DNA Structure:
60 Years Ago:
60 years ago, when the DNA structure was first discovered, all people knew about it was that it was supposed to be “the main molecule of life” and it had four different bases – A, T, G and C. Since this was all that was known about it, many people believed that the structure was too simple to the main macromolecule of life. When the structure of DNA was fully uncovered, the people who doubted Watson and Crick became more accepting of the idea that this was the main macromolecule of life. With the discovery of the double helix structure of DNA, scientists could fully piece together the DNA structure. They figured out that there was a sugar-phosphate backbone that ran along the outside of the structure that contained the A, T, G and C bases on the inside. Scientists then realized that the bases that fit inside could only fit together if they ran anti-parallel (opposite directions) of each other. The idea of the bases – adenine, thymine, cytosine, and guanine was already discovered. Using these bases, they were able to then pair the bases together; adenine with thymine, and cytosine with guanine. Putting all of this information together made it apparent that DNA was the molecule that carried the genetic code
DNA has four different bases; A, G, T, and C. These are sugar phosphates.
- A and T pair together and are bonded by 2 hydrogen bonds
- G and C pair together and are bonded by 3 hydrogen bonds
- A and G are purines, which means they are double ringed
- T and C are permidines, which means they are single ringed
- The strands that make up DNA structure run anti-parallel to each other. They are mirror images of each other
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DNA Replication:
DNA replication begins with a sequence of nucleotides inside the DNA helix known as the origin of replication. An enzyme known as helicase ‘unzips’ the DNA strands at the point of origin by breaking apart the hydrogen bonds between each base. The origin of replication can occur anywhere along the nucleotide sequence, and where these origins are, it forms a bubble where the DNA has been unzipped. Each end of the bubble is known as the
replication fork, where the helicase enzyme is present. After DNA is unwound from its double helix form, binding proteins attach to the now single strands of
DNA to help stabilize the new structure. Next, DNA polymerase 3 adds nucleotides to the 3’ end of the DNA strand. Since the DNA polymerase 3 cannot initiate a new DNA strand, we need another enzyme to ‘prime’ the DNA for replication and formation of a new nucleotide sequence, and this is the job of RNA polymerase. RNA polymerase is responsible for constructing and attaching a proper primer to the original DNA strand. RNA polymerase lays down a sequence of RNA nucleotides at random sections of the single DNA strand. These nucleotides are the primer that DNA polymerase binds to. RNA polymerase then attaches new nucleotides to the 3’ OH end. After this, DNA polymerase 3 can then come and add deoxyribonucleotides which will synthesize a new strand of DNA. There needs to be a few RNA primers to start up the chain for the DNA polymerase. DNA polymerase cannot bind to a single strand DNA, so RNA primer must be laid down before DNA polymerase can attach. Since DNA strands run antiparallel to each other, they have to be elongated in different ways. The ‘leading strand’ is synthesized by DNA polymerase by continuously adding nucleotides to the 3’ end of the strand – which is the only end that nucleotides can be added to. The ‘lagging strand’ is differently synthesized; it’s synthesized in short segments called okazaki fragments. When DNA polymerase 3 replaces the RNA polymerase, it’s replaced with DNA polymerase 1 which removes the entire RNA enzyme and replaces it with DNA. The DNA of the lagging strand is again unzipped and continues this process. After DNA polymerase runs into a completed portion of DNA, it ‘drops’ the strand. There will always be a portion at the beginning or end of the strand that still has RNA primer, but no spot for DNA polymerase to come up and add DNA nucleotides to, and because of this, enzymes eat away the RNA primer. Finally, the telomerase enzyme restores the strand to the original length.
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Fork
Biology, Eigth Edition by Neil A. Campbell, Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert B. Jackson
replication fork, where the helicase enzyme is present. After DNA is unwound from its double helix form, binding proteins attach to the now single strands of
DNA to help stabilize the new structure. Next, DNA polymerase 3 adds nucleotides to the 3’ end of the DNA strand. Since the DNA polymerase 3 cannot initiate a new DNA strand, we need another enzyme to ‘prime’ the DNA for replication and formation of a new nucleotide sequence, and this is the job of RNA polymerase. RNA polymerase is responsible for constructing and attaching a proper primer to the original DNA strand. RNA polymerase lays down a sequence of RNA nucleotides at random sections of the single DNA strand. These nucleotides are the primer that DNA polymerase binds to. RNA polymerase then attaches new nucleotides to the 3’ OH end. After this, DNA polymerase 3 can then come and add deoxyribonucleotides which will synthesize a new strand of DNA. There needs to be a few RNA primers to start up the chain for the DNA polymerase. DNA polymerase cannot bind to a single strand DNA, so RNA primer must be laid down before DNA polymerase can attach. Since DNA strands run antiparallel to each other, they have to be elongated in different ways. The ‘leading strand’ is synthesized by DNA polymerase by continuously adding nucleotides to the 3’ end of the strand – which is the only end that nucleotides can be added to. The ‘lagging strand’ is differently synthesized; it’s synthesized in short segments called okazaki fragments. When DNA polymerase 3 replaces the RNA polymerase, it’s replaced with DNA polymerase 1 which removes the entire RNA enzyme and replaces it with DNA. The DNA of the lagging strand is again unzipped and continues this process. After DNA polymerase runs into a completed portion of DNA, it ‘drops’ the strand. There will always be a portion at the beginning or end of the strand that still has RNA primer, but no spot for DNA polymerase to come up and add DNA nucleotides to, and because of this, enzymes eat away the RNA primer. Finally, the telomerase enzyme restores the strand to the original length.
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Fork
Biology, Eigth Edition by Neil A. Campbell, Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert B. Jackson
Mr. Anderson makes the best instructional YouTube videos. He has them for almost every biology topic you can think of!
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DNA Replication:
Helicase: unzips bases (hydrogen bonds) and creates "bubble"
sugar = 3'
phosphate = 5'
RNA Polymerase aka: Primase - lays down RNA primer.
DNA Polymerase always adds a nucleotide to the 3' OH.
https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcTi1rwkrJOn0tuRY3IfcTjlJPB8zSX1_N0mPjamKiIf6SDNqehW
sugar = 3'
phosphate = 5'
RNA Polymerase aka: Primase - lays down RNA primer.
DNA Polymerase always adds a nucleotide to the 3' OH.
https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcTi1rwkrJOn0tuRY3IfcTjlJPB8zSX1_N0mPjamKiIf6SDNqehW