If cells don’t replicate their DNA or don’t do it completely, the daughter cell will end up with no DNA or only part of the DNA. This cell will likely die. Cells also copy their DNA right before a special cell division event called meiosis, which results in special cells called gametes (also known as eggs and sperm.)

The lardger a cell becomes, the more demands the cell places on its DNA. As a cell increases in size, it usually does not make extra copies of DNA.

Cell growth is the process by which cells accumulate mass and increase in physical size. In some cells, size is proportional to DNA content. For instance, continued DNA replication in the absence of cell division (called endoreplication) results in increased cell size.

Interphase is the longest part of the cell cycle. This is when the cell grows and copies its DNA before moving into mitosis. During mitosis, chromosomes will align, separate, and move into new daughter cells. The prefix inter- means between, so interphase takes place between one mitotic (M) phase and the next.

Each time a cell divides, the two resulting daughter cells must contain exactly the same genetic information, or DNA, as the parent cell. To accomplish this, each strand of existing DNA acts as a template for replication.

Cells divide for two reasons: Growth. When one cell divides into two, both must have a copy of the genetic information. Therefore, before cell division occurs, the genes must also make duplicates of themselves so that all of the important genetic information ends up in each of the new cells.

Replication is an essential process because, whenever a cell divides, the two new daughter cells must contain the same genetic information, or DNA, as the parent cell. Once the DNA in a cell is replicated, the cell can divide into two cells, each of which has an identical copy of the original DNA.

The point where the double helix is opened up and the DNA is copied is called a replication fork. Once the strands are separated, an enzyme called DNA polymerase copies each strand using the base-pairing rule. The two strands are not exactly copied the same way.

DNA polymerases

Complementary deoxyribonucleic acid (DNA) is DNA in which the sequence of the constituent molecules on one strand of the double stranded structure chemically matches the sequence on the other strand. A useful analog is to picture a key and a lock. Complementary DNA (cDNA) is a copy of a region of a strand of DNA.

Enzymes involved in DNA replication are:

• Helicase (unwinds the DNA double helix)
• Gyrase (relieves the buildup of torque during unwinding)
• Primase (lays down RNA primers)
• DNA polymerase III (main DNA synthesis enzyme)
• DNA polymerase I (replaces RNA primers with DNA)
• Ligase (fills in the gaps)

There are four basic components required to initiate and propagate DNA synthesis. They are: substrates, template, primer and enzymes.

One of the key players is the enzyme DNA polymerase, also known as DNA pol. In bacteria, three main types of DNA polymerases are known: DNA pol I, DNA pol II, and DNA pol III. It is now known that DNA pol III is the enzyme required for DNA synthesis; DNA pol I and DNA pol II are primarily required for repair.

Terms in this set (5)

• DNA polymerase III. leading and lagging strand synthesis.
• Ligase. seals the nick in the phosphodiester bond between two Okazaki fragments.
• Helicase. breaking the hydrogen bond between nitrogenous bases, generating single stranded DNA templates.
• Topoisomerase.
• Primase.

DNA is always synthesized in the 5′-to-3′ direction, meaning that nucleotides are added only to the 3′ end of the growing strand. As shown in Figure 2, the 5′-phosphate group of the new nucleotide binds to the 3′-OH group of the last nucleotide of the growing strand.

DNA replication employs a large number of proteins and enzymes, each of which plays a critical role during the process. One of the key players is the enzyme DNA polymerase, also known as DNA pol, which adds nucleotides one by one to the growing DNA chain that are complementary to the template strand.