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2. 13/11/11
DNA and proteins 2
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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
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4. 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
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5. Four types of nitrogenous bases
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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
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6. The double-helix structure of DNA
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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
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7. Complementary and antiparallel strands
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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
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8. DNA replication is semiconservative
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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
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9. Phases of DNA replication
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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
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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
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11. Genes and polypeptides
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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
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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
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13. Translation and the genetic code
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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
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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
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15. Translation phases Translation is completed in three phases:
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Translation phases
Translation is completed in three phases:
initiation;
elongation;
termination.
It occurs in the ribosomes with intervention from rRNA and tRNA. 15
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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
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17. Different types of mutations
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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
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18. Chromosomal anomalies
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Chromosomal anomalies
Chromosomal anomalies affect entire DNA segments.
gene 1
gene 2
gene 3
gene duplication
gene 1
gene 2
gene 3 18
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19. Spontaneous and induced mutations
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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
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20. Transformation in bacteria
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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
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21. Transduction and conjugation in bacteria
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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
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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
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23. Transcription regulation in bacteria
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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
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