DNA StructureWill FaganIB 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• Note that all nucleotides are exactly the same except for the nitrogenous base.

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.

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

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.

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

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

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.

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.

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.

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.

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)

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.

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.

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.

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

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.

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.

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.

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