28.10 Replication of DNA

1. 28.10Replication of DNA

2. Fig. 28.8 DNA Replication • The DNA to be copied is a double helix, shown here as flat for clarity. The two strands begin to unwind. (next slide)

3. Fig. 28.8 DNA Replication • Each strand will become a template for construction of its complement.

4. Fig. 28.8 DNA Replication • Two new strands form as nucleotides that are complementary to those of the original strands are joined by phosphodiester linkages. Polynucleotide chains grow in the 5'-3' direction—continuous in the leading strand, discontinuous in the lagging strand.

5. Fig. 28.8 DNA Replication • Two duplex DNAs result, each of which is identical to the original DNA.

6. Elongation of the Growing DNA Chain • The free 3'-OH group of the growing DNA chain reacts with the 5'-triphosphate of the appropriate nucleotide.

7. OH O O O Adenine, Guanine, Cytosine, or Thymine CH2 O– O P P P O O O– O– O– •• OH • • Poly-nucleotidechain O Adenine, Guanine, Cytosine, or Thymine CH2OPO O– O O Fig. 28.9: Chain Elongation

8. OH – O O O– Adenine, Guanine, Cytosine, or Thymine CH2 O P P O O O P O– O– O– • • O • • Poly-nucleotidechain O Adenine, Guanine, Cytosine, or Thymine CH2OPO O– O O Fig. 28.9: Chain Elongation

9. 28.11Ribonucleic Acids

10. DNA and Protein Biosynthesis • According to Crick, the "central dogma" of molecular biology is: "DNA makes RNA makes protein." • Three kinds of RNA are involved.Messenger RNA (mRNA)Transfer RNA (tRNA)Ribosomal RNA (rRNA) • There are two main stages.TranscriptionTranslation

11. Transcription • In transcription, a strand of DNA acts as a template upon which a complementary RNA is biosynthesized. • This complementary RNA is messenger RNA (mRNA). • Mechanism of transcription resembles mechanism of DNA replication. • Transcription begins at the 5' end of DNA and is catalyzed by the enzyme RNA polymerase.

12. Fig. 28.10: Transcription Only a section of about 10 base pairs in the DNAis unwound at a time. Nucleotides complementaryto the DNA are added to form mRNA.

13. The Genetic Code • The nucleotide sequence of mRNA codes for the different amino acids found in proteins. • There are three nucleotides per codon. • There are 64 possible combinations of A, U, G, and C. • The genetic code is redundant. Some proteins are coded for by more than one codon.

14. U C A G U C First letter A Second letter Third letter G Table 28.3 • UUU Phe UCU Ser UAU Tyr UGU Cys U • UUC Phe UCC Ser UAC Tyr UGC Cys C • UUA Leu UCA Ser UAA Stop UGA Stop A • UUG Leu UCG Ser UAG Stop UCG Trp G • U • C • A • G • U • C • A • G • U • C • A • G

15. U C A G U C A G • UUU Phe UCU Ser UAU Tyr UGU Cys U • UUC Phe UCC Ser UAC Tyr UGC Cys C • UUA Leu UCA Ser UAA Stop UGA Stop A • UUG Leu UCG Ser UAG Stop UCG Trp G • CUU Leu CCU Pro CAU His CGU Arg U • CUC Leu CCC Pro CAC His CGC Arg C • CUA Leu CCA Pro CAA Gln CGA Arg A • CUG Leu CCG Pro CAG Gln CCG Arg G • AUU Ile ACU Thr AAU Asn AGU Ser U • AUC Ile ACC Thr AAC Asn AGC Ser C • AUA Ile ACA Thr AAA Lys AGA Arg A • AUG Met ACG Thr AAG Lys ACG Arg G • GUU Val GCU Ala GAU Asp GGU Gly U • GUC Val GCC Ala GAC Asp GGC Gly C • GUA Val GCA Ala GAA Glu GGA Gly A • GUG Val GCG Ala GAG Glu GCG Gly G

16. U C A G U C A G UAA, UGA, and UAG are "stop" codons thatsignal the end of the polypeptide chain. • U • C • UAA Stop UGA Stop A • UAG Stop G • U • C • A • G • AUU Ile ACU Thr AAU Asn AGU Ser U • AUC Ile ACC Thr AAC Asn AGC Ser C • AUA Ile ACA Thr AAA Lys AGA Arg A • AUG Met ACG Thr AAG Lys ACG Arg G • U • C • A • G AUG is the "start" codon. Biosynthesis of allproteins begins with methionine as the first aminoacid. This methionine is eventually removed afterprotein synthesis is complete.

17. Transfer tRNA • There are 20 different tRNAs, one for each amino acid. • Each tRNA is single stranded with a CCA triplet at its 3' end. • A particular amino acid is attached to the tRNA by an ester linkage involving the carboxyl group of the amino acid and the 3' oxygen of the tRNA.

18. UUC AAG 5' 3' 3' 5' The complementary sequence in tRNA is calledthe anticodon. Transfer RNA • Example—Phenylalanine transfer RNA One of the mRNA codons for phenylalanine is:

19. O OCCHCH2C6H5 NH3 + Anticodon Fig. 28.11: Phenylalanine tRNA 3' 3' 5' 5'

20. Ribosomal RNA • Most of the RNA in a cell is ribosomal RNA. • Ribosomes are the site of protein synthesis. They are where translation of the mRNA sequence to an amino acid sequence occurs. • Ribosomes are about two-thirds RNA and one-third protein. • It is believed that the ribosomal RNA acts as a catalyst—a ribozyme.

21. 28.12Protein Biosynthesis

22. Protein Biosynthesis • During translation the protein is synthesized beginning at its N-terminus. • mRNA is read in its 5'-3' direction. Begins at the start codon AUG Ends at stop codon (UAA, UAG, or UGA)

23. Fig. 28.12: Translation Methionine at N-terminusis present as its N-formylderivative. • Reaction that occurs is nucleophilic acyl substitution. Ester is converted to amide.

24. Fig. 28.12: Translation

25. Fig. 28.12: Translation • Ester at 3' end of alanine tRNA is Met-Ala. • Process continues along mRNA until stop codon is reached.