Telomeres are structures at the ends of DNA strands that get longer in the DNA of sperm cells as males age. That phenomenon is different for most other types of cells, for which telomeres get shorter as organisms age. In 1992, scientists showed that telomere length (TL) in sperm increases with age in contrast to most cell of most other types. Telomeres are the protective caps at the end of DNA strands that preserve chromosomal integrity and contribute to DNA length and stability. In most cells, telomeres shorten with each cell division due to incomplete replication, though the enzyme telomerase functions in some cell lines that undergo repetitive divisions to replenish any lost length and to prevent degradation. Cells, and therefore organisms, with short telomeres are more susceptible to mutations and genetic diseases. While TL increases in a subset of sperm cells and longer telomeres may prevent early disintegration of DNA, it may also prevent natural mechanisms of apoptosis, or cell death, from occurring in abnormal sperm.

The Hayflick Limit is a concept that helps to explain the
mechanisms behind cellular aging. The concept states that a normal human
cell can only replicate and divide forty to sixty times before it
cannot divide anymore, and will break down by programmed cell death
or apoptosis. The concept of the Hayflick Limit revised Alexis
Carrel's earlier theory, which stated that cells can replicate
themselves infinitely. Leonard Hayflick developed the concept while
at the Wistar Institute in Philadelphia,
Pennsylvania, in 1965. In his 1974 book Intrinsic
Mutagenesis, Frank Macfarlane Burnet named the concept after
Hayflick. The concept of the Hayflick Limit helped scientists study
the effects of cellular aging on human populations from embryonic
development to death, including the discovery of the effects of
shortening repetitive sequences of DNA, called telomeres, on the
ends of chromosomes. Elizabeth Blackburn, Jack Szostak and Carol
Greider received the Nobel Prize in Physiology or Medicine in 2009
for their work on genetic structures related to the Hayflick
Limit.

Theophilus Shickel Painter studied the structure and
function of chromosomes in the US during in the early to mid-twentieth century. Painter worked at
the University of Texas at Austin in Austin, Texas. In the 1920s
and 1930s, Painter studied the chromosomes of the salivary gland
giant chromosomes of the fruit fly (Drosophila
melanogaster), with Hermann J. Muller. Muller and Painter
studied the ability of X-rays to cause changes in the chromosomes
of fruit flies. Painter also studied chromosomes in mammals.
He investigated the development of the male gamete, a process
called spermatogenesis, in several invertebrates and vertebrates,
including mammals. In addition, Painter studied the role the
Y-chromosome plays in the determination and development of the male
embryo. Painter's research concluded that egg cells fertilized by
sperm cell bearing an X-chromosome resulted in a female embryo,
whereas egg cells fertilized by a sperm cell carrying a
Y-chromosome resulted in a male embryo. Painter's work with
chromosomes helped other researchers determine that X- and
Y-chromosomes are responsible for sex determination.

Telomeres are sequences of DNA on the ends of chromosomes that protect chromosomes from sticking to each other or tangling, which could cause irregularities in normal DNA functions. As cells replicate, telomeres shorten at the end of chromosomes, which correlates to senescence or cellular aging. Integral to this process is telomerase, which is an enzyme that repairs telomeres and is present in various cells in the human body, especially during human growth and development. Telomeres and telomerase are required for normal human embryonic development because they protect DNA as it completes multiple rounds of replication.

Telomerase is an enzyme that regulates the lengths of telomeres in the cells of many organisms, and in humans it begins to function int the early stages of embryonic development. Telomeres are repetitive sequences of DNA on the ends of chromosomes that protect chromosomes from sticking to each other or tangling. In 1989, Gregg Morin found that telomerase was present in human cells. In 1996, Woodring Wright and his team examined human embryonic cells and found that telomerase was active in them. Scientists manipulate telomerase in cells to give cells the capacity to replicate infinitely. Telomerase is also necessary for stem cells to replicate themselves and to develop into more specialized cells in embryos and fetuses.

Cold Spring Harbor Laboratory (CSHL) is a non-profit research institution that specializes in cancer, neuroscience, plant biology, quantitative biology, and genomics. The organization is located on the shores of Cold Spring Harbor in Laurel Hollow, New York. The Brooklyn Institute of Arts and Sciences established the CSHL in 1890, to provide scientists with facilities to research Charles Darwin's evolutionary theory. The first mission of CSHL was biological science education. Since 1998, CSHL has housed the Watson School of Biological Sciences, a PhD program dedicated to scientific research. Nobel Laureates who conducted experiments at the CSHL include Barbara McClintock, Alfred Hershey, James Watson, Francis Crick, and Sydney Brenner. Throughout its history, researchers at CSHL have studied embryology, reproductive medicine, and genetics.