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DNA Structure

DNA Structure. Will Fagan IB Biology 2011. 3.3 DNA Structure. DNA- Deoxyribonucleic acid Each nucleotide of DNA is composed of a phosphate group, a sugar called deoxyribose and a molecule that is called a nitrogenous base. . Nitrogenous Bases . Adenine Thymine Guanine Cytosine

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DNA Structure

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  1. DNA Structure Will Fagan IB Biology 2011

  2. 3.3 DNA Structure • DNA- Deoxyribonucleic acid • Each nucleotide of DNA is composed of a phosphate group, a sugar called deoxyribose and a molecule that is called a nitrogenous base.

  3. Nitrogenous Bases • Adenine • Thymine • Guanine • Cytosine • Note that all nucleotides are exactly the same except for the nitrogenous base.

  4. Linking DNA Nucleotides • DNA takes the shape of a double helix. • Double Helix- DNA is composed of two strands and each strand is shaped like a spiral staircase.

  5. Linking DNA Nucleotides Cont. • Complementary Base Pairs • Adenine bonds to Thymine • Cytosine bonds to Guanine.

  6. Adenine & Thymine • These two nitrogenous bases are bonded together using two hydrogen bonds. • Adenine and Guanine are relatively large so the only way that the strand maintains uniform length is through complimentary base pairing.

  7. Guanine and Cytosine • These two nitrogenous bases are held together by a triple hydrogen bond.

  8. 7.1 DNA Structure HL • In order to understand bonding within DNA, one should understand the structure of a five carbon sugar.

  9. Chain Composition • Each strand is composed of a backbone alternating phosphate and deoxyribose molecules. • The two molecules are held together by a covalent bond known as a phosphodiester bond/linkage.

  10. Resulting Reaction • These linkages produce a chain of DNA. • The reaction between the phosphate group on the 5’ carbon and the hydroxyl group on the 3’ carbon is known as a condensation reaction. • At the end of this bonding, each 2 unit polymer still has a 5’ carbon free at one end and a 3’ carbon free at the other.

  11. Order of Nucleotides • Does the sequence of nucleotides matter? Absolutely. • As nucleotides are linked together with phosphodiester bonds, a definite sequence develops. • This sequence makes up the genetic code upon which life is based.

  12. Holding the Strands Together • The two chains run in opposite directions and are described as anti-parallel. • One strand has the 5’ carbon on the top and the 3’ carbon on the bottom.

  13. Interaction between Nitrogenous Bases • Adenine and Guanine are double-ring structures that are known as purines. • Cytosine and Thymine are single-ring structures known as pyrimidines. • A single ring always pairs with a double ring. (Complementary base pairing)

  14. DNA Packaging • The DNA molecules of eukaryotic cells are paired with a type of protein called a histone. • Nucleosomes consists of two molecules of each of four different histones. • The DNA wraps twice around these eight protein molecules.

  15. DNA Packaging • DNA is negatively charged and histones are positively charged, therefore they are naturally attracted to each other. • There is a fifth histone which leads to further wrapping (supercoiling) of the DNA molecule.

  16. DNA Packaging • When DNA is wrapped around the histones and into more elaborate structures, it is inaccessible to the transcription of enzymes. • This allows only certain areas to be involved in protein synthesis.

  17. Types of DNA Sequences • Some DNA consists of highly repetitive sequences, some code for genes, and some are structural.

  18. Highly Repetitive Sequences • These sequences account for between 5% and 45% of the total genome. • Composed of 5-300 base pairs per repetitive sequences. • Repetitive sequences that are in random areas are known as satellite DNA.

  19. Protein- Coding Genes • Within the DNA molecule, there is a single copy of genes that has a coding function. • They provide base sequences essential to produce proteins at the ribosome. • Base sequences are carried from the nucleus to the ribosome by mRNA. • Coding fragments are known as exons and non-coding fragments are introns.

  20. Structural DNA • Highly coiled DNA that does not have a coding function. • One also sees the presence of pseudogenes that likely do not have a function due to mutation. • Gene research is known as genomics.

  21. QUICK BRAIN BREAK!

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