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A gene is a segment of DNA that codes for a polypeptide or a functional RNA. Many times the word "gene" is used interchangeably with the term "protein-coding gene", but a gene does not need to code for a protein (polypeptide).
There are two main steps in making a protein from DNA: transcription and translation.
TRANSCRIPTION
During the process of transcription, an enzyme called RNA polymerase binds to DNA at a gene's promoter, then begins unwinding the DNA and making a complementary strand of RNA from the exposed DNA template. Depending on the gene being transcribed, the result can be a molecule of mRNA (messenger RNA), tRNA (transfer RNA), or rRNA (ribosomal RNA). Each type of RNA performs a specific function later in translation.
In eukaryotes, mRNA and tRNA, as well as the ribosomal subunits made using rRNA, must be exported out of the nucleus through the nuclear pore complexes. In addition, the original primary transcripts for mRNA have introns spliced out, a 5' cap added, and a poly-A tail tacked on before the mRNA is exported from the nucleus.
TRANSLATION
Protein synthesis occurs on 'workbenches' called ribosomes in a process called translation. In translation, all 3 primary types of RNA cooperate. mRNA (messenger RNA) carries the genetic information transcribed from DNA to the ribosome; tRNA (transfer RNA) molecules carry specific amino acids to the ribosome to be added to the growing chain; and rRNA (ribosomal RNA), along with proteins, make up ribosomes.
RIBOSOME
A ribosome has 3: the A site, the P site, and the E site. The A (aminoacyl) site is where amino acids arrive at the ribosome, carried by their respective tRNA molecules. The P (peptidyl) site is where the growing polypeptide is located, attached to the tRNA docked there. The E (exit) site is where a tRNA that has no amino acid attached it is positioned just before it leaves the ribosome.
GENETIC CODE
A ribosome "reads" an mRNA molecule 3 bases at a time; a set of 3 consecutive mRNA bases that are read as a single unit is called a codon. Because there 4 possibilities (A, C, U, or G) at each of 3 positions, there are a total of 4^3 = 64 possible codons. The genetic code is the mapping of each of the 64 possible codons to its corresponding amino acid; in a few cases, a codon maps instead to either a START or STOP signal
Translation consists of 3 stages: initiation, elongation, and termination.
INITIATION
There are two separate subunits for a ribosome: a large subunit and a small subunit. The two are separate most of the time, but when mRNA binds to one of them, the other subunit binds to the complex, forming a ribosome and initiating translation.
ELONGATION
We can break elongation down into steps.
(1) Codon recognition: A tRNA carrying the correct amino acid (determined by complementarity between the tRNA?s anitcodon and the mRNA?s codon) docks in the A site.
(2) Peptide bond formation: The existing, growing polypeptide is transferred from the tRNA molecule in the P site to the amino acid on the new tRNA in the A site.
(3)Translocation: The ribosome moves 3 bases along the mRNA, moving the uncharged tRNA in the P site into the E (exit) site, the tRNA in the A site (with the polypeptide now attached) into the P site, and positioning a new codon into association with the now-empty A site.
(4) Back to step (1)
TERMINATION
Eventually, a STOP codon is reach, positioned in the A site. A STOP codon does not code for an amino acid, but for a release factor. The release factor binds, dissociating the ribosome into its two subunits and releasing the mRNA and the polypeptide. The polypeptide will then fold into a stable tertiary structure to become a protein (or, it may associate with other such folded polypeptides to form a protein with quaternary structure).
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