The work of Gregor Mendel, an Austrian monk, provided evidence than an organism's traits are passed from parents to offspring by way of genes. Genes are discrete sequences in an organism's DNA (genetic code) used by cells to make specific proteins. These various proteins produce the traits of an organism.
The ribosome, a structure composed of RNA (nucleic acid that carries instructions from DNA for protein synthesis) and protein, constructs proteins based on the instructions provided by DNA; ribosomes may be free floating in cytoplasm or attached to form rough endoplasmic reticulum. It is here that the code from the DNA is interpreted and a specific sequence of amino acids, which are the building blocks of proteins, is produced. The following is a summary of the three main types of RNA and how they behave during the two main parts of protein synthesis, called transcription and translation.
There are three main types of RNA that work together to build proteins.
- Messenger RNA, or mRNA, is the molecule made from DNA that contains the complementary gene sequence (the sequence opposite to the DNA strand). It transcribes the DNA code in the nucleus and carries it out into the cytoplasm.
- Transfer RNA, or tRNA, is the molecule that carries each amino acid to the strand of mRNA during translation of protein synthesis. It is a looped structure that recognizes a particular sequence of three nucleotides (basic structural genetic units) on the mRNA and binds to the specific amino acid coded by that mRNA sequence. This form of RNA then brings the amino acids to the ribosomes for production of the protein chain (the protein formed when peptide bonds are catalyzed between each amino acid).
- Ribosomal RNA, or rRNA, is the RNA component of ribosomes that catalyzes peptide bond formation. Together with protein molecules, it forms a ribosome, which is a site of protein synthesis.
Types of RNA
When reading strands of RNA, particular sequences create each of the amino acids. A series of three nucleotides on an mRNA strand that codes for a particular amino acid is called a codon, such as AGC. There are 64 possible combinations of A, T, C, and G that can occur. Three combinations halt protein synthesis (called stop codons), while the remaining 61 combinations code for the 20 amino acids. Most amino acids are coded by more than one codon. For example, both AGA and AGG code for the amino acid arginine.
The first part of protein synthesis is called transcription. In general, transcription is the formation of mRNA from the template DNA strand to be used to build proteins. The template strand provides the sequence, and the complementary strand is its mirror; for example, if the template strand reads AGCCGA, the complementary strand will read UCGGCU. This is because A binds with U or T, and G combines with C. In order for transcription to happen, the enzyme RNA polymerase (a protein) must be present, along with RNA nucleotides. Transcription happens in three main steps: initiation, elongation, and termination. Initiation is the beginning of transcription, when RNA polymerase binds to a gene promoter, thereby signaling the DNA to unwind. Elongation occurs as nucleotides are added to the mRNA strand. When RNA polymerase encounters a stop codon, transcription is terminated. Once the mRNA has been transcribed in the nucleus, it undergoes several modifications before it is ready to leave. The last step in the preparation of the mRNA is RNA splicing.In eukaryotes, genes contain both coding and noncoding regions of sequence. The DNA sequence within a gene that codes for a specific protein that must be combined during mRNA processing is called an exon. The DNA sequence within a gene that does not code for a specific protein and must be removed from the mRNA during processing is called an intron. These contain regulatory sequences and, if left in place, would produce nonfunctional protein chains. A large molecule made of RNA and proteins that removes introns and joins the adjacent exon ends to form the mature strand of mRNA is called a spliceosome. It attaches to intron sequences, releasing the intron. The spliceosome then joins the adjacent exon ends to form the mature strand of mRNA, ready to leave the nucleus.