DNA Structure & Replication Notes DNA (Deoxyribose Nucleic Acid)
DNA is found only in the nucleus. It is composed of subunits called nucleotides, which are the building blocks of all nucleic acids. The DNA nucleotide it made up of three different components: deoxyribose sugar, a phosphate, and one of four nitrogen bases: adenine, guanine, thymine, and cytosine.
Adenine & Guanine are double-ringed molecules known as purines, while cytosine & thymine are single-ringed pyrimidines. The general structure of a nucleotide is shown in Figure 1. Notice where the phosphate and the base bond to the sugar. Figure 1
The DNA Molecule It is double-stranded. It is made up of two strands of nucleotides paired with one another, giving it a ladder-like structure. The sides of the ladder are composed of alternating units of deoxyribose sugar & phosphates connected together by covalent bonds to form the backbone of the molecule.
The phosphates connect the 5 of one sugar to the 3 of the next sugar. The two sides of the molecules are identical except that they run in opposite directions. The two strands are said to be anti-parallel to one another. The rungs of the DNA ladder are composed of the nitrogen bases which are bonded to the deoxyribose sugar & project out into the center of the molecule where they pair with one another.
Chargaff s Rule The chemical structure of the nitrogen bases limits their pairing to very specific combinations. This pairing of the bases is referred to as complementary base pairing. Adenine on one strand will always pair only with Thymine on the opposite strand. Guanine on one strand will always pair with Cytosine on the opposite strand.
Base Pairings
The two strands are held together by weak hydrogen bonds that form between the bases. Adenine & Thymine form 2 hydrogen bonds, while guanine & cytosine form 3 hydrogen bonds. Watson and Crick described DNA as a Double-Helix because the two strands of nucleotides are twisted around one another in a spiral.
DNA Discovery
PART 2: REPLICATION OF DNA DNA is the master blueprint for an organism. The specific arrangement or sequence of the paired nitrogen bases in the DNA molecule makes up the genetic code in the cell.
The DNA molecules must be able to produce exact copies of itself prior to cell division in order to ensure that: 1. The heredity information is carried from generation to generation 2. And each new cell will have the exact same genetic information.
The process by which the DNA molecule is copied is called DNA replication. DNA replication has been shown experimentally to be a semi-conservative process which means that at the completion of replication each new DNA molecule consists of one strand from the original DNA molecule and one newly synthesized strand.
THE STEPS OF DNA REPLICATION : The two original strands composing the DNA double helix uncoil. The weak hydrogen bonds are broken by enzymes and the two strands separate from one another, leaving the bases exposed. As this occurs, each of the two strands acts as a template or mold for the replication of a new strand. New nucleotide bases are attracted to their complementary partners on each of the separated strands and pair with them forming hydrogen bonds. Two identical molecules of DNA are formed, each made of a new and old strand (semiconservative).
Double Helix uncoils Bases are left exposed acting as a template for new strand New bases are attracted to the complimentary partner and join together forming hydrogen bonds Two new molecules formed- each with one new and one old strand
In step 3 of replication, as the new nucleotides come in and pair, an enzyme, DNA polymerase, moves along each newly forming strand, catalyzing the formation of covalent bonds between the sugars and phosphates of the new nucleotides. DNA polymerase can only move in a 5 to 3 direction. (See Figure 4).
One of the new strands is synthesized continuously, in one long unbroken piece in the 5 to 3 direction. This is called the leading strand and has continuous replication. Leading strand
The DNA polymerase does not act on the second strand until a stretch of about 100-300 nucleotides has been exposed. Then it starts at the replicating fork and moves along, in the opposite direction, until the 100-300 nucleotides are paired. This is the lagging strand and it has discontinuous replication. Lagging Strand
When another such section of nucleotides of the DNA has opened, the process is repeated. This results in a series of short replicated pieces called Okazaki fragments, which are later connected together.
SUMMARY of Replication: During replication, the leading strand has continuous replication (into fork) and the lagging strand has discontinuous replication (out of fork) in the opposite direction.
Replication Videos YouTube - DNA Replication Process (serious) YouTube - DNA Replication Song