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Replication

Replication

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Tutorial

DNA Structure

The basic unit of DNA and RNA is a nucleotide. It is composed of a phosphate, sugar and nitrogen base. The phosphate and sugar make up the backbone and the bases connect off of that. In DNA, they form the rungs of the double helix ladder. The bases in DNA consist of Adenine, Guanine, Cytosine and Thymine. A and G are purines (2 rings) while C and T are pyrimidines (1 ring). A + T always pair by forming 2 Hydrogen bonds and C + G pair by forming 3 Hydrogen bonds. RNA is single stranded and its bases consist of A, C, G, and Uracil. The two strands of DNA run antiparallel to one another. The left side runs in a 5' to 3' direction and the right strand runs 3' to 5'. 

Replication

DNA replication starts at the origin of replication. Helicase seperates the DNA strands and forms a replication bubble. This is called the initiation stage. Elongation is the next step. DNA polymerase adds new nucleotides to the new strands in a 5' to 3' direction. This happens continuously on the 5' to 3' or leading strand. The other strand (3' to 5) is called the lagging strand and it replicates in smaller segments called Okazaki fragments. These pieces are put together by DNA ligase. The lagging strand starts with RNA primer being placed and then DNA polymerase puts down nucleotides until it reaches the next primer. DNA polymerase then replaces the primer and ligase ties the strands together. The process ends with termination which is when the strand is done being replicated. 

 

 

Since new nucleotides can only be added on the 3' end, the 5' end of the strand can never be completely finished. This causes a small section of the chromosome to be removed each time it is replicated. To prevent losing genes, the ends of chromosomes have telomeres which are repeating bases sequences that do not code for any genes. 

Source: The Summary of DNA Replication. N.d. Photograph. DNA & the Central DogmaWeb. 5 Jan 2014. .