In the image, start from the 5' end of the mRNA molecule and move right. Then a second tRNA "train car" enters the ribosome with its own specific anticodon to bond with the codon on mRNA, and so on. It passes through the ribosome where it meets up with a tRNA molecule that has a complementary anticodon to the first codon triplet of mRNA. If you examine the above image, the messenger RNA has come from the nucleus with the complementary copy of the DNA molecule. The process of translation is like a train that picks up codon triplets and drops off amino acids to form polypeptides. When a triplet is found in mRNA, it is called a codon or a codon triplet. It was since discovered that the genetic code is written in a "language" of three bases that contain enough information to code for one of the 20 amino acids. The message written on mRNA is the message that determines the order of the amino acids. The sequence of nucleotides making up the mRNA is enough to make one polypeptide. The sequence of mRNA nucleotides is the transcribed version of the original DNA sequence. The mRNA molecule is a complementary copy of one gene of DNA. T hings to note about transcription: (a) One DNA strand is copied, (b) mRNA is always shorter than the DNA it is copied from because it is a complementary copy of one gene, (c) the presence of thymine identifies it as DNA, and (d) the presence of uracil identifies it as RNA. mRNA (in red) begins transcribing the separated strand using free nucleotides in the cell. The center oval is the gene that's been split by RNA polymerase. The image above shows the transcription process. mRNA is a short strand that is a complementary copy of a single gene. Uracil, a nitrogenous base specific only to RNA, pairs with adenine. Free RNA nucleotides float into place by complementary base pairing. RNA, a single-stranded molecule, will link with one of the strands to form the mRNA molecule with RNA polymerase used to catalyze the reaction. The process of transcription goes as follows: A section of DNA (a gene) unzips into two single strands. This process ensures that an exact copy of DNA is produced. Obviously, it takes a little longer for the lagging strand to complete. There is a leading strand and a lagging strand the lagging strand forms in the opposite direction of the leading strand, from 5' to 3'. New DNA strands are rebuilt only from the 5' end, so the two new strands are not constructed at the same rate. This process is catalyzed by DNA polymerase. The free nucleotides hanging out in the nucleus can now bond to the single strands, but they only bond in complementary base pairs: A to a T, G to a C. The unpaired nucleotides on each single strand is now a template to create two new strands. Helicase begins at some random point within the molecule or at the end of it and splits one complementary base pair (A-T, C-G) at a time. Remember that the helix is held together by extremely weak hydrogen bonds. Helicase is responsible for separating the double helix into two single strands. Hanging out in the nucleus during this time are two kinds of molecules to facilitate this process: the enzymes helicase and DNA polymerase, and free nucleotides. Recall that DNA replication occurs during the S sub-phase of interphase.
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