DNA Replication Flashcards
DNA replication follows the ____ model. Meaning each new cell receives ____
DNA replication follows the semi-conservative model. Meaning each new cell recieives one parental strand and one daughter strand.
State the 3 basic stages of semi-conservative replication.
- Inititation - Proteins bind to the origin of replication. The parental strands being to seperate (become unzipped) at replication forks.
- Elongation - The parent strands serve as template strands. Free floating nucelotides begin pairing with compimentary bases on the template strand. Nucleotides bond to form the sugar phosphate backbone on the daughter strand
- Termination - Primers replaced with DNA and ligase repairs gaps in the sugar phosphate backbone. The newly formed DNA strands seperate and form double helices with their template strand, creating two new double helices.
Desribe the seperation of the parent strands in detail.
During initation, single stranded proteins bind to the oroigin of replication and recruit more proteins + 2 Helicase enzymes.The enzyme Helicase is responsible for the “unzipping of the parent strands. Helicase breaks the hydrogen bonds between bases creating an area known as the replication fork as it moves along the strands. The 2 helicase enzymes move in seperate directions from one another along the strands.
Describe the role of a primer in DNA replication.
Once the parent strands have been unwound, their bases are exposed and can serve as templates. A primer is needed to act as a starting point for the addition of DNA nucleotides.
A short section of RNA complimentary to the template strand is synthesized from RNA nucleotides which hydrogen bond with the template strand. The enzyme RNA polymerase(primase) forms the RNA nucleotides into a strand. This short RNA section is the primer.
Describe the role of DNA polymerase III
DNA polymerase III catalyses the addition of DNA nucelotides to the daughter strand in a 5’ to 3’ direction. Using the parent strand as a template and using the primer as a starting point.
Describe the DNA nucleotides and how they become bonded to the daughter strand.
The pool of free DNA nucleotides is in the form of nucleotide triphosphates (i.e. each nucleotide carries three phosphates -ATP, TTP, GTP and CTP).
They become bonded to the daughter strand by the formation of the sugar phosphate backbone, which is formed by a phosphodiester bond between the C’3 of the deoxyribose sugar of the nucelotide in the daughter strand and the first phosphate group of the free nucleotide.
The energy for this reaction is supplied by the breaking of the bond between the first and second phophate group, releasing pyrophosphate.
The leading strand is the daughter strand which is complimentary to the _ to _ template strand
It is complimentary to the 3’ to 5’ template strand.
The lagging strand is the daughter strand which is complimentary to the _ to _ template strand.
It is complimentary to the 5’ to 3’ template strand.
Whilst nucelotides are added ____ to the leading strand. Nucleotides must be added ____ to the lagging strand.
Whilst nucelotides are added continously to the leading strand. Nucleotides must be added dsicontinously to the lagging strand.
Descibe addition of nucleotides to the lagging strand.
Around 1000 nucelotides are exposed on the template strand beofre an RNA primer is added. The daughter strand is then synthesised away from the replication fork in a 5’ to 3’ direction in segments of around 1000 nucelotides, known as Okazaki fragments.
Each Okazaki fragment requires its own primer, so the lagging strand will contain numerous small stretches of RNA. DNA Polymerase I catalyses the replacement of these RNA stretches by addition of DNA nucleotides. This leaves small gaps in the sugar phosphate backbone of the daughter strand, the enzyme ligase catalyses the formation of phosphodiester bonds between nucleotides, repairing these gaps.
