
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