When one cell divides into two daughter cells, the DNA, all 46 chromosomes, for example, in humans, must be replicated. The specificity of base pairing between A/T and C/G is essential for the synthesis of new DNA strands that are identical to the parental DNA. Each strand of DNA serves as a template for DNA synthesis. Synthesis occurs by adding bases that exactly mirror the template strand. So, as each strand is copied, two sets of DNA are made that are identical to the original two strands. The order of nucleotide bases along a DNA strand is known as the sequence.
If a problem occurs during DNA replication, this can lead to a disruption of gene function. For example, if the wrong base is inserted during replication (a
mutation ) and this mistake happens to be in the middle of an important gene, it could result in a non-functional protein. Fortunately, we have evolved various mechanisms to ensure that such mutations are detected, repaired, and not propagated. However, these mechanisms sometimes fail, and uncorrected mutations will occur. If the resulting alteration in gene function, through its interplay with the environment, sufficiently disrupts metabolism or structure, clinical disease can result.
Some viruses store genetic information in rna
DNA was believed to be the sole medium for genetic information storage. Furthermore, Watson and Crick's central dogma assumed that information flowed "one-way" from DNA to RNA to protein. So it came as a surprise in 1971 when it was discovered that some viruses’ genetic information is RNA.
Even so, these viruses ultimately make proteins in the same way as higher organisms. During infection, the RNA code is first transcribed "back" to DNA - then to RNA to protein, according to the accepted scheme. The initial conversion of RNA to DNA - going in reverse of the central dogma - is called reverse transcription, and viruses that use this mechanism are classified as retroviruses. A specialized polymerase, reverse transcriptase, uses the RNA as a template to synthesize a complementary and double stranded DNA molecule as shown in the picture.
Lecture 4. genes can replicate themselves
As genes are made of DNA, they can make themselves when DNA is replicated. The specificity of base pairing between A/T and C/G helps explain how DNA is replicated prior to cell division. Enzymes unzip the DNA by breaking the hydrogen bonds between the base pairs. The unpaired bases are now free to bind with other nucleotides with the appropriate complementary bases. The enzyme primase begins the process by synthesizing short primers of RNA nucleotides complementary to the unpaired DNA. DNA polymerase now attaches DNA nucleotides to one end of the growing complementary strand of nucleotides. Replication proceeds continuously along one strand, called the leading strand, which is shown here on the right. The process occurs in separate short segments called Okazaki fragments next to the other, or lagging, strand on the left. This difference is due to the fact that DNA polymerase can only add new nucleotides to the 3 prime end of a nucleotide strand in a 5’ 3’ direction. A primer begins any new strand, including each Okazaki fragment. An enzyme replaces the RNA primer with DNA nucleotides. Then an enzyme called DNA ligase binds the fragments to one another.
