DNA Structure - IBDPBiology-Dnl

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
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
 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
 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
 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
 another histone molecule
holds the nucleosome(s)
 the DNA and proteins are
 The DNA has a negatively
charged backbone
(because of the phosphate
 The proteins (histones) are
positively charged
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
 Genes contains
information for the
synthesis of one
polypeptide & also regulate
how other genes are
 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
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
 exons are unique
 smaller proportion of
 introns may be repetitive
 higher proportion of genome
Exons and Introns
 eukaryotic genes can
contain exons and
 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
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
 State the role of nucleosomes.
 Distinguish between unique and highly repetitive
sequences in nuclear DNA.

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