Biology Reference
In-Depth Information
FIGURE 2.1. Individual genes lie next to one another (and in some cases, even
overlap) on chromosomes. The ability of DNA to be compacted in the nucleus of
each cell is a remarkable feat. DNA molecules wrap themselves around histone pro-
teins to form chromatin, then further condensed to form the chromosomes. A nucle-
osome is composed of histones and 2 1/2 turns of DNA. When DNA is transcribed
to RNA, it unwinds from the histone only in the particular site where transcription
is occurring. The RNA is then translated into a functional protein. The gene that
encodes the gap junction protein connexin 26 lies on human chromosome 13. Thirty
to fifty percent of NSHL is due to mutations in the connexin 26 (
GJB2
) gene.
and human are identical in size at 3,000 Mb. There are exceptions to this
rule, however; the onion genome size is 15,000 Mb.
Human cells have 23 sets of chromosomes. In the haploid human cell, 22
chromosomes are autosomal, or non-sex chromosomes, while the remain-
ing one is an X or Y sex chromosome. The fusion of two haploid cells to
form a diploid cell during fertilization enables one homologue from each
pair of chromosomes to join to form paired chromosomes containing nearly
identical DNA. The basic material of chromosomes is termed chromatin,
and it is composed of DNA and its associated histone proteins.
The histone proteins serve to compact the DNA so that it may fit into a
nucleus. An assemblage of eight histone proteins is encircled by a stretch
