Transcript DNA - Zanichelli

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Sylvia S. Mader
Immagini e
concetti
della biologia
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
B2 - Molecular
genetics
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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.
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DNA and inheritance
Alfred Hershey and Martha Chase (1952) used the T2
bacteriophage and Escherichia coli for their experiments.
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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.
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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.
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Nucleic Acids
DNA is a polynucleotides (polymers of nucleotides). Each
nucleotide is composed of three parts:
• a C5 sugar called deoxyribose;
• a phosphate group;
• a nitrogen-containing base.
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Nucleic Acids
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.
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Nucleic Acids
RNA (ribonucleic acid) contains a C5 sugar called ribose
and the bases A,C,G. Uracil (U) replaces T.
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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
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DNA structure
DNA is composed by a double strand of nucleotides
where C always pairs with G and T pairs with A.
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DNA structure
James Watson and Francis Crick constructed the first
model of DNA using Rosalind Franklin and Maurice
Wilkins’s X-ray diffraction data.
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DNA structure
In Watson and Crick double helix model weak
hydrogen bonds between the bases hold the two
chains to one another.
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DNA is suitable for replication
DNA duplication (or replication) is
semiconservative as each new
filament contains an old strand and
a new strand.
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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.
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DNA is suitable for replication
In the C5 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.
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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 noncodifying repetitive nucleotide
sequences.
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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.
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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.
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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.
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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.
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Transcription
During transcription,
complementary base pairing
occurs, RNA polymerase
joins bases and the coded
information passes from the
gene to the mRNA.
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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.
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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.
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Ribosomes
Translation occurs at ribosomes in the cytoplasm.
A ribosome has a binding site for mRNA and three binding
sites for tRNA.
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Polypeptide synthesis
Polypeptide synthesis occurs as a ribosome moves down
mRNA. A polyribosome is a complex of ribosomes
translating the same mRNA.
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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.
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II step
During elongation, a tRNA at the P site passes a peptide to
an amino acid-carrying tRNA at the A site.
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.
translocation
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Gene expression
The processes of elongation and translocation occur 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
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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.
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Point mutations
Point mutations affect only one or a few nucleotides.
Depending on the DNA sequence, those mutations can be
harmful or without consequence.
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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.
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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.
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Virus in genetics
Bacteriophages reproduce inside bacteria and show two
different life cycles: lytic and lysogenic cycle.
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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.
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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)
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