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DNA structure & replication

Topic 7
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coli were used 
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A ​bacteriophage virus ​is a virus composed of a protein outer coat and 
an inner core of DNA, or sometimes RNA 

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When this virus infects a cell, it takes over the metabolism of the cell 
resulting in multiple viruses of its kind being formed  

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Phosphoru marked the DNA of the virus and sulfur the protein of which the 
virus was made/had on the outer core 

➢ One culture had radioactive phosphorus and the other radioactive sulfur 
➢ Then they were allowed to infect the bacterium E
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coli would contain phosphorus, but if it 
contained sulfur, then the protein would be the genetic material 

➢ They put the cultures in centrifugal and observed that the pellet which had formed 
on the bottom, contained phosphorus 
 
DNA structure 
➢ Each strand of DNA is composed of a backbone of alternating phosphate and 
deoxyribose molecules 
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These two molecules are held together by a covalent bond called 

phosphodiester bond o
​ r ​linkage  
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It forms between a hydroxyl group of the 3’ carbon of deoxyribose and the 
phosphate group attached to the 5’ carbon of the deoxyribose 
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Condensation reaction 

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Each nucleotide is attached to the previous one by this kind of bond 

➢ New nucleotides are always added to the 3’ end  
➢ DNA molecules always have a non-bonding 5’ carbon at the top and a free 3’ carbon 
at the bottom with which other nucleotides can bond 

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DNA structure & replication

Topic 7
...
1

Biology HL 

 
Types of DNA sequences 

➢ Genomics i​ nvolves the science of sequencing, interpreting and comparing whole 
genomes 

➢ The International Human Genome Project, started in 1970, had the assignment to 
determine the whole human genome 
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First publication in 2001 

➢ It was found that only 2% of our genes code for proteins 
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The rest are: 
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Regulators for gene expression 
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Sequences of DNA regulating if and when DNA should be 
unwinded from the histones 

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Introns (24%) 
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Highly repetitive sequences that do not have any coding 
function 

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5-300 base pairs per repetitive sequence 

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Can be as many as 100 000 replicates of a certain type per 
genome 
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If it is in discrete areas, it is referred to as s
​ atellite 

DNA  
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●

●
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They are mostly dispersed  

They are transportable elements, called ​jumping genes 
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Can move from one genome location to another 

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Never detach from the DNA molecule they are part of 

The centromere of chromosomes is largely made up of them 

Telomeres 
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Highly coiled DNA that does not have a coding function as 

pseudogenes​, due to mutation involving base sequence 
change 
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Occur on the ends of chromosomes  

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Consist of 6-8 base pair sequence that is repeated up to 
hundreds of thousands of times 
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Number of pairs and number of their repetition 
depend of the species  

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Genes for tRNA 
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The genetic code to produce tRNA molecules 

 
3 

DNA structure & replication

Topic 7
...
1

Biology HL 

 
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The RNA primer consists of RNA nucleotides and it is 5-10 
nucleotides long 

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DNA polymerase III​ can now add nucleotides in the 5’ to 3’ direction  
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One strand will be formed continuously and fast 
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One strand will be formed in fragments and slower 
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●

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The l​ eading strand 
The l​ agging strand  

The fragments are called O
​ kazaki fragments 

When DNA polymerase III has formed the Okazaki fragments, ​DNA 

ligase​ joins the backbones of the fragments 
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When DNA polymerase III is done, ​DNA polymerase I​ comes and exchanges 
the RNA primer to DNA 

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The nucleotides that are added to the elongating DNA molecule are actually 

deoxynucleoside triphosphate (dNTP) ​molecules 
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Contain deoxyribose, a nitrogenous base and 3 phosphate groups 

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When added, 2 phosphate groups leave providing enough energy for 
the chemical bonding of the nucleotide to occur 

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As
​ ingle strand binding protein ​ binds to the 2 strands to prevent the double 
strands from reforming 

 
DNA profiling 
➢ The process of obtaining a specific DNA pattern from an organism 
➢ Most of our own DNA is identical to the DNA of other people 
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There are specific regions called p
​ olymorphisms ​that show significant 
variation 
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Polymorphisms are analysed with DNA profiling  

➢ To profile polymorphisms we look at a group of 13 very specific ​loki (​ locations on a 
chromosome) referred to as s
​ hort tandem repeats (STRs) 

➢ STRs are short, repeating sequences of DNA that do not code for any protein 
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Composed of 2-5 base pairs 

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Example: GATA 
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CTAACGATAG ATAGATAGAT AGATAGATAG ATAGATAGATA 

 
 
 

5 

DNA structure & replication

Topic 7
...
Sanger in 1970 
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Used fragments of DNA copied through PCR 

➢ Sequence process: 
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Single stranded fragments are placed in 4 different test tubes 
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Each test tube contains primers, the DNA polymerases and DNA 
nucleotides 

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Each tube contains a special type of nucleotide called ​dideoxynucleotide​ , 
derived from a dideoxynucleic acid 
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Has a different sugar with no OH- group on the 3’ carbon 

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This means that after it has been added by DNA polymerase, it will 
prevent any further nucleotide addition to the chain 

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There are 4 different dideoxynucleotides, A*, T*, C*, G* 

The synthesis of each new DNA strand begins at the 3’ end of the primer and 
continues until a dideoxynucleotide is added 
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These dideoxynucleotides are available in limited amounts but allow 
chains of various lengths to be assembled 

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The sequences produced are separated using ​electrophoresis​
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Watson and F

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