25.6 Replication of DNA DNA replication begins in the nucleus with partial unwinding of the double helix; this process involves enzymes known as helicases. The unwinding occurs simultaneously in many specific locations known as origins of replication. The DNA strands separate, exposing the bases. These branch points, called replication forks, provide a bubble into which the replication process can begin. Chapter Twenty Six 1
(a) DNA unwinds, exposing single strands. (b) Single-stranded DNA is exposed at numerous replication forks as DNA unwinds. (c) DNA polymerase enzymes facilitate copying of the single-stranded DNA. Chapter Twenty Six 2
DNA polymerase catalyzes the reaction between the 5 phosphate on an incoming nucleotide and the free 3 OH on the growing polynucleotide. The template strand can only be read in the 3 to 5 direction, and the new DNA strand can grow only in the 5 to 3 direction. Chapter Twenty Six 3
Only the leading strand grows continuously from 5 to 3 towards the fork. The lagging strand is replicated from 5 to 3 in short segments called Okazaki fragments. These short sections are joined later by DNA ligase. Chapter Twenty Six 4
Two identical copies of the DNA double helix are produced during replication. In each new double helix, one strand is the template and the other is the newly synthesized strand. We describe the result as semiconservative replication (one of the two strands is conserved). Chapter Twenty Six 5
25.7 Structure and Function of RNA Ribosomal RNAs: Outside the nucleus but within the cytoplasm of a cell are the ribosomes, small granular organelles where protein synthesis takes place. Each ribosome is a complex consisting of about 60% ribosomal RNA (rrna) and 40% protein, with a total molecular weight of approximately 5,000,000 amu. The transfer RNAs (trna) are smaller RNAs that deliver amino acids one by one to protein chains growing at ribosomes. Each trna carries only one amino acid. Chapter Twenty Six 6
The messenger RNAs (mrna) carry information transcribed from DNA. They are formed in the cell nucleus and transported out to the ribosomes, where proteins will be synthesized. These polynucleotides carry the same code for proteins as does the DNA. Chapter Twenty Six 7
25.8 Transcription: RNA Synthesis Only one of the two DNA strands is transcribed during RNA synthesis. The DNA strand that is transcribed is the template strand; its complement in the original helix is the informational strand. The mrna molecule is complementary to the template strand, which makes it an exact RNAduplicate of the DNA informational strand, with the exception that a U replaces each T in the DNA strand. Chapter Twenty Six 8
The transcription process begins when RNA polymerase recognizes a control segment in DNA that precedes the nucleotides to be transcribed. The sequence of nucleic acid code that corresponds to a complete protein is known as a gene. The RNA polymerase moves down the DNA segment to be transcribed, adding complementary nucleotides one by one to the growing RNA strand as it goes. Transcription ends when the RNA polymerase reaches a codon triplet that signals the end of the sequence to be copied. Chapter Twenty Six 9
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Some of these bases, however, do not code for genes. It turns out that genes occupy only about 10% of the base pairs in DNA The code for a gene is contained in one or more small sections of DNA called an exon. The code for a given gene may be interrupted by a sequence of bases called an intron. Introns are sections of DNA that do not code for any part of the protein to be synthesized. Chapter Twenty Six 11
The initial mrna strand contains both exons and introns, and is known as heterogeneous nuclear RNA (or hnrna). In the final mrna molecule released from the nucleus, the intron sections have been cut out and the remaining pieces are spliced together through the action of a structure known as a spliceosome. Chapter Twenty Six 12
25.9 The Genetic Code Codon: A sequence of three ribonucleotides in the messenger RNA chain that codes for a specific amino acid; also a three-nucleotide sequence that is a stop codon and stops translation. Genetic code: The sequence of nucleotides, coded in triplets (codons) in mrna, that determines the sequence of amino acids in protein synthesis. Of the 64 possible three-base combinations in RNA, 61 code for specific amino acids and 3 code for chain termination. Chapter Twenty Six 13
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25.10 Translation: Transfer RNA and Protein Synthesis Overview of protein synthesis: The codons of mature mrna are translated in the ribosomes, where trnas deliver amino acids to be assembled into proteins (polypeptides). The three stages in protein synthesis are initiation, elongation, and termination. Chapter Twenty Six 15
Structure of trna. (a) The cloverleaf shaped trna contains an anticodon triplet and a covalently bonded amino acid at its end. (b) A computergenerated model of the serine trna molecule. (c) The 3-D shape of a trna molecule. Chapter Twenty Six 16
Initiation: Protein synthesis begins when an mrna, the first trna, and the small subunit of a ribosome come together. The first codon on the end of mrna, an AUG, acts as a start signal for the translation machinery and codes for a methionine carrying trna. Initiation is completed when the large ribosomal subunit joins the small one and the methioninebearing trna occupies one of the two binding sites on the united ribosome. If it is not needed, the methionine from chain initiation is removed by post-translational modification before the new protein goes to work. Chapter Twenty Six 17
The three elongation steps now repeat: The next trna binds to the ribosome. Peptide bond formation attaches the new amino acid to the chain and the first trna is released. Ribosome position shifts to free the second binding site for new trna. Chapter Twenty Six 18
Termination: A stop codon signals the end of translation. An enzyme called a releasing factor then catalyzes cleavage of the polypeptide chain from the last trna. The trna and mrna molecules are released from the ribosome, and the two ribosome subunits again separate. Chapter Twenty Six 19
Chapter Summary Nucleic acids are polymers of nucleotides. Each nucleotide contains a sugar, a base, and a phosphate group. A nucleoside contains a sugar and a base, but not the phosphate group. Nucleotides are connected by phosphate diester linkages between the 3 OH group of one and the 5 phosphate group of the next. DNA consists of two polynucleotide strands twisted together in a double helix. The sugar phosphate backbones are on the outside, and the bases are in the center of the helix. The bases are complementary, opposite every T is an A, opposite every G is a C. The base pairs are connected by hydrogen bonds. Chapter Twenty Six 20