生化课件summary of nucleic acids

1、Composition , structure and function of nucleic acids,Summary,The nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are polymers of nucleotide units.,Nitrogenous Base + Sugar Nucleosides + phosphate group Nucleotide nucleic acids Nucleosides Are Formed by Joining a Nitrogenous B

2、ase to a Sugar through a N-glycosidic bond Nucleotides Are Nucleoside Phosphates Nucleic acids are linear polymers of nucleotides linked 3 to 5 by phosphodiester bridges,Each nucleotide unit is made of,a nitrogenous base AAdenine purine T Thymine pyrimidine G Guanine purine C Cytosine pyrimidine U U

3、racil pyrimidine (RNA only) a pentose sugar D-ribofuranose for RNA and 2-deoxy-D-ribofuranose for DNA. a phosphate group,Nucleoside 5-Triphosphates Are Carriers of Chemical Energy ATP has been termed the energy currency of the cell. GTP is the major energy source for protein synthesis . CTP is an es

4、sential metabolite in phospholipid synthesis . UTP forms activated intermediates with sugars that go on to serve as substrates in the biosynthesis of complex carbohydrates and polysaccharides.,Adenosine 3,5-cyclic monophosphate (cAMP), guanosine 3,5-cyclic monophosphate (cGMP) function as secondary

5、messengers in cell signal transductions. The Bases of Nucleotides Serve as “Information Symbols”,RNA molecules are categorized into several major types:,messenger RNA -mRNA, ribosomal RNA -rRNA, Transfer RNA-tRNA, small nuclear RNA -snRNA, (Eukaryotic cells).,Messenger RNA -mRNA, are copies of DNA (

6、synthesized by DNA transcription), that carry the information of one or a few genes to the ribosomes, where the corresponding protein(s) is(are) synthesized. Ribosomal RNA -rRNA, are structural components of ribosomes (the protein synthesis machine in cells).,Transfer RNA (tRNA) are adapter molecule

7、s that faithfully translate the information in a mRNA molecule into the specific amino acid sequences in a polypeptide chain. Some RNA molecules, named as Ribozymes, have catalytic activities functioning in the processing (cleavage) of precursor RNA molecules,DNA stores genetic information,The amino

8、 acid sequence of every protein and the nucleotide sequence of every RNA molecule in a cell are all specified by the nucleotide sequence of that cells DNA molecule. A segment of DNA that contains the information required for the synthesis of a functional protein or RNA is referred as a gene.,DNA str

9、ucture,Double helix AT/GC base pairs Antiparallel strands Major grooveminor groove A-, B-, and Z-DNA,The sequential order of nucleotides in a polynucleotide, is the so-called primary structure of nucleic acids. Two basic protocols for nucleic acid sequencing are in widespread use: the chain terminat

10、ion or dideoxy method of F. Sanger and the base-specific chemical cleavage method developed by A. M. Maxam and W. Gilbert. Chargaffs rules: A = T; C = G; pyrimidines = purines.,Watson and Cricks Double Helix (B-DNA),Right-handed double helix Antiparallel strands The phosphate-deoxyribose backbones a

11、re on the outside of the helix, whereas the purine and pyrimidine bases are stacked inside The planes of the bases are perpendicular to the helix and the planes of the deoxyribose rings A = T; C G The diameter 20 adjacent bases 3.4 10 residues (each chain) , rotation 360 per turn. Major grooveminor

12、groove,DNA is remarkably flexible molecule with many rotatable bonds. Different double-helical DNA structures can be formed by rotating various bonds that connect the structure. A DNA is wider and shorter than the B-form helix. Z-DNA is a left-handed double helix in which backbone phosphates zigzag

13、The conformation of DNA (A, B, or Z) depends on the temperature and salt concentration as well as the base composition of the DNA. Z-DNA appears to be favored in certain regions of DNA in which the sequence is rich in G and C base pairs.,DNA STABILITY Melting is denaturation. Annealing is renaturati

14、on. Hydrophobic stacking provides stability. Intercalating agents stack between bases. STABILITY INCREASED BY Decreased temperature Increased GC content (three hydrogen bonds) Increased salt (ionic strength),Intact DNA molecules from bacteria, some viruses, mitochondria, and chloroplasts are circula

15、r and supercoiled. The axis of the double helix can be twisted to form a superhelix (the circular DNA without any superhelical turns is known as a relaxed molecule). Supercoiling makes the DNA molecule more compact thus important for its packaging in cells.,RNA SECONDARY STRUCTURE,Stem: A stretch of

16、 double-stranded RNA Loop: A loop of RNA Hairpin loop: A very short loop Pseudoknot: Interaction between one secondary structure element and another part of the same RNA molecule Bulges,RNA SECONDARY STRUCTURE,mRNA,In prokaryotes, a single mRNA may contain the information for the synthesis of severa

17、l polypeptide chains within its nucleotide sequence. (polycistronic) In contrast, eukaryotic mRNAs encode only one polypeptide (monocistronic). eukaryotic mRNAs are synthesized in the nucleus in the form of much larger precursor molecules called heterogeneous nuclear RNA, or hnRNA. hnRNA molecules c

18、ontain stretches of nucleotide sequence that have no protein-coding capacity.,These noncoding regions are called intervening sequences or introns because they intervene between coding regions, which are called exons. Introns must be spliced out before the message can be translated. Eukaryotic hnRNA

19、and mRNA molecules have a run of 100 to 200 adenylic acid residues attached at their 3-ends, so-called poly(A) tails. This polyadenylylation occurs after transcription has been completed and is believed to contribute to mRNA stability.,Transfer RNA,Transfer RNA (tRNA) serves as a carrier of amino ac

20、id residues for protein synthesis. TRNA Secondary Structure cloverleaf These four segments are designated the acceptor stem (amino acid arm), the D loop, the anticodon loop, and the TC loop. Extra or Variable loop All tRNA molecules possess a “CCAOH” 3 terminal. The amino acid is attached to the fre

21、e 3-OH of the terminal catalyzed by aminoacyl-tRNA synthetase. TRNA Tertiary Structure - L-conformation,Ribosomal RNA,the protein-synthesizing machinery of cells, are composed of two subunits, called small and large, and ribosomal RNAs are integral components of these subunits.,The different rRNA mo

22、lecules of both prokaryotes and eukaryotes are generated from single pre-rRNAs,The 16S, 23S and 5S rRNAs in bacteria are all generated from a single 30S pre-rRNA The 18S, 28S and 5.8S rRNAs in eukaryotes are generated from a single 40S pre-rRNA). The 5S rRNA in eukaryotic cells is generated separate

23、ly,Denaturation and Renaturation of DNA,Duplex nucleic acids unwind to form two single strands at extreme pH and high temperature with changed physical properties. Viscosity decreases sharply. UV absorption at 260 nm increases significantly, called hyperchromism or hyperchromic effect. (base stackin

24、g decreases absorption). hypochromic effect,The rise in absorbance coincides with strand separation, and the midpoint of the absorbance increase is termed the melting temperature, Tm . (half of the duplex chain is separated) The unwinding (i.e., denaturation) of the double helix is called melting be

25、cause it occurs abruptly at a certain temperature. Each species of DNA has a characteristic melting temperature (Tm or tm) .,DNA Renaturation,DNA denaturation is reversible. Denatured DNA will renature to reform the duplex structure if the denaturing conditions are removed (that is, if the solution is cooled, the pH is returned to neutrality, or the denaturants are diluted out). When cooled under appropriate conditions, the two strands find each other, pair corre