%。 form a NA is a c If a plece of NA in one strand has the squence-AGCTACGAT 配荷 物 A一T一G- Aiguapan 26-10 Protein Synthesis Through RNA ach 2w 9 9 The combination of sugar and base is called a nucleoside. A sugar, base, and one or more phosphates is called a nucleotide. The sequence of individual nucleotides (4) within a particular strand of DNA taken three at a time (64) specify the sequence of amino acids (20) in a particular polypeptide chain. Nucleic acids form a double helix. DNA is a double helix consisting of two polynucleotide strands hydrogen-bonded to each other through their organic bases and running in opposite directions. If a piece of DNA in one strand has the sequence –AGCTACGATC-: The individual base pairing energies are: GC: 27.7 kcal mol-1 and AT: 15.1 kcal mol-1. DNA replicates by unwinding and assembling new complementary strands. The double helical nature of DNA suggested to Watson and Crick a mechanism for the replication of DNA during cell division. Each of the two strands in a parent DNA molecule serve as a template for the assembly of a new complementary strand in a new DNA molecule. This process occurs with an error frequency of less than 1 in 10 billion base pairs during DNA replication. 26-10 Protein Synthesis Through RNA The expression of the protein sequence information contained in DNA occurs in two steps, transcription and translation. Transcription is the process of copying a short sequence of DNA coding for one or a small number of proteins into a separate complementary molecule of mRNA. Translation is the process of reading the bases of the mRNA in groups of three and converting this information into a polypeptide sequence by means of small adapter molecules called transfer RNAs (tRNAs) and large catalytic assemblies called ribosomes