Description of the DNA Replication system.pdf
- 2. • DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. • Replication is an essential process because, whenever a cell divides, the two new daughter cells must contain the same genetic information, or DNA, as the parent cell.
- 3. Why Replicate DNA? • DNA is the genetic material that defines every cell. Before a cell duplicates and is divided into new daughter cells through either mitosis or meiosis, biomolecules and organelles must be copied to be distributed among the cells. • DNA, found within the nucleus, must be replicated in order to ensure that each new cell receives the correct number of chromosomes. • The process of DNA duplication is called DNA replication. Replication follows several steps that involve multiple proteins called replication enzymes and RNA. • In eukaryotic cells, such as animal cells and plant cells, DNA replication occurs in the S phase of interphase during the cell 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 replication is 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 complementary base 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).
- 7. Stages of DNA Replication Step 1: Replication Fork Formation Step 2: Primer Binding Step 3: Elongation Step 4: 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 called Adenine (A), Thymine (T), Cytosine (C) and Guanine (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 DNA Helicase. • DNA Helicase disrupts the hydrogen bonding between base pairs to separate the strands into a Y shape known as the Replication fork. This area will be the template for replication to begin.
- 9. Cont….. • DNA is directional in both strands, signified by a 5' and 3' end. • The 5' end has a phosphate (P) group attached, while the 3' end has 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 of RNA called a primer binds to the 3' end of the strand. • The primer always binds as the starting point for replication. • Primers are generated by the enzyme DNA primase.
- 11. Step 3: DNA Replication: Elongation • Enzymes known as DNA polymerases are responsible for creating the new strand by a process called elongation. • There are five different known types of DNA polymerases in bacteria and human cells. • In bacteria such as E. coli, polymerase III is 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. • The lagging strand begins replication by binding with multiple primers. Each primer is only several bases apart. • DNA polymerase then adds pieces of DNA, called Okazaki 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 called exonuclease removes 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 called DNA ligase joins 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 called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA. • Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape. • In the end, replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.
- 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 new complementary strand to be created. Complementary bases attach to one another (A-T and C-G).
- 17. Cont… • The primary enzyme involved in this is DNA polymerase which 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, the leading strand, runs 5' to 3' towards the fork and is made continuously. • The other, the lagging strand, runs 5' to 3' away from the fork and is made in small pieces called Okazaki fragments.
- 19. Some Definitions • Replication fork: The replication fork is a structure that forms within the long helical DNA during DNA replication. It is created by the enzyme DNA helicases, which break the hydrogen bonds holding the two DNA strands 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: The enzymes play 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 ligases play an essential role in maintaining genomic integrity by joining breaks in the phosphodiester backbone of DNA that occur during replication and recombination, and as a consequence of DNA damage and its repair.
- 22. • SSB proteins (single stand binding protein) - During DNA replication, SSB molecules bind to the newly separated individual DNA strands, keeping the strands separated by holding them in place so that each strand can serve as a template for new DNA synthesis. • Primase: Primase is an enzyme that synthesizes short RNA sequences called primers. These primers serve as a starting point for DNA synthesis. Since the enzyme primase produces 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 breaking hydrogen bonds between 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 adding nucleotides to leading and lagging DNA strands. • Topoisomerase or 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 phosphodiester bonds between nucleotides.
- 24. Nitrogenous Bases of DNA
- 25. Hydrogen Bonding Between Nitrogenous Bases of DNA
- 27. 5´ -3´ structure of DNA Stand