Biology HL Topics 3.3 & 7.1 Pp 55 – 57 & 193 - 196 . . . DNA Introduction DNA :- deoxyribonucleic acid A DNA molecule is very long and packed into compact structure called chromosomes. Each DNA molecule consists of two twisted polynucleotide strands that forming a double helix. The two strands are held together by hydrogen bonds between complementary pairs of nitrogenous bases. DNA Structure DNA has three main components deoxyribose (a pentose) sugar base (there are four different ones) phosphate Organic (Nitrogenous) Bases They are divided into two groups Pyrimidines and purines Pyrimidines (made of one ring) Thymine Cytosine Purines (made of two rings fused together) Adenine Guanine (specific formulas and structures of the rings are not required) Nucleotide Structure Nucleotides are formed by the condensation of a deoxyribose sugar, phosphate and one of the 4 bases The following illustration represents one nucleotide Polynucleotide Structure Molecular structure of DNA Deoxyribose sugar Phosphate Organic Base Hydrogen Bonds Covalent Bond Formation of DNA Double Helix DNA is a double helix molecule made up of two strands polynucleotide that are twisted over each other The two polynucleotides stands are held together by hydrogen bonding Hydrogen bonding occurs as a result of complimentary base pairing Adenine pair up with thymine Cytosine pair up with guanine Each pair is connected through hydrogen bonding Hydrogen bonding always occurs between one pyrimidine and one purine DNA Double Helix & Hydrogen Bonding • Adenine always pairs with thymine because they form two H bonds with each other • Cytosine always pairs with guanine because they form three hydrogen bonds with each other The ‘backbones’ of DNA molecules are made of alternating sugar and phosphates The ‘rungs on the ladder’ are made of bases that are hydrogen bonded to each other Antiparallel strands, 3’–5’ linkages The two polynucleotide strands run in opposite direction of each other i.e. they are 'anti-parallel’. When the covalent bonds are formed between nucleotides, they attach in the direction of 5’→3’ The 5’ end of one nucleotide attaches to the 3’ end of the previous nucleotide The 5’ end always has the phosphate attached. Chromosome structure Structure of nucleosomes Chromosomes are made up of DNA & proteins (histones) Nucleosomes are the basic unit of chromatin organization Nucleosome is made of DNA strand wound twice around the core of 8 histone molecules like a bead another histone molecule holds the nucleosome(s) together the DNA and proteins are The DNA has a negatively charged backbone (because of the phosphate groups) The proteins (histones) are positively charged electromagnetically attracted to each other to form chromatin nucleosomes help to supercoil chromosomes & to regulate transcription Supercoiling condenses the DNA molecule by a factor of X 15,000 Histones are responsible for the packaging of DNA at the different levels Genes Gene is a unit of genetic information Genes contains information for the synthesis of one polypeptide & also regulate how other genes are expressed Order of nucleotides make up the genetic code All cells of an organism contain the same genetic information but they do not all express the same genes Unique sequences & highly repetitive sequences in nuclear DNA Unique sequences Highly repetitive sequences occur once in genome occur many times in a genome long base sequences short sequences (5–300 bases) they may be genes they are not genes may be translated they are ever translated small differences between can vary greatly between individuals exons are unique sequences smaller proportion of genome individuals introns may be repetitive higher proportion of genome Exons and Introns eukaryotic genes can contain exons and introns exons are the 'gene coding region' that codes for the synthesis of a polypeptide introns are non-coding regions within the gene, they are edited out to form mature mRNA Revision Questions Draw a labelled simple diagram of the molecular structure of DNA.  The structure of the DNA double helix was described by Watson and Crick in 1953. Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together.  Outline the structure of the nucleosomes in eukaryotic chromosomes.  State the role of nucleosomes.  Distinguish between unique and highly repetitive sequences in nuclear DNA.