CONCEPT 13.2: Replicating the Ends of DNA Molecules

Introduction

In eukaryotic cells, the replication of linear chromosomes presents unique challenges, particularly at the ends of DNA molecules. This section explores the structure and function of telomeres, the role of telomerase, and their implications for cell division and cancer.

Problems with Replicating Linear Chromosomes

• End Replication Problem: DNA polymerases cannot fully replicate the 3' ends of linear DNA, leading to progressive shortening of chromosomes with each cell division.

• Consequences: Over time, essential genetic information may be lost, affecting cell viability and function.

Telomeres: Structure and Function

• Definition: Telomeres are repetitive nucleotide sequences at the ends of eukaryotic chromosomes that protect genetic data during replication.

• Sequence: In humans, the telomere sequence is typically TTAGGG repeated thousands of times.

• Function: Telomeres do not code for genes; instead, they act as buffers to prevent the loss of important DNA during cell division.

Telomere Shortening and Cell Populations

• Cell Division and Telomere Length: Each time a cell divides, its telomeres become shorter.

• Example: If population A has been grown and propagated for 6 months and population B for 2 months, population A will have shorter telomeres due to more cell divisions.

Telomerase: The Enzyme that Maintains Telomeres

• Definition: Telomerase is an enzyme that adds telomeric repeats to the ends of chromosomes, counteracting telomere shortening.

• Function: Telomerase is active in certain cell types, allowing them to divide many times without losing essential DNA.

• Equation: Telomerase catalyzes the addition of the sequence:

Fill in the Blanks

• The more times a cell divides, the shorter its telomeres and this plays a role in the process of aging.

• Germ cells, the stem cells that divide over and over within ovaries and testis to produce eggs and sperm, produce the telomerase enzyme to prevent the loss of genes.

Cell Types and Telomerase Expression

The following table compares telomerase expression in different cell types:

Telomerase and Cancer

• Role in Cancer: Cancer cells often reactivate telomerase, allowing them to divide indefinitely and become 'immortal.'

• Therapeutic Application: Anti-telomerase drugs may be useful in treating cancer by limiting the ability of cancer cells to maintain their telomeres, thus restricting their capacity for unlimited division.

• Example: Telomerase inhibitors are being researched as potential cancer therapies.

Additional info: Telomere length is also associated with cellular aging and some age-related diseases. In most somatic cells, telomerase is inactive, leading to gradual telomere shortening and eventual cell senescence.