DNA and the Genome
Unit 1: CFE Higher Biology
June – October
Unit Assessment – before October break
Unit 1: DNA and the Genome
Key area 1: The structure of
DNA
CFE Higher Biology
DNA and
the
Genome
Take a moment…think…
What have you learned about DNA so far?
In groups, collect an sheet of poster
paper.
DNA
Nominate a scribe. Write down as many
things you can remember about DNA from
your school/life so far.
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DNA and
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Carousel
Pass the sheets around.
Read the facts/comments. Put a tick next
to any you had on your sheet.
Add any you had that are missing.
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DNA and
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Genome
Share with class
Share what is on your sheet with the
class.
The aim is to fill a sheet of your jotter
with “DNA background information”.
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DNA and
the
Genome
Key concepts
• DNA is inherited.
• DNA is the genetic material of living things.
• DNA is located within the nucleus of all cells apart from
red blood cells.
• DNA is a long chemical sequence and this sequence
contains the information needed for that living thing to
develop, survive and pass its genetic information on to the
next generation.
• The DNA chemical sequence differs between individuals.
The pattern of this sequence is called the genotype.
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DNA and
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The structure of DNA
Unit 1: DNA and the Genome
Prior knowledge
• DNA is the genetic material.
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DNA and
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Deoxyribonucleic acid
The DNA molecule is comprised of two
chains of nucleotides.
The nucleotides are comprised of a sugar,
a phosphate and a base.
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DNA and
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Nucleotides
5’ end
Base
Phosphate
3’ end
Sugar
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5’ pronounced
“5 prime”
DNA and
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Why 5’ and 3’?
This is what a nucleotide
looks like at the molecular
level…
…lets zoom in to look
at the sugar in more
detail…
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Carbon 5
Carbon 4
Carbon 1
Carbon 2
Carbon 3
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Nucleotides in a
chain are joined
together by sugarphosphate bond to
make long chains of
nucleotides – called
polynucleotides.
(poly = many)
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The bases
In DNA there are four different bases:
Adenine
Thymine
Cytosine
Guanine
or A, T, C and G
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Base pairing
These bases are
described as
being
complementary to
each other. This
means their
shapes match up.
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DNA and
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DNA base pair rules
Adenine always joins to Thymine
Cytosine always joins with Guanine
The bases are joined by hydrogen bonds.
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DNA and
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The double helix
The chains of polynucleotides are joined
together by the bases, by hydrogen
bonds, for form a double helix structure.
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DNA and
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Genome
Imagine the
sugars and
phosphates joined
together making
the side of a
ladder, with the
pairs of bases
forming the rungs.
Then the ladder
gets twisted…
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DNA and
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DNA and
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The double helix is described as having
two anti-parallel chains of nucleotides
because one side goes from 5’ to 3’ and
the opposite side goes from 3’ to 5’.
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DNA and
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Genome
How was all this discovered?
Who would have thought that science could produce
such a story of intrigue and characters?
Like all major scientific discoveries, the discovery
of the structure of DNA by James Watson and
Francis Crick in 1953 was the result of years of
work from a great number of scientists.
Not only is the story one of great scientific
interest, it is also one of great human interest and
controversy.
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DNA and
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Genome
Your task
This task will see you research one of the individuals or
groups of scientists whose work contributed to the
discovery. There are six people to choose from and your
teacher will help you decide which one to focus on:
•Griffiths
•Avery et al.
•Hershey and Chase
•Chargaff
•Franklin and Wilkins
•Watson and Crick
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DNA and
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Genome
The results of your research will then be
produced as a scientific poster, which you
will present to the class.
• who the scientist(s) is, where they worked and
when
• the aims of the experiments
• what the experiments were, including a diagram
• what the conclusions were
• other interesting information that you have
gathered that lends interest to the story.
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DNA and
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Title – The title of the work or discovery
By – your names
Introduction – who the
scientist was, where
they worked, some
background information
The experiment – what did they do? What were the results?
Include diagrams
Their conclusions
Read the
instruction
sheet for
more
details.
And you will never guess... some
interesting piece of information
about the scientist(s) and the
story of the discovery of the
structure of DNA.
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DNA and
the
Genome
Key concepts
DNA is composed of two polynucleotide chains.
Nucleotides consist of a sugar, phosphate and base.
Nucleotides bond to form a sugar–phosphate backbone.
The two polynucleotide chains run antiparallel, with a
deoxyribose sugar at the 3′ end and a phosphate group at
the 5′ end.
• The nucleic acid bases are paired by hydrogen bonding in
the centre to form a double helix.
• Base pairing is specific, with adenine pairing with thymine
and cytosine pairing with guanine.
•
•
•
•
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DNA and
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Organisation of DNA in
prokaryotes and eukaryotes
Unit 1: DNA and the Genome
Prior knowledge
• DNA is the genetic material of living
things.
• DNA structure.
• Difference between a prokaryote and
eukaryote.
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Prokaryotes v Eukaryotes
Prokaryotes are organisms lacking a
nucleus.
Think back to National 5…which organisms
did not have a nucleus?
e.g. bacteria
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DNA organisation in prokaryotes
Prokaryotes usually have a single circular
double stranded chromosome.
Some prokaryotes have a second
chromosome which can carry extra nonessential genes – this is called a plasmid.
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The DNA is tightly packaged with
proteins to form a nucleoid.
How long is the DNA?
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DNA and
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An Escherichia coli (E. coli) cell is 1 μm wide
by 2 μm long. (1 μm = 1000th of a mm)
The chromosome is approximately 1 mm
long.
So the chromosome is 1000 times the width
of the cell.
How do you cram it all in there?
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DNA and
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Supercoiling
Take a large elastic band.
Hold both ends and begin to twist it…what
happens?
Write a description in your jotter.
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DNA and
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Eukaryotes
Eukaryotes are organisms with a nucleus
containing several linear chromosomes.
Eukaryotes also have extra DNA out with
the nucleus – mitochondrial and
chloroplast DNA.
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Mitochondrial and chloroplast
DNA
Mitochondrial DNA is found in the
mitochondria of both plants and animals.
Chloroplast DNA is found only in plants.
These are inherited solely from the mother
along with the other cell organelles during
cell division.
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Mitochondrial DNA (mtDNA)
Circular, double stranded DNA.
Varies in size (15, 569 bp in humans, 80,000
bp in yeast to 2 million bp in some plants)
Codes for transfer RNA (tRNA), ribosomal
RNA and some proteins in the mitochondria.
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DNA and
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Chloroplast DNA (cpDNA)
Also, circular, double stranded DNA.
Between 80,000 and 600, 000 bp in size.
Chloroplasts can have multiple copies.
Codes for rRNA, tRNAs, proteins required
for transcription, translation and
photosynthesis.
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Endosymbiont theory
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DNA packaging in eukaryotes
One human chromosome – if pulled out – is
approximately 4 cm.
The cell packages this into a bundle of 1.2 –
2 μm long.
But you have 46 chromosomes – this is
approximately 1.84 metres of DNA in
every cell of your body.
That’s enough in your body to stretch to the
DNA and
moon and back!
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the
Genome
Stages of mitosis
The organisation of DNA in a eukaryotic
cell depends on the stage of mitosis they
are in.
Think back to National 5…what happens
during cell division…
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The stages of the mitosis have different
names.
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Level 1: Nucleosomes
DNA double helix is wrapped around histone
proteins forming nucleosomes (beads on a
string)
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DNA and
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The pieces of DNA between the
nucleosomes is known as linker DNA and
is a constant length.
The combination of DNA and protein is
called chromatin.
This level of organisation is seen through
out the cell cycle and mitosis.
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DNA and
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Level 2: Thick chromatin fibre
The chain of nucleosomes then folds into
a thicker chromatin fibre. Seen during
interphase.
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DNA and
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Level 3: Looped fibres
The thick chromatin fibre then folds
again, on a non-histone protein scaffold,
to form looped fibres. Seen in prophase.
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Level 4: More folds to make
chromosome
The folded chromatin then folds further.
To produce a condensed chromosome –
seen in metaphase.
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DNA and
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Genome
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DNA and
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Genome
Let’s package some DNA…
Need:
• 4 m string (to represent 4 cm DNA in
each cell)
• 80 milk bottle tops
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DNA and
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To represent the first level of packaging
– nucleosomes – thread beads at regular
intervals of 5 cm along the string, making
a knot before and after each bead to
keep it in place and to also demonstrate
the reduction in size of DNA as it wraps
around the histones.
Work through the rest of the stages
from memory or using the beads on a
string image sheet.
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DNA and
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Genome
Key concepts
•
•
•
•
•
•
•
DNA exists in very long molecules that are packaged and organised in
cells.
The organisation of DNA is different in prokaryotes and eukaryotes.
Prokaryotes usually have a single circular chromosome.
Eukaryotes usually have several linear chromosomes, which are
packaged.
Eukaryotic cells also contain mitochondrial DNA, and chloroplast DNA in
green plants.
The DNA in chromosomes undergoes four stages of packaging to
achieve the most condensed state, seen during metaphase.
DNA combines with proteins to achieve its packaged state.
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DNA and
the
Genome