> Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes . Telomeres act as protective caps at their ends just like the plastic tips at the end of shoelaces
-without the coating , shoelaces become frayed until they can no longer do their job ; similarly , without telomeres , DNA strands become damaged and cells can not do their job . Telomeres lose a bit of their DNA every time a cell divides . Eventually , when all of the telomere DNA is gone , the cell can no longer replicate , and dies . Telomeres are shortened with age , but can also be shortened by stress , smoking , obesity , lack of exercise , and a poor diet .
> Genes are distinct sequences of nucleotides , linearly arranged on a chromosome and each occupying a specific location . For most genes , one copy is inherited from the mother and one copy is inherited from the father . The version of a gene that a parent passes down to their child is known as an allele . Alleles are different forms of the same gene , i . e . there are slight variations in the sequence of nucleotides .
For example , one variation of a gene might begin with the sequence A , and another variation might begin with the sequence a . These different variations of genes are called alleles .
Some cells have the ability to reverse telomere shortening by expressing telomerase , an enzyme that maintains and even extends the telomeres of chromosomes . Telomerase is not usually active in somatic cells ( cells of the body ), but it is active in germ cells ( the cells that make sperm and eggs ) and some adult stem cells ( cells that can give rise to many cell lines ). These are cell types that need to undergo many divisions . Similarly , the chromosomes of cancer cells also do not usually lose their telomeres , accounting for the uncontrolled growth of malignant cells ; telomerase is active in 90 % of cancer cells .
● A person is said to be homozygous for a gene if they have two identical versions of the gene : two copies of A or two copies of a .
● A person is said to be heterozygous if they have two different versions of the gene : one copy of A plus one copy of a .