2/11/24, 8:46 AM Summary Campbell Biology - Chapter 16, 17
Chpt 16: The Molecular Basis of Inheritance
Key concepts:
16.1 DNA is the genetic material
16.2 Many proteins work together in DNA replication and repair
16.3 A chromosome consists of a DNA molecule packed together with proteins
- Watson and crick proposed double-helical model of DNA
- Nucleic acids are unique in their ability to direct their own replication from monomers
Concept 16.1: DNA is the genetic material
- Griffith: studied two bacterial strains, one pathogenic and one non-pathogenic
o He found that when he heat-killed some of the pathogenic bacteria, then mixed the
dead cells with the living non-pathogenic bacteria, and some of the living cells
became pathogenic
o These newly pathogenic cells gave rise to offspring that were also pathogenic
o He concluded that some chemical component of the dead pathogenic bacteria
must’ve caused this trait
o This phenomenon was called transformation, which is now defined as a change in
genotype and phenotype due to the assimilation of external DNA by a cell
- Despite Griffith’s experiment, scientists still thought proteins were the genetic material
- Another study into bacteriophages (bacteria-infecting viruses) by Hershey and Chase used
radioactivate phosphorus found only in DNA and radioactive sulphur found only in proteins,
and determined DNA to be the genetic material
o Concluded that phages must have injected the genetic molecule DNA into the new
cells to produce more viral DNA and proteins
- Chargaff observed that the base composition of DNA varies from one species to another,
and that there was a ratio of the number of bases; that is, the number of A=T and G=C
(approximately at least)
- Chargaff’s rules:
o The base composition of DNA varies between species
o For each species, the percentage of A and T bases are roughly equal and the
percentages of G and C bases are roughly equal
- Franklin contributed to the double helix model
o Her model put the relatively hydrophobic nitrogenous bases in the molecule’s
interior, away from the surrounding aqueous solution, and the negatively charged
phosphate groups wouldn’t be forced together in the interior
o The two sugar-phosphate backbones are antiparallel: their subunits run in opposite
directions
o Adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C)
- Adenine and guanine are purines, nitrogenous bases with two organic rings, while cytosine
and thymine are nitrogenous bases called pyrimidines, which have just one single ring
- Therefore, purine pairs with pyrimidine
- A-T bond has two hydrogen bonds, G-C bond has three hydrogen bonds
about:blank 1/20
,2/11/24, 8:46 AM Summary Campbell Biology - Chapter 16, 17
Chpt 16: The Molecular Basis of Inheritance
Figure 1 - chemical nature of nitrogenous bases
Figure 2- base pairing and number of hydrogen bonds in specific bonds
Concept 16.2: many proteins work together in DNA replication and
repair
- The basic principle: base pairing produces a template strand
- Semiconservative model: Type of DNA replication in which the replicated double helix
consists of one old strand, derived from the parental molecule, and one newly formed
strand
- This model differs from the conservative model in which the two parental strands
reassociate after acting as templates for new strands, restoring the parental double helix,
and also the dispersive model in which each strand of both daughter molecules contains a
mixture of old and newly synthesised DNA
DNA Replication: a closer look
- The copying of DNA is remarkably accurate and fast
- Many enzymes and proteins participate in DNA replication
- Replication models are based off E. coli but can be extended to other prokaryotes and even
eukaryote
Getting started
- The replication of a chromosome begins at particular sites called origins of replication, short
stretches of DNA having a specific sequence of nucleotides
- E. coli only has one circular chromosome. It only has a single origin of replication
about:blank 2/20
, 2/11/24, 8:46 AM Summary Campbell Biology - Chapter 16, 17
Chpt 16: The Molecular Basis of Inheritance
- Proteins recognise this site and attach to the DNA, separating the two strands and opening
up a ‘replication bubble’
- Replication then proceeds in both directions until the entire molecule is copied
- In contrast to prokaryotes, eukaryotes have hundreds of replication origins, which allows for
speedier replication of longer DNA molecules
- At the end of each replication bubble is a replication fork, a Y-shaped region where the
parental strands of DNA are being unwound
- Helicase is an enzyme that untwists the double helix at the replication forks, separating the
two parental strands to make them available as template stands.
- Single-strand binding proteins bind to the unpaired DNA strands to stop them from
repairing. Topoisomerase regulates the unwinding before the replication fork.
- But, the enzymes that synthesis DNA cannot initiate the synthesis of a polynucleotide, they
can only add DNA nucleotides to the end of an already existing chain
- The initial nucleotide chain that is produced during DNA synthesis is actually a short stretch
of RNA, called a primer, and is synthesised by the enzyme primase
- Primase starts a complementary RNA chain, adding the RNA nucleotides one at a time. The
new DNA strand will start from the 3’ end of the RNA primer
Synthesising a New DNA Strand
- Enzymes called DNA polymerases catalyse the synthesis of new DNA by adding nucleotides
to a pre-existing chain.
- There are several different types of DNA pols but two are key: DNA pol III and DNA pol I
- DNA pols. require a primer and a DNA template strand, along with complementary
nucleotides lined up
- DNA pol III adds a DNA nucleotide to the RNA primer and then continues adding DNA
nucleotides, complementary to the parental DNA template strand
- Each nucleotide to be added to a growing DNA strand consists of a sugar attached to a base
and to three phosphate groups
o ATP has a ribose sugar, whereas dATP (adenine nucleotide) has a deoxyribose sugar
instead. The triphosphate group makes the monomer unstable and reactive
- As each monomer joins the growing end of a DNA strand, two phosphate groups are lost as a
molecule of pyrophosphate.
about:blank 3/20
Chpt 16: The Molecular Basis of Inheritance
Key concepts:
16.1 DNA is the genetic material
16.2 Many proteins work together in DNA replication and repair
16.3 A chromosome consists of a DNA molecule packed together with proteins
- Watson and crick proposed double-helical model of DNA
- Nucleic acids are unique in their ability to direct their own replication from monomers
Concept 16.1: DNA is the genetic material
- Griffith: studied two bacterial strains, one pathogenic and one non-pathogenic
o He found that when he heat-killed some of the pathogenic bacteria, then mixed the
dead cells with the living non-pathogenic bacteria, and some of the living cells
became pathogenic
o These newly pathogenic cells gave rise to offspring that were also pathogenic
o He concluded that some chemical component of the dead pathogenic bacteria
must’ve caused this trait
o This phenomenon was called transformation, which is now defined as a change in
genotype and phenotype due to the assimilation of external DNA by a cell
- Despite Griffith’s experiment, scientists still thought proteins were the genetic material
- Another study into bacteriophages (bacteria-infecting viruses) by Hershey and Chase used
radioactivate phosphorus found only in DNA and radioactive sulphur found only in proteins,
and determined DNA to be the genetic material
o Concluded that phages must have injected the genetic molecule DNA into the new
cells to produce more viral DNA and proteins
- Chargaff observed that the base composition of DNA varies from one species to another,
and that there was a ratio of the number of bases; that is, the number of A=T and G=C
(approximately at least)
- Chargaff’s rules:
o The base composition of DNA varies between species
o For each species, the percentage of A and T bases are roughly equal and the
percentages of G and C bases are roughly equal
- Franklin contributed to the double helix model
o Her model put the relatively hydrophobic nitrogenous bases in the molecule’s
interior, away from the surrounding aqueous solution, and the negatively charged
phosphate groups wouldn’t be forced together in the interior
o The two sugar-phosphate backbones are antiparallel: their subunits run in opposite
directions
o Adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C)
- Adenine and guanine are purines, nitrogenous bases with two organic rings, while cytosine
and thymine are nitrogenous bases called pyrimidines, which have just one single ring
- Therefore, purine pairs with pyrimidine
- A-T bond has two hydrogen bonds, G-C bond has three hydrogen bonds
about:blank 1/20
,2/11/24, 8:46 AM Summary Campbell Biology - Chapter 16, 17
Chpt 16: The Molecular Basis of Inheritance
Figure 1 - chemical nature of nitrogenous bases
Figure 2- base pairing and number of hydrogen bonds in specific bonds
Concept 16.2: many proteins work together in DNA replication and
repair
- The basic principle: base pairing produces a template strand
- Semiconservative model: Type of DNA replication in which the replicated double helix
consists of one old strand, derived from the parental molecule, and one newly formed
strand
- This model differs from the conservative model in which the two parental strands
reassociate after acting as templates for new strands, restoring the parental double helix,
and also the dispersive model in which each strand of both daughter molecules contains a
mixture of old and newly synthesised DNA
DNA Replication: a closer look
- The copying of DNA is remarkably accurate and fast
- Many enzymes and proteins participate in DNA replication
- Replication models are based off E. coli but can be extended to other prokaryotes and even
eukaryote
Getting started
- The replication of a chromosome begins at particular sites called origins of replication, short
stretches of DNA having a specific sequence of nucleotides
- E. coli only has one circular chromosome. It only has a single origin of replication
about:blank 2/20
, 2/11/24, 8:46 AM Summary Campbell Biology - Chapter 16, 17
Chpt 16: The Molecular Basis of Inheritance
- Proteins recognise this site and attach to the DNA, separating the two strands and opening
up a ‘replication bubble’
- Replication then proceeds in both directions until the entire molecule is copied
- In contrast to prokaryotes, eukaryotes have hundreds of replication origins, which allows for
speedier replication of longer DNA molecules
- At the end of each replication bubble is a replication fork, a Y-shaped region where the
parental strands of DNA are being unwound
- Helicase is an enzyme that untwists the double helix at the replication forks, separating the
two parental strands to make them available as template stands.
- Single-strand binding proteins bind to the unpaired DNA strands to stop them from
repairing. Topoisomerase regulates the unwinding before the replication fork.
- But, the enzymes that synthesis DNA cannot initiate the synthesis of a polynucleotide, they
can only add DNA nucleotides to the end of an already existing chain
- The initial nucleotide chain that is produced during DNA synthesis is actually a short stretch
of RNA, called a primer, and is synthesised by the enzyme primase
- Primase starts a complementary RNA chain, adding the RNA nucleotides one at a time. The
new DNA strand will start from the 3’ end of the RNA primer
Synthesising a New DNA Strand
- Enzymes called DNA polymerases catalyse the synthesis of new DNA by adding nucleotides
to a pre-existing chain.
- There are several different types of DNA pols but two are key: DNA pol III and DNA pol I
- DNA pols. require a primer and a DNA template strand, along with complementary
nucleotides lined up
- DNA pol III adds a DNA nucleotide to the RNA primer and then continues adding DNA
nucleotides, complementary to the parental DNA template strand
- Each nucleotide to be added to a growing DNA strand consists of a sugar attached to a base
and to three phosphate groups
o ATP has a ribose sugar, whereas dATP (adenine nucleotide) has a deoxyribose sugar
instead. The triphosphate group makes the monomer unstable and reactive
- As each monomer joins the growing end of a DNA strand, two phosphate groups are lost as a
molecule of pyrophosphate.
about:blank 3/20
