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DNA Structure & DNA Replication
Transcript of DNA Structure & DNA Replication
These bases are held together by hydrogen bonding
This pairing is called complementary based pairing. The bases of one strand are paired with the bases of the other strand and are facing inward towards each other. The two strands are anti-parallel to each other
Anti-parallel is when the strands of DNA are parallel but they run in opposite directions. One strand of DNA runs in the direction 5’ to 3’ and the other runs in the opposite direction, 3’ to 5’. This means they are parallel but run in opposite directions Semi-Conservative Replication When the DNA molecule replicates, it is made of one old strand and one new strand of DNA
DNA replicates like this so each of the old strands of DNA can have a newly synthesized strand of DNA DNA Replication The most important job of DNA is to replicate (to copy or reproduce itself)
DNA replication gets its energy from bonds being broken between phosphate molecules
The starting points of DNA replication are called “Origins of Replication”
It is important to have many different starting points on the same DNA molecule because it greatly increases the speed of replication Did You Know DNA polymerase adds nucleotides at a rate of 50 per second in humans. At this rate, it would take approximately 700 days to replicate the 3 billion base pairs of the human genome, yet the whole process takes place in approximately 5 to 10 hours. The accelerated rate indicates that replication is occurring at many points at once. Six Key Players in DNA Replication The 6 key players in DNA replication are:
1. DNA helicase
2. Single strand binding proteins (SSBs)
3. DNA gyrase
4. DNA polymerase I & III
5. RNA primase
6. DNA ligase http://highered.mcgraw-hill.com/sites/dl/free/0072437316/120076/micro04.swf DNA Helicase The first step to DNA replication, is to unzip/unwind the DNA into two different strands
This step is called “Unwinding”
In order to unzip/unwind the DNA helix, we need to break the hydrogen bonds that keep the helix together
DNA helicase does this. It is an enzyme that breaks apart the hydrogen bonds Single Strand Binding Proteins The strands of DNA need to stay separated
Single strand binding proteins or SSBs, are a protein that bind to the newly “unzipped” DNA strands and blocks the hydrogen bonding from happening DNA Gyrase As the DNA helix unwinds, there could be tension on the DNA strands, so the enzyme DNA gyrase relieves any tension from the unwinding DNA Polymerase iii Now that the DNA is being separated into two different strands, the DNA polymerase can start to synthesize (add complementary strands of DNA) to the DNA
This step is called “Elongation”
DNA is always synthesized in the 5’ to 3’ direction
One of the DNA strands goes from 5’ to 3’ and synthesizes towards the replication fork. This strand is called the “leading strand”
The other strand goes from 3’ to 5’ and synthesizes away from the replication fork. This strand is called the “lagging strand” Replication Fork & Bubble A replication fork is the area where the DNA is being split into its two separate strands
As stated earlier, there are many different starting points on the same DNA molecule, so that means there are many different replication forks on the same DNA molecule. When two of these replication forks come into close contact with one another, the form a replication bubble The Lagging Strand While the leading strand will be continuously synthesized by the DNA polymerase III, the lagging strand requires a special primer, called RNA primase, to be able to synthesize
The job of RNA primer is to add temporary start sequences from 5’ to 3’ direction
Then, DNA polymerase III can start to synthesize from the RNA primers and create short fragments called “Okazaki fragments”
Once the DNA polymerase III reaches the next RNA primer site, it is replaced by DNA polymerase I
DNA polymerase I then takes off the RNA primer and synthesizes that spot on the lagging strand
The last step is to connect all of the Okazaki fragments together. The enzyme DNA ligase is responsible for this
DNA ligase forms a special bond called a “phosphodiester bond”, to connect all the Okazaki fragments on the lagging strand Differences Between the Leading & Lagging Strands The leading is continuously synthesized from 5’ to 3’, while the lagging strand is discontinuously synthesized from 5’ to 3'
The lagging strand requires RNA primers and the leading strand does not
The lagging strand has Okazaki fragments, while the leading strand does not
The leading strand in synthesized in the direction of the replication fork, while the lagging strand is synthesized in the opposite direction Proofreading Once DNA replication is complete, DNA polymerase I & III proofread the DNA to ensure there are no mistakes
If there any mistakes, the DNA polymerase removes the incorrect nucleotide(s) and adds the right nucleotide(s) http://highered.mcgraw-hill.com/sites/dl/free/0072437316/120076/bio23.swf Bibliography Anti-Parallel Strands. (n.d.). Academic Brooklyn Cuny Education. Retrieved March 12, 2013, from academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/dsDNA.jpg
DNA Double Helix. (n.d.).http://www.ynaija.com. Retrieved March 12, 2013, from www.ynaija.com/wp-content/uploads/2012/07/DNA-helix.jpg
DNA Replication Fork. (n.d.). Highered Mcgraw-Hill. Retrieved March 12, 2013, from highered.mcgraw-hill.com/sites/dl/free/0072437316/120076/micro04.swf
DNA Replication Proofreading. (n.d.).Virology. Retrieved March 12, 2013, from www.virology.ws/wp-content/uploads/2009/05/dna-polymerase-3.jpg
Giuseppe, M. (2003). DNA: The Molecular Basis of Life. Nelson biology 12 (pp. 204-223). Toronto: Nelson Thomson Learning.
How Nucleotides Are Added in DNA Replication. (n.d.). Highered Mcgraw-Hill. Retrieved March 12, 2013, from highered.mcgraw-hill.com/sites/dl/free/0072437316/120076/bio23.swf
Nitrogenous Bases. (n.d.). Blogspot. Retrieved March 12, 2013, from 2.bp.blogspot.com/_T-ZaNqYdy-Q/SfWRZ-v_0EI/AAAAAAAAADE/-sRYG1cAmDU/s400/DNA_STR2.png
Semi-Conservative Replication. (n.d.).Wikimedia. Retrieved March 12, 2013, from upload.wikimedia.org/wikibooks/en/4/48/Semiconservative_replication*.png
Structure of a Nucleotide. (n.d.).Wikispaces. Retrieved March 12, 2013, from dna02.wikispaces.com/file/view/i_am_a_w