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13/11/11 DNA and proteins 2 © Zanichelli editore 2016 2 2

13/11/11 The discovery of DNA While doing experiments to produce a vaccine against pneumonia, Frederick Griffith discovered an unknown substance, called transforming principle. This substance could be transferred from dead virulent bacteria to live nonvirulent bacteria. Experiments by Oswald Avery on the same bacteria, as well as those of Alfred D. Hershey and Martha Chase on bacteriophage T2, finally proved that the transforming principle was actually DNA. 3 © Zanichelli editore 2016 3 3

13/11/11 The structure of DNA In all living organisms, DNA is made up of nucleotides, formed by: a molecule of deoxyribose; a phosphate group; a nitrogenous base. nitrogenous base P phosphate group deoxyribose 4 © Zanichelli editore 2016 4 4

Four types of nitrogenous bases 13/11/11 Four types of nitrogenous bases There are different nitrogenous bases in DNA: adenine (A) thymine (T) guanine (G) cytosine (C). The bases are paired in a certain way: A with T and C with G. T C G A 5 © Zanichelli editore 2016 5 5

The double-helix structure of DNA 13/11/11 The double-helix structure of DNA Watson and Crick described the DNA molecule as a double-helix structure. DNA is formed by two long strands of nucleotides paired and wrapped in a spiral around an imaginary axis. 6 © Zanichelli editore 2016 6 6

Complementary and antiparallel strands 13/11/11 Complementary and antiparallel strands The strands are complementary since the pairing occurs in a particular way (A-T and C-G) and antiparallel, since the strands orient in opposite directions. The sugar-phosphate backbone creates structural stability while genetic information is contained in the linear sequence of nitrogenous bases. A A C T A G C G T T 7 © Zanichelli editore 2016 7 7

DNA replication is semiconservative 13/11/11 DNA replication is semiconservative DNA replication is called semiconservative: in the new molecules, there is one original strand used as a template and one newly synthesized strand. replication original strands newly synthesized strands 8 © Zanichelli editore 2016 8 8

Phases of DNA replication 13/11/11 Phases of DNA replication DNA replication occurs in two phases: opening and synthesis. In the opening phase, DNA separates its strands at the site of the origin of replication where a Y-shaped replication fork is created. In the synthesis phase, new nucleotides link with those displayed on the template strand and the DNA polymerase binds them together in a strand construct. 9 © Zanichelli editore 2016 9 9

13/11/11 DNA polymerases DNA polymerases are enzymes that add one new nucleotide to the forming DNA strand, starting from a small single-strand RNA called a primer. The synthesis direction is always 5’-3’. 5’ 3’ 3’ 5’ primer 10 © Zanichelli editore 2016 10 10

Genes and polypeptides 13/11/11 Genes and polypeptides Genes are DNA fragments characterized by a specific sequence of bases. They contain the information for synthesizing a specific polypeptide chain. For a gene to be expressed as a protein, three types of RNA must first intervene: messenger RNA (mRNA); transfer RNA (tRNA): ribosomal RNA (rRNA). 11 © Zanichelli editore 2016 11 11

13/11/11 Transcription Transcription leads to the synthesis of a single strand of mRNA, which is complementary to the nucleotide sequence of the gene. It occurs in four stages: recognition and initiation; elongation; termination; maturation. DNA RNA 12 © Zanichelli editore 2016 12 12

Translation and the genetic code 13/11/11 Translation and the genetic code During translation the sequence of nucleotides is decoded into a sequence of amino acids. This process is made possible through the genetic code: three nitrogenous bases of RNA (called codons) encodes a specific amino acid. The genetic code is universal and redundant. A A A Lysine A A C Asparagine A G C Serine 13 © Zanichelli editore 2016 13 13

13/11/11 tRNA and translation During translation, tRNAs work as adapters, carrying the correct amino acid that corresponds to a specific codon. amino acid tRNA codon G A U A A C U A G C G U U 14 © Zanichelli editore 2016 14 14

Translation phases Translation is completed in three phases: 13/11/11 Translation phases Translation is completed in three phases: initiation; elongation; termination. It occurs in the ribosomes with intervention from rRNA and tRNA. 15 © Zanichelli editore 2016 15 15

13/11/11 Genes and mutation During the process of DNA transcription and translation, mutations may occur. Mutations are permanent alterations in the base sequence of one or more genes in the composition of the genome, which is the set of genes of an organism. When mutations occur in gametes, they are transmitted from parents to the next generation. 16 © Zanichelli editore 2016 16 16

Different types of mutations 13/11/11 Different types of mutations Point mutations affect small portions of the DNA. There are different types of point mutations: the substitution of nucleotides; the insertion of nucleotides; the deletion of nucleotides. mutation by substitution  17 © Zanichelli editore 2016 17 17

Chromosomal anomalies 13/11/11 Chromosomal anomalies Chromosomal anomalies affect entire DNA segments. gene 1 gene 2 gene 3 gene duplication gene 1 gene 2 gene 3 18 © Zanichelli editore 2016 18 18

Spontaneous and induced mutations 13/11/11 Spontaneous and induced mutations Spontaneous mutations are caused by errors during DNA replication or during the distribution of chromosomes in the new cells in mitosis. Induced mutations are caused by mutagens, such us chemicals, radiation or viruses. 19 © Zanichelli editore 2016 19 19

Transformation in bacteria 13/11/11 Transformation in bacteria DNA recombination in prokaryotes allows the transfer of a copy of one or more genes from one cell to another. Through the process of transformation, bacteria incorporate stretches of free DNA from dead cells into their genome. receiving cell free DNA 20 © Zanichelli editore 2016 20 20

Transduction and conjugation in bacteria 13/11/11 Transduction and conjugation in bacteria In transduction, bacterial genes transfer from one bacterium to another with the help of viruses. Conjugation is the transfer of a portion of the bacterial chromosome from a donor to a recipient through sex pili. sex pilus 21 © Zanichelli editore 2016 21 21

13/11/11 Plasmids Plasmid to replicate and transfer Plasmids are small circular DNA molecules that contain from two to thirty genes. They are present in bacteria and can confer additional properties to a cell. During conjugation, plasmids can be replicated and then transferred from one bacterial cell to another. 22 © Zanichelli editore 2016 22 22

Transcription regulation in bacteria 13/11/11 Transcription regulation in bacteria Only when required, bacteria can also control gene expression to produce proteins. Transcription regulation occurs through signal molecules coming from the environment and regulatory proteins (transcription factors) in the cell. Lac-operon and trp-operon are examples of transcription regulation. 23 © Zanichelli editore 2016 23 23