The work of Gregor Mendel, an Austrian monk, provided evidence that an organism's traits are passed from parents to offspring by way of genes (Mendel called them factors). 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. In addition, proteins are continuously produced while involved in the functioning of each cell; for example, membrane transport proteins such as ion channels that move ions into and out of cells.
The ribosome, a structure composed of RNA 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 two main parts of this process in eukaryotes called transcription and translation. Ribonucleic acid (RNA) is an organic molecule that carries genetic messages out of the nucleus. It consists of a single strand of nucleotides. Transcription is the formation of messenger mRNA from the template DNA strand to be used to build proteins. Messenger RNA (mRNA) is an RNA molecule made from a DNA template and contains the complementary gene sequence—that is, the sequence opposite to the DNA strand. The mRNA leaves the nucleus, the organelle that houses DNA, and goes into the cytoplasm, the watery interior of the cell, where the ribosomes are located. Translation is the assembly of amino acids into a proteins in the ribosomes through the reading of mRNA by transfer RNA and the ribosome. Transfer RNA (tRNA) is the molecule that carries each amino acid to the strand of mRNA during translation of protein synthesis.
The concept that genetic information flows from DNA to RNA, and from RNA to proteins is called the central dogma of biology. While this idea has undergone much revision as new information becomes available, the general premise has remained the same. The original thinking was that there was "one gene for one protein," with the idea that each small piece of DNA would code for a particular protein. Research has shown that many proteins are the result of more than one chain of amino acids. Hemoglobin, the protein in blood that carries oxygen, is composed of four different proteins. In addition, modification can occur that allows multiple proteins to be produced from one gene sequence. These details do not alter the directional flow of genetic information from DNA to protein.
Types of RNA
Three Types of RNA
- mRNA: This is messenger RNA. It is the molecule made from a DNA template that contains the complementary gene sequence, that is, the sequence opposite to the DNA strand. It transcribes the DNA code in the nucleus and carries it out into the cytoplasm. It is a single-strand sequence complementary to the DNA gene sequence.
- tRNA: This is transfer RNA. It 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.
- rRNA: Ribosomal RNA (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.
The structure of an RNA molecule is similar to that of a DNA molecule except for a few differences: RNA is single-stranded rather than double-stranded; it contains a ribose sugar rather than a deoxyribose sugar, and the base uracil is used in place of thymine.
Comparison of DNA and RNA
| Trait | DNA | RNA |
|---|---|---|
| Structure | double-stranded (forms a double helix) | single-stranded |
| Sugar backbone | deoxyribose | ribose |
| Nitrogen-containing bases | A, T, G, C | A, U, G, C |
DNA and RNA are both molecules that carry information. DNA is double stranded, has the nitrogen bases adenine, guanine, cytosine, and thymine, and has a backbone made from the sugar deoxyribose. RNA, on the other hand, is single stranded, has the nitrogen base uracil instead of thymine, and has a backbone made from the sugar ribose.
The Genetic Code
One factor tying all living things together is that they all use the same genetic code. This means the DNA and RNA found in every organism has the same nitrogen-containing bases, ring-shaped compounds that bind specifically to one other base. The order of these bases is different so each organism can form and express its individual traits, but the components of the genetic material are the same. All living things having the same genetic code adds significant evidence to the theory of evolution, which states all living things arose from a single common ancestor.
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 stop protein synthesis ("stop codons"), the remaining 61 combinations code for the 20 amino acids. Most amino acids are coded for by more than one codon. For example, both AGA and AGG code for the amino acid arginine. The illustration shows which amino acids are encoded by each codon sequence. To read the chart, start on the left side with the first nitrogen-containing base in the codon. This is the first one on the 5′ end of the strand. Then go across the top to the second nitrogen-containing base in the codon. Finally, go to the right and line up the third nitrogen-containing base in the sequence to identify the amino acid that codon codes for. The sequence AUG represents methionine, or the "start" codon. When proteins are being synthesized, this codon always appears first on the strand to show the enzymes and other synthesis components where they need to start their efforts. Conversely, UAA, UAG, and UGA are the "stop" codons. These appear as the last sequence being translated to show the enzymes where they are to cease their work. There are some minor differences in codon assignment in protozoans (a group of unicellular organisms) and in mitochondrial DNA (DNA found within mitochondria, the organelles that produce energy in the cell).