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ZOOLOGY: SEM- V, PAPER- C11T: MOLECULAR BIOLOGY, UNIT 2: DNA REPLICATION

P P r r o o k k a a r r y y o o t t i i c c v v / / s s E E u u k k a a r r y y o o t t i i c c D D N N A A R R e e p p l l i i c c a a t t i i o o n n

B BY B Y Y

D D R R . . P P O O U U L L A A M M I I A A D D H H I I K K A A R R Y Y M M U U K K H H E E R R J J E E E E A A S S S S I I S S T T A A N N T T P P R R O O F F E E S S S S O O R R

D D E E P P A A R R T T M M E E N N T T O O F F Z Z O O O O L L O O G G Y Y

N N A A R R A A J J O O L L E E R R A A J J C C O O L L L L E E G G E E

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ZOOLOGY: SEM- V, PAPER- C11T: MOLECULAR BIOLOGY, UNIT 2: DNA REPLICATION

Comparisons between prokaryotic and eukaryotic DNA replication

 When compared to prokaryotic DNA replication, the completion of eukaryotic DNA replication is more complex involving multiple origins of replication.

 Prokaryotic DNA is arranged in a circular shape, and has

only one replication origin when replication starts. By

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contrast, eukaryotic DNA is linear. When replicated, there are as many as one thousand origins of replication.

 Eukaryotic DNA is bidirectional. Here the meaning of the

word bidirectional is different. Eukaryotic linear DNA has

many origins (called O) and termini (called T). "T" is

present to the right of "O". One "O" and one "T" together

form one replicon. After the formation of pre-initiation

complex, when one replicon starts elongation, initiation

(4)

starts in second replicon. Now, if the first replicon moves

in clockwise direction, the second replicon moves in

anticlockwise direction, until "T" of first replicon is

reached. At "T", both the replicons merge to complete the

process of replication. Meanwhile, the second replicon is

moving in forward direction also, to meet with the third

replicon. This clockwise and counter-clockwise movement

of two replicons is termed as bidirectional replication.

(5)

 Eukaryotic DNA replication requires precise coordination

of all DNA polymerases and associated proteins to

replicate the entire genome each time a cell divides. This

process is achieved through a series of steps of protein

assemblies at origins of replication, mainly focusing the

regulation of DNA replication on the association of the

MCM helicase with the DNA. These origins of replication

direct the number of protein complexes that will form to

initiate replication. In prokaryotic DNA replication

(6)

regulation focuses on the binding of the DnaA initiator protein to the DNA, with initiation of replication occurring multiple times during one cell cycle.

 Both prokaryotic and eukaryotic DNA use ATP binding and

hydrolysis to direct helicase loading and in both cases the

helicase is loaded in the inactive form. However,

eukaryotic helicases are double hexamers that are loaded

(7)

onto double stranded DNA whereas prokaryotic helicases are single hexamers loaded onto single stranded DNA.

 Segregation of chromosomes is another difference

between prokaryotic and eukaryotic cells. Rapidly dividing

cells, such as bacteria, will often begin to segregate

chromosomes that are still in the process of replication. In

eukaryotic cells, chromosome segregation into the

daughter cells is not initiated until replication is complete

(8)

in all chromosomes. Despite these differences, however, the underlying process of replication is similar for both prokaryotic and eukaryotic DNA.

Followings are the differences between Prokaryotic and

Eukaryotic DNA Replication:

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ZOOLOGY: SEM- V, PAPER- C11T: MOLECULAR BIOLOGY, UNIT 2: DNA REPLICATION

Prokaryotic DNA Replication

Eukaryotic DNA replication

Process occurs in prokaryotic cells.

Process occurs in eukaryotic cells.

It is a continuous process. This process occurs in the S- phase of cell cycle and in late

G1 phase.

(10)

Occurs inside the cytoplasm

Occurs inside the nucleus

Circular, double-stranded DNA

Linear, double-stranded DNA with end

Only one origin of

replication per molecule of DNA

Have many origins of replication in each

chromosome

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Origin of replication is about 100-200 or more

nucleotides in length

Each origin of replication is formed of about 150

nucleotides Replication occurs at one

point in each chromosome

Replication occurs at several points simultaneously in each

chromosome

Only have one origin of Has multiple origins of

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replication replication

Small amount of DNA The DNA is 25-50 times more than prokaryotic DNA

Initiation is carried out by protein Dna A and Dna B

Initiation is carried out by the Origin Recognition Complex DNA polymerase I and III

are involved

DNA polymerase ɑ, δ and ε

are involved.

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ZOOLOGY: SEM- V, PAPER- C11T: MOLECULAR BIOLOGY, UNIT 2: DNA REPLICATION

Large okazaki fragments Small okazaki fragments Two circular chromosomes

are obtained

Two sister chromatids are obtained

DNA gyrase is required DNA gyrase is not required Replication is very rapid,

2000 base pairs per second

Replication is very slow, 100

base pairs per second

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ZOOLOGY: SEM- V, PAPER- C11T: MOLECULAR BIOLOGY, UNIT 2: DNA REPLICATION

Similarities between Prokaryotic and Eukaryotic Replication

The similarities between prokaryotic and eukaryotic replication are as follows:

Both the replication processes occur before nuclear division.

The DNA involved in both processes are double-stranded.

The replication occurs in 5’ to 3’ direction.

(15)

The single-strand binding proteins stabilizes the unwound DNA.

The RNA primer is synthesised by the enzyme primase.

Both DNA replications require topoisomerase and helicase.

Both the DNA replications are bi-directional.

(16)

ZOOLOGY: SEM- V, PAPER- C11T: MOLECULAR BIOLOGY, UNIT 2: DNA REPLICATION

T T H H A A N N K K Y Y O O U U