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Curs 4
DNA replication
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DNA and RNA are polymers of
nucleotides
Nucleotide
Phosphategroup
Nitrogenousbase
Sugar
Polynucleotide Sugar-phosphate backbone
DNA nucleotide
Phosphategroup
Nitrogenous base(A, G, C, or T)
Thymine (T)
Sugar(deoxyribose)
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Each strand of thedouble helix is
oriented in theopposite direction
5 end 3 end
3 end 5 end
P
P
P
P
P
P
P
P
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CICLUL CELULARMitozaeste procesul de diviziune celular, specific
celulelor eucariote, prin care dintr-o celul-mam rezultdoua celule-fiice, identice din punct de vedere genetic
decondensarea cromozomilordublarea cantitii de ADN, ARN i
proteine i condensarea cromozomilor
sinteza unor proteine necesare formrii fusului
de diviziune i sinteza de ATP
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DNA replication begins at many specific sites
How can entire chromosomes be replicated during S phase?
Parental strandOrigin of replication
Bubble
Two daughter DNA molecules
Daughter strand
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Replication of DNA:
base pairing allows eachstrand to serve as a template
for a new strand
new strand is 1/2 parent
template and 1/2 new DNA
Replicarea ADN este semiconservativa
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DNA Polymerase
Bidirectional synthesis of the DNA double
helix
Corrects mistaken base pairings
Requires an established polymer (small RNA
primer) before addition of more nucleotides
Other proteins and enzymes necessary
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Energy of Replication
The nucleotides arrive as nucleosides DNA bases with PPP
P-P-P = energy for bonding
DNA bases arrive with their own energysource forbonding
bonded by enzyme: DNA polymerase III
ATP GTP TTP CTP
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Limits of DNA polymerase III can only build onto 3 end of an
existing strand
Leading & Lagging strands
5
5
5
5
3
3
3
53
53 3
Leading strand
Lagging strand
ligase
Leading strand
continuous synthesis
Lagging strand
Okazaki fragments
joined by ligase
DNA polymerase III
3
5
growingreplication fork
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DNA Replication
Priming:
1. RNA primers: before new DNA strands can
form, there must be small pre-existingprimers (RNA)present to start the addition of
new nucleotides (DNA Polymerase).
2. Primase: enzyme that polymerizes(synthesizes) the RNA Primer.
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DNA Replication
Synthesis of the new DNA Strands:
1. DNA Polymerase: with a RNA primerin
place, DNA Polymerase (enzyme) catalyze
the synthesis of a new DNA strand in the 5
to 3 direction.
RNA
PrimerDNA Polymerase
Nucleotide
5
5 3
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DNA Replication
2. Leading Strand: synthesized as a
single polymerin the 5 to 3 direction.
RNA
PrimerDNA PolymeraseNucleotides
35
5
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DNA Replication
4. Okazaki Fragments: series of short
segments on the lagging strand.
Lagging Strand
RNA
Primer
DNA
Polymerase
3
3
5
5
Okazaki Fragment
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DNA Replication
5. DNA ligase: a linking enzyme that
catalyzes the formation of a covalent bond
joining fragments
Example: joining two Okazaki fragments together.
Lagging Strand
Okazaki Fragment 2DNA ligase
Okazaki Fragment 1
5
5
3
3
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DNApolymeraseworks in
only onedirection:
5 to 3
5 end
P
P
Parental DNA
DNA polymerasemolecule
5
3
3
5
3
5
Daughter strandsynthesizedcontinuously
Daughter
strandsynthesizedin pieces
DNA ligase
Overall direction of replication
5
3
Telomeresequencesare lostwith eachreplication.
Cancer,aging
telomeres
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Repeating, non-coding sequences at the end ofchromosomes = protective cap
limit to ~50 cell divisions
Telomerase
enzyme extends telomeres
can add DNA bases at 5 end
different level of activity in different cells
telomerase
Telomeres
5
5
5
5
3
3
3
3
growingreplication fork
TTAAGGGTTAAGGG
M t ti h th i f
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Mutations are changes inthe DNA base sequence
caused by errors in DNA
replication or by mutagens
change of a single DNA
nucleotide causes sickle-
cell disease
Mutations can change the meaning of
genes
Th i f ti tit ti i
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The DNA is transcribed into RNA, which istranslated into the polypeptide
DNA
RNA
Protein
TRANSCRIPTION
TRANSLATION
The information constituting an organismsgenotype is carried in its sequence of bases
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John B. Gurdon eliminated the nucleus of a
frog egg cell and replaced it with the nucleus
from a specialised cell taken from a tadpole.
The modified egg developed into a normal
tadpole.
Subsequent nuclear transfer experiments have
generated cloned mammals
The Nobel Prize in Physiology or Medicine 2012
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Shinya Yamanaka
Shinya Yamanaka studied genes that areimportant for stem cell function. When he
transferred four such genes into cells takenfrom the skin, they were reprogrammed intopluripotent stem cells that could develop intoall cell types of an adult mouse. He namedthese cells induced pluripotent stem (iPS)cells.
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iPS cells can now be generated from humans,
including patients with disease. Mature cells
including nerve, heart and liver cells can be
derived from these iPS cells, thereby allowing
scientists to study disease mechanisms in new
ways.