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1 2 We study DNA for many reasons: its central importance to all life on Earth, medical benefits such as cures for diseases, better food crops. 3 Rosalind Franklin’s photo Watson and Crick with their famous model Our genes are on our chromosomes. Chromosomes are made up of a chemical called DNA. 7 DNA Structure DNA consists of two molecules that are arranged into a ladder-like structure called a Double Helix. A molecule of DNA is made up of millions of tiny subunits called Nucleotides. Each nucleotide consists of: 1. Phosphate group 2. Pentose sugar 3. Nitrogenous base The backbone of the molecule is alternating phosphates and deoxyribose sugar The teeth are nitrogenous bases. phosphate deoxyribose bases 9 Nucleotides Phosphate Nitrogenous Base Pentose Sugar Nucleotides The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs”. There are four types of nitrogenous bases. Nucleotides A Adenine C Cytosine T Thymine G Guanine Nucleotides Each base will only bond with one other specific base. Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Form a base pair. Form a base pair. 5 O 3 3 P 5 O O C G 1 P 5 3 2 4 4 P 5 P 2 3 1 O T A 3 O 3 5 O 5 P P DNA Structure Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand. A C T C A G T T G A G T C A Two Kinds of Bases in DNA N C C N C N C N N C C C N C C 17 N N N C C C N C C C C C N C 18 N N N C C N C N C C N C C C C C 19 Remember, DNA has two strands that fit together something like a zipper. The teeth are the nitrogenous bases but why do they stick together? 20 C N N N C C C C other because of hydrogen bonds. Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA. The bonds between cytosine and guanine are shown here with dotted lines N The bases attract each C N C C C N 21 When making hydrogen bonds, cytosine always pairs up with guanine Adenine always pairs up with thymine Adenine is bonded to thymine here N C C C C N C 22 • Adenine and Thymine always join together A T • Cytosine and Guanine always join together C G 23 24 25 Each cell has about 2 m of DNA. The average human has 75 trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only 0.000000002 m. The earth is 150 billion m or 93 million miles from the sun. 26 DNA Structure To crack the genetic code found in DNA we need to look at the sequence of bases. The bases are arranged in triplets called codons. AGG-CTC-AAG-TCC-TAG TCC-GAG-TTC-AGG-ATC DNA Structure A gene is a section of DNA that codes for a protein. Each unique gene has a unique sequence of bases. This unique sequence of bases will code for the production of a unique protein. It is these proteins and combination of proteins that give us a unique phenotype. Advantages of Double-Stranded Nature of DNA Forms a stable structure Hydrophobic (water repelling) bases stack on top of one another, away from solvent Negatively charged phosphate backbone is on the outside accessible to solvent Each strand of DNA molecule serves as the template: For a new strand of DNA (replication) For an RNA molecule (transcription) Nucleic Acids DNA and RNA are nucleic acids Comprised of: A 5-carbon sugar (deoxyribose or ribose) Nucleotides DNA has two strands bonded together RNA has one strand ONLY C, H, N, O, P atoms Nucleotide Structure Three different components Phosphate group Nitrogen containing base (A,T,G,C,U) Five-carbon sugar (ribose or deoxyribose) DNA Gene Protein Trait 33