DNA Replication: Leading/Lagging Strands & Telomeres

Replicating DNA Molecule Ends

• DNA molecule ends get replicated through specialized mechanisms.
• Eukaryotic chromosomes with linear DNA face challenges due to DNA polymerase limitations.
• Standard replication is not effective at completing the 5' ends of new DNA strands.
• DNA molecules become progressively smaller after each replication cycle.

Leading Strand Reproduction

• The leading strand reproduces in its entirety

Lagging Strand Issues

• The lagging strand doesn't complete fully
• The final RNA primer removes without DNA replacement.
• Gaps must be filled at the lagging strand's ends to uphold chromosome integrity during cell division.
• Eukaryotic chromosomal DNA ends shorten with each replication round.

End Replication Problem

• Telomere presence avoids gene exposure to the environment, which can causes mutations

Consequences of End Replication

• One strand replicates fully to the end
• The other strand has an 8-12 base pair gap at the 5' end.
• Chromosomes in dividing cells shorten over time.
• Shortening can lead to gene loss from chromosome ends.
• Prokaryotes, with circular DNA, circumvent this issue.
• Eukaryotic telomeres lack genes.

Telomeres: Structure

• Typically made of multiple repeats of a simple nucleotide sequence.
• Human telomeres usually feature the TTAGGG sequence, repeated 100-1,000 times.
• Repetitive DNA sequences are present at human chromosome ends.
• Thousands of repeats of the TTAGGG sequence are present.
• Telomeric interacting proteins bind it.

Telomeres: Function

• Telomeres prevent gene erosion during DNA replication.
• Telomeric DNA becomes shorter in dividing somatic cells of older individuals and cultured cells.
• Without telomeres, chromosome ends may get "repaired", leading to fusion and genomic instability.
• They separate one chromosome from another in the DNA sequence
• Telomeres regulate the number of times a cell can divide, acting as a "clock."
• Telomeric sequences shorten through DNA replication.
• Telomere shortening may tie into tissue aging and aging in general.

Telomerase

• Eukaryotic cells use a mechanism to restore shortened telomeres in germ cells, which then give rise to gametes.
• Telomerase, an enzyme, lengthens telomeres in eukaryotic germ cells.
• Cells featuring telomerase have a greater division capacity.
• Active in germ cells, in vitro immortalized cells, most cancer cells, and potentially stem cells in humans.
• Cells with short lifespan have no telomerase activity.
• Telomeres shorten over time.
• Cell division functions as a mitotic clock in replicative senescence.
• Telomerase includes RNA and protein.
• The RNA subunit has a sequence complementary to the DNA repeat sequence.
• RNA serves as the template for telomere DNA synthesis.
• It demonstrates reverse transcriptase activity.
• Uses a short RNA molecule to extend the 3' end of the telomere.
• Primase and DNA polymerase get room to extend the 5' end.
• Telomerase extends the telomere, although it does not repair the 3' end overhang.
• The template strand extends beyond the DNA to be copied.
• Telomerase adds repeat sequences to the 3' end of the template strand to preserve genetic information.
• DNA polymerase completes the lagging strand.
• Australian scientist Elizabeth Blackburn discovered telomerase
• The 5' strand extends through lagging strand mechanisms.
• An overhang remains on the 3' end, sometimes tucking and capping.
• Special capping proteins protect ends from nucleases.

Telomeres and Disease

• Dividing somatic and cultured cell DNA shortens with generations.
• Telomere length may limit the lifespan of certain tissues and organisms.
• Normal telomere shortening helps to protect against cancer by restricting cell divisions.

Telomerases as Drug Targets

• High telomerase activity is related to the great proliferative ability of numerous cancer cells
• Unlimited cell division are strains of cultured cells which are considered immortal
• Telomerase a worthwhile cancer diagnostic and chemotherapy target.
• Telomere and telomerase offer ideal anti-cancer targets.

Telomerases and Cancer

• Telomerase is active in 80-90% of cancers.
• Targeting the RNA component with antisense oligodeoxynucleotides and RNaseH and reverse transcriptase inhibitors such as AZT, and catalytic protein subunit inhibitors are ways to target telomerases for cancer treatments.

Summary

• Telomeres protect chromosomes and chromosomes ends
• Telomerase maintains telomeres.
• Telomere shortening leads to cell death or senescence.
• Short telomeres lead to less tissue renewal.
• Telomere length may predict age-related diseases.