DNA Structure and Function

The model for DNA replication suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied. The parental DNA strand is conserved, and the daughter DNA is newly synthesized. This semi-conservative method suggests that each of the two parental DNA strands acts as template for new DNA to be synthesized. After replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand.

Replication in eukaryotes occurs in the nucleus and starts at multiple origins of replication. An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between base pairs. ATP is required for this process. As the DNA opens up, replication forks are formed. A primer is required to initiate synthesis, which is then extended by DNA polymerase as it adds nucleotides one by one to the growing chain. The leading strand is synthesized continuously, whereas the lagging strand is synthesized in short stretches called Okazaki fragments. DNA remains in one continuous strand by DNA ligase linking the DNA fragments. The ends of the chromosomes pose a problem as polymerase is unable to extend them without a primer. Telomerase, an enzyme with an inbuilt RNA template, extends the ends by copying the RNA template and extending one end of the chromosome. DNA polymerase can then extend the DNA using the primer. In this way, the ends of the chromosomes are protected.

DNA polymerase can make mistakes while adding nucleotides. It edits the DNA by proofreading every newly added base. Most mistakes are corrected by proofreading. Those not corrected by proofreading may result in a mutation defined as a permanent change in the DNA sequence. Mutations can be of many types, such as substitution, deletion, insertion, and translocation. If an insertion or deletion results in the alteration of the translational reading frame (a frameshift mutation), the resultant protein is usually nonfunctional. Mutations in repair genes have been known to cause cancer. Many mutated repair genes have been implicated in certain forms of pancreatic cancer, colon cancer, and colorectal cancer. If many mutations accumulate in a somatic cell, they may lead to problems such as the uncontrolled cell division observed in cancer. Mutations can be induced or may occur spontaneously and have the potential to be helpful or harmful.

Nucleotide excision repairs thymine dimers. When exposed to UV light, thymines lying adjacent to each other can form thymine dimers. In normal cells, they are excised and replaced. Credit: Rao, A., Fletcher, S. and Tag, A. Department of Biology, Texas A&M University.

The Central Dogma states DNA codes for RNA codes for proteins. The genetic code refers to DNA (ATCG), RNA (AUCG) and the 20 amino acids. The Central Dogma describes the flow of genetic information in the cell from DNA containing genes to mRNA to proteins. Genes are used to make mRNA by transcription. Then, mRNA is used to synthesize proteins by translation at the ribosome. Almost every species on the planet uses the same genetic code.

Transcription in eukaryotes involves mRNA and occurs in the nucleus. The players in translation include the mRNA template, ribosomes, tRNAs, and various enzymatic factors. During translation, the mRNA template provides specific information in the form of codons (sets of three bases that each code for a single amino acid). As the ribosome moves along the mRNA, each mRNA codon comes into place to bind with the appropriate tRNA for polypeptide formation. Translation begins at the initiating AUG on the mRNA, specifying the amino acid methionine. The formation of peptide bonds occurs between sequential amino acids specified by the mRNA template according to the genetic code. Charged tRNAs enter the ribosomal A site, and their amino acid bonds with the amino acid at the P site. The entire mRNA is translated in three-nucleotide “steps” at the ribosome. When a stop codon is encountered, a release factor binds and dissociates the components to free the new protein. Folding of the protein occurs during and after translation.