DNA Replication

Key Words

-Antiparallel -DNA Ligase -DNA Polymerase -Lagging Strand -Leading Strand -Okazaki Fragments -Replication Fork -RNA Primer -Semiconservative

Objective

-Summarize the events of DNA replication.

The Replication Process

Before a cell divides, it duplicates its DNA in a copying process called replication. This process, which occurs during late interphase of the cell cycle, ensures that each resulting cell has the same complete set of DNA molecules. During replication, the DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand. The two strands of the double helix have separated, or "unzipped," allowing two replication forks to form. As each new strand forms, new bases are added following the rules of base pairing. If the base on the old strand is adenine, then thymine is added to the newly forming strand. Likewise, guanine is always paired with cytosine. For example, a strand that has the base sequence TACGTT produces a strand with the complementary base sequence ATGCAA. The result is two DNA molecule identical to each other and to the original molecule. Note that each DNA molecule resulting from replication has one original strand and one new strand.

Three-Prime and Five-Prime

DNA polynucleotides have directionality, meaning that they have a three-prime end and a five-prime end. These three- and five-prime designations correspond to the numbered carbons of the nucleotide sugar component. Each nucleotide has a phosphate group attached to the five-prime carbon of the sugar and a hydroxyl group attached to the three-prime carbon of the sugar. When nucleotides bond together to form a polymer, or polynucleotide, the phosphate groups at the five-prime carbon of one sugar forms a bond to the three-prime carbon of the adjacent nucleotide where the hydroxl group is located. This type of bonds occurs throughout the DNA molecule. For this reason, each DNA strand has a five-prime phosphate group at one end and a three-prime hydroxl group at the other end. Since DNA is double stranded, each DNA molecule is made up of two DNA strands that are held together by hydrogen bonds between the nitrogenous bases. These two DNA strands are arranged in an antiparallel fashion, meaning that one DNA strand of the double helix runs in the five-prime to three-prime direction, while the other complementary stand runs in the opposite direction, from three prime to five-prime.

DNA Replication

DNA replication begins at a special nucleotide sequence called the origin of replication. Long DNA can contain many of these special sites. Two special protein bond to the DNA at the origin of replicatioin to begin the process of replication. When replication begins, the double helix first needs to untwist. Then a molecule called an RNA primer attaches to the template strand. This provides a three-prime end for the enzyme DNA polymerase to begin attaching nucleotides for the newly made DNA strand that are complementary to the template strand. Because of the role of DNA polymerase, one new DNA strand called the leading strand grows continuously, while the other new DNA strand grows discontinuously. The strand that grows discontinuously is called the lagging strand is first made up of new DNA fragments called Okazaki fragments. The Okazaki fragments are later made into one continuous strand of DNA by an enzyme called ligase.

Where does DNA replication begin?

DNA replication begins at origins or replication.

The Role of Enzymes

DNA replication is carried out by a series of enzymes. These enzymes first "unzip" a molecule of DNA by breaking the hydrogen bonds between base pairs and unwinding the two strands of the molecule. Each strand then serves as a template for the attachment of complementary bases. Enzymes are proteins with highly specific functions. For this reason, they are often named for the reactions they catalyze. The principal enzyme involved in DNA replication is called DNA polymerase. DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA. Besides producing the sugar-phosphate bonds that join nucleotides together, DNA polymerase also "proofreads" each new DNA strand, so that each molecule is a near-perfect copy of the original.

Why is there a leading and lagging strand?

DNA synthesis can only go in the 5' to 3' direction and the two strands are antiparallel. This means that only one strand is replicated as the leading strand and the other as the lagging strand.

DNA Replication Speed and Mutations

Each of the DNA strands is copies simultaneously at a high rate of speed. While mistakes are rare, they do occur. A gene can be fron 2,000 to 1 million base pairs, and mistakes can occur on an average of one mistake per 1 million base pairs. Changes in the number of nucleotides or the sequence of nucleotides in DNA are called mutations. These changes can be detrimental, beneficial, or of no consequences to an organism. Beneficial changes are seen as the fuel for evolutionary change. Detrimental changes can lead to genetic disorders and cancer.

What do RNA primers do?

RNA primers bind the DNA to start a new strand.

What directs the sequence of bases used to construct new strands?

The bases on the two original strands direct the sequence of the new strands.

How are the two strands in the DNA double helix related to each other?

They are antiparallel.

Semiconservative

Watson and Crick first explained that DNA replication occurs in a semiconservative fashion. This means that each new DNA molecule contains one DNA strand from the original cell and one newly made DNA strand. The original strands of DNA serve as templates for making the new DNA strands.

Central Dogma

Watson and Crick proposed what they called the central dogma. This describes the flow of information from DNA, where genetic information is stored, to mRNA, which carries the genetic code (transcription). In addition, the dogma describes how the code from mRNA is decoded into a protein in the process of translation.

Copying the Code`

When Waton and Crick discovered the structure of DNA, they immediately recognized one genuinely surprising aspect of the structure. Base pairing in the double helix explains how DNA can be copied, or replicated, because each base on one strand pairs with one--an only one--base on the opposite strand. Each strand of the double helix therefore has all the informatioin needed to reconstruct the other half by the mechanism of base pairing. Because ech strand can be used to make the other strand, the strands are said to be complementary.

Euchromatin

When a cell receives a signal to divide, the DNA within that cell must replicate so that the daughter cells can each receive an exact copy of the DNA found in the original cell. This occurs during the S phase of interphase during the cell cycle since, during this phase, the DNA is in its uncondense form called euchromatin.

Are both strands replicated?

Yes.