DNA and RNA differ in their chemical structures. While DNA is double-stranded, RNA is single-stranded. There are four main types of RNA - messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), and small stable RNA. mRNA carries genetic information from DNA to the ribosome. It is modified with a 5' cap and 3' poly-A tail. tRNA transfers amino acids to the growing polypeptide chain during protein synthesis. rRNA makes up the ribosomal subunits and is involved in protein synthesis.

1. DNA and RNA June.19.2010

2. Chemical structure of DNA

7. The chemical nature of RNA differs from that of DNA 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

9. DNA OH¯ OH¯ DNA RNA DNA RNA

10. Types of RNA Messenger RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA (rRNA) Small stable RNA (sRNA)

11. Messenger RNA (mRNA) The most heterogeneous in size and stability. It is synthesized as large precursor forms known as heterogenous nuclear RNA or hnRNA . All members of the class function as messengers conveying the information in a gene to the protein synthesizing machinery, where it serve as a template on which a specific sequence of amino acids is polymerized to form a specific protein molecule. Newly synthesized mRNA combines with protein to form mRNA-protein complex (mRNA), which come out through nuclear pore by simple diffusion into the cytoplasm to serve as templates for protein synthesis.

12. The 5’ terminal of mRNA is “capped” by a 7-methylguanosine triphosphate that is linked to an adjuacent 2’ – O -methy ribonucleoside at its 5’-hydroxyl through the three Phosphates. The mRNA molecules frequently contain internal 6-methyladenylates and other 2’ – O -methylated nucletides. The cap is involved in the recognition of mRNA by the translating machinery, and it probably helps stabilize the mRNA by preventing the attack of 5’-exonucleases. The protein-synthesizing machinery begins translating the mRNA into proteins beginning downstream of the 5’ or capped terminal. The other end of most mRNA molecules, the 3’-hydroxyl terminal, has an attached polymer of adenylate residuces 20-250 nucleotides in length. (Poly(A) tail).

14. Posttranscriptional modification of mRNA showing the 7-methylguanosine cap and poly-A tail.

15. Transfer RNA (tRNA) tRNA molecules vary in length from 74 to 95 nucleotides, have clover leaf like structure. The tRNA molecule serve as adapters for the translation of the information in the sequence of nucleotides of the mRNA into specific amino acids. There are at least 20 species of tRNA molecules in every cell, at least one corresponding to each of the 20 amino acids required for protein synthesis. All tRNA molecules have four main arms. The acceptor arm terminates in the nucleotides CCA.

16. Thymidine, pseudouridine, cytidine

17. A. Characteristic tRNA secondary structure. B. Folded (tertiary) tRNA structure found in cells. D = dihydro-uracil. ψ = pseudouracil.

18. Primary tRNA transcript. B. Functional tRNA after posttranscriptional modification. Modified bases include D (dihydrouracil), ψ ( pseudouracil), and m , which means that the base has been methylated.

19. AUG

20. Genetic Code

21. Ribosomal RNA (rRNA) Ribosomal RNAs are synthesized in large precursor form known as pre rRNA. Four rRNA molecules 5, 5.8, 18 and 28 S are present in eukaryotes formed from 40S and 60S subunit. rRNA are required for the formation of ribosomes. 16S RNA is involved in initiation of protein synthesis.

22. Posttranscriptional processing of eukaryotic ribosomal RNA by ribonucleases (RNases)

23. Prokaryotic and eukaryotic rRNAs

26. Structure of eukaryotic messenger RNA

27. Alternative splicing patterns in eukaryotic mRNA.

31. Differences between DNA and RNA DNA RNA 1. Sugar moiety is deoxyribose. Sugar moiety is ribose. 2. Uracil is absent. Thymine is absent. . 3. Double stranded molecules. Single stranded molecules. 4. Sum of purine bases is equal to sum of pyrimidine base. (A+G)=(C+T) Sum of purine bases is not equal to sum of pyrimidine base. (A+G)#(C+T) 5. Resistant to hydrolysis by alkali because of absence of hydroxyl group on 2 carbon atom of deoxyribose. Because of presence of hydroxyl group on 2 carbon atom of ribose, RNA is easily hydrolyzed by alkali.

33. 以上です。