DNA Replication
Biology: Cambridge International A Level, topic 6.1.6
DNA Replication
When a cell divides, it forms two daughter cells. This means that before cell division, the cell must duplicate its DNA so that each daughter cell can inherit the full set of DNA.
DNA replication
- DNA is made up of two polynucleotide strands that form a double helix.
- During DNA replication, each of the two strands are used as a template from which new strands are copied.
Semi-conservative model
- After replication, the new DNA is made up of one original polynucleotide strand and a new, complementary strand.
- This explains why DNA replication is described as semi-conservative because one original strand is conserved.
The process of replication (1)
- DNA helicase binds to DNA and breaks the hydrogen bonds between the two strands.
- The DNA helix unwinds and the two strands separate.
The process of replication (2)
- Free-floating nucleotides form hydrogen bonds with the complementary bases.
- Each strand is used as a template to produce complementary strands.
- Two hydrogen bonds form between A & T nucleotides. Three hydrogen bonds form between C & G nucleotides.
- DNA polymerase forms phosphodiester bonds between the nucleotides.
- Two new DNA strands are synthesised.
The process of replication (3)
- DNA polymerase can only extend in the 5' to 3' direction. This poses a slight problem as one strand of DNA is in the 5' to 3' direction and the other is oriented in the 3' to 5' direction.
- The leading strand, which is complementary to the 3' to 5' parental DNA strand, is synthesized continuously. This is because the polymerase can add nucleotides in the 5'-3' direction.
- The other strand, complementary to the 5' to 3' parental DNA, is extended in small fragments.
The process of replication (4)
- These fragments are known as Okazaki fragments. Each Okazaki fragments is synthesized in the 5'-3' direction.
- The fragments are joined together by an enzyme called DNA ligase. They are joined together to form an overall strand that runs in the 3'-5' direction.
Strands of the Double Helix
Nucleotides join together to form a polynucleotide strand. Two polynucleotide strands coil together to form a double helix.
DNA strands
- The carbon atoms of the sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”).
- The phosphate residue attaches to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide.
- The bond is called a 5′–3′ phosphodiester linkage.
Antiparallel
- This is called an antiparallel orientation because the helix's two strands run in opposite directions.
- The 5′ carbon end of one strand will face the 3′ carbon end of its matching strand.
DNA polymerase
- DNA polymerase catalyses the condensation reaction between the nucleotides in a DNA strand.
- DNA polymerase forms phosphodiester bonds between adjacent nucleotides.
Test yourself on DNA Replication
What is true of new DNA strands formed during replication?
Which of the following statements about semi-conservative replication are true?
Which enzyme separates the two strands of DNA?
What is the name given to the orientation of DNA strands in the double helix?
Which of the following statements about DNA polymerase are true?