Principles of Anatomy and Physiology 14e with Atlas of the Skeleton Set (14th Edition)

Published by Wiley
ISBN 10: 1-11877-456-6
ISBN 13: 978-1-11877-456-4

Chapter 3 - The Cellular Level of Organization - Checkpoint: 26


The information for making the protins indispensable for life is contained in coded form in the genes of the DNA molecules in the nucleus. Transcription is the copying of that coded information in single stranded mRNA molecules. This process takes place in the nucleus. The template is the single stranded DNA and the raw material used to make the complementary strand is the set of ribonucleotides ATP, GTP, UTP and CTP. Several types of RNA are produced by the process; they include mRNA, tRNA , rRNA and nom-coding RNA. The vital catalyst in the assembly of RNAs is RNA polymerase. Translation is the reading and applying the copied DNA information in the process of protein synthesis. This process occurs in the cytoplasm. The template is mRNA, and the raw material assembled on the template is the set of 20 amino acids. ribosomes , both small and large subunits, play vital roles in the process of translation.

Work Step by Step

Transcription occurs in the nucleus. First the double stranded DNA molecule unwinds --only one strand serves as a template. The base triplet nucleotide code is copied to form a new single strand of RNA with a complementary sequence of bases. Transcription starts when RNA polymerase attaches near a special sequence of bases called a promoter. .Bases are paired in a complementary manner-- G-C, C-G, A-U, andT-A. The RNA polymerase transcribes until it come to a terminator sequence of bases that marks the end of the gene . At this point the enzyme detaches from the new RNA molecule and the DNA template strand . RNA is of several kinds: transcription produces mRNA, tRNA , rRNA and noncoding RNA. After the RNA molecules are synthesized, they move into the cytoplasm through pores in the nuclear membrane. Translation involves all types of RNA . Ribosomes are partly composed of rRNA and they play essential roles in the translation of the message of mRNA. Each ribosomes has a small and a large subunit. The small subunit hlods the mRNA strand in place while it is being read.The large subunit has a much tougher job; It has three binding sites named A , P and E binding sites.; The of the P site is to bind the peptide chain as it is being assembled. The A site binds the t-RNA molecules as they bring up aa's for attachment to the peptide chain. The E site holds the pepetide chain about to be released into the cytoplasm. The first codon mRNa is always AUG--which is also the codon for methionine(Me). The translation starts when the t-RNA anticodon pairs up with AUG. . The complex initiator t-RNA with its amino acid (Me) attaches to the P site. Then the next t-RNA anticodon with its attached amino acid , moves in and pairs with the appropriate codon; this attaches to the A site of the large subunit. Next, an enzyme of the large subunit catalyzes a peptide bond formation between Me and the newly arrived aa brought in by a tRNA. The next step is that the process shifts to the next mRNA codon. The the tRNA in the P-site enters the E site and is released. At this point, a two amino acid peptide chain has been formed. The chain lengthens when another t-RNA with its attached amino acid binds to an exposed codon in the A site. . Codons and anti-codons continue to pair ; new peptide bonds are formed at he A site; the mRNA shifts along the ribosome and the new peptide is released by the E site. The repetition of these processes results in the lengthening of the peptide. The process of adding aa's to the peptide chain ends when a STOP codon is reached. The peptide chain then detaches from the-RNA, and the t-RNA leavers the ribosomal T-site . Finally the two-unit ribosome disassembles..
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