DNA structure & replication Topic 7.1 Biology HL
Hershey & Chase
➢ 1952, experiments that confirmed that DNA is the genetic material of life
➢ They used radioisotopes of phosphorus and sulfur to mark the protein and the DNA
of bacteriophage viruses
○ Radioisotopes are radioactive forms of elements that decay over time at a
predictable rate
■ For the experiment 32
P and 35
S were used
○ T2 bacteriophage virus and E.coli were used
■ A bacteriophage virus is a virus composed of a protein outer coat and
an inner core of DNA, or sometimes RNA
■ When this virus infects a cell, it takes over the metabolism of the cell
resulting in multiple viruses of its kind being formed
○ 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.coli
○ In this way the virus could reproduce and the researchers could see whether
the genetic material is the DNA or the protein
■ If it was the DNA, then E.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
○ These two molecules are held together by a covalent bond called
phosphodiester bond o
r linkage
○ It forms between a hydroxyl group of the 3’ carbon of deoxyribose and the
phosphate group attached to the 5’ carbon of the deoxyribose
■ Condensation reaction
■ 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
1
,DNA structure & replication Topic 7.1 Biology HL
➢ The DNA molecule is negatively charged
➢ The two strands of DNA are a
ntiparallel to each other
○ They run in opposite directions
○ One strand has the 5’ carbon on the top and the 3’ carbon on the bottom and
the other strand has the 3’ on top and the 5’ on the bottom
➢ The nitrogenous bases on the nucleotides form hydrogen bonds with the bases of
the other strand
○ A with T => double bond and C and G => triple bond
■ Complementary bases
Complementary bases
➢ Adenine and guanine are double-ring structures known as purines
➢ Cytosine and thymine are single-ring structures known as pyrimidines
➢ A double-ring nitrogenous base always pairs with a single-ring nitrogenous base
➢ Complementary pairing occurs, because of the specific distance that exists between
the two sugar-phosphate chains
DNA packaging
➢ In eukaryotic cells, DNA molecules are paired with a type of protein called h
istone
○ There are 4 different histones and one fifth for a different use
○ DNA and histones make up a n
ucleosome
➢ A nucleosome consists of two molecules of each of the 4 different histones
➢ The DNA wraps twice around these 8 histones and one molecule of the fifth type is
attached to the nucleosome acting as tape which helps the structure to maintain its
shape
○ The DNA is attracted to the histones, because DNA is negatively charged
while histones are positively charged
➢ Between the nucleosomes is a single string of DNA
➢ The fifth histon leads to further wrapping of the DNA
➢ When DNA is wrapped around histones and then in more elaborate structures, it
cannot be transcribed
○ This regulates the process
○ Allows only certain areas of DNA to be involved in protein synthesis
2
, 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
○ First publication in 2001
➢ It was found that only 2% of our genes code for proteins
○ The rest are:
■ Regulators for gene expression
● Sequences of DNA regulating if and when DNA should be
unwinded from the histones
■ Introns (24%)
● Highly repetitive sequences that do not have any coding
function
● 5-300 base pairs per repetitive sequence
● Can be as many as 100 000 replicates of a certain type per
genome
○ If it is in discrete areas, it is referred to as s
atellite
DNA
○ They are mostly dispersed
● They are transportable elements, called jumping genes
○ Can move from one genome location to another
○ Never detach from the DNA molecule they are part of
● The centromere of chromosomes is largely made up of them
■ Telomeres
● Highly coiled DNA that does not have a coding function as
pseudogenes, due to mutation involving base sequence
change
● Occur on the ends of chromosomes
● Consist of 6-8 base pair sequence that is repeated up to
hundreds of thousands of times
○ Number of pairs and number of their repetition
depend of the species
■ Genes for tRNA
● The genetic code to produce tRNA molecules
3
Hershey & Chase
➢ 1952, experiments that confirmed that DNA is the genetic material of life
➢ They used radioisotopes of phosphorus and sulfur to mark the protein and the DNA
of bacteriophage viruses
○ Radioisotopes are radioactive forms of elements that decay over time at a
predictable rate
■ For the experiment 32
P and 35
S were used
○ T2 bacteriophage virus and E.coli were used
■ A bacteriophage virus is a virus composed of a protein outer coat and
an inner core of DNA, or sometimes RNA
■ When this virus infects a cell, it takes over the metabolism of the cell
resulting in multiple viruses of its kind being formed
○ 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.coli
○ In this way the virus could reproduce and the researchers could see whether
the genetic material is the DNA or the protein
■ If it was the DNA, then E.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
○ These two molecules are held together by a covalent bond called
phosphodiester bond o
r linkage
○ It forms between a hydroxyl group of the 3’ carbon of deoxyribose and the
phosphate group attached to the 5’ carbon of the deoxyribose
■ Condensation reaction
■ 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
1
,DNA structure & replication Topic 7.1 Biology HL
➢ The DNA molecule is negatively charged
➢ The two strands of DNA are a
ntiparallel to each other
○ They run in opposite directions
○ One strand has the 5’ carbon on the top and the 3’ carbon on the bottom and
the other strand has the 3’ on top and the 5’ on the bottom
➢ The nitrogenous bases on the nucleotides form hydrogen bonds with the bases of
the other strand
○ A with T => double bond and C and G => triple bond
■ Complementary bases
Complementary bases
➢ Adenine and guanine are double-ring structures known as purines
➢ Cytosine and thymine are single-ring structures known as pyrimidines
➢ A double-ring nitrogenous base always pairs with a single-ring nitrogenous base
➢ Complementary pairing occurs, because of the specific distance that exists between
the two sugar-phosphate chains
DNA packaging
➢ In eukaryotic cells, DNA molecules are paired with a type of protein called h
istone
○ There are 4 different histones and one fifth for a different use
○ DNA and histones make up a n
ucleosome
➢ A nucleosome consists of two molecules of each of the 4 different histones
➢ The DNA wraps twice around these 8 histones and one molecule of the fifth type is
attached to the nucleosome acting as tape which helps the structure to maintain its
shape
○ The DNA is attracted to the histones, because DNA is negatively charged
while histones are positively charged
➢ Between the nucleosomes is a single string of DNA
➢ The fifth histon leads to further wrapping of the DNA
➢ When DNA is wrapped around histones and then in more elaborate structures, it
cannot be transcribed
○ This regulates the process
○ Allows only certain areas of DNA to be involved in protein synthesis
2
, 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
○ First publication in 2001
➢ It was found that only 2% of our genes code for proteins
○ The rest are:
■ Regulators for gene expression
● Sequences of DNA regulating if and when DNA should be
unwinded from the histones
■ Introns (24%)
● Highly repetitive sequences that do not have any coding
function
● 5-300 base pairs per repetitive sequence
● Can be as many as 100 000 replicates of a certain type per
genome
○ If it is in discrete areas, it is referred to as s
atellite
DNA
○ They are mostly dispersed
● They are transportable elements, called jumping genes
○ Can move from one genome location to another
○ Never detach from the DNA molecule they are part of
● The centromere of chromosomes is largely made up of them
■ Telomeres
● Highly coiled DNA that does not have a coding function as
pseudogenes, due to mutation involving base sequence
change
● Occur on the ends of chromosomes
● Consist of 6-8 base pair sequence that is repeated up to
hundreds of thousands of times
○ Number of pairs and number of their repetition
depend of the species
■ Genes for tRNA
● The genetic code to produce tRNA molecules
3
