Prokaryote means "before the nucleus;" it includes bacteria and blue-green algae. A Prokaryotic Deoxyribonucleic acid is one large circular deoxyribonucleic acid chromosome located in the cytoplasm-since there is no nuclear membrane to hold on to. Prokaryotes have 1 large chromosome which replicates and divides at the time of cell division The Plasmids are small circular Deoxyribonucleic acid molecules also found in the cytoplasm of some bacteria code for extra chromosomal genes. Replication and inheritance of these genes is independent from that of the chromosome. More than on than type of plasmid may be present in a bacterium and a single bacterium can have hundreds of plasmids. Plasmid genes have often been found to carry genes encoding for antibiotic resistance. Under certain circumstances, the plasmid of a Deoxyribonucleic acid is incorporated into the large circular chromosome and its deoxyribonucleic acid is then replicated and inherited with it. A mitochondrial Deoxyribonucleic acid is a circular deoxyribonucleic acid chromosome similar to the large bacterial chromosome.
Eukaryote means "true nucleus." Eukaryotes are highly organized organisms; plants, animals and single-celled organisms, except bacteria and blue-green algae. Their genetic material is contained within an intracellular nuclear membrane. Eukaryotes have much more Deoxyribonucleic acid, which is divided into a number of chromosomes. Every organism has a specific number of chromosomes. There are 46 chromosomes in humans. Each chromosome is replicated and then evenly divided during cell division assuring that each cell gets the right number of chromosomes. Humans are diploid. This means that there are 2 of each of the 23 types of chromosomes. One inherited from the father and one from the mother. This leads us to another genetic event which is the Packing of the Deoxyribonucleic acid.
The Deoxyribonucleic acid is organized into clumps called nucleosomes by complexing with histones, giving it the appearance of "thread beads." It then wraps around the histone 2 times. A nucleosome is a DNA-wrapped histone core. Histone core is formed from 8 histone molecules around 140 base pairs. Further packing of the Deoxyribonucleic acid due to hydrophobic interactions and in association with other non-histone proteins will pack it into a chromatin. A Hetechromatin is very densely packed and inactive chromatin. A Euchromatin is an active chromatin.
After the packing of Deoxyribonucleic acids, Replication of DNA follows. The Replication is by semiconservative replication. After replication, each of the new double helixes is made of one original strand and one newly synthesized strand. Double stranded DNA unwinds and 2 single strands are exposed forming a "replication fork" which moves along the chain as DNA is replicated. This occurs in both directions with 2 replication forks in an anti-parallel, 5' to 3' direction. These processes are facilitated by the following enzymes:
• DNA Helicase binds to single stranded DNA at the replication fork and opens the double stranded DNA similar to pushing a zipper apart. This unwinds the DNA, and it requires Adenosine Triphosphate to open the DNA.
• Helix-destabilizing proteins then bind to and stabilize single strands of DNA. The helix-destabilizing proteins are also called single stranded DNA-binding proteins. As the replication fork proceeds, it causes twisting which is relieved by topoisomerases.
• Type I Topoisomerase or swivelase binds to and cleaves one of the single strands of DNA. This is called nuclease activity. This allows the DNA to untwist around the axis of the phosphodiester bond of the intact DNA strand and then reconnects the single strand. This is called a ligase activity.
• Type IITopoisomerase or gyrase binds to and cleaves both single strands at the same time and results in a negative twisting which relaxes both strands and then reseals the strands. Quinolones inhibit DNA gyrase.
• Ligases connect two strands of DNA end to end. This enzyme seals the DNA strands. The process requires Adenoaine Triphosphate or Nitric Acid Dehydrogenase.
The next step involves a Primase. The Primase is an RNA polymerase which uses triphosphate ribonucleotides to form a short strand of Ribonucleic Acid complementary to DNA near the replication fork which serves as the double stranded primer necessary for DNA polymerase III. Later, DNA polymerase I, an exonuclease, removes the RNA primer and replaces it with DNA. A Deoxyribonucleic Acid polymerase III reads each of the old strands in the 3' to 5' direction and uses the appropriate triphosphate deoxyribonucleotides to synthesize new complementary strands in the 5' to 3' direction. The energy to drive these reactions is provided by the high energy triphosphate.
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