DNA Replication:
Replication of DNA is the process that occurs before a cell can reproduce. It is when a DNA helix is duplicated into two more identical DNA helixes. The location of where DNA is replicated in a cell also depends on the type of cell, either prokaryotic or eukaryotic. DNA replication within prokaryotic cells occurs in the cytoplasm, whereas DNA replication within eukaryotic cells takes place in its nucleus.
1. The first step to DNA replication is for a DNA Helix to be ‘unzipped’. During this process, the original DNA helix (also known as the parent or template strand) is split down the middle to create two separate strands (also known as the daughter strands). The unzipping of DNA occurs at several locations in a single DNA strand which allows effective DNA replication. By doing this, each strand serves as a ‘template’ for the creation of two new helixes. This method of replication is known as ‘semi conservative replication’. An enzyme called ‘DNA helicase’ is responsible for ‘unzipping’.
The location at where the DNA helix is being split into two new strands is called the ‘replication fork’.
2. Once the DNA helix has been unzipped, ‘Single-Strand Binding proteins’ are temporarily fixed onto each side of the unzipped helix. This ensures that the strands don't twist around and come back together.
3. A DNA helix is comprised of two polynucleotide strands arranged in an anti-parallel manner. So when the helix is split, this means that one of the strands is arranged “5’ to 3’” whilst the other strand is arranged “3’ to 5’”.
(5’ represents the phosphate end of the nucleotide. 3’ represents the sugar end.)
After unzipping the DNA helix, another enzyme called DNA polymerase travels along the daughter strands in order to add complementary nucleotides to the existing ones. By doing this, the daughter strands would have complete pairs of nucleotides. In addition to this, since the double helix is arranged in an anti-parallel manner, this means that one polynucleotide strand would have the ‘sugar end’ on the outside, and the other would have the ‘phosphate end’ on the outside. Nucleotides can only be ‘added’ to the ‘sugar end’.
DNA polymerase only synthesises (adds nucleotides) in a 5’ to 3’ direction (moves from the phosphate end of the strand to the sugar end of the strand). Therefore, the 5’ to 3’ daughter strand can easily be replicated. The 5’ to 3’ daughter strand is known as the ‘leading strand’.’
1. The first step to DNA replication is for a DNA Helix to be ‘unzipped’. During this process, the original DNA helix (also known as the parent or template strand) is split down the middle to create two separate strands (also known as the daughter strands). The unzipping of DNA occurs at several locations in a single DNA strand which allows effective DNA replication. By doing this, each strand serves as a ‘template’ for the creation of two new helixes. This method of replication is known as ‘semi conservative replication’. An enzyme called ‘DNA helicase’ is responsible for ‘unzipping’.
The location at where the DNA helix is being split into two new strands is called the ‘replication fork’.
2. Once the DNA helix has been unzipped, ‘Single-Strand Binding proteins’ are temporarily fixed onto each side of the unzipped helix. This ensures that the strands don't twist around and come back together.
3. A DNA helix is comprised of two polynucleotide strands arranged in an anti-parallel manner. So when the helix is split, this means that one of the strands is arranged “5’ to 3’” whilst the other strand is arranged “3’ to 5’”.
(5’ represents the phosphate end of the nucleotide. 3’ represents the sugar end.)
After unzipping the DNA helix, another enzyme called DNA polymerase travels along the daughter strands in order to add complementary nucleotides to the existing ones. By doing this, the daughter strands would have complete pairs of nucleotides. In addition to this, since the double helix is arranged in an anti-parallel manner, this means that one polynucleotide strand would have the ‘sugar end’ on the outside, and the other would have the ‘phosphate end’ on the outside. Nucleotides can only be ‘added’ to the ‘sugar end’.
DNA polymerase only synthesises (adds nucleotides) in a 5’ to 3’ direction (moves from the phosphate end of the strand to the sugar end of the strand). Therefore, the 5’ to 3’ daughter strand can easily be replicated. The 5’ to 3’ daughter strand is known as the ‘leading strand’.’
On the other hand, the 3’ to 5’ daughter strand cannot be easily replicated because DNA polymerase does not travel in the right direction (nucleotides are only added to sugar ends). This strand is known as the ‘lagging strand’. In order for the ‘lagging strand’ to replicate, this strand replicates in small sections called Okazaki fragments. The lagging strand is replicated in the reverse direction towards the replication fork.
As shown in the diagram on the right, the first Okazaki fragment which is synthesised (process of the DNA polymerase ‘adding’ complementary nucleotides to the strand) starting furthest from the replication fork. It then synthesizes towards the replication fork. Since each segment is being synthesized separately, each section is then sealed together by an enzyme called ‘ligase’.
5. A subunit of DNA polymerase also known as DNA Polymerase III Holoenzyme is responsible for ‘proofreading’ the synthesizing process of the DNA replication. This process ensures that no mutations or mistakes are made during DNA replication.
6. After each daughter strand is successfully synthesized by the DNA polymerase, the two complete copies of DNA automatically twist to create two new DNA helixes. As a result of DNA Replication, each complementary new strand is made up of half of a parent strand (which was split from the template strand), and the other half a daughter strand (the other half of the strand which was created from DNA Polymerase). This is represented in the diagram below.
As shown in the diagram on the right, the first Okazaki fragment which is synthesised (process of the DNA polymerase ‘adding’ complementary nucleotides to the strand) starting furthest from the replication fork. It then synthesizes towards the replication fork. Since each segment is being synthesized separately, each section is then sealed together by an enzyme called ‘ligase’.
5. A subunit of DNA polymerase also known as DNA Polymerase III Holoenzyme is responsible for ‘proofreading’ the synthesizing process of the DNA replication. This process ensures that no mutations or mistakes are made during DNA replication.
6. After each daughter strand is successfully synthesized by the DNA polymerase, the two complete copies of DNA automatically twist to create two new DNA helixes. As a result of DNA Replication, each complementary new strand is made up of half of a parent strand (which was split from the template strand), and the other half a daughter strand (the other half of the strand which was created from DNA Polymerase). This is represented in the diagram below.
Picture Sources:
Craig Freudenrich, P. 2014. HowStuffWorks "DNA Replication". [online] Available at: http://science.howstuffworks.com/life/cellular-microscopic/dna3.htm [Accessed: 9 Mar 2014].
Sparknotes.com. 2014. SparkNotes: DNA Replication and Repair: DNA Replication. [online] Available at: http://www.sparknotes.com/biology/molecular/dnareplicationandrepair/section1.rhtml [Accessed: 11 Mar 2014].
Craig Freudenrich, P. 2014. HowStuffWorks "DNA Replication". [online] Available at: http://science.howstuffworks.com/life/cellular-microscopic/dna3.htm [Accessed: 9 Mar 2014].
Sparknotes.com. 2014. SparkNotes: DNA Replication and Repair: DNA Replication. [online] Available at: http://www.sparknotes.com/biology/molecular/dnareplicationandrepair/section1.rhtml [Accessed: 11 Mar 2014].
