DNA REPLICATION. DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Slide 1
DNA REPLICATION
Slide 2
DNA replication is a biological process that occurs in all
living organisms and copies their DNA; it is the basis for
biological inheritance. The process starts with one double-stranded
DNA molecule and produces two identical copies of the molecule.
Each strand of the original double-stranded DNA molecule serves as
template for the production of the complementary strand. Cellular
proofreading and error toe-checking mechanisms ensure near perfect
fidelity for DNA replication.living organismsDNAbiological
inheritanceproofreadingnear perfect In a cell, DNA replication
begins at specific locations in the genome, called "origins".
Unwinding of DNA at the origin, and synthesis of new strands, forms
a replication fork. In addition to DNA polymerase, the enzyme that
synthesizes the new DNA by adding nucleotides matched to the
template strand, a number of other proteins are associated with the
fork and assist in the initiation and continuation of DNA
synthesis.celloriginsreplication forkDNA
polymeraseenzymenucleotidesproteins DNA replication can also be
performed in vitro (artificially, outside a cell). DNA polymerases,
isolated from cells, and artificial DNA primers are used to
initiate DNA synthesis at known sequences in a template molecule.
The polymerase chain reaction (PCR), a common laboratory technique,
employs such artificial synthesis in a cyclic manner to amplify a
specific target DNA fragment from a pool of DNA.in vitroDNA
polymerasespolymerase chain reaction
Slide 3
DNA STRUCTURE DNA usually exists as a double-stranded
structure, with both strands coiled together to form the
characteristic double-helix.double-helix Each single strand of DNA
is a chain of four types of nucleotides having the bases: adenine,
cytosine, guanine, and
thymine.nucleotidesadeninecytosineguaninethymine A nucleotide is a
mono-, di-, or triphosphate deoxyribonucleoside; that is, a
deoxyribose sugar is attached to one, two, or three phosphates.
Chemical interaction of these nucleotides forms phosphodiester
linkages, creating the phosphate-deoxyribose backbone of the DNA
double helix with the bases pointing inward.phosphodiester linkages
Nucleotides (bases) are matched between strands through hydrogen
bonds to form base pairs. Adenine pairs with thymine, and cytosine
pairs with guanine.hydrogen bondsbase pairs
Slide 4
a)Key features of DNA b) Partial chemical structure c)
Space-filling model structure
Slide 5
Slide 6
DNA strands have a directionality, and the different ends of a
single strand are called the "3' (three-prime) end" and the "5'
(five-prime) end". These terms refer to the carbon atom in
deoxyribose to which the next phosphate in the chain attaches. In
addition to being complementary, the two strands of DNA are
antiparallel. They are orientated in opposite directions. This
directionality has consequences in DNA synthesis, because DNA
polymerase can synthesize DNA in only one direction by adding
nucleotides to the 3' end of a DNA strand. The pairing of bases in
DNA through hydrogen bonding means that the information contained
within each strand is redundant. The nucleotides on a single strand
can be used to reconstruct nucleotides on a newly synthesized
partner strand
Slide 7
Slide 8
DNA REPLICATION AND REPAIR The relationship between structure
and function is manifest in the double helix. The idea that there
is specific pairing of nitrogenous bases in DNA was the flash of
inspiration that led Watson and Crick to the correct double helix.
At the same time, they saw the functional significance of the base-
pairing rules. They ended their classic paper with this wry
statement : It has not escaped our notice that the specific pairing
we have postulated immediately suggests a possible copying
mechanism for the genetic mechanism for the genetic material.
Slide 9
Slide 10
The figure below is an illustrates by Watson n Cricks basic
idea. To make it easier to follow, we show only a short section of
double helix in untwisted form. The two strands are complementary;
each stores the information necessary to reconstruct the other.
When a cell copies a DNA molecule, each strand serves as a template
for ordering nucleotides into a new strand. Nucleotides line up
along the template strand according to the base-pairing rules and
are linked to form the new strand. Where there was one
double-stranded DNA molecule at the beginning of the process, there
are soon two, each an exact replica of the parent molecule. The
copying mechanism is analogous to using a photographic negative to
make a positive image, which can in turn be used to make another
positive, and so on.
Slide 11
Slide 12
This model of DNA replication remained untested for several
years following publication of the DNA structure. The requisite
experiments were simple in concept but difficult to perform. Watson
and Cricks model predicts that when a double helix replicates, each
of the two daughter molecules will have one old strand, derived
from the parent molecules, and one newly made strand. This
semiconservative model can be distinguish from a conservative model
of replication, in which the two parent strands somehow come back
together after the process( that is, the parent molecule is
conserved). In yet a third model, called the dispersive model, all
four strands of DNA following replication have a mixture of old and
new DNA. Although mechanisms for conservatives or dispersive DNA
replication are not easy devise, these models remained
possibilities until they could be ruled out.
Slide 13
Slide 14
Slide 15
STEPS OF REPLICATION
Slide 16
The replication of DNA molecule begins at special sites called
origin of replication. Multiple replication bubbles form and
eventually fuse, thus speeding up the copying of the very long DNA
molecules. At each end of a replication bubble is a replication
fork, a Y-shaped region where the parental strands of DNA are being
unwound. Helicases are enzymes that untwist the double helix at the
replication forks, separating the two parental strands and making
them available as template strands. After parental strands
separation, single-strand binding proteins bind to the unpaired DNA
strands, stabilizing them. The untwisting of the double helix
causes tighter twisting and strain ahead of the replication fork.
Topoisomerase helps releive this strain by breaking, swiveling, and
rejoining DNA strands.
Slide 17
Slide 18
Slide 19
The unwound sections of parental DNA strands are now available
to serve as templates for the synthesis of new complementary DNA
strands. However, the enzyme that synthesize DNA cannot initiate
the synthesis of a polynucleotide; they can only add nucleotides to
the end of an already existing chain that is base-paired with the
template strand. The initial nucleotide chin that is produced
during DNA synthesis ia actually a short stretch of RNA, not DNA.
This RNA chain is called primer and is synthesized by the enzyme
primase. Primase starts an RNA chain from a single nucleotide,
adding RNA nucleotides one at a time, using the parental DNA strand
as a template. The completed primer, generally 5 to 10 nucleotides
long, is thus base- paired to the template strand. The new DNA
strand will start from the 3end of the RNA primer.