When a cell copies a DNA molecule, each strand serves as a template for ordering nucleotides into a new complementary strand. DNA Replication The nucleotides are linked to form new strands One at a time, nucleotides line up along the template strand according to the base-pairing rules.

The replication of a DNA molecule begins at special sites, origins of replication. Replication proceeds in both directions until the entire molecule is copied. These enzymes separate the strands, forming a “replication bubble”. In bacteria, this is a single specific sequence of nucleotides that is recognized by the replication enzymes.

An enzyme, DNA helicase untwists and separates the template DNA strands at the replication fork.

DNA polymerases catalyze the elongation of new DNA at a replication fork. Each has a nitrogen base, deoxyribose, and a triphosphate tail. As nucleotides align with complementary bases along the template strand, they are added to the growing end of the new strand by the polymerase.

DNA polymerases can only add nucleotides to the free 3’ end of a growing DNA strand. At the replication fork, one parental strand (3’-> 5’ into the fork), the leading strand, can be used by polymerases as a template for a continuous complementary strand. This creates a problem at the replication fork because one parental strand is oriented 3’->5’ into the fork, while the other antiparallel parental strand is oriented 5’->3’ into the fork. A new DNA strand can only elongate in the 5’->3’ direction.

Okazaki fragments, each about nucleotides, are joined by DNA ligase to form the sugar-phosphate backbone of a single DNA strand The other parental strand (5’->3’ into the fork), the lagging strand, is copied away from the fork in short segments (Okazaki fragments).

Are we pre-programmed for aging? The burning question of telomerases and their role in cell death The central premise: all chromosomes have sections at their ends called telomeres, which act like the plastic end on a shoelace. A telomere is a repeating DNA sequence, often reaching lengths of 15,000 base-pairs. TTAGGGTTAGGGTTAGGG

So how do they work? Each time a DNA molecule replicates, it loses a few sections of telomere due to the fact that the DNA of the lagging strand is not able to complete the full sequence. “So, why can’t it? I thought DNA made a perfect copy EVERY TIME!” Because the lagging strand makes Okizaki fragments, the last section at the end will be shorter, and unable to replicate fully.

Fluorescent stained chromosomes with telomeres counter-stained

Each time a cell divides, it loses from 20 to 250 base-pairs of the telomere. The telomere does not actually code for anything, so no genetic information is lost. HOWEVER, once the telomere becomes too short, the chromosome reaches a “critical length”, and can no longer replicate.

This means that the cell becomes “old” and dies by a process called apoptosis. Cell aging, also called senescence, is the process by which a cell becomes old and dies. Telomere activity is controlled by two factors: 1) erosion (losing sections) and 2) addition, which is determined by an enzyme, telomerase.

Telomerase, also called telomere terminal transferase, is an enzyme made of protein and RNA subunits that elongates terminal ends of DNA molecules. Telomerase is rare, and found only in a few types of cells: Fetal cells Adult reproductive cellsTumor cells It is almost undetectable in somatic cells.

Because somatic cells do not have any of the enzyme, they reach a certain number of cell divisions and then age and die. Cell aging was first described by Leonard Hayflick in The “Hayflick Limit” is the limit of cell proliferation, based on the number of times a cell has divided.

Cellular immortality refers to the ability to reproduce and divide indefinitely. Since cancer cells grow uncontrollably, and have telomerase present, it is thought that the telomerase is allowing the cells to continue to divide without limit. If telomerase activity could be turned off, cancer cells would act like normal body cells, and could not reproduce.

Senescent cells not only are incapable of dividing, but also exhibit altered patterns of gene expression. Ex: Senescent skin cells produce lesser amounts of elastin and collagen, contributing to the atrophy of the skin and increase in age-related skin disorders. Ex: metabolic changes in senescent retinal cells are considered contributors to age-related macular degeneration (AMD).

In the case of AMD, 33% of the population age 70 is affected. The delay or prevention of cell senescence through the extension of cell life-span is expected to have important beneficial effects. Laboratory culturing of human cells will be made easier if the life-spans of individual cells is made longer.

Because of the large number of cell divisions required by gene therapy, cell senescence is a major limiting factor in their success. Ex: in reconstitution of the immune system after chemotherapy, the cells exhaust the equivalent of an estimated 40 years of their life-span. This restricts the use in older patients, and may produce problems later in life for younger patients