Sunday, April 24, 2011

Genetics: DNA Replication (2)

To remember: DNA has direction (a 5 prime end, where the phosphate group is attached, and a 3 prime end where a hydroxyl group is attached)

Replications begin at many points along the DNA called points of origin:

Here are a summary of the basic steps involved in DNA replication:

1) Unwind DNA: Helicase is the enzyme responsible for unwinding the DNA and breaking the hydrogen bonds between the nitrogen bases. Gyrase is the enzyme which relieves tension during this process. To prevent the DNA from recoiling, Single-stranded Binding Proteins bind to the individual chains of nucleic acids.

2) Primer is added: RNA Primase adds a RNA primer to the DNA chain so that DNA polymerase can function.

NOTE: DNA polymerase has two restrictions: It cannot add to the the 5 prime end, and it needs an RNA primer to start it off.

3) Daughter strand is built: DNA Polymerase 3 is the enzyme responsible adds nucleotides to the 3 prime end of the new chain forming polydiester bonds.

NOTE: The new strand must grow from 5 prime to 3 prime due to energy needs.
Forming a new DNA chain is a analytic reaction which is endergonic.
The energy comes from the phosphate groups attached to nuclosides (which are basically nucleotides with 2 additional phosphate groups attached)

Ex. ATP (nucleoside shown below) turns to AMP (the nucleotide used in DNA). During this process, 2 phosphate groups are released in addition to energy which is used to synthesize the new DNA.
The nucleotide must add to the 3 prime side so that the bond may be broken to produce energy.

NOTE: Because of the need for nucleotides to grow from 5 prime to 3 prime, one strand of DNA is produced continuously (the leading strand), where as the other is produced in pieces (the lagging strand). The pieces are called Okazaki Fragments.

4) Primer is removed, segments produced on lagging strand are connected: The enzyme ligase is responsible for sealing the holes between the segments. DNA polymerase serves as a proof reader; it removes RNA primer and replaces it with DNA, it also checks for errors. Once again, ligase seals the holes.

Here is a link to a helpful youtube video that may help you better visualize the process: CLICK HERE!

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