How do researchers trace mitochondrial DNA over centuries?
—M. Sivak, Irvington, N.Y.

Bert Ely, a biologist at the University of South Carolina, offers this explanation:

Mitochondrial DNA (mtDNA) does not change very much, if at all, from generation to generation. The mtDNA passes only from a mother to her children; fathers cannot impart their mtDNA.

Mutations (changes) do occur in mtDNA but not often— less frequently than once per 100 descendants. Therefore, a person's mtDNA is probably identical to that of his or her direct maternal ancestor a dozen generations back, and this shared inheritance can be used to connect people across large spans of time. For example, if a particular type of mtDNA were found primarily in Africa, then we could conclude that people from elsewhere in the world who had that type of mtDNA had a maternal ancestor from Africa.

Unlike most DNA, mitochondrial DNA is not found in our chromosomes or even in the nucleus (the central enclosure that contains all the chromosomes) of our cells. Small membrane- bound structures present in all plant and animal cells, mitochondria are responsible for generating most of the energy needed for cell function. Each mitochondrion contains its own DNA and its own protein-synthesizing machinery.

Before people started to travel around the world, the rare changes that occurred in mtDNA resulted in unique types of mtDNA on every continent. Therefore, scientists can assign most contemporary mitochondrial DNAs to a continent of origin based on the region of the mtDNA—the so-called HvrI—where mutations are most likely to occur. Scientists can analyze the HvrI to find a record of all the past mutations as the mtDNA was transmitted from mother to daughter from one generation to another. These accumulated mutations are the basis for distinctive mitochondrial DNAs found on every continent.

Within continents, regional mtDNA variation exists as well. Because a woman's descendants are likely to settle nearby, mutations originating in her mtDNA generally will be confined to the local area where she lived. Whenever people moved from one place to another, of course, they took their mtDNA with them. People have moved extensively over time in sub-Saharan Africa, for example. As a result, a recent study has shown that approximately half of all African mtDNAs are shared among people from multiple countries in Africa.

How are seashells or snail shells formed?

Francis Horne, a biologist at Texas State University, provides an answer:

Shells, which are the exoskeletons of mollusks such as clams, oysters, snails and many others, are not made up of cells like typical animal structures. They are composed mostly of calcium carbonate with a small quantity of protein— usually less than 2 percent.

Mantle tissue located under and in contact with the shell secretes proteins and minerals to form the shell. First, an uncalcified layer of conchiolin—protein and chitin, a strengthening, naturally produced polymer—forms. Then comes the highly calcified prismatic layer, followed by the final pearly layer, or nacre. The process is analogous to laying down steel (protein) and pouring concrete (mineral) over it.

Whereas the bones of land animals such as humans grow with them, snails and clams have to gradually enlarge and extend their shells by adding new material at the margins. The newest part of a snail's shell, for example, is located around the cavity where the animal pokes out. The outer edge of its mantle continuously adds new shell at this opening.

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