Description of the DNA Replication system.pdf

Description about dna replication system.

1. DNA Replication

2. •DNA replicationis the process by which a double-strandedDNAmolecule
is copied to produce two identicalDNAmolecules.
•Replicationis an essential process because, whenever a cell divides, the
two new daughter cells must contain the same genetic information,
orDNA, as the parent cell.

3. Why Replicate DNA?
•DNAis the genetic material that defines every cell. Before acellduplicates and is
divided into newdaughter cellsthrough eithermitosisormeiosis, biomolecules
andorganellesmust be copied to be distributed among the cells.
•DNA, found within thenucleus, must be replicated in order to ensure that each new
cell receives the correct number ofchromosomes.
•The process of DNA duplication is calledDNA replication. Replication follows several
steps that involve multipleproteinscalled replication enzymes andRNA.
•In eukaryotic cells, such asanimal cellsandplant cells, DNA replication occurs in the
S phase of interphaseduring thecell cycle.
•The process of DNA replication is vital for cell growth, repair, and reproduction in
organisms.

4. Key points
•Deoxyribonucleic acid, commonly known as DNA, is a nucleic acid that has
three main components: A deoxyribose sugar, A phosphate molecule, and a
nitrogenous base.
•Since DNA contains the genetic material for an organism, it is important
that it be copied when a cell divides into daughter cells. The process that
copies DNA is called replication.
•Replication involves the production of identical helices of DNA from one
double-stranded molecule of DNA.
•Enzymes are vital to DNA replication since they catalyze very important
steps in the process.
•The overall DNA replication process is extremely important for both cell
growth and reproduction in organisms. It is also vital in the cell repair
process.

5. DNA Replication
•DNA replicationis semi-conservative. This means that each of the
two strands in double-stranded DNA acts as a template to produce
two new strands.
•Replication relies on complementarybase pairing, that is the
principle explained by Chargaff's rules: adenine (A) always bonds with
thymine (T) and cytosine (C) always bonds with guanine (G).

6. •https://www.yourgenome.org/video/dna-replication

7. Stages of DNA Replication
Step1:ReplicationForkFormation
Step2:PrimerBinding
Step3:Elongation
Step4:Termination

8. Step 1: Replication Fork Formation
•Before DNA can be replicated, the double stranded molecule must be “unzipped”
into two single strands. DNA has four bases calledAdenine (A),Thymine
(T),Cytosine (C)andGuanine (G)that form pairs between the two strands.
•Adenine only pairs with Thymine and Cytosine only binds with Guanine.
•In order to unwind DNA, these interactions between base pairs must be broken.
This is performed by an enzyme known as DNAHelicase.
•DNA Helicase disrupts thehydrogen bondingbetween base pairs to separate the
strands into a Y shape known as theReplication fork. This area will be the
template for replication to begin.

9. Cont…..
•DNAis directional in both strands, signified by a 5' and 3' end.
•The5' endhas a phosphate (P) group attached, while the3' endhas a
hydroxyl (OH) group attached. This directionality is important for
replication as it only progresses in the 5' to 3' direction.
•However, the replication fork is bi-directional; one strand is oriented
in the 3' to 5' direction(leading strand)while the other is oriented 5'
to 3'(lagging strand). The two sides are therefore replicated with two
different processes to accommodate the directional difference.

10. Step 2: Primer Binding
•The leading strand is the simplest to replicate.
•Once the DNA strands have been separated, a short piece
ofRNAcalled aprimerbinds to the 3' end of the strand.
•The primer always binds as the starting point for replication.
•Primers are generated by the enzymeDNA primase.

11. Step 3:
DNA Replication: Elongation
•Enzymes known asDNA polymerasesare responsible for creating the
new strand by a process called elongation.
•There are five different known types of DNA polymerases
inbacteriaandhuman cells.
•In bacteria such as E. coli,polymerase IIIis the main replication
enzyme, while polymerase I, II, IV and V are responsible for error
checking and repair.
•DNA polymerase III binds to the strand at the site of the primer and
begins adding new base pairs complementary to the strand during
replication.

12. Cont….
•In eukaryotic cells, polymerases alpha, delta, and epsilon are the
primary polymerases involved in DNA replication.
•Because replication proceeds in the 5' to 3' direction on the leading
strand, the newly formed strand is continuous.
•Thelagging strandbegins replication by binding with multiple
primers. Each primer is only several bases apart.
•DNA polymerase then adds pieces of DNA, calledOkazaki fragments,
to the strand between primers. This process of replication is
discontinuous as the newly created fragments are disjointed.

13. Step 4: Termination
•Once both the continuous and discontinuous strands are formed, an
enzyme calledexonucleaseremoves all RNA primers from the original
strands. These primers are then replaced with appropriate bases.
•Another exonuclease“proofreads” the newly formed DNA to check,
remove and replace any errors.
•Another enzyme calledDNA ligasejoins Okazaki fragments together
forming a single unified strand.

14. Cont….
•The ends of the parent strands consist of repeated DNA sequences
called telomeres. Telomeres act as protective caps at the end of
chromosomes to prevent nearby chromosomes from fusing.
•A special type of DNA polymerase enzyme
calledtelomerasecatalyzes the synthesis of telomere sequences at
the ends of the DNA.
•Once completed, the parent strand and its complementary DNA
strand coils into the familiardouble helixshape.
•In the end, replication produces twoDNA molecules, each with one
strand from the parent molecule and one new strand.

15. The Replication Processes

16. The Replication Processes
•DNA replication occurs through the help of several enzymes. These enzymes
"unzip" DNA molecules by breaking the hydrogen bonds that hold the two
strands together.
•Each strand then serves as a template for a newcomplementary strandto be
created. Complementary bases attach to one another (A-T and C-G).

17. Cont…
•The primary enzyme involved in this isDNA polymerasewhich joins
nucleotides to synthesize the new complementary strand.
•DNA polymerase also proofreads each new DNA strand to make sure
that there are no errors.

18. Leading and lagging strands
•DNA is made differently on the two strands at a replication fork.
•One new strand, theleading strand, runs 5' to 3' towards the fork and is made
continuously.
•The other, thelagging strand, runs 5' to 3' away from the fork and is made in
small pieces calledOkazaki fragments.

19. Some Definitions
•Replication fork: Thereplication forkis a structure that forms within
the long helicalDNAduringDNA replication. It is created by the
enzyme DNA helicases, which break the hydrogen bonds holding the
twoDNAstrands together in the helix.
•

20. •Enzymes involve in DNA replication:

21. •Function of DNA helicase: There are DNA and RNA helicases. DNA helicases are essential during
DNA replication because they separate double-stranded DNA into single strands allowing each
strand to be copied.
•Function of DNA polymerase: Theenzymesplay an essential role in DNA replication, usually
working in pairs to produce two matching DNA strands from a single DNA molecule.
•Function of DNA ligase: DNA ligasesplay an essentialrolein maintaining genomic integrity by
joining breaks in the phosphodiesterbackbone ofDNAthat occur during replication and
recombination, and as a consequence ofDNAdamage and its repair.

22. •SSB proteins (single stand binding protein) -DuringDNA replication, SSB
moleculesbindto the newly separated individualDNA strands, keeping
thestrandsseparated by holding them in place so that eachstrandcan serve as a
template for newDNA synthesis.
•Primase: Primase is an enzyme that synthesizes short RNA sequences called
primers. These primers serve as a starting point for DNAsynthesis. Since the
enzyme primaseproduces RNA molecules, the enzyme is a type of RNA
polymerase.

23. Replication Enzymes
•DNA replication would not occur without enzymes that catalyze various steps in the
process. Enzymes that participate in the eukaryotic DNA replication process include:
•DNA helicase-unwinds and separates double stranded DNA as it moves along the DNA.
It forms the replication fork by breakinghydrogen bondsbetween nucleotide pairs in
DNA.
•DNA primase-a type of RNA polymerase that generates RNA primers. Primers are short
RNA molecules that act as templates for the starting point of DNA replication.
•DNA polymerases-synthesize new DNA molecules by addingnucleotidesto leading and
lagging DNA strands.
•Topoisomeraseor DNA Gyrase-unwinds and rewinds DNA strands to prevent the DNA
from becoming tangled or supercoiled.
•Exonucleases-group of enzymes that remove nucleotide bases from the end of a DNA
chain.
•DNA ligase-joins DNA fragments together by forming phosphodiesterbonds between
nucleotides.

24. Nitrogenous Bases of DNA

25. Hydrogen Bonding Between Nitrogenous Bases of DNA

27. 5´-3´structure of DNA Stand