37. Telomeres consist of

(1) Simple tandem repeats

(2) Nucleolar organizer region

(3) GC rich sequences

(4) Single copy repeats

Introduction

Telomeres are specialized structures located at the ends of linear chromosomes in eukaryotic cells. They play an essential role in maintaining chromosome integrity and stability. At their core, telomeres consist of simple tandem repeats—short, repetitive DNA sequences repeated thousands of times. This article delves into the composition of telomeres, their structural features, and their biological significance.

What Are Telomeres?

Telomeres are repetitive nucleotide sequences that cap the ends of chromosomes, preventing them from being recognized as DNA breaks and protecting them from degradation or fusion with other chromosomes. They act like the plastic tips on shoelaces, preventing chromosome fraying.

Composition: Simple Tandem Repeats

• Telomeres are composed of short DNA sequences repeated in tandem many times.

• In most vertebrates, including humans, the repeat sequence is the hexanucleotide 5′-TTAGGG-3′. This sequence is repeated thousands of times, forming arrays that can extend from a few kilobases up to tens of kilobases in length.

• These repeats are arranged head-to-tail in a tandem fashion, creating a uniform repetitive region at chromosome ends.

Structural Features of Telomeric DNA

• The telomeric repeat region is double-stranded, except for a single-stranded G-rich 3′ overhang.

• This G-rich overhang can invade the double-stranded telomeric DNA to form a T-loop, a lariat-like structure that protects the chromosome end.

• Telomeric DNA can also form G-quadruplex structures, stabilized by stacked guanine quartets, which contribute to telomere stability and regulation.

Biological Functions of Telomeres

1. Protect Chromosome Ends:

   • Telomeres prevent chromosome ends from being mistaken for DNA breaks, thus avoiding inappropriate repair activities like end-to-end fusions.

2. Prevent DNA Loss During Replication:

   • Due to the end-replication problem, DNA polymerases cannot fully replicate the 3′ ends of linear chromosomes. Telomeres buffer this loss by providing non-coding repetitive sequences that can be shortened without affecting essential genes.

3. Regulate Cellular Aging:

   • Telomere length acts as a mitotic clock; progressive shortening with each cell division leads to replicative senescence, influencing aging and cell lifespan.

4. Facilitate Telomerase Action:

   • The G-rich overhang serves as a substrate for telomerase, an enzyme that extends telomeres, counteracting shortening in germline and stem cells.

Variation Across Species

• While the TTAGGG repeat is common in vertebrates, other organisms have different telomeric repeats, such as TTGGGG in Tetrahymena or irregular repeats in yeast.

• The length and exact sequence of telomeric repeats vary but share the common feature of being simple tandem repeats.

Why Telomeres Are Not Other DNA Elements

• Not nucleolar organizer regions: These contain rRNA gene repeats, distinct from telomeric repeats.

• Not single-copy repeats: Telomeres consist of highly repetitive sequences, not unique single-copy DNA.

• Not merely GC-rich sequences: Although telomeric repeats are G-rich, their defining characteristic is the tandem repetition of specific short sequences.

Summary Table

Conclusion

Telomeres are composed of simple tandem repeats of short DNA sequences repeated thousands of times at chromosome ends. These repetitive sequences, such as the TTAGGG hexamer in humans, are essential for protecting chromosomes from degradation, preventing fusion, and maintaining genomic stability. Their unique structure and sequence composition enable critical cellular functions including regulation of aging and facilitation of telomerase activity.

Correct Answer:
(1) Simple tandem repeats