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Lecture 11

Lecture 11-DNA Structure

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Biology 1202B
Brenda Murphy

Lecture 11: DNA Structure and Organization Human Genome 1952 – Hershey & Chase – discovered DNA inherited material 1953 – Watson, Crick & Franklin – discovered the DNA structure double helix 1956 – Tijo & Levan – found there were 46 linear chromosomes in the nucleus 1963 – Margit & Sylvan Nass – discovered circular chromosomes in mitochondria DNA Structure  Watson and Crick won a Nobel peace prize for discovering the double helix structure of DNA  Rosalind Franklin played an indirect role in this discovery because she actually discovered the helix by x-ray diffraction. However, she died at a young age from exposure to the substances she was working with. Her reports made their way to Watson and Crick. Nucleotides  Nucleotides are the building blocks of nucleic acids  A nucleic acid is composed of a phosphate group, a pentose (5) sugar, and a nitrogenous base  These three basic components stay constant in nucleic acids but can differ in many other ways Differences between RNA and DNA 1. Ribose Sugar – DNA lacks an OH 2. Thymine (T) and Uracil (U) 3. Strand – DNA = double stranded - RNA = single stranded DNA  Phosphate group  Deoxyribose Sugar  4 nitrogenous bases: o Guanine (G) o Adenine (A) o Thymine (T) o Cytosine (C) RNA  A phosphate group  Ribose Sugar  4 nitrogenous bases o Guanine (G) o Adenine (A) o Uracil (U) o Cytosine (C) DNA & RNA are both made of dNTP – (deoxynucleoside triphosphate)  Ie, dATP, dCTP, dGTP, dTTP, dUPT (in RNA instead of dTTP) Deoxynucleoside Triphosphates (dNTP) What are they? Nucleotide = Nucleoside + phosphate - A nucleoside triphosphate is a nitrogenous base linked to a sugar (deoxyribose), which is linked to a chain of three phosphate groups  Deoxyadenosie = dATP  Deoxyguanosine = dGTP  Deoxythymidine = dTTP  Deoxycytidine = dCTP  Deoxyuridine = dUTP - 4 nucleotide subunits of DNA, linked into a polynucleotide chains Purines: double ring structures - Adenine and guanine Pyrimidines: single ring structures - Thymine and cytosine - sugar phosphate backbone - Phosphate bridges 3’ carbon to 5’ carbon from two different sugars - Two sugars are bound between the 3’ and 5’ ends, this is called a phosphodiesther bond The Double Helix  The two strands of DNA run antiparallel to each other. This means that the one side starts at the 5’ end and goes to the 3’ end, whereas the other side starts at the 3’ end and goes to the 5’ end.  Chargaff’s Rule: The amount of purines (adenine and guanine) = the amount of pyrimidine’s. (Cytosine and thymine) Complimentary base pairing  Guanine is always bound to cytosine  Adenine is always bound to thymine  The two polynucleotide chains twist around each other in a right handed way o Start at lower left to upper right = right handed helix o Start at lower right to upper left = left handed helix Watson and Crick’s model for DNA replication  Complimentary base pairing in the DNA double helix  A pairs with T, G pairs with C  The two chains unwind and separate  Each “old strand” is a template for the addition of complementary bases  The base pairs are stabilized by hydrogen bonds: o A = T have 2 hydrogen bonds joining them together
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