Allison Dame

DNA Think quest

In complete sentences, briefly summarize the process of DNA replication. Make sure you include the following terms: polymerase, primase, RNA primer, replication fork, 5’ – 3’, helicase, ligase, leading strand, lagging strand, complementary strand.

DNA replication begins when helicase unwinds a segment of the DNA and breaks the hydrogen bonds between the two complementary strands of DNA. DNA polymerase can only add new nucleotides to a free 3’ end of a growing chain. Synthesis of one strand of the DNA, called the leading strand, proceeds continuously in the 5’ to 3’ direction. Synthesis of the complementary strand, called the lagging strand, is more complex. DNA polymerase can add new deoxyribonucleotides only to a free 3’ OH. To provide a free 3’ OH starting point on the lagging strand, RNA primase attaches to the DNA and synthesizes a short RNA primer. DNA polymerase III then adds deoxyribonucleotides to the 3’ end of the RNA primer. DNA polymerase I replace DNA polymerase III, removes the RNA and replaces it with DNA. Finally, the enzyme DNA ligase forms a phosphodiester bond between the 3’ OH of the growing strand and the 5’ phosphate in front of it. During DNA replication, the leading strand is synthesized continuously white the lagging strand is synthesized discontinuously.

 

Briefly describe the structure of a chromosome in complete sentences.  Include the major building blocks and how they are put together.

Chromosomes are made of 50% proteins (structure, protection, turn genes on and off, and 50% DNA (deoxy-ribonucleic acid). Chromosomes are made of many chromatin threads, each containing DNA and proteins. The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones. Without such packaging, DNA molecules would be too long to fit inside cells. For an organism to grow and function properly, cells must constantly divide to produce new cells to replace old, worn-out cells. During cell division, it is essential that DNA remains intact and evenly distributed among cells. Chromosomes are a key part of the process that ensures DNA is accurately copied and distributed in the vast majority of cell divisions.

Briefly outline the contributions of the following people in complete sentences: Wilkins, Watson, Crick, Franklin.

Wilkins: Wilkins studied biological molecules like DNA and viruses using a variety of microscopes and spectrophotometers. He eventually began using x-rays to produce diffraction images of DNA molecules. The x-ray diffraction images produced by him, Rosalind Franklin, and Raymond Gosling led to the deduction by James Watson and Francis Crick of the 3-dimensional helical nature of DNA

 

Watson: James Watson was a research fellow at Cambridge University. After attending one of Franklin’s lectures, Watson and Crick made a failed model based on the information presented at the lecture. Watson and Crick suggested that the molecule was made of two chains of nucleotides, one going up and the other going down. They added to the model findings about matching base pairs after Crick learned of Chargaff’s findings. Watson and Crick showed that each strand of the DNA molecule was a template for the other. They found that during cell division the two strands separate and on each strand a new “other half” is built, just like the one before. This way DNA can reproduce itself without changing its structure – except for occasional errors or mutations.

 

Crick: Francis Crick was a graduate student at Cambridge University. Crick began working at a Cambridge University laboratory, learning biology, organic chemistry, and x-ray diffraction technology. Watson joined the lab shortly after Crick began working at the laboratory. They were convinced that if the three-dimensional structure of a molecule known to play a role in passing genetic information- DNA- could be determined, then the way genes are passed might also be revealed. Crick and Watson shared the Nobel Prize in physiology/medicine in 1962.

 

Franklin: Franklin began working in x-ray defraction right after getting her PhD. She then started studying living cells at the King’s College in London. She made many advance in x-ray defraction with DNA and extracted finer DNA fibers than ever before and arranged them in parallel bundles. She found that DNA could exist in two forms depending on humidity in the surrounding air. This helped her deduce that the phosphate part of the molecule was on the outside. All of these discoveries were crucial keys to discovering DNA’s structure. Because she left King’s College, she turned her work to viruses and laid foundation for structural virology.

Describe two controversies surrounding the race to discover the structure of DNA. What is your opinion about each?  Would the same controversies exit today?  Why or why not?

Two controversies surrounding the race to discover the structure of DNA