curs 4 replicarea adn

8/12/2019 Curs 4 Replicarea ADN

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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.

curs 4 replicarea adn

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