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Biology HL Topics 3.3 & 7.1 Pp 55 – 57 & 193 - 196
Assessment Statements 3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose),
base and phosphate.
3.3.2 State the names of the four bases in DNA.
3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand.
3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds.
3.3.5 Draw and label a simple diagram of the molecular structure of DNA.
7.1.1 Describe the structure of DNA, including the antiparallel strands, 3’–5’ linkages and hydrogen bonding between purines and pyrimidines.
7.1.2 Outline the structure of nucleosomes.
7.1.3 State that nucleosomes help to supercoil chromosomes and help to regulate transcription.
7.1.4 Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA.
7.1.5 State that eukaryotic genes can contain exons and introns.
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 bondsbetween 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 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
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
long base sequences
they may be genes
may be translated
small differences between individuals
exons are unique sequences
smaller proportion of genome
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
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. [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]