Vocabulary

chromosome histone chromatid centromere chromatin sex chromosome autosome homologous chromosome karyotype diploid haploid figure 8-1

As a cell prepares to divide, its DNA coils around histones and twists into rod-shaped chromosomes.

Cell

Coil within chromosome

Cell

Coil within chromosome

Further coiling within coil

DNA and histones

DNA and histones

DNA double helix

DNA double helix

figure 8-2

Chromosomes, such as this one isolated from a dividing human cell, consist of two identical chromatids. (TEM 12,542 X)

figure 8-2

Chromosomes, such as this one isolated from a dividing human cell, consist of two identical chromatids. (TEM 12,542 X)

Figure 8-2 shows a chromosome that was isolated from a dividing cell. Notice that the chromosome consists of two identical halves. Each half of the chromosome is called a chromatid. Chromatids form as the DNA makes a copy of itself before cell division. When the cell divides, each of the two new cells will receive one chromatid from each chromosome. The two chromatids of a chromosome are attached at a point called a centromere. The centromere holds the two chromatids together until they separate during cell division. As you will learn in the next section, centromeres are especially important for the movement of chromosomes during cell division.

Between cell divisions, DNA is not so tightly coiled into chromosomes. Regions of DNA uncoil in between cell divisions so they can be read and so the information can be used to direct the activities of the cell. The less tightly coiled DNA-protein complex is called chromatin.

As you might expect, chromosomes are simpler in prokaryotes than in eukaryotes. The DNA of most prokaryotes consists of only one chromosome, which is attached to the inside of the cell membrane. Prokaryotic chromosomes consist of a circular DNA molecule. As with eukaryotic chromosomes, prokaryotic chromosomes must be very compact to fit into the cell.

table 8-1 Chromosome Numbers of Various Species

Number of Organism chromosomes

Adder's tongue fern

1,262

Carrot

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