DNA REPLICA
DNA molecules are very long. They wrap around proteins and wind tightly, forming structures called chromosomes. A human somatic (non-sex) cell has 23 pairs of chromosomes. Twenty-two pairs are autosomes, which do not differ between the sexes. The autosomes are numbered from 1 to 22, with 1 the largest. The other two chromosomes, the X and the Y, are sex chromosomes. The Y chromosome bears genes that determine maleness. In humans, a female has two X chromosomes and a male has one X and one Y. Charts called karyotypes display the chromosome pairs from largest to smallest.
A human cell has two complete sets of genetic information. The 20,325 or more protein-encoding genes are scattered among 3.2 billion DNA bases in each set of 23 chromosomes.
A human body consists of approximately 50 to 100 trillion cells. All cells except red blood cells contain the entire genome, but cells differ in appearance and activities because they use only some of their genes—and which ones they access at any given time depends upon environmental conditions both inside and outside the body.
The genome is like the Internet in that it contains a wealth of information, but only some of it need be accessed. The expression of different subsets of genes drives the differentiation, or specialization, of distinctive cell types. An adipose cell is filled with fat, but not the collagen and elastin proteins of connective tissue cells. Both cell types, however, have complete genomes. Groups of differentiated cells assemble and interact with each other and the nonliving material that they secrete to form aggregates called tissues.
The body has four basic tissue types, composed of more than 260 types of cells. Tissues intertwine and layer to form the organs of the body, which in turn connect into organ systems. The stomach shown at the center of figure 1.3, for example, is a sac made of muscle that also has a lining of epithelial tissue, nervous tissue, and a supply of blood, which is a type
A human cell has two complete sets of genetic information. The 20,325 or more protein-encoding genes are scattered among 3.2 billion DNA bases in each set of 23 chromosomes.
A human body consists of approximately 50 to 100 trillion cells. All cells except red blood cells contain the entire genome, but cells differ in appearance and activities because they use only some of their genes—and which ones they access at any given time depends upon environmental conditions both inside and outside the body.
The genome is like the Internet in that it contains a wealth of information, but only some of it need be accessed. The expression of different subsets of genes drives the differentiation, or specialization, of distinctive cell types. An adipose cell is filled with fat, but not the collagen and elastin proteins of connective tissue cells. Both cell types, however, have complete genomes. Groups of differentiated cells assemble and interact with each other and the nonliving material that they secrete to form aggregates called tissues.
The body has four basic tissue types, composed of more than 260 types of cells. Tissues intertwine and layer to form the organs of the body, which in turn connect into organ systems. The stomach shown at the center of figure 1.3, for example, is a sac made of muscle that also has a lining of epithelial tissue, nervous tissue, and a supply of blood, which is a type