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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

DNA Structure

Will Fagan

IB Biology 2011

3 3 dna structure
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
Nitrogenous Bases
  • Adenine
  • Thymine
  • Guanine
  • Cytosine
  • Note that all nucleotides are exactly the same except for the nitrogenous base.
linking dna nucleotides
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
Linking DNA Nucleotides Cont.
  • Complementary Base Pairs
  • Adenine bonds to Thymine
  • Cytosine bonds to Guanine.
adenine thymine
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
Guanine and Cytosine
  • These two nitrogenous bases are held together by a triple hydrogen bond.
7 1 dna structure hl
7.1 DNA Structure HL
  • In order to understand bonding within DNA, one should understand the structure of a five carbon sugar.
chain composition
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
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
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
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
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
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 packaging1
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 packaging2
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
Types of DNA Sequences
  • Some DNA consists of highly repetitive sequences, some code for genes, and some are structural.
highly repetitive sequences
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
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
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.