DNA POLYMERASE

Maria khadijaAnum razq

DNA POLYMERASES The DNA polymerases are enzymes that create

DNA molecules by assembling nucleotides, the building blocks of DNA. These enzymes are essential to DNA replication and usually work in pairs to create two identical DNA strands from one original DNA molecule.

CONTI… During this process, DNA polymerase “reads” the

existing DNA strands to create two new strands that match the existing ones.

FUNCTION.. Every time a cell divides, DNA polymerase is

required to help duplicate the cell’s DNA, so that a copy of the original DNA molecule can be passed to each of the daughter cells. In this way, genetic information is transmitted from generation to generation.

Naturally occurring DNA exists in single-stranded and double-stranded forms, both of which can exist in linear and circular forms

Difficult to generalize about all cases of DNA replication

We will study the replication of circular double-stranded DNA and then of linear double-stranded DNA most of the details we discuss were first investigated in

prokaryotes, particularly E. coli

FLOW OF GENETIC INFORMATION IN THE CELL Mechanisms by which information is transferred in

the cell is based on “Central Dogma”

PROKARYOTIC REPLICATION Challenges in duplication of circular double-stranded

DNAachievement of continuous unwinding and

separation of the two DNA strandsprotection of unwound portions from attack

by nucleases that attack single-stranded DNA

synthesis of the DNA template from one 5’ -> 3’ strand and one 3’ -> 5’ strand

efficient protection from errors in replication

Replication involves separation of the two original strands and synthesis of two new daughter strands using the original strands as templates

WHICH DIRECTION DOES REPLICATION GO?

DNA double helix unwinds at a specific point called an origin of replication

Polynucleotide chains are synthesized in both directions from the origin of replication; DNA replication is bidirectional in most organisms

At each origin of replication, there are two replication forks, points at which new polynucleotide chains are formed

There is one origin of replication and two replication forks in the circular DNA of prokaryotes

In replication of a eukaryotic chromosome, there are several origins of replication and two replication forks at each origin

DNA POLYMERASE DNA is synthesized from its 5’ -> 3’ end (from the 3’ -

> 5’ direction of the template) the leading strand is synthesized continuously in the

5’ -> 3’ direction toward the replication fork the lagging strand is synthesized

semidiscontinuously (Okazaki fragments) also in the 5’ -> 3’ direction, but away from the replication fork

lagging strand fragments are joined by the enzyme DNA ligase

DNA POLYMERASE REACTION The 3’-OH group at the end of the growing DNA

chain acts as a nucleophile. The phosphorus adjacent to the sugar is attacked,

and then added to the growing chain.

PROPERTIES OF DNA POLYMERASES There are at least five types of DNA polymerase

(Pol) in E coli, three of which have been studied extensively

FUNCTION OF DNA POLYMERASE DNA polymerase function has the following

requirements: all four deoxyribonucleoside triphosphates: dTTP,

dATP, dGTP, and dCTP Mg2+

an RNA primer - a short strand of RNA to which the growing polynucleotide chain is covalently bonded in the early stages of replication

DNA-Pol I: repair and patching of DNA DNA-Pol III: responsible for the polymerization of the

newly formed DNA strand DNA-Pol II, IV, and V: proofreading and repair

enzymes

PROOFREADING AND REPAIR

DNA replication takes place only once each generation in each cell

Errors in replication (mutations) occur spontaneously only once in every 109 to 1010 base pairs

Can be lethal to organisms

Proofreading - the removal of incorrect nucleotides immediately after they are added to the growing DNA during replication (Figure 10.10)

Errors in hydrogen bonding lead to errors in a growing DNA chain once in every 104 to 105 base pairs

EUKARYOTIC REPLICATION

Not as understood as prokaryotic. Due in no small part to higher level of complexity.

Cell growth and division divided into phases: M, G1, S, and G2

EUKARYOTIC REPLICATION Best understood

model for control of eukaryotic replication is from yeast.

DNA replication initiated by chromosomes that have reached the G1 phase

THE EUKARYOTIC REPLICATION FORK

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