Biochemistry, 4th Edition P35. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Are the Different Classes of Nucleic Acids 303 histone chromosomal proteins many of which are involved in regulating which genes in DNA are transcribed at any given moment. The amount of DNA in a diploid mammalian cell is typically more than 1000 times that found in an E. coll cell. Some higher plant cells contain more than 50 000 times as much. Various Forms of RNA Serve Different Roles in Cells Unlike DNA cellular RNA molecules are almost always single-stranded. However all of them typically contain double-stranded regions formed when stretches of nucleotides with complementary base sequences align in an antiparallel fashion and form canonical A U and G C base pairs. Compare Figures and to convince yourself that U would pair with A in the same manner T does. Such base pairing creates secondary structure. Messenger RNA Carries the Sequence Information for Synthesis of a Protein Messenger RNA mRNA serves to carry the information or message that is encoded in genes to the sites of protein synthesis in the cell where this information is translated into a polypeptide sequence. That is mRNA molecules are transcribed copies of the protein-coding genetic units of DNA. Prokaryotic mRNAs have from 75 to 3 000 nucleotides mRNA constitutes about 2 of total prokaryotic RNA. Messenger RNA is synthesized during transcription an enzymatic process in which an RNA copy is made of the sequence of bases along one strand of DNA. This mRNA then directs the synthesis of a polypeptide chain as the information that is contained within its nucleotide sequence is translated into an amino acid sequence by the protein-synthesizing machinery of the ribosomes. Ribosomal RNA and tRNA molecules are also synthesized by transcription of DNA sequences but unlike mRNA molecules these RNAs are not subsequently translated to form proteins. In prokaryotes a single mRNA may contain the information for the synthesis of several polypeptide chains within its nucleotide sequence Figure
