Chapter 9: Genes, chromosomes
and DNA
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9-1
Tracking the genetic material
•
1869—chromatin isolated by Miescher, containing
nucleic acid and protein
• Chromosomes consist of DNA and proteins
• 1900—concept of ‘Mendelian inheritance’
controlled by ‘genes’
• 1910—Morgan and others noted parallel
inheritance of ‘genes’ with chromosomes,
suggesting that genes were ‘on’ the chromosomes
(cont.)
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Tracking the genetic material
(cont.)
•
The transforming principle in Streptococcus
pneumoniae, where virulence can be transferred
by cellular extracts containing DNA (Avery,
McLeod & McCarty 1944)
– mice injected with live non-virulent bacteria and heatkilled virulent bacterial material died
– neither preparation on its own killed the mice
– non-virulent strain was ‘transformed’ by the virulent
material
– the virulence acquired from the heat-killed strain was
passed on to progeny of the transformed bacteria
(cont.)
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9-3
Fig. 9.5: Transforming principle in
Streptococcus pneumoniae
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Tracking the genetic material
(cont.)
•
DNA, not protein, is the genetic information
(Hershey & Chase 1952)
– bacteriophage DNA or protein was specifically
radioactively labelled
– bacteriophage infected bacteria—new bacteriophage
produced by infected organisms
– the presence of radiolabel inside infected bacteria was
only detected when the DNA was radiolabelled
– no radiolabelled protein was found inside the bacteria
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9-5
Fig. 9.6: Radioactive labelling of DNA with
32P or protein with 35S
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9-6
Chromosomes
•
DNA is organised into chromosomes
• Each chromosome is a single DNA molecule
• In eukaryotic cells, chromosomes are located in
the nucleus
• Each species has a unique chromosome
complement—shape, size and number
• Centromere essential for segregation during cell
division
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Fig. 9.1: Stained human chromosomes
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Chromosome structure
•
Multiple levels of DNA folding
– nucleosome: 146 base pairs (bp) are coiled in 1.75
turns around a core of histone proteins (H2A, H2B, H3,
H4) 10 nm diameter
(cont.)
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9-9
Fig. 9.3: Model of a nucleosome particle
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Chromosome structure (cont.)
•
•
•
This string of nucleosome ‘beads’ is then further
coiled into chromatin fibres 30 nm diameter
Metaphase chromosomes are further condensed
to about 1/10 000 of their full length
Loops of 20–100 kb are attached to a central
protein scaffold
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9-11
Fig. 9.4: A condensed chromosome in
metaphase
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9-12
DNA structure
•
•
•
DNA is a double-stranded molecule twisted into a
helix
Each strand, comprising a sugar-phosphate
backbone and attached bases, is connected to a
complementary strand by non-covalent hydrogen
bonding between paired bases
The bases are adenine (A), thymine (T), cytosine
(C) and guanine (G)
(cont.)
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9-13
DNA structure (cont.)
•
DNA consists of four different nucleotides
• Each nucleotide has three parts: a phosphate
group, a pentose sugar and an organic base
(cont.)
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
9-14
Fig. 9.7: Molecular structure of DNA
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DNA structure (cont.)
•
Bases are purines (A and G) and pyrimidines (C
and T)
• Purines have a pair of fused rings; pyrimidines
only have one
• A and T are connected by two hydrogen bonds; G
and C are connected by three hydrogen bonds
• The number of bonds is the basis of specific
pairing between the bases
(cont.)
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DNA structure (cont.)
•
•
Nucleotides are linked together by phosphodiester
bonds
Nucleic acids have distinct ends
– the 3’ end has a free hydroxyl group on the 3’ carbon of a
sugar
– the 5’ end has a free phosphate group at the 5’ carbon of
the sugar
•
The two strands of the helix are antiparallel: the 5’
end of one strand is directly apposed to the 3’ end
of the other strand
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DNA replication
•
DNA is replicated semi-conservatively—each
separate strand provides the template for new
strand synthesis by the base-pairing rules
• Semi-conservative replication allows synthesis of
new strands with high fidelity
• New DNA molecules consist of one ‘old’ strand
from the original molecule and one newly
synthesised strand
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Fig. 9.8a: Semiconservative replication
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Fig. 9.8b: Sequence-based representation of
replicating DNA
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DNA replication in prokaryotes
•
Bacteria have a single circular chromosome
• Replication begins at a single origin of replication
• A nick is made in at least one strand and the
molecule unwinds
• A replication fork is formed on each side of the
origin as small lengths of DNA separate for
synthesis of new strands
• The two replication forks eventually meet at the
terminus
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Fig. 9.10: DNA synthesis in circular
chromosomes
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Enzymes in replication
•
•
Requires gyrases to unwind the supercoiled
helices and helicases to separate the strands
New strand synthesis is performed by DNA
polymerases
– DNA polymerase III attaches bases in the 5’ 3’ direction
– DNA polymerase I checks the added base and corrects it
by 3’ to 5’ exonuclease activity—also removes RNA
primers used to initiate replication
•
DNA polymerases require priming to initiate strand
extension
– a short RNA primer with a 3’ OH group is added to the
template strand by a primase
(cont.)
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Fig. 9.13: Initiation of DNA synthesis
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Enzymes in replication (cont.)
•
Synthesis always proceeds 5’ 3’ on the strand
being produced therefore
– one strand is synthesised continuously (leading strand)
– the other (lagging strand) is synthesised discontinuously
as the replication fork moves along the template strand
– primases attach a series of primers along the template
strand
– DNA polymerase extends the primers away from the
replication fork
– the resulting Okazaki fragments are then ligated by DNA
ligase
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Fig. 9.11: Replication fork of Escherichia
coli
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Replication in eukaryotes
•
Chromosomes have many origins of replication
• Two replication forks are formed at each origin
• Synthesis proceeds 5’ to 3’ at each unit of
replication (replicon) with leading and lagging
strands
(cont.)
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Fig. 9.14: DNA synthesis in a chromosome
of a eukaryote
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Replication in eukaryotes (cont.)
•
•
•
•
Okazaki fragments are shorter than in prokaryotes
Leading and lagging strand synthesis in human
cells is performed by different DNA polymerases
Multiple replicons are necessary due to the large
size of eukaryote chromosomes
Replicons are initiated at different times
– chromosomes have early-, mid- or late-replicating regions
– gene-rich regions tend to be replicated first
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Telomeres during replication
•
•
•
DNA polymerases only replicate DNA 5’ to 3’ and
need a primer
When the primer is removed from the 5’ end of the
new strand a gap is left from which DNA
polymerase cannot extend
At each round of cell division chromosomes would
become shorter
(cont.)
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Telomeres during replication
(cont.)
•
To overcome this problem
– chromosomes have telomeres repeat DNA sequences up
to 10–15 kb
– added to chromosome ends by telomerase
– priming provided by RNA molecule within the telomerase
complex
– chromosome length is maintained
(cont.)
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Fig. 9.15: Completion of replication at ends
(telomeres) of eukaryotic chromosomes
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Telomeres during replication
(cont.)
•
•
•
•
•
Mammalian somatic cells have no telomerase
activity so become shorter with age
This limits the number of divisions each cell can
undergo
Essential sequences are eventually lost and the
cell dies
Restoration of telomerase activity allows cells to
proliferate indefinitely
Telomerase is important in ageing and cancer
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