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7.1: Chromosomes

In eukaryotes, chromosomes consist of a single molecule of DNA associated with many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negatively-charged phosphates in DNA. Cchromosomes have a small number of copies of many different kinds of non-histone proteins. Most of these are transcription factors that regulate which parts of the DNA will be transcribed into RNA.

Structure

For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope. However, before a cell is ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle). As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope. These duplicated chromosomes are called dyads.

Fig. 7.1.1 Dyads

When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains 1–10 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays. While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.

The kinetochore is a complex of >80 different proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase. The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm. Staining with the trypsin-giemsa method reveals a series of alternating light and dark bands called G bands. G bands are numbered and provide "addresses" for the assignment of gene loci.

Chromosome Numbers

All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number. These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined. The gametes contain the haploid number (n) of chromosomes. In plants, the haploid stage takes up a larger part of its life cycle.

Table 7.1.1: Diploid numbers of some commonly studied organisms
Homo sapiens (human) 46
Mus musculus (house mouse) 40
Drosophila melanogaster (fruit fly) 8
Caenorhabditis elegans (microscopic roundworm) 12
Saccharomyces cerevisiae (budding yeast) 32
Arabidopsis thaliana (plant in the mustard family) 10
Xenopus laevis (South African clawed frog) 36
Canis familiaris (domestic dog) 78
Gallus gallus (chicken) 78
Zea mays (corn or maize) 20
Muntiacus reevesi (the Chinese muntjac, a deer) 23
Muntiacus muntjac (its Indian cousin) 6
Myrmecia pilosula (an ant) 2
Parascaris equorum var. univalens (parasitic roundworm) 2
Cambarus clarkii (a crayfish) 200
Equisetum arvense (field horsetail, a plant) 216

Karyotypes

The complete set of chromosomes in the cells of an organism is its karyotype. It is most often studied when the cell is at metaphase of mitosis when all the chromosomes are present as dyads. The karyotype of the human female contains 23 pairs of homologous chromosomes: 22 pairs of autosomes and an additional 1 pair of X chromosomes. In contrast, the karyotype of the human male contains the same 22 pairs of autosomes with one X chromosome and one Y chromosome. A gene on the Y chromosome designated SRY is the master switch for making a male. Both X and Y chromosomes are called the sex chromosomes.

Figure 7.1.2: Human Karyotype

Above is a human karyotype (of which sex?). It differs from a normal human karyotype in having an extra #21 dyad. As a result, this individual suffered from a developmental disorder called Down Syndrome. The inheritance of an extra chromosome, is called trisomy, in this case trisomy 21. It is an example of aneuploidy

Translocations

Karyotype analysis can also reveal translocations between chromosomes. A number of these are associated with cancers, for example

  • the Philadelphia chromosome (Ph1) formed by a translocation between chromosomes 9 and 22 and a cause of Chronic Myelogenous Leukemia (CML)
  • a translocation between chromosomes 8 and 14 that causes Burkitt's lymphoma
  • a translocation between chromosomes 18 and 14 that causes B-cell leukemia

Fluorescence in situ Hybridization (FISH)

Figure 7.1.3 provides dramatic evidence of the truth of the story of chromosomes. A piece of single-stranded DNA was prepared that was complementary to the DNA of the human gene encoding the enzyme muscle glycogen phosphorylase. A fluorescent molecule was attached to this DNA. The dyads in a human cell were treated to denature their DNA; that is, to make the DNA single-stranded. When this preparation was treated with the fluorescent DNA, the complementary sequences found and bound each other. This produced a fluorescent spot close to the centromere of each sister chromatid of two homologous dyads (of chromosome 11, upper right). This analytical procedure, which here revealed the gene locus for the muscle glycogen phosphorylase gene, is called fluorescence in situ hybridization or FISH.

Figure 7.1.3: Location of the gene for muscle glycogen phosphorylase on human chromosome 11 courtesy of David C. Ward

DNA Content

The molecule of DNA in a single human chromosome ranges in size from 50 x 106 nucleotide pairs in the smallest chromosome (stretched full-length this molecule would extend 1.7 cm) up to 250 x 106nucleotide pairs in the largest (which would extend 8.5 cm). Stretched end-to-end, the DNA in a single human diploid cell would extend over 2 meters. In the intact chromosome, however, this molecule is packed into a much more compact structure. The packing reaches its extreme during mitosis when a typical chromosome is condensed into a structure about 5 µm long (a 10,000-fold reduction in length).

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