Telomeres and Childhood Adversity
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Within each of our cells are 46 chromosomes, exquisitely complex structures of DNA and protein that carry the coding instructions for all of our traits. These chromosomes are passed down from our parents and constitute the basis of heredity, the passage of traits from one generation to the next. But chromosomes don’t just carry a record of our genetic legacy. They also carry a record—in the lengths of their telomeres—of the stresses we encounter.
Telomeres are special protective structures found at the end of each of our chromosomes. Like the small plastic tips that keep the ends of a shoelace from unraveling, telomeres prevent chromosomes from being degraded at their ends. In spite of the protection of the telomeres, chromosomes of most cells shorten progressively with each cell division. Due to a quirk of DNA replication, most cells are unable to copy the very end of each linear chromosome (see Chapter 12 for a full discussion of the end-replication problem). Hence, with each round of replication, a chromosome becomes shorter, until it is so reduced that the cell stops dividing, becomes inactive, and eventually dies. For most cells, this shortening of telomeres limits the number of divisions possible. Exceptions occur in germ-line cells that produce future generations, certain stem cells, and—unfortunately—many cancer cells that have escaped normal constraints on cell division.
Because telomeres become shorter with each cell division, much research has focused on determining if telomere length is indicative of biological aging. Although the relationship between telomere length and aging is complex and not fully understood, considerable evidence suggests that telomeres do shorten with age, and that processes which lead to premature telomere shortening are associated with features of aging. In 2011, geneticists observed that hardships encountered early in life can play a part in shortening our telomeres.
To study the effects of early life experience on telomere length, geneticists studied 100 children living in state-run orphanages in Romania. At an early age, some of these children were placed in foster homes; others remained in the orphanages. Previous studies demonstrated that children in such orphanages receive less individual attention and care compared to children growing up with natural or foster parents, and institutional care is assumed to be more stressful than foster care.
When the children were 6 to 10 years old, the researchers collected samples of their DNA and measured the length of their telomeres. The results were striking: children who remained in the orphanages had significantly shorter telomeres than those that spent time in foster care. The researchers concluded that telomere length is affected by childhood adversity: children reared in stressful environments are more likely to have shorter telomeres than those raised in less stressful environments. Several other studies have found a similar association between telomere length in adults and early childhood stresses, such as abuse and chronic illness. How stress affects telomeres and results in their shortening is not known, but the research documents that chromosomes are more than just a repository of our genetic information—their structure is also affected by our environment.
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In this chapter, we examine the molecular structure of chromosomes and the DNA found in cytoplasmic organelles. The first part of the chapter focuses on a storage problem: how to cram tremendous amounts of DNA into the limited confines of a cell. Even in those organisms having the smallest amounts of DNA, the length of genetic material far exceeds the length of the cell. Thus, cellular DNA must be highly folded and tightly packed, but this packing creates problems: it renders the DNA inaccessible, unable to be copied or read. Functional DNA must be capable of partly unfolding and expanding so that individual genes can undergo replication and transcription. The flexible, dynamic nature of DNA packing is a major theme of this chapter. We first consider supercoiling, an important tertiary structure of DNA found in both prokaryotic and eukaryotic cells. After a brief look at the bacterial chromosome, we examine the structure of eukaryotic chromosomes. We pay special attention to the working parts of a chromosome—specifically, centromeres and telomeres. We also consider the types of DNA sequences present in many eukaryotic chromosomes.
The second part of this chapter focuses on the organization of DNA sequences found in mitochondria and chloroplasts. Uniparental inheritance exhibited by genes found in these organelles was discussed in Chapter 5; here we examine molecular aspects of organelle DNA. We briefly consider the structures of mitochondria and chloroplasts, the inheritance of traits encoded by their genes, and the evolutionary origin of these organelles. We then examine the general characteristics of mitochondrial DNA (mtDNA), followed by a discussion of the organization and function of different types of mitochondrial genomes. Finally, we turn to chloroplast DNA (cpDNA) and examine its characteristics, organization, and function.