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Published byTamsin Jennings Modified over 8 years ago
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DNA: STRUCTURE AND REPLICATION
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DNA: The Code of Life DNA is the molecule that contains all of the hereditary material for an organism It is found ONLY in the nucleus of eukaryotic cells DNA is also a nucleic acid, which means it is composed of thousands of subunits called nucleotides Each nucleotide in DNA consists of A phosphate group Deoxyribose (5-carbon sugar) A nitrogenous base
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It’s a Double Helix! In 1953, Dr. James Watson and Dr. Francis Crick developed a model of the DNA molecule that showed DNA is a “double helix,” or a twisted ladder Discovery of the Double Helix Discovery of the Double Helix
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Composition of the Ladder The sides of the “ladder” are formed by a backbone of deoxyribose and phosphate molecules The “rungs” consist of 2 nitrogenous bases joined together in the middle by hydrogen bonds Phosphate Sugar
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Meet the Nitrogenous Bases! The nitrogenous bases that make up the “rungs” of the DNA ladder are: Adenine (A) Thymine (T) Cytosine (C) Guanine (G) These bases are the “letters” that spell out the genetic code
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Base Pairing Two bases (a pair) make up each “rung” of the DNA ladder In base pairing: Adenine always pairs with Thymine (A-T) Cytosine always pairs with Guanine (C-G)
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The Formation of DNA To make DNA, nucleotides are joined together in long, continuous strands of nucleotide building blocks Two DNA strands bind together to form the “double” helix The binding of two DNA strands to each other requires that the bases pair in a very controlled and restricted manner
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Complimentary Strands! An A on one strand means that the opposite strand must contain a T at the same position A G on one strand means that the opposite strand has a C at the same position In this way, the strands are said to be complimentary!
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Determine the Complimentary Strand! The following sequence of bases is found on one strand of DNA: ATGCCAATGGGATC What is the sequence of bases on the complementary strand? ATGCCAATGGGATC TACGGTTACCCTAG
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Practice Problem A strand of DNA has 100 bases. Thirty-five of these bases are adenine. How many bases are cytosine? Adenine and Thymine are complimentary, so if 35 adenines are present, 35 thymines must also be present. 35 + 35 = 70 bases 100 – 70 = 30 bases leftover to represent cytosine and guanine Cytosine and guanine are complimentary, so equal numbers of both bases should be present. 30/2 = 15 Cytosine Bases
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DNA Replication We have learned that during mitosis, chromosomes duplicate. This means that DNA must be able to make a copy of itself! To replicate the genetic material, the two strands of DNA are separated one base at a time and a new strand of DNA is made using the old strand
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DNA Replication DNA literally unwinds and “unzips” along the weak hydrogen bonds that hold the base pairs together Once the DNA “unzips,” free nucleotides floating in the cytoplasm pair with the “free” bases of both DNA strands Because the nitrogenous bases can only pair in one way (A T and C G), two new double strands of DNA are formed These two new strands are identical to each other and the original DNA molecule Replication AnimationSecond Animation
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