DNA Replication -Summarize the events of DNA replication. - Compare DNA replication in prokaryotes with that of eukaryotes.

Copying the Code Base pairing explained how DNA could be copied, or replicated, because each base on one strand pairs with only one base on the opposite strand. Because each strand can be used as a template to make the other strand, the strands are said to be complementary.

DNA Replication During replication, the DNA molecule separates into two strands (known as the Parent strands) and produce two new complementary strands (known as the Daughter strands) following the rules of base pairing. Each of the original strands of the double helix of DNA serves as a template for a new strand.

The Replication Process Step 1: The two strands of DNA must unwind before they can separate, the enzymes DNA topoisomerase and DNA helicase are responsible for the unwinding of the DNA strands. Then the DNA strands unzip through the action of the enzymes DNA helicase and DNA polymerase which allow the replication forks to form.

The Replication Process Step 2: As each new strand forms, new bases are added following the rules of base pairing by DNA polymerase. If the base on the old strand is adenine, then thymine is added to the newly forming strand. Likewise, guanine is always paired to cytosine.

The Finished Product The result of replication is typically two DNA molecules identical to each other and to the original molecule. Each DNA molecule resulting from replication has one original strand and one new strand.

The Role of Enzymes in DNA Replication The principal enzymes involved in DNA replication are topoisomerase, DNA helicase, telomerase* and DNA polymerase. * Telomerase replaces short bits and protects the ends of the nucleotides of DNA known as telomeres, which are otherwise shortened or exposed when a cell divides via mitosis.

DNA Polymerase DNA polymerase is an enzyme that has many roles. Aids DNA Helicase in the “unzipping” of the strands of DNA for Replication Joins individual nucleotides to each parent strand to produce a new (daughter) strand of DNA “Proofreads” each new DNA strand, only making an error of 1 nucleotide per 100,000 (approximate).

Prokaryotic DNA Replication In prokaryotes, DNA replication starts when a regulatory protein binds to a single starting point on the chromosome. This triggers the beginning of DNA replication. Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied.

Eukaryotic DNA Replication Eukaryotic chromosomes are generally much bigger than those of prokaryotes. In eukaryotic cells, replication begins at dozens, or even hundreds, of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied. Each short section of DNA formed during replication are called Okazaki fragments.

What are Okazaki fragments? Okazaki fragments allow for the replication of the 3’ 5’ strand, also known as the “lagging strand”. The 5’  3’ strand, also known as the “leading strand”, is replicated in one continuous piece. These exist because DNA replication occurs in the 5'  3' direction because DNA polymerase acts on the 3'-OH of the existing strand for adding free nucleotides.

PCR (Polymerase Chain Reaction) The polymerase chain reaction (PCR) is a technology in molecular biology used to amplify a single copy or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence. *This process has many applications, some examples include use at crime scenes or to determine possible maternity/paternity.