DNA Replication

DNA Function Recall: DNA is the carrier of hereditary information for all living organisms. DNA controls the activity of all cells by coding for proteins that are produced. DNA contains the full ‘blue-print’ for an individual of a species, including the instructions for replication or making more DNA.

DNA Replication When DNA is being replicated it is described as being ‘Semi-Conservative’. ‘Some of the DNA is kept in the new strand.’ What this means is that each parent strand of a DNA molecule acts as a template for the formation of a new complimentary strand. Both strands are conserved (used) but end up as part of two different DNA molecules.

Mechanisms of DNA Replication DNA is a polymer (combination of molecules) of nucleotides. Each parent strand acts as a template for a new complimentary strand.

Challenges that are found with DNA Replication DNA is very long (6 feet long in humans cells). DNA must be unwrapped from it’s double helix shape before replication can take place. Once unwrapped, each strand has a different polarity.

The 5’ to 3’ Direction Rule Polarity of DNA strands can change depending on whether they end/begin in a phosphate or a deoxyribose sugar. The 5’ (5 prime) end of the DNA ends in a carbon from the deoxyribose sugar part of the DNA. Remember the sugar is a 5 carbon sugar. The 3’ (3 prime) end of the DNA ends in a phosphate from the phosphate group (PO 4 ) part of the DNA.

Enzymes Involved In DNA replication, the process is controlled by 6 enzymes. These enzymes each have a specific job involved in creating the DNA molecule. They include: o DNA POLYMERASE o RNA PRIMER o PRIMASE o TOPOISOMERASE o HELICASE o LIGASE

DNA Polymerase Primary enzyme involved in DNA synthesis. Using a single parent DNA strand as a template to make a new complimentary strand. Adds nitrogenous bases (A,T,C,G) to the complimentary strand.

LIMITATIONS 1) Can only add nucleotides to the 3’ end of an growing DNA chain. Remember the Rules! – DNA can only be replicated in a 5’ to 3’ direction. 2) Cannot add the first 2 nucleotides.

RNA Primer Creates the starting point for the first 2 strands to be added. Does not actually add the bases, only starts the process.

Primase Adds the first 2 nucleotides after RNA primer has prepped the site. Once the first 2 are added, DNA polymerase can add continually along the strand.

Topoisomerase Binds to the double helix of the DNA. Clips the ‘backbone’ of the DNA to allow it to unravel. Continually unwinds all parts until the DNA is a flat ladder and is no longer twisted into a double helix.

Helicase Think scissors! Helicase cuts along all the hydrogen bounds between the 2 parent strands. This exposes them to allow more bases to be added to create the complimentary strand.

Ligase Think glue! This sticks the hydrogen bonds back together to create the 2 new DNA strands.

2 Types of Strands When complimentary strands are being created for the parent strands, there are two types: 1) Leading strand o Easier to figure out. o Goes in the 5’-3’ direction easily, therefore is continuously synthesized. 2) Lagging strand o More complicated o Same steps as leading strand but goes against 5’-3’ direction rule o Additional steps must be taken to replicate this strand, as it cannot be continuously synthesized.

Leading Strand

Steps to Replication: Leading Strand 1) DNA is unwound by Topoisomerase. 2) Helicase cuts the hydrogen bounds, continuously, between the nitrogenous bases to open up the 2 parent strands. 3) RNA primer is created at the 3’ end of leading strand. This is at the end of the strand, away from the ‘fork’ of the DNA molecule. This forms the spot for the first 2 nucleotides.

4) Primase places the first 2 nucleotides down. 5) DNA polymersase adds the rest of the bases, in a 5’-3’ direction, creating the complimentary strand. 6) Bases are added continually until the full strand is complete. 7) Ligase then attaches all hydrogen bonds back together.

Lagging Strand

Steps to Replication: Lagging Strand 1) DNA is unwound by Topoisomerase. 2) Helicase cuts the hydrogen bounds, continuously, between the nitrogenous bases to open up the 2 parent strands. 3) RNA primer is created at the 3’ end of lagging strand. This is at the middle of the strand, at the opening of the ‘fork’ of the DNA molecule. This forms the spot for the first 2 nucleotides. 4) Primase places the first 2 nucleotides in a short piece of DNA called an OKAZAKI FRAGMENT.

5) DNA polymerase adds the rest of the bases to the fragment, in a 5’-3’ direction. 6) The OKAZAKI FRAGMENT is placed down beside the parent strand. 7) OKAZAKI FRAGMENTS are continually made until the entire strand is complete. 8) Ligase not only attaches the hydrogens back together, its also glues all the OKAZAKI FRAGMENTS together between bases.

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Directions of DNA Replication does not begin at one end of the strand to another. Replication begins at many points along the strand, using Replication bubbles. Replication is bidirectional as well, decreasing the time it takes to make a full, new DNA molecule.