Chapter 2 Structure and Function of Nucleic Acids. Introduction
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The two strands of the double-helical molecule, each of which possesses a polarity, are antiparallel; ie, one strand runs in the 5’to 3’ direction and the other in the 3’ to 5’direction. The two strands, in which opposing bases are held together by hydrogen bonds, wind around a central axis in the form of a double helix. The genetic information resides in the sequence of nucleotides on one strand, the template strand. This is the strand of DNA that is copied during nucleic acid synthesis.
A always pairs with T, and G with C. This complementary base-paring enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix.
Three kinds of DNA double-helix
Figure 4-8. The denaturation and renaturation of double-stranded DNA molecules.
RNA is a polymer of purine and pyrimidine ribonucleotides linked together by 3’, 5’-phosphodiester bridges analogous to those in DNA. Although sharing many features with DNA, RNA possesses several specific differences.
Those cytoplasmic RNA molecules that serve as templates for protein synthesis are designated messenger RNAs, or mRNA. Many other cytoplasmic RNA molecules (ribosomal RNAs, or rRNA) have structural roles wherein they contribute to the formation of ribosomes ( the organellar machinery for protein synthesis) or serve as adapter molecules (transfer RNAs; tRNAs) for the translation of RNA information into specific sequences of polymerized amino acids.
Some RNA molecules have intrinsic catalytic activity. The activity of these ribozymes often involves the cleavage of a nucleic acid. An example is the role of RNA in catalyzing the processing of the primary transcript of a gene into mature messenger RNA.
The mRNA molecules present in the cytoplasm are not the RNA products immediately synthesized from the DNA template but must be formed by processing from a precursor molecule before entering the cytoplasm. Thus, in mammalian nuclei, the immediate products of gene transcription constitute a fourth class of RNA molecules. These nuclear RNA molecules are very heterogeneous in size and are quite large. The heterogeneous nuclear RNA (hnRNA) molecules may have a molecular weight in excess of 107, whereas the molecule weight of mRNA molecules is generally less than 2×106.
(Only on some caps)
7-methylguanosine triphosphate cap
Figure 4-18. Structure of the 5′ methylated cap of eukaryotic mRNA.
Figure 4-19. Overview of RNA processing in eukaryotes using β-globin gene as an example. The β-globin gene contains three protein-coding exons (red) and two intervening noncoding introns (blue). The introns interrupt the protein-coding sequence between the codons for amino acids 31 and 32 and 105 and 106. Transcription of this and many other genes starts slightly upstream of the 5′ exon and extends downstream of the 3′ exon, resulting in noncoding regions (gray) at the ends of the primary transcript. These regions, referred to as untranslated regions (UTRs), are retained during processing. The 5′ 7-methylguanylate cap (m7Gppp; green dot) is added during formation of the primary RNA transcript, which extends beyond the poly(A) site. After cleavage at the poly(A) site and addition of multiple A residues to the 3′ end, splicing removes the introns and joins the exons. The small numbers refer to positions in the 147-aa sequence of β-globin.
Figure 4-30. Recognition of a tRNA by aminoacyl synthetases. Aspartyl-tRNA synthetase (AspRS) is a class II enzyme, and arginyl-tRNA synthetase (ArgRS) is a class I enzyme.
Figure 6-11. Amino acid activation. The two-step process in which an amino acid (with its side chain denoted by R) is activated for protein synthesis by an aminoacyl-tRNA synthetase enzyme is shown. As indicated, the energy of ATP hydrolysis is used to attach each amino acid to its tRNA molecule in a high-energy linkage. The amino acid is first activated through the linkage of its carboxyl group directly to an AMP moiety, forming an adenylated amino acid;the linkage of the AMP, normally an unfavorable reaction, is driven by the hydrolysis of the ATP molecule that donates the AMP. Without leaving the synthetase enzyme, the AMP-linked carboxyl group on the amino acid is then transferred to a hydroxyl group on the sugar at the 3' end of the tRNA molecule. This transfer joins the amino acid by an activated ester linkage to the tRNA and forms the final aminoacyl-tRNA molecule. The synthetase enzyme is not shown in these diagrams.
Figure 4-29. Aminoacylation of tRNA. Amino acids are covalently linked to tRNAs by aminoacyl-tRNA synthetases.
Figure 4-22. Assigning codons using synthetic mRNAs containing a single ribonucleotide. Addition of such a synthetic mRNA to a bacterial extract that contained all the components necessary for protein synthesis except mRNA resulted in synthesis of polypeptides composed of a single type of amino acid as indicated.
A ribosome is a cytoplasmic nucleoprotein structure that acts as the machinery for the synthesis of proteins from the mRNA templates. On the ribosomes, the mRNA and tRNA molecules interact to translate into a specific protein molecule information transcribed from the gene.
Figure 4-32. The general structure of ribosomes in prokaryotes and eukaryotes.
Figure 4-33. Two-dimensional map of the secondary structure of the small (16S) rRNA from bacteria, showing the location of base-paired stems and loops. In general, the length and position of the stem-loops are very similar in all species, although the exact sequence varies from species to species. The most highly conserved regions are represented as red lines, and the numbered stem-loops unique to prokaryotes are preceded by a P. Eukaryotic small (18S) rRNAs exhibit a generally similar pattern of stem-loops, although, as with prokaryotes, a few are unique.
Figure 4-25. Translation of nucleic acid sequences in mRNA into amino acid sequences in proteins requires a two-step decoding process. First, an aminoacyl-tRNA synthetase couples a specific amino acid to its corresponding tRNA. Second,a three-base sequence in the tRNA (the anticodon) base-pairs with a codon in the mRNA specifying the attached amino acid. If an error occurs in either step, the wrong amino acid may be incorporated into a polypeptide chain.
Figure 6-17. The genetic code. The standard one-letter abbreviation for each amino acid is presented below its three-letter abbreviation. Codons are written with the 5'-terminal nucleotide on the left. Note that most amino acids are represented by more than one codon and that variation is common at the third nucleotide (see also Figure 3-16).
Figure 4-35. Two types of methionine tRNA are found in all cells. One, designated tRNAiMet, is used exclusively to start protein chains, and the other, designated tRNAMet, delivers methionine to internal sites in a growing protein chain. In bacteria, a formyl group (CHO) is added to methionyl-tRNAiMet, forming fMet-tRNAiMet
Some are capable of hydrolyzing both strands of a double-stranded molecules, whereas others can only cleave single strands of nucleic acids. Some nuclease can hydrolyze only unpaired single strands, while others are capable of hydrolyzing single strands participating in the formation of double-stranded molecule.
Palindromic: The nucleotide sequence is the same if the helix is turned 180 degrees around the center of the short region of the helix that is recognized.
1. The element that could be used in nucleic acid quantitation is ( )
A. Ribose and deoxyribose
B. phosphoric acid and pentaglucose
C. Pentaglucose and basic group
E. phosphoric acid，pentose and basic group
C. 2’,5’ －磷酸二酯键
7. 核酸分子中储存、传递遗传信息的关键部分是( )
14. DNA Tm值较高是由于下列哪组核苷酸含量较高所致?
A. Golgi's body
B. rough endoplasmic reticulum
A. optimum temperature
B. hydrolytic temperature
C. Renaturation temperature
D. melting temperature
E. denaturation temperature