Biology HL Topics 3.3 & 7.1
Pp 55 – 57 & 193 - 196

 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 has three main components
 deoxyribose (a pentose) sugar

 base (there are four different ones) phosphate

 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)

 Nucleotides are formed by the condensation of a deoxyribose sugar, phosphate and one of the 4 bases
 The following illustration represents one nucleotide

Deoxyribose sugar
Phosphate
Organic Base
Hydrogen Bonds
Covalent Bond

 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

• 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

 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.

 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 has a negatively charged backbone
(because of the phosphate groups)
 The proteins (histones) are positively charged
 the DNA and proteins are 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

 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
 occur once in genome
 long base sequences
 they may be genes
 may be translated
 small differences between individuals
 exons are unique sequences
 smaller proportion of genome
Highly repetitive sequences
 occur many times in a genome
 short sequences (5–300 bases)
 they are not genes
 they are ever translated
 can vary greatly between individuals
 introns may be repetitive
 higher proportion of genome

 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

 Draw a labelled simple diagram of the molecular structure of DNA. [5]
 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. [8]
 Outline the structure of the nucleosomes in eukaryotic chromosomes. [4]
 State the role of nucleosomes. [2]
 Distinguish between unique and highly repetitive sequences in nuclear DNA. [5]