Cells and Molecules - Brutes or Angels: Human Possibility in the Age of Biotechnology - page 31

their chromosomes once or twice without subsequently undergoing cell division.
in such cases, the cell ends up with 92 or 184 chromosomes, respectively.
13. The most common way that non-gene DnA regulates the activity of genes
is by serving as a binding site for proteins that turn on or turn off the activity
of certain genes. elucidating the function of non-gene, human DnA is an objec-
tive of the enCoDe Project: http://www.genome.gov/10005107 (accessed no-
vember 11, 2012).
14. To avoid confusion, i have written that humans have 23 pairs of chromo-
somes, 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes. But
since there are two types of sex chromosomes, X and y, humankind as a species
actually has 24 types of chromosomes. male body cells have both X and y chro-
mosomes, so they contain 24 types of chromosomes; human female cells lack the
y chromosome and have 23 types of chromosomes.
15. The first meiotic division is called m1 and the second division is called
m2. During m1, chromatids remain unseparated and one version of each repli-
cated chromosome enters each daughter cell. During m2, chromatids separate
and cell divisions produce four haploid germ cells, either four small, functional
sperm cells or one large, functional egg and polar bodies that eventually degen-
erate. Polar bodies are so named because they usually remain associated at one
“pole” of the larger cell after cell division. not depicted in igure 1.10 is the fact
that during egg formation the first polar body forms during m1 and another one
forms when the future egg cell divides again during m2. sometimes the first po-
lar body does not divide again, in which case only two polar bodies are present
at the end of m2. But if the first polar body does divide, there will be three polar
bodies at the end of m2.
16. here are three excellent topics on the scientiic approach to understanding
how life may have originated on earth: frye 2000 ( The Emergence of Life on Earth:
A Historical and Scientific Overview ); hazen 2005 ( Genesis: The Scientific Quest for Life's
Origins ); and Jastrow and rampino 2008 ( Origins of Life in the Universe ).
17. “When did eukaryotic cells (cells with nuclei and other internal organelles)
first evolve? What do we know about how they evolved from earlier life forms?”
octo ber 21, 1991. http://www.scientificamerican.com/article.cfm?id=when- did
-eukaryotic-cells&page=2 (accessed July 15, 2010).