**(a) Telomere**

Telomeres are repetitive sequences of DNA that are located at the ends of chromosomes. They consist of short nucleotide sequences that do not encode any specific genetic information. Telomeres play a crucial role in the ageing process in humans.

**Structure and Function of Telomeres:**

- Telomeres consist of tandem repeats of the nucleotide sequence TTAGGG in humans. These repeats are usually present in thousands of copies.
- The main function of telomeres is to protect the integrity of the chromosome. They act as protective caps that prevent the erosion or degradation of the important genetic information encoded in the DNA molecule.
- During DNA replication, the enzyme DNA polymerase cannot fully replicate the ends of linear chromosomes. This leads to the gradual shortening of telomeres with each cell division.
- Telomeres also prevent the fusion of chromosome ends, which can result in genomic instability and DNA damage.
- Telomeres help in maintaining the stability and integrity of the genome by protecting the coding regions of DNA from degradation and rearrangement.
- Telomeres also play a role in regulating the lifespan of cells. When telomeres become critically short, they trigger a cellular response known as replicative senescence, leading to cell cycle arrest and eventually cell death.

**Telomeres and Ageing:**

- As cells divide and telomeres shorten, they eventually reach a critical length where they can no longer protect the chromosome ends effectively. This triggers cellular senescence or apoptosis.
- Telomere shortening is associated with the ageing process. Studies have shown that telomeres gradually shorten with age in most human tissues.
- Telomere shortening is also accelerated by factors such as oxidative stress, inflammation, and exposure to certain environmental toxins.
- Shorter telomeres have been linked to various age-related diseases, including cardiovascular disease, cancer, and neurodegenerative disorders.
- Conversely, individuals with longer telomeres have been found to have a decreased risk of age-related diseases and increased longevity.

In conclusion, telomeres play a critical role in the ageing process in humans. Their gradual shortening with each cell division contributes to cellular senescence and the eventual decline in tissue function associated with ageing. Understanding the mechanisms involved in telomere maintenance and exploring strategies to preserve telomere length may have implications for promoting healthy ageing and preventing age-related diseases.

SAT DNA present on satellite chromosomes is responsible for ageing in human.