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3.3 and 3.4 DNA Structure and Replication

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3.3 Assessment Statements 3.3.1 Outline DNA nucleotide structure 3.3.2 State the names of the 4 bases in DNA 3.3.3 Outline how DNA nucleotides are linked together 3.3.4 Explain how a DNA double helix is formed using complementary base pairing 3.3.5 Draw and label a simple diagram of the molecular structure of DNA

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A little bit of DNA History

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Francis Crick and James Watson

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The Discovery of DNA (& other stuff)

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DNA and RNA Structure In 1868, Meischer first isolated deoxyribonucleic acid from cells in pus and from fish sperm. No one knew its function.

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In 1928, Griffith was working with smooth (S,virulent) and rough (R, non-virulent) strains of a pneumonia-causing bacterium.

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In 1944, Avery showed that the substance was DNA.

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Hershey and Chase confirmed that DNA is the hereditary material (not protein).

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3.3.1 Outline DNA nucleotide structure

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3.3.1 DNA Nucleotide Structure

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DNA is composed of four kinds of nucleotides. Each consists of a five carbon sugar (deoxyribose) a phosphate group (PO4-) one of four bases adenine (A) thymine (T) guanine (G) cytosine(C)

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3.4.2 Name the 4 bases in DNA The nucleotides are similar, but T and C are single ringed pyrimidines A and G are double ringed purines.

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Edwin Chargaff, in 1949, noted two critical bits of data. 1)The four kinds of nucleotide bases making up a DNA molecule differ in relative amounts from species to species

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2) The amount of A = T, and the amount of C = G.

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3.3.4 Explain how a DNA double helix is formed using complementary base pairing Complementary base pairing

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Rosalind Franklin used x-ray diffraction techniques to produce images of DNA molecules.

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She concluded: DNA exists as a long, thin molecule of uniform diameter The structure is highly repetitive DNA is helical

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Franklins Story

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Watson and Crick used numerous sources of data to build models of DNA.

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The following features were incorporated into their model: The bases were hydrogen bonded (a weak bond) together in the center of the helix T (a pyrimidine) had two hydrogen bonds with A (a purine) C (a pyrimidine) had three hydrogen bonds with G (a purine)

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The nucleotides are joined together using phosphodiester bonds

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Each sugar of the backbone is covalently bonded to the nitrogenous base off of carbon 1 of the ribose sugar. covalent bond carbon 1

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There are 10 base pairs per turn of the helix.

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The two sides are anti parallel, meaning that the sugar and phosphates are running in opposite directions. Each side ends in a phosphate (5 end) and a sugar (3 end) Since the sides are anti parallel, one side goes in the 3' to 5' direction, and the other goes in the 5' to 3' direction.

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3.3.5 Draw and label a simple diagram of the molecular structure of DNA

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3.4 Assessment Statements 3.4.1 Explain DNA replication in terms of unwinding of the double helix and separation of strands by helicase followed by formation of new complementary strands by DNA polymerase 3.4.2 Explain the significance of complementary base pairing in conservation of the base sequence of DNA 3.4.3 State that DNA replication is semiconservative.

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3.4.1 Explain DNA replication Helicase Breaks hydrogen bonds between bases, unzips and unwinds the double helix. Is an enzyme (a protein that speeds up chemical reactions) Is made during G1

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DNA replication in real time! Overview of DNA replication

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DNA polymerase adds DNA nucleotides in a 5 to 3 direction. The enzyme is very specific and can only add nucleotides to the 3 end. Helicase enzyme unzips the double helix DNA polymerase adds nucleotides using the original strand as a template replication fork 3 5 5 3 3 5 Free nucleotides from cytoplasm are added using complementary base pairing

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3.4.3 State that DNA replication is semiconservative. The mechanism of semi-conservative replication produces two descendent double helices that each contain one of the original polynucleotide chains

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Meselson and Stahl carried out an experiment which showed the semi conservative nature of DNA replication.

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The DNA molecule is copied precisely from one cell generation to the next. In a unicellular organism this means that the total genome is successfully copied into each new generation. In a multicellular organism all cells contain an exact copy of the total genome (even though not all genes are expressed in every cell). Genes are faithfully passed from one generation to the next. 3.4.2 Explain the significance of complementary base pairing

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1.Which molecules form the nucleotide marked in the diagram? A. phosphate, deoxyribose and nitrogenous base B. phosphorus, ribose and nitrogenous base C. phosphorus, deoxyribose and guanosine D. phosphate, ribose and guanine

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2. Which of the following are connected by hydrogen bonds? A. Hydrogen to oxygen within a molecule of water B. Phosphate to sugar in a DNA molecule C. Base to sugar in a DNA molecule D. Hydrogen to oxygen between two different molecules water

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3. (a) Draw a labelled diagram to show how two nucleotides are joined together in a single strand of DNA (3)

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4. What principle is necessary to preserve the sequence of DNA during replication? A. Base pairing is complementary. B. One gene codes for one polypeptide. C. Substrates are specific to enzymes. D. The genetic code is universal.

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5.(a) Explain why DNA must be replicated before mitosis and the role of helicase in DNA replication (4)

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(b) Explain how the base sequence of DNA is conserved during replication (5)