The Chromosomal DNA
The DNA molecule may be circular or linear and can be composed of 100,000–10,000,000,000 nucleotides in a long chain. Typically, eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. In addition, cells may contain more than one type of chromosome; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.
The Autosomal DNA
Most of the DNAs are autosomal/chromosomal DNAs. Half of the autosomal/chromosomal DNAs are from each of the parents. This is the DNA that can uniquely identify a specific individual. The autosomal DNA contains almost all of our health/medical information. The autosomal DNA is used in maternity/paternity tests and for forensic/crime purposes.
Y-DNA
Only males have the Y chromosome, Y-DNA, which rarely changes (mutates slowly) and is passed down to sons from the father’s direct paternal/male line.
Mitochondrial DNA (mtDNA)
Mitochondrial DNA (mtDNA) is the DNA located in mitochondria and is circular in shape. In mammals, each double-stranded circular mtDNA molecule consists of 15,000–17,000 base pairs. The nuclear and mitochondrial DNAs are believed to be of separate evolutionary origin. The mtDNAs are thought to be derived from the circular genomes of the bacteria that were engulfed by the early ancestors of today’s eukaryotic cells. Each mitochondrion is estimated to contain 2–10 mtDNA copies. About 100–10,000 separate copies of mtDNA are usually present per cell (egg and sperm cells are exceptions) in humans (and probably in metazoans in general).
In most multi-cellular organisms, the mtDNA is inherited from the mother (maternally inherited). Both males and females have the mtDNA.
The two strands of the mtDNA are differentiated by their nucleotide content. The guanine-rich strand referred to as the heavy strand and the cytosine-rich strand is referred to as the light strand. The heavy strand encodes 28 genes and the light strand encodes nine genes for a total of 37 genes. The 37 genes of the mtDNA encode various molecules. In total, 13 genes encode for proteins (polypeptides), 22 genes are for tRNA and two genes are for the small and large subunits of rRNA. The mtDNA is replicated by the DNA polymerase gamma complex.
Chloroplast DNA
Chloroplast genomes are relatively large, usually ∽140 kb in higher plants and <200 kb in lower eukaryotes. This is comparable to the size of a large bacteriophage, e.g., T4 of ∽65 kb. There are multiple copies of the genome per organelle, typically 20–40 in a higher plant.
The chloroplast genome codes for all the rRNA and tRNA species needed for protein synthesis. The ribosomes include two small rRNAs in addition to the major species. The tRNA set resembles that of mitochondria. The chloroplast genome codes for ∽50 proteins, including RNA polymerase and some ribosomal proteins. The chloroplast genome of the higher plants varies in length (Table 1.6).
Table 1.6 Chloroplast DNA: The chloroplast genes codes for 4 rRNAs, 30 tRNAs and ∽40 proteins
| Genes (RNA coding) | Genes (Thylakoid membrane) |
|---|---|
| 16S rRNA | PS-I |
| 23S rRNA | PS-II |
| 4.5S rRNA | Cyt b/f |
| 5S rRNA | H+-ATPase |
| tRNA | Others |
| Gene Expression | NADH dehydrogenase |
| R Proteins | Ferridoxin |
| RNA polymerase | Ribulose BisPhosphate cyclooxygenase |
All these gene products are used within the chloroplast, but all the chloroplast structures also depend on proteins encoded by nuclear genes, translated in the cytosol, and imported into the chloroplast.
Plasmid DNA
A ‘plasmid’ is a small DNA molecule that is separate from and can replicate independently of the chromosomal DNA. They are double-stranded and, in many cases, circular. Plasmids usually occur naturally in bacteria; however, sometimes, they are found in eukaryotes (e.g., the 2-µm-ring in Saccharomyces cerevisiae). Plasmid sizes vary from 1 to over 1,000 kb pairs. Plasmids are capable of autonomous replication within a suitable host.