Structure and Function of DNA
A million bytes can record a few hundred pages of text or an image from a digital camera. 600 million bytes are required to store music on a CD. Similar to computers, living cells also store information, and they have been evolving and diversifying for over 3.5 billion years. All living cells on Earth store their hereditary information in the form of double-stranded molecules of DNA. A deoxyribonucleic acid (DNA) molecule consists of two long polynucleotide chains (DNA strands) composed of four types of nucleotide subunits. The chains run antiparallel to each other and are held together by hydrogen bonds. The nucleotides in DNA are composed of a five-carbon sugar (deoxyribose) attached to a single phosphate group and to a nitrogen-containing base, which can be either adenine (A), cytosine (C), guanine (G), or thymine (T).
The nucleotides are covalently linked to each other in a chain through their sugars and phosphates. In the three-dimensional structure of DNA—the DNA double helix—the sugar-phosphate backbones are on the outside, whereas the bases are held together via hydrogen bonds on the inside of the double helix. In each case, a bulkier two-ring base (a purine) is paired with a single-ring base (a pyrimidine): A always pairs with T, and G with C. Thus, each DNA chain contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner chain. Each chain can therefore act as a template for the synthesis of a new complementary chain, which enables a cell to replicate its genome before passing it on to its descendants. The complete sequence of nucleotides in the human genome, written out in the four-letter nucleotide alphabet, would fill more than a million pages of text. (Reference: Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Morgan, David; Raff, Martin; Roberts, Keith; Walter, Peter. Molecular Biology of the Cell. W. W. Norton & Company.)
Chromosomes and Genomes
Eukaryotic DNA is packaged into a set of chromosomes. Each chromosome consists of a single, enormously long linear DNA molecule associated with many proteins and RNA molecules required for packaging the DNA in a more compact structure, but also for the processes of gene expression, DNA replication, and DNA repair. The complex of DNA and tightly bound protein is called chromatin. In contrast to eukaryotes, bacteria lack a nuclear compartment enclosing the DNA, and they generally carry their genes on a single DNA molecule, which is often circular. With the exception of the gametes (eggs and sperm) and a few specialised cell types, each human cell nucleus contains two copies of each chromosome, one homologous chromosome inherited from the mother and one from the father. The sex chromosomes in males are the only nonhomologous chromosome pairs, where a Y chromosome comes from the father and an X chromosome from the mother. Thus, each human cell contains 22 pairs common to both males and females, plus two sex chromosomes (X and Y in males, two Xs in females), summing up to a total of 46 chromosomes.
If the double helices of all 46 chromosomes contained in a human cell would be laid out end to end, they would reach about 2 meters; yet the nucleus, containing the DNA, is only about 6 um in diameter. This is equivalent to packing 40 km of fine thread into a tennis ball. Chromosomes carry genes—the functional units of heredity. While some simple bacteria have only 500 genes, the human genome contains about 30,000. Astonishing differences in genome size between species arise, however, mostly from different amounts of DNA interspersed between genes. The human genome is 200 times larger than that of the yeast Saccharomyces cerevisiae, and 200 times smaller than that of a species of amoeba. How the genome is divided into chromosomes also varies greatly among different species. For example, while human cells have 46 chromosomes, those of the common carp contain over 100.
The genetic information of eukaryotic cells has a hybrid origin, derived from both the ancestral anaerobic archaeal and the aerobic bacteria that it adopted as symbionts. Most of the information is stored in the nucleus, but a small amount remains inside small membrane-enclosed organelles called mitochondria and, for plant and algal cells, in the chloroplasts. Mitochondria take up oxygen and harness energy from the oxidation of food molecules (such as sugars) to produce most of the ATP that powers the cell’s activities, but also generate metabolic intermediates and have important biosynthetic and signaling functions. Through photosynthesis, chloroplasts are using sunlight energy to synthesise carbohydrates from atmospheric carbon dioxide and water, and deliver the products to the host cell as food. Similarly to bacteria, mitochondria store their information in a circular DNA genome, have their own ribosomes that are distinct from the rest found in the eukaryotic cell, and their own transfer RNAs. Both mitochondria and chloroplasts have their own genomes, which are degenerate cut-down versions of the corresponding bacterial genomes. The mitochondrial genome in a human cell, for example, consists of only 16,569 nucleotide pairs, and codes for only 13 proteins, 2 ribosomal RNA components, and 22 transfer RNAs.
The human genome spans over 3.2 x 10^9 nucleotide pairs and contains about 21,000 genes coding for proteins, approximately 9000 noncoding RNA genes, and over 20,000 pseudogenes—sequences closely resembling functional genes, but containing mutations preventing proper expression or function. Only about 2% of the human genome codes for protein, while 50% consists of high-copy-number repetitive elements—mobile pieces of DNA that have gradually inserted themselves and multiplied in the chromosomes over evolutionary time. If the nucleotide pairs were drawn as vertical lines separated by a 1 mm space, the human genome would extend 3200 km, far enough to stretch across the center of Africa. (Reference: Alberts, B.; Johnson, A.; Lewis, J.; Morgan, D.; Raff, M.; Roberts, K.; Walter, P. Molecular Biology of the Cell. W. W. Norton & Company.)
Author: Sebastian Florescu, PhD
Published online: Feb 2020