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DNA and proteins
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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.
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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
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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
G
C
A
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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.
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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
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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
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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 Yshaped 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.
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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
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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).
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Transcription
DNA
RNA
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.
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Translation and the genetic code
A
A
A
A
A
C
A
G
C
Lysine
Asparagine
Serine
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.
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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
A
A
G
A
U
C
U
A
G
C
G
U
U
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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.
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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.
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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

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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
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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.
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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.
free DNA
receiving cell
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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
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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.
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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.
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