Higher Biology - Oronsay

Higher Biology Replication of DNA G r M R Da v o s d i

n DNA Replication During cell division, the genes must be able to replicate in order that each new cell gets a full chromosome complement. In order for replication to occur, the following must be present in the nucleus of the cell: 1. 2. 3. 4. 5. 6.

3/2/20 a supply of nucleotides (4 types) ATP a DNA molecule to copy DNA polymerase (enzyme) Ligase (enzyme) primer G R Davidson Slide 2 DNA Replication

Replication begins with the uncoiling of the DNA helix. Once this has happened, the weak hydrogen bonds joining the two strands break, causing the DNA molecule to unzip. Once the bases are exposed, a primer attaches to the end of each strand. 3/2/20 G R Davidson Slide 3 DNA Replication

The primer then causes the DNA polymerase to start adding free nucleotides to the 3 end of the strand. These nucleotides are joined together at the sugar-phosphate bond. Hydrogen bonds form between the bases. Ligase then joins the fragments to complete the strand. 3/2/20 G R Davidson Slide 4

DNA Replication Because DNA can only have nucleotides added from the 3 end, replication of the 2 strands is slightly different. The 3 end is continuous and forms the leading strand of the new DNA. 3/2/20 G R Davidson 5 DNA Replication The 5 end must have the nucleotides added in fragments starting at the

available bases of the exposed 3 end. Ligase is then used to join these fragments together. This forms the lagging strand of the new DNA molecule and is described as discontinuous (because it is built in sections). 3/2/20 G R Davidson 6 DNA Replication 3 end of parental strand 5 end of

parental strand OpenStax College 3/2/20 G R Davidson 7 DNA Replication When particularly long chromosomes need to be copied, there are many replication forks operating at the same time. This ensures that the DNA molecule is copied as quickly as possible. This DNA carries the genetic code of the organism which makes up its

genotype. 3/2/20 G R Davidson 8 Polymerase Chain Reaction The polymerase chain reaction (PCR) is a process used in laboratories to create multiple pieces of DNA outside the organism. This is called DNA amplification. A primer is used which is complementary to the specific target DNA. 3/2/20

G R Davidson 9 Polymerase Chain Reaction The DNA is heated to 95oC which breaks the hydrogen bonds between the original DNA strands. The sample is then cooled to around 55oC to allow the addition of primers which bind to the start of each strand. The sample is then heated to 72oC and thermostable DNA polymerase is added. (Obtained from bacteria which live next to hot springs and can cope with high temperatures.) 3/2/20 G R Davidson

10 Polymerase Chain Reaction Complementary free nucleotides are added to the 3 end of the new strands. The number of the original strands is now doubled, then doubled again, and so on, showing exponential growth. 3/2/20 G R Davidson 11

Polymerase Chain Reaction This provides sufficient DNA to carry out a number of applications such as: Genetic fingerprinting in forensics. Archaeobiology. Research. Early infection detection. Phylogenetics. 3/2/20 G R Davidson 12

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