Replication: DNA copied into DNA

Created by George Rice, Montana State University

Figure depicting DNA replication.
From DNA Replication at ThinkQuest.
In general, DNA is replicated by uncoiling of the helix, strand separation by breaking of the hydrogen bonds between the complementary strands, and synthesis of two new strands by complementary base pairing. Replication begins at a specific site in the DNA called the origin of replication. DNA replication is bidirectional from the origin of replication. To begin DNA replication, unwinding enzymes called DNA helicases cause the two parent DNA strands to unwind and separate from one another at the origin of replication to form two "Y"-shaped replication forks. These replication forks are the actual site of DNA copying. Helix destabilizing proteins bind to the single-stranded regions so the two strands do not rejoin. Enzymes called topoisimerases produce breaks in the DNA and then rejoin them in order to relieve the stress in the helical molecule during replication. As the strands continue to unwind and separate in both directions around the entire DNA molecule, the hydrogen bonding of free DNA nucleotides with those on each parent strand produces new complementary strands. As the new nucleotides line up opposite each parent strand by hydrogen bonding, enzymes called DNA polymerases join the nucleotides by way of phosphodiester bonds. Actually, the nucleotides lining up by complementary base pairing are deoxynucleoside triphosphates, composed of a nitrogenous base, deoxyribose, and three phosphates. As the phosphodiester bond forms between the 5' phosphate group of the new nucleotide and the 3' OH of the last nucleotide in the DNA strand, two of the phosphates are removed providing energy for bonding. In the end, each parent strand serves as a template to synthesize a complementary copy of itself, resulting in the formation of two identical DNA molecules.

Transcription: DNA copied into RNA

Ribbon diagram of transcription factors on a DNA strand.
See RNA synthesis to learn more about transcription.

Transcription is the process through which a DNA sequence is enzymaticaly copied by an RNA polymerase to produce a complementary RNA. Or, in other words, the transfer of genetic information from DNA into RNA. In the case of protein-encoding DNA, transcription is the beginning of the process that ultimately leads to the translation of the genetic code (via the mRNA intermediate) into a functional peptide or protein. Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for DNA; therefore, transcription has a lower copying fidelity than DNA replication. Like DNA replication, transcription proceeds in the 5' to 3' direction (ie the old polymer is read in the 3' to 5' direction and the new, complementary fragments are generated in the 5' to 3' direction). Transcription is divided into 3 stages: initiation, elongation and termination.

Types of RNA:

  • mRNA - messenger RNA is a copy of a gene. It acts as a photocopy of a gene by having a sequence complementary to one strand of the DNA and identical to the other strand. The mRNA acts as a busboy to carry the information stored in the DNA in the nucleus to the cytoplasm where the ribosomes can make it into protein.
  • tRNA - transfer RNA is a small RNA that has a very specific secondary and tertiary structure such that it can bind an amino acid at one end, and mRNA at the other end. It acts as an adaptor to carry the amino acid elements of a protein to the appropriate place as coded for by the mRNA.
  • rRNA - ribosomal RNA is one of the structural components of the ribosome. It's sequence is the compliment of regions in the mRNA so that the ribosome can match with and bind to an mRNA it will make a protein from.