Proteins are the result of gene expression. They are often referred to as gene products because of the central dogma of biochemistry: DNA to RNA to protein. Genes are expressed when they are “on”, meaning that their protein product is made and used in the cell or organism.
Gene expression in humans and other animals begins with the DNA sequence. Deoxyribonucleic acid (DNA) is a double-stranded molecule made with complementary sequences. The sequence consists of four nucleotides adenine (A), guanine (G), cytosine (C), and thymine (T). Genes are blocks of nucleotide sequence flanked by repeated sequences recognized by the cellular machinery (enzymes and binding proteins). The transcription factors, which are themselves proteins, bind to the DNA to allow the RNA polymerase, an enzyme that builds RNA strands, to read the sequence of one of the DNA strands after the double strand is opened.
Ribonucleic acid (RNA) is a single-stranded molecule with a similar backbone to DNA and made of four nucleotides. The difference in the RNA and DNA nucleotides is that T is replaced by Uracil (U). When an RNA strand has been completely transcribed, it is processed and transported out of the nucleus to the cytosol of the cell. This is a simplified explanation of the process because there are various regulatory mechanisms in place that can prevent gene expression by limiting RNA production or preventing it altogether. The RNA transcribed from DNA is known as messenger RNA (mRNA). Other types of cellular RNA include RNA genomes (as that found in retroviruses), transfer RNA (tRNA) that plays a role in amino acid transport and placement in protein synthesis, and short interfering RNA (siRNA) that play a role in the regulation of gene expression.
The processed mRNA is bound by a ribosome, a protein subunit assembly that differs from species to species. The ribosome exposes two codons at a time. Codons are triplet nucleotide sequences. Protein synthesis begins with the start codon AUG. The first step in protein synthesis is initiation. The tRNA brings in the first amino acid, the building blocks of proteins, and holds it in the first position of the ribosome. Another tRNA, each being specific for the amino acid they carry and codon they recognize, brings in the next amino acid based on the codon in the second ribosome position. The amino acids are joined by peptidyl transferase, the primary enzymatic function of the ribosome. The first tRNA detaches and the second moves into the first position. The next tRNA comes into the second position to repeat the process.
Elongation (the second step of protein synthesis) continues until the stop codon is reached, the amino acid chain hanging off the ribosome like a tail. Inframe and out of frame translation refers to the placement of the codons. If there is a mutation in the RNA sequence, due to either a mistake in transcription or a mutation in the DNA, it can cause the codon sequences to change and alter the protein sequence. This can have a number of effects on gene expression the protein may not fold correctly and be destroyed by the cell, the protein may not function, the protein may be shortened, the protein may be elongated, or there may be no effect at all.
Termination often requires a release factor to allow the peptide to release from the ribosome. A peptide is an amino acid chain. Peptides are processed in many ways before they are functional: joining with other peptides to become polypeptides, folding, being processed by the golgi apparatus and endoplasmic reticulum to have groups added, or being relocated to the membrane.
In this manner, each gene encodes a different protein, which can be measured to determine the activity of the gene, known as gene expression.