DNA structure and DNA replication Flashcards
Describe the key features of the nucleotide structure.
A nucleotide is made of a sugar (deoxyribose or ribose), a phosphate group, and a nitrogenous base (purine or pyrimidine)1.
Describe the key feature of the DNA structure.
DNA is a double helix, composed of two antiparallel strands of nucleotides joined by phosphodiester bonds2. The strands are held together by complementary base pairing between A and T, and G and C3.
Describe the key feature of DNA that allowed Watson and Crick to propose the
double helix model
Watson and Crick used the X-ray diffraction data from Rosalind Franklin to deduce that DNA had a helical structure with a constant diameter of 20 nm and a periodicity of 34 nm. They also used the Chargaff’s rule to infer that A and T, and G and C were paired in a specific way.
Describe the key steps of DNA replication.
DNA replication starts at the origin of replication, where the DNA helicase unwinds the double helix and forms a replication bubble2. The DNA primase synthesizes a short RNA primer on each strand, which serves as a starting point for the DNA polymerase4. The DNA polymerase adds new nucleotides to the 3’ end of the primer, following the base pairing rules. The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments. The RNA primers are removed by another DNA polymerase and replaced by DNA5. The DNA ligase joins the Okazaki fragments and seals the nicks in the sugar-phosphate backbone.
Describe what are the key component of the DNA replication
DNA template
DNA polymerase
RNA primer
deoxyribonucleotides
Mg2+ ions
Other accessory factors include the DNA helicase, the single-strand binding proteins, the DNA primase, and the DNA ligase.
What does it mean that the replication is semiconservative?
Semiconservative replication means that each daughter DNA molecule contains one parental strand and one newly synthesized strand6. This preserves the genetic information and ensures its accurate transmission.
What is the end replication problem?
The end replication problem refers to the inability of the DNA polymerase to complete the synthesis of the lagging strand at the end of the linear chromosome, due to the removal of the last RNA primer7. This results in the shortening of the telomeres, the repetitive sequences at the chromosome ends, with each cell division.
How is the end-replication problem solved?
The end-replication problem is solved by the enzyme telomerase, which uses an RNA template to add new telomeric repeats to the 3’ end of the G-rich strand. This extends the telomere and allows the DNA polymerase to fill in the gap on the C-rich strand.
When is telomerase activated?
Telomerase is activated in cells that need to maintain their telomere length, such as germ cells, stem cells, and cancer cells. In most somatic cells, telomerase is not expressed or is repressed, leading to telomere shortening and cellular senescence8.
Describe possible syndromes affecting telomerase activity
Defects in telomere-related genes can cause a group of rare disorders called telomeropathies, which affect highly dividing tissues and cause premature aging9. Some examples are dyskeratosis congenita, Hoyeraal Hreidarsson syndrome, Revesz syndrome, Coat Plus syndrome, aplastic anemia, and pulmonary fibrosis.
Describe the Meselson-Stahl’s experiment and what it demonstrated
Meselson and Stahl used isotopes of nitrogen to label the DNA of E. coli and followed its replication in a density gradient. They showed that after one round of replication, the DNA had an intermediate density, and after two rounds, the DNA had both intermediate and light density. This supported the semiconservative model of replication over the conservative and dispersive models.
Describe Avery experiment on DNA transformation
Avery and his colleagues used different enzymes to treat the extract of S. pneumoniae, a bacterium that can transform from a non-virulent to a virulent form. They showed that only the treatment with DNase, which degrades DNA, prevented the transformation. This suggested that DNA was the transforming principle and the carrier of genetic information
Describe the Chase and Harshey experiment on genomic structure of phage.
Chase and Hershey used radioactive isotopes of sulfur and phosphorus to label the proteins and DNA of T2 phage, a virus that infects bacteria. They showed that after the infection, only the DNA, not the proteins, entered the bacterial cells and directed the production of new viruses. This confirmed that DNA was the genetic material of the virus10.
