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2. 2 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Sylvia S. Mader Immagini e concetti della biologia

3. 3 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 B2 - Molecular genetics

4. 4 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA and inheritance Fredrick Griffith’s experiments with Streptococcus pneumoniae (1931) demonstrated that a “transforming substance” was used as replicator.

5. 5 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA and inheritance Alfred Hershey and Martha Chase (1952) used the T2 bacteriophage and Escherichia coli for their experiments.

6. 6 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA and inheritance Hershey and Chase showed that DNA, not proteins, enters bacterial cells and directs the phage reproduction. Hershey and Chase’s first experiment - virus DNA is labeled.

7. 7 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA and inheritance Hershey and Chase showed that DNA, not proteins, enters bacterial cells and directs the phage reproduction. Hershey and Chase’s second experiment - virus capsid is labeled.

8. 8 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Nucleic Acids DNA is a polynucleotides (polymers of nucleotides). Each nucleotide is composed of three parts: a C 5 sugar called deoxyribose; a phosphate group; a nitrogen-containing base.

9. 9 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 There are four possible nitrogen bases. Adenine (A) and guanine (G) have a double-ring structure and belong to a group called purines. Thymine (T) and cytosine (C) have a single-ring structure and are called pyrimidines. Nucleic Acids

10. 10 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 RNA (ribonucleic acid) contains a C 5 sugar called ribose and the bases A,C,G. Uracil (U) replaces T. Nucleic Acids

11. 11 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA DNA is a functional genetic material as it: varies between species and individuals can store information remains constant within a species replicates undergoes mutations

12. 12 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA structure DNA is composed by a double strand of nucleotides where C always pairs with G and T pairs with A.

13. 13 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 James Watson and Francis Crick constructed the first model of DNA using Rosalind Franklin and Maurice Wilkins’s X-ray diffraction data. DNA structure

14. 14 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA structure In Watson and Crick double helix model weak hydrogen bonds between the bases hold the two chains to one another.

15. 15 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA is suitable for replication DNA duplication (or replication) is semiconservative as each new filament contains an old strand and a new strand.

16. 16 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA is suitable for replication DNA duplication involves three steps: 1.Unrolling and unwinding; 2.Complementary base paring; 3.Joining. DNA polymerase is an enzyme that assists steps 2 and 3.

17. 17 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA is suitable for replication In the C 5 deoxyribose, carbons are numbered 1', 2', 3', 4', and 5' to distinguish from the atoms of the nitrogen base ring. The 5' carbon binds to the P-group. The P-group of a nucleotide is bonded to the 3' C of the adjacent nucleotide.

18. 18 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA polymerase DNA polymerase adds nucleotides to the 3' end of the DNA and needs a primer to start the replication of a strand. Telomeres are special non- codifying repetitive nucleotide sequences that.

19. 19 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 DNA replication The second strand of DNA is duplicated in opposite direction of the replication fork. Replication is continuous for the leading strand but discontinuous for the lagging strand.

20. 20 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genes and protein synthesis Genes are expressed by proteins. The “one gene one protein” hypothesis is based on the observation that a defective gene causes a defective enzyme.

21. 21 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Protein synthesis The making of a protein requires two steps. 1.Transcription: DNA is transcribed into mRNA. 2.Translation: the transcript mRNA directs the amino acid sequence.

22. 22 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genetic code The genetic code is the set of information encoded in DNA that is translated into amino acid sequences. A three-nucleotides codon (i.e. AUC) in a sequence specifies a single amino acid. With few exceptions, the genetic code is universal.

23. 23 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Transcription During transcription, complementary base pairing occurs, RNA polymerase joins bases and the coded information passes from the gene to the mRNA.

24. 24 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Transcription In eukaryotes, exon is a DNA sequence that will be expressed while intron is DNA that has regulatory function but it will not be expressed. Both introns and exons are transcript to primary mRNA.

25. 25 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Translation Amino acids are transferred by tRNA (transfer) to the cytoplasm where mRNA is translated into proteins. tRNA’s anticodon base-pairs with mRNA’s codon.

26. 26 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Ribosomes Translation occurs at ribosomes in the cytoplasm. A ribosome has a binding site for mRNA and three binding sites for tRNA.

27. 27 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Polypeptide synthesis Polypeptide synthesis occurs as a ribosome moves down mRNA. A polyribosome is a complex of ribosomes translating the same mRNA.

28. 28 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 I step Initiation begins the process of polypeptide synthesis. The starting codon is UAG. The ribosome tRNA binding sites are: E (exit) site, P (peptide) site and A (amino acid) site.

29. 29 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 II step During elongation, a tRNA at the P site passes a peptide to an amino acid-carrying tRNA at the A site. translocation elongation Then translocation occurs: the ribosome moves forward and the peptide bearing tRNA is at the P site. The used tRNA exits from the E site.

30. 30 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Gene expression The processes of elongation and translocation occurs over and over again. At termination, the ribosome reaches a stop codon and the polypeptide is released. Transcription and translation make the gene expression possible. Resume: participants in gene expression

31. 31 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mutations A mutation is a permanent change in the genomic sequence. It affects the gene expression. Germ line mutations occur in gametes and can be passed on to offsprings. Somatic mutations involve body cells and are not transmitted to descendants. Insertions add nucleotides into the DNA, deletions remove one or more nucleotides from the DNA.

32. 32 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Point mutations Point mutations affect only one or a few nucleotides. Depending on the DNA sequence, those mutations can be harmful or without consequence.

33. 33 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Frameshift mutations Frameshift mutations involve insertions or deletions of several nucleotides. Frameshift mutations can lead to the expression of a new non-functional protein and cause genetic disorders as the Cystic fibrosis.

34. 34 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Transposons Discovered by Barbara McClintock in 1981. A transposon (or “jumping gene”) is a sequence of DNA that can change position within the chromosome. Transposition can block transcription and be a source of translocation, deletions, inversion or duplication.

35. 35 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Virus in genetics Bacteriophages reproduce inside bacteria and show two different life cycles: lytic and lysogenic cycle.

36. 36 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 HIV and AIDS HIV is the cause of AIDS and, as many other viruses, uses reverse transcription (from RNA to DNA) to insert a copy of its genome into the host genome.

37. 37 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Bacteria in genetics Bacteria can exchange genes by: Transformation (bacteria pick up external DNA) Conjugation (DNA exchanges via sex pilus) Transduction (DNA exchange via viruses)