Chapter 16 – The Molecular Basis of Inheritance
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DNA replication – the process by which a DNA molecule is copied and how
cells repair their DNA
16.1: DNA is the genetic material
The search for the genetic material: Scientific inquiry
- Role of DNA in heredity was first worked out while studying bacteria and
viruses that infect them
o Much simpler than fruit flies or pea plants
Evidence that DNA can transform bacteria
- See figure 16.2
- Transformation – change in genotype and phenotype due to assimilation of
external DNA by a cell
- Transforming agent is DNA
Evidence that viral DNA can program cells
- Bacteriophages (phages) –viruses that infect bacteria
- Virus – DNA enclosed by a protective coat, often just a protein
o To produce more viruses, a virus must infect a cell and take over the
metabolic machinery
- See figure 16.4
- DNA functions as the genetic material of phage T2 (one of many phages that
infect E. coli)
o Phage DNA entered host cells, phage protein did not (proteins contain
sulfur, DNA does not)
Additional evidence that DNA is the genetic material
- DNA is a polymer of nucleotides
o Nitrogenous base (A, T, C, G) (hydrophobic)
o Sugar (Deoxyribose)
o Phosphate group (negatively charged)
- Base composition of DNA varies from species to species
- A and T, C and G are found in ~ same compositions in species
Building a structural model of DNA: Scientific inquiry
- Question: How did the structure of DNA account for the role in inheritance
- Double helix – two strands of DNA wind around each other in a helical shape,
discovered by Watson and Crick
- See figure 16.7
- Sugar-phosphate backbone is antiparallel – the nitrogenous bases face the
interior, sugar-phosphate forms the backbone
- A pairs with T, and C pairs with G (purine pairs with pyrimidine)
- A forms 2 H-bonds with T, C forms 3 H-bonds with G
16.2: Many proteins work together in DNA replication and repair
The basic principle: Base pairing to a template strand
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See figure 16.9
Two strands of DNA are complimentary – each strand stores the information
necessary to reconstruct the other
- Semiconservative model – one strand of the parent molecule will be present
in the daughter molecule
- See figure 16.10 and 16.11
DNA replication: A closer look
- E. Coli has a single chromosome that is about 4.6 million nucleotide pairs
- In a favorable environment, can copy all DNA in less than an hour
- Human DNA has about 6 billion nucleotide pairs, takes a few hours to copy
- Very few errors occur, only about one per 10 billion nucleotides
- Most processes are fundamentally similar in prokaryotes and eukaryotes
Getting started
- Origins of replication – where replication of DNA begins
o Short stretch of DNA with a specific nucleotide sequence
- Most bacterial chromosomes have single origin, circular in shape
- Proteins that initiate DNA replication recognize the sequence and attach to
DNA separating the two strands and open a replication bubble
- Replication then proceeds in both directions until entire molecule is copied
- A eukaryotic chromosome may have hundreds of replication origins, which
can speed up the replication process
- Replication fork – Y-shaped region where the parental strands of DNA are
unwound, found at the end of the replication bubble
- Helicases – enzymes that untwist the double helix at the replication fork
- Single-strand binding proteins – bind to unpaired DNA strands to keep them
from re-pairing
- Topoisomerase – helps relieve the strain of tighter twisting ahead of the
replication fork by breaking, swiveling and rejoining DNA strands
- See figure 16.13
- Unwound sections can serve as templates for the synthesis of new
complementary DNA strands
- Initial nucleotide chain that is produced during DNA synthesis is actually a
short stretch of RNA called a primer
- Primase – starts a complementary RNA chain from a single RNA nucleotide
- New DNA strand starts from the 3’ end of the RNA primer
Synthesizing a new DNA strand
- DNA polymerase – enzyme that catalyzes the synthesis of new DNA by
adding nucleotides to a preexisting chain
o At least 11 different types in eukaryote cells
o Require a primer and a DNA template strand
o Adds DNA nucleotide to the RNA primer and then continues adding
DNA nucleotides to the RNA primer and continues adding DNA
nucleotides, complementary to the parental DNA template strand
o ~500/second in bacteria, ~50/second in humans
Antiparallel Elongation
- Two ends of a DNA strand are different, each strand has directionality
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Two strands are antiparallel – they are originated in opposite directions to
each other
- New strands formed in DNA replication must also be antiparallel
- See figure 16.14
- DNA polymerase adds nucleotides only to the free 3’ end of a primer and
never to the 5’ end
o New DNA strand elongates from 5’ to 3’ direction
- See figure 16.15
- Leading strand – new complementary DNA strand synthesized continuously
along the template strand toward the replication fork
- To elongate the other new strand of DNA, DNA polymerase must work along
the other template away from the replication fork – lagging strand
- Okazaki fragments – lagging strand is synthesized discontinuously in
segments
- See figure 16.16
- DNA ligase – joins the sugar-phosphate backbone of Okazaki fragments into a
continuous DNA strand
- See figure 16.17
The DNA replication complex
- Various proteins that participate in DNA replication form a single large
complex
- DNA replication complex may not move along the DNA
o DNA may move through the complex during the replication process
Proofreading and Repairing DNA
- Initial pairing errors between incoming nucleotides and those on the
template are more common than errors in a completed DNA molecule
- DNA polymerases proofread each nucleotide against its template
o Removes incorrectly paired nucleotides and resumes synthesis
- Mismatch repair – mismatched nucleotide evades proofreading, so other
enzymes remove and replace incorrectly paired nucleotides
- Defect in repair enzyme that is associated with form of colon cancer
- Maintenance of genetic information requires repair of various kinds of
damage to existing DNA
o Subject to harmful chemical and physical agents
o DNA bases can undergo spontaneous chemical changes under normal
cellular conditions
- Mutations – permanent change
- Nuclease – DNA-cutting enzyme that takes out damaged DNA segment
o Damaged gap is filled in using undamaged strand as template
- Nucleotide excision repair – filling gap involves DNA ligase and DNA
polymerase
- See figure 16.19
Evolutionary significance of altered DNA nucleotides
- Error rate after proofreading is extremely low, but mistakes do slip through
- Mutation can change phenotype of organism
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If mutation occurs in germ cell (gives rise to gamete), mutation can be passed
from generation to generation
Replicating ends of DNA molecules
- In linear DNA (eukaryotic), adding nucleotides to the 3’ end of a preexisting
polynucleotide by DNA polymerase can lead to a problem
o Usual replication machinery provides no way to complete the 5’ end
of the daughter DNA strands
o Causes shortening of the DNA molecule with staggered ends
- Telomeres – special nucleotide sequences at the end of DNA molecules
o Do not contain genes, just repetitions of short nucleotide sequence
 Eg. TTAGGG is repeated between 100-1000 times
o Acts as a buffer zone that protects genes
o Provide protective function by postponing erosion of genes located
near the ends of DNA
o See figure 16.20
o Telomeric DNA tends to be shorter in older individuals
 Suggested this could be connected to ageing process
- Telomerase – catalyzes the lengthening of telomeres in eukaryotic germ cells
- Normal shortening of telomeres may protect from cancer by limiting the
number of divisions somatic cells undergo
o Cells from large tumors often have unusually short telomeres
16.3: A chromosome consists of a DNA molecule packed together with proteins
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See figure 16.22
Nucleoid – region in a bacterium where the DNA is found
Each DNA molecule in a human is ~4cm long
Chromatin – DNA and protein complex found in nucleus of eukaryotic cell