DNA STRUCTURE AND REPLICATION

Central Dogma Francis Crick theorized the central dogma of molecular biology after he and Watson deciphered the structure of DNA. This dogma has held true for all organisms and is universal. “It hasn’t escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” The central dogma explains how genes (DNA) become gene products (proteins).

Nucleotides DNA (deoxyribonucleic acid) is a polymer of nucleotide monomers, there are 4 nucleotide monomers in DNA, adenine, guanine, cytosine and thymine. Cytosine and Thymine are single ring pyrimidines while adenine and guanine are double ring purines. Chargaff’s rule states that there is a 1:1 ratio of purines: pyrimidines in each bas pair. A-T and G-C

The double helix The right handed double helix structure of DNA is made of two anti-parallel strands of DNA that are hydrogen bonded together. The strand ends are labeled 5’ and 3’ so they are distinguishable. The backbone of each strand is made of alternating sugars (deoxyribose) and phosphate groups which are linked together by a phosphodiester bond.

Complimentary and Antiparallel Each strand of DNA in the double helix is said to be complimentary and antiparallel. The nitrogenous bases are complimentary to each other in that they fit together. Adenine's fit with thymine's etc. The sugar phosphate backbone runs in an antiparallel manner. That is one strand runs 5’- 3’ and the other runs 3’-5’

DNA Replication Semi- conservative replication

1. Unzip the double helix The enzyme helicase disrupts the hydrogen bonds that hold the double helix together. This separates the strands and exposes the nucleotides so that both strands can act as template strands for leading and lagging synthesis.

2. Complementary base pairing DNA polymerase can only base pair nucleotides in the 5’-3’ manner. This means that each template strand of DNA will be base paired in a different manner. Leading strand synthesis- The 3’-5’ template strand undergoes continuous synthesis of the complimentary new strand in the 5’-3’ direction. Lagging strand synthesis- The 5’-3’ template strand undergoes discontinuous synthesis of the complimentary new strand in the 5’’-3’ direction. Okazaki fragments- the short discontinuously synthesized new lagging strand DNA fragments that will be ligated together at the end of replication.

3. Ligate the backbone DNA ligase ligates (connects) the sugar phosphate backbone of the newly synthesized DNA strands by completing the phosphodiester bonds.

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