The Structure of DNA • • • • • Inherited characteristics are determined by genes. Genes are found on chromosomes. Chromosomes are made of protein and DNA. DNA stands for deoxyribonucleic acid. DNA is the genetic material that determines inherited characteristics • DNA is made of nucleotides. • A nucleotide consists of: A sugar (ribose) A Phosphate A base • There are 4 bases: Adenine Guanine Thymine Cytosine • Each base has a different shape (like puzzle pieces) • Chargaff was a biochemist who paved the way to the discovery of the double helix structure of DNA. • Chargaff used chemistry to discover that the amounts of Adenine and Thymine were always present in the same proportion in DNA and that Guanine and Cytosine were also always present in DNA in the same proportional amounts. • In other words, there is always a 1:1 ratio of A to T and of C to G. • These are now known as “Chargaff’s Rules.” Rosalind Franklin used X-Ray diffraction to discover the “double helix” structure of DNA. Her assistant, Maurice Wilkins, showed the x-ray image to James Watson – without her knowledge or permission. She died in 1958 at age 37 of cancer – possibly due to radiation exposure from her work with X-ray diffraction. Rosalind Franklin http://www.yourgenome.org/stories/giants-ingenomics-rosalind-franklin Watson and Crick used Rosalind Franklin’s x-ray diffraction work Chargaff’s 1:1 ratio, to build a model of DNA and prove the chemical makeup and double helix structure of DNA. This discover led to an understanding of how DNA is copied and how it works in the cell. They won a Nobel Prize for their work. Neither Erwin Chargaff nor Rosalind Franklin were awarded the prize. A HELIX is a 3 dimensional spiral. A DOUBLE HELIX then is TWO spirals. • The two sides of the ladder are made of alternating sugar parts and phosphate parts. • The rungs of the ladder are the base pairs. • Adenine always pairs with Thymine and Guanine always pairs with Cytosine. • Knowing the base pairs allows the cell to REPLICATE or make copies. • Each base ALWAYS pairs with its COMPLEMENT. Thus each pair of bases are complementary. • For example, AGC will always pair with TCG. • What is the complement of AAT? • Copies of DNA are made by splitting the DNA molecule down the middle. • Then, the bases on each side are used as the pattern for the new strand. • Half of EACH DNA molecule is OLD DNA and the other half is NEW DNA. • DNA is copied every time the cell divides so that the new cell gets a complete copy of DNA. 1. Who is this and what is this a picture of? Rosalind Franklin; The picture is a n X-ray diffraction of DNA. 2. Who are these two scientists and what did they do? Watson and Crick proved the double helix shape of DNA. 3. What are the 3 components of DNA? Sugar, phosphate, nitrogen base 4. What are the 4 bases? Adenine, Guanine, Thymine and Cytosine 5. What bases always pair together? Adenine and Thymine Guanine and Cytosine
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