Chloroplast and mitochondria are membrane bound organelles found in eukaryotic cells’ cytoplasm. Chloroplasts are present in all multicellular plants and some algae species while mitochondria are found in most eukaryotic cells. Their main similarity is that both of these organelles generate the universal cell energy carrier known as ATP (Adenosine Triphosphate).
- Chloroplast DNA
Geneticists have long established that many chloroplast associated traits display cytoplasmic inheritance. This suggests that nuclear genes are not responsible for encoding of these traits. In 1963, it was discovered that chloroplasts have their own deoxyribonucleic acid (DNA) known as chloroplast DNA (cpDNA). In various plants, the chloroplast genome size ranges from 120,000-160,000 base pairs. The cpDNA is found on a single, double stranded, circular DNA molecule lacking related histone proteins. Each chloroplast has multiple copies of the chloroplast genome, and since each cell has multiple organelles, then several hundred to thousands of cpDNA copies in a normal plant cell exist.
Gene organization and structure of cpDNA:
The sequencing of various plant and algae species chloroplast genomes has determined that the basic organization of cpDNA is eubacterial: the order of some gene groups is similar to that observed in Escherichia Coli and many chloroplast genes are organized into operon-like clusters.
In vascular plants, the chloroplast chromosomes are organized similarly in gene order and gene content. Typically, a chloroplast genome encodes 4 rRNA (ribosomal RNA) genes, 30-35 tRNA (transfer RNA) genes, a few ribosomal proteins, many proteins involved in photosynthetic processes and several others involved in non-photosynthetic processes. One of the key proteins that cpDNA encodes is ribulose-1, 5-bisphosphate carboxylase-oxygenase (abbreviated as RuBisCO). This protein participates in carbon fixation during photosynthesis and makes up to 50% of all protein found in green plants. It is thus considered the most abundant protein in the world. The protein is complex and consists of eight identical small sub-units and eight identical large sub-units. Each small sub-unit is encoded by nuclear DNA while each large sub-unit is encoded by cpDNA. The chloroplast genome is circular with genes on both strands. Some of these genes have been identified on a start-stop basis on the codon in the same reading frame, but still no proteins have been isolated for the genes. These gene sequences are known as open reading frames.
A salient feature in most chloroplast genomes is the presence of a large inverted repeat. In rice, the repeat includes the genes for 23S rRNA, 4.5S rRNA, 5S rRNA, and several other genes for proteins and tTRNAs. In some plants, the repeats account for the majority of the genome and are entirely absent in others. Lastly, most cpDNA sequences show similarity to those in equivalent eubacterial genes.
- Mitochondrial DNA:
In all animals and most fungi, the mitochondrial genome comprises a single, highly coiled circular DNA molecule. Mitochondrial genomes in plants are often a complex collection of multiple circular molecules of DNA. Each mitochondrion has multiple copies of this genome, and a cell usually has many mitochondria. Like most eubacterial chromosomes, mitochondrial DNA (mtDNA) lacks histone proteins normally associated with nuclear DNA in eukaryotic cells. The GC (Guanine-Cytosine) content of mtDNA significantly differs from that of nuclear DNA and for this reason, mtDNA can be separated from nuclear DNA using density gradient centrifuging.
Gene organization and structure of mtDNA:
The nucleotide sequences for the mitochondrial genomes of various organisms such as plants, animals and fungi have been established. The genes for most enzymes and structural proteins found in mitochondria are actually nuclear DNA encoded, translated onto cytoplasmic ribosomes and transported into the mitochondria. The mtDNA typically encodes a few tRNA and rRNA molecules that are needed for mitochondrial protein synthesis.
Mitochondrial DNA is essentially double stranded with the exception of the displacement loop (D-Loop) which is the non-coding segment of the mitochondrial genome that maintains elements for initiating replication and transcription but does not code for gene products. The D-Loop contains an extra 7S DNA and is triple stranded.
The mtDNA has 16,569 nucleotide pairs. The heavy (H) strand is rich in Guanine and encodes for 28 genes while the light (L) strand is rich in Cytosine and encodes for 9 genes: ND6 and 8 tRNA genes. In total mtDNA encodes for 37 genes, 13 of these genes encode mitochondrial peptide units, 2 are rRNA’s and 22 are tRNA’s located between every 2 rRNA. However, despite this general overview of mtDNA, the organization of mitochondrial genes and their expression across various organisms is extremely diverse.
Works Cited:
- Chapter 20 Organelle DNA . N.p.. Web. 28 Mar 2013. http://w3.ualg.pt/~rcastil/GPE/Main_files/0_OrganelleDNA_book_chapter.pdf
- Bock, Ralph, and Volker Knoop. Genomics of Chloroplasts and Mitochondria. Dordrecht: Springer, 2012. Internet resource.
- "Genetics - Genetics of Mitochondria and Chloroplasts." Rapid Learning Center - Science and Math Visually in 24 Hours. N.p., n.d. Web. 28 Mar. 2013.
- "Mitochondrial DNA." National Forensic Science Technology Center . N.p., n.d. Web. 28 Mar. 2013.
- "Mitochondrial DNA - Genetics Home Reference." Genetics Home Reference - Your guide to understanding genetic conditions. N.p., n.d. Web. 28 Mar. 2013.
- Proteins, one of these, and mitochondrial transcription factor (mtTFA). "Mitochondrial Disorders." Neuromuscular Home Page. N.p., n.d. Web. 28 Mar. 2013.
