Genomic DNA

The cellular DNA, packaged as a double-stranded DNA molecule, is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in a loop located within the nucleoid(Figure 1).

Figure 1. Prokaryotes, including bacteria and archaea, have a single, circular chromosome located in a central region called the nucleoid.

Figure 2. There are 23 pairs of homologous chromosomes in a female human somatic cell. The condensed chromosomes are viewed within the nucleus (top), removed from a cell in mitosis and spread out on a slide (right), and artificially arranged according to length (left); an arrangement like this is called a karyotype. In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes. A method of staining called “chromosome painting” employs fluorescent dyes that highlight chromosomes in different colors. (credit: National Human Genome Project/NIH)

In eukaryotes, the genome consists of several double-stranded linear DNA molecules (Figure 2). Each species of eukaryotes has a characteristic number of chromosomes in the nuclei of its cells.  Diploid describes a nucleus containing two copies of genetic material or a complete set of chromosomes.  For humans, diploid would be shown as 2n = 46.  Haploid, found in our sex cells, would have only one copy of the genetic material or a half a set of chromosomes.  So, n = 23 for humans.

Matched pairs of chromosomes in a diploid organism are called homologous (“same knowledge”) chromosomes. Homologous chromosomes are the same length and have genes in exactly the same location, or locus. Genes, the functional units of chromosomes, determine specific characteristics.  Traits are the variations of those characteristics. An example would be hair color with traits that are blonde, brown, or black.

 The variation of individuals within a species is due to the specific combination of the genes inherited from both parents. Even a slightly altered sequence within a gene can result in an alternative trait.  For example, there are three possible gene sequences on the human chromosome coding for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome for blood type, the blood type(trait) is determined by which two versions of the gene is inherited.

Minor variations of traits, such as blood type and eye color, contribute to the natural variation found within a species. The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity.  The X and Y chromosomes do have variations to accurately produce gametes, but other slight variations do exist.

Eukaryotic Chromosomal Structure and Compaction

Figure 3. Double-stranded DNA wraps around histone proteins to form nucleosomes that have the appearance of “beads on a string.” The nucleosomes are coiled into a 30-nm chromatin fiber. When a cell undergoes mitosis, the chromosomes condense even further.

DNA must be tightly packaged to fit in the cell’s nucleus. At the same time, it must also be readily accessible for gene expression.  During some stages of the cell cycle, the long strands of DNA are condensed into compact chromosomes.  Chromosomes are compacted in a number of ways to include:

(1)  short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the chromosome forming nucleosome(Figure 3);

(2)  nucleosomes coil into chromatin fiber;  further shortens the chromosome; forms chromatin fiber

(3)  proteins are used to pack the chromatin;  ensure each chromosome (in a non-dividing cell) occupies a particular area of the nucleus that does not overlap with any other chromosome (see the top image in Figure 3).

DNA replication occurs in interphase during the S phase of the cell cycle.  After replication, a chromosome is composed of two linked sister chromatids held together at a compacted region called the centromere.

Section Summary

Prokaryotes have a single looped chromosome composed of double-stranded DNA in the nucleoid.  Eukaryotes have double-stranded  linear chromosomes composed of chromatin in the nucleus.  A full set of matched chromosomes is referred to as diploid.  Haploid is having a half a set of chromosomes. Genes are segments of DNA that code for a specific protein. An organism’s traits are determined by the genes inherited from each parent. Duplicated chromosomes are composed of two sister chromatids held together by the centromere.  Chromosomes are compacted using a variety of mechanisms during the cell cycle which is necessary for chromosomal segregation during mitosis.