Starter/Task

• On to a show me board, write down as much as you know about DNA!

Higher BiologyCfE

• Unit 1: DNA and the Genome• Unit 2: Metabolism and Survival• Unit 3: Sustainability and

Interdependence

Course Content

Key Area 1.1 The Structure of DNAKey Area 1.2 Replication of DNAKey Area 1.3 Control of Gene ExpressionKey Area 1.4 Cellular DifferentiationKey Area 1.5 The Structure of the GenomeKey Area 1.6 MutationKey Area 1.7 EvolutionKey Area 1.8 Genomic Sequencing

Unit 1: DNA and the Genome

(a)The Structure of DNA(b)Organisation of DNA

Key Area 1.1 The Structure of DNA

Key Area 1.1(a) The Structure of DNA

• Name the molecules in a DNA nucleotide and identify them in a diagram.

• Name the type of bond on the backbone of the DNA molecule.

• Give the full names of the 4 DNA bases.• Describe the base pairing rule for DNA bases.• Describe the role of hydrogen bonds in the DNA

structure.• State the name of the coiled structure adopted by DNA.• Identify the positions of 3’ and 5’ carbons on a DNA

nucleotide.• Identify the positions of 3’ and 5’ ends on a DNA strand.• Describe how 2 strands of DNA align themselves to each

other.

Key Area 1.1 (a) Learning Outcomes

• DNA (deoxyribonucleic acid) is a complex molecule present in all living cells.

• DNA stores genetic information in its sequence of bases which determines the organisms genotype and the structure of its proteins.

The Structure of DNA

Task

• On a show me board, draw and label the structure of a nucleotide.

• A molecule of DNA consists of two strands each made up of repeating units called nucleotides.

A nucleotide is made up of a molecule of deoxyribose sugar joined to a phosphate group and an organic base.

• The shape of the deoxyribose sugar in a nucleotide is determined by the arrangement of carbon atoms in the molecule. Each carbon atom is numbered. This can help us describe the arrangement of a DNA molecule.

Note3C = 3’ (3 prime) carbon atom5C = 5’ (5 prime) carbon atom

• To produce a strand of DNA, a chemical bond forms between the phosphate group of one nucleotide and carbon 3 on the deoxyribose sugar of another nucleotide.

• This forms the molecule’s sugar-phosphate backbone.

Sugar-phosphate backbone

• There are four different types of nucleotide in a DNA molecule. They differ from each other because they have a different base.

The base attaches to carbon 1.

• Two strands of nucleotides become joined together by hydrogen bonds forming between their bases.

• Hydrogen bonds are weak so the strands can easily become separated.

• Each base can only join up with one other type of base:– Adenine (A) always bonds with Thymine

(T)– Guanine (G) always bonds with Cytosine

(C)

• This is called the base-pairing rule.

• DNA takes the form of a double helix.

• DNA is made up of two antiparallel strands. This means that the strands run in opposite directions to each other.

• Using the numbered carbon atoms on the deoxyribose sugar allows us to show the antiparallel structure.

Left: 5’ 3’Right 3’ 5’

1. Name the molecules in a DNA nucleotide and identify them in a diagram.

2. Name the type of bond on the backbone of the DNA molecule.

3. Give the full names of the 4 DNA bases.4. Describe the base pairing rule for DNA bases5. Describe the role of hydrogen bonds in the DNA

structure.6. State the name of the coiled structure adopted by DNA.7. Identify the positions of 3’ and 5’ carbons on a DNA

nucleotide.8. Identify the positions of 3’ and 5’ ends on a DNA strand.9. Describe how 2 strands of DNA align themselves to each

other.

Key Area 1.1(a) You should now be able to . . .

Starter/Task

On to a show me board:1) Describe the base pairing rule.2) Name the carbon that a base

attaches to.3) Name the type of bond that holds

bases together.

Key Area 1.1(b) The organisation of DNA

• Identify prokaryotes and eukaryote cells from diagrams.

• Describe the key similarities and differences between prokaryote and eukaryote cells.

• Describe structure of a plasmid and name the types of cells where they are found.

• Describe structure of circular chromosomes and identify the location and types of cells where they are found.

• Compare the DNA found in mitochondria and nucleus of eukaryote cells.

• Describe the DNA in linear chromosomes found in nucleus of eukaryote cells.

Key Area 1.1(b) Learning Outcomes

Prokaryotes and Eukaryotes

What are Prokaryotes?• Prokaryotes are organisms that lack

a true membrane-bound nucleus.• Bacteria are examples of

prokaryotes.• Their DNA is found in the cytoplasm

of the cell.

What are Eukaryotes?• Eukaryotes are organisms which

have a membrane bound nucleus that stores their genetic material.

• Animals, plants and fungi are examples of eukaryotes.

• Double stranded DNA can be either circular or linear.

Circular DNA

Linear DNA

• Prokaryotes have a large circular chromosome.

• They sometimes also have smaller rings of DNA called plasmids.

Circular plasmids may also be found in yeast (a

fungus), which is classified as a eukaryote!

A bacterial cell

• In eukaryotes, the DNA is found tightly coiled into linear chromosomes.

• DNA can also be found in mitochondria (mtDNA) and chloroplasts (cpDNA) where it forms circular chromosomes. This DNA is used to make proteins essential to the functioning of the organelle.

Some scientists think that mitochondria and chloroplasts originated from prokaryotic cells that were at some point engulfed by larger cells!

• In linear chromosomes found in eukaryotes, the DNA strand would be several times longer than the length of the cell to which it belongs.

• Therefore, the DNA is actually tightly coiled and packaged around bundles of proteins in order to store it efficiently.

Summary – Prokaryotic DNA vs. Eukaryotic DNA

Characteristic Prokaryotic cell Eukaryotic cell

Organism that has this type of cell

Bacteria Fungi, green plants and animals

True nucleus bound by membrane

Absent Present

Organisation of chromosomal DNA

Composed of a ring of DNA associated with few or no proteins

Composed of DNA in linear form associated with proteins

Plasmids (each consisting of a small ring of DNA)

Present in many types of bacteria cell

Present in some yeasts; absent in plant and animal cells.

Chloroplasts (each containing several small circular chromosomes)

Absent Present in green plant cells

Mitochondria (each containing several small circular chromosomes)

Absent Present

Ribosomes Present Present

QQT task

• Using your notes from key area 1.1(a) and 1.1(b), make question cards with the answer.

• Find a partner and quiz them using your question cards (it’s important you tell them the correct answer if they get it wrong).

• They will quiz you using their question cards.• Swap all cards with each other and then move to a new

partner to repeat the process.• You will now have 7 minutes to quiz as many people as you

can.• At the end of the 7 minutes, your teacher will collect in your

quiz cards and use these to quiz the class.

Q: What does DNA stand for?

A: Deoxyribonucleic acid

• Identify prokaryotes and eukaryote cells from diagrams.

• Describe the key similarities and differences between prokaryote and eukaryote cells.

• Describe structure of a plasmid and name the types of cells where they are found.

• Describe structure of circular chromosomes and identify the location and types of cells where they are found.

• Compare the DNA found in mitochondria and nucleus of eukaryote cells.

• Describe the DNA in linear chromosomes found in nucleus of eukaryote cells.

Key Area 1.1(b) You should now be able to..

Practical Technique Extracting DNA

• DNA can be isolated from cells of pea seeds.

• You may have tried this with kiwi fruit.• When kiwi fruit is used instead of peas,

most often white strands that form as a precipitate in the upper layer of cold ethanol are made of pectin, not DNA.

• The DNA is obscured by pectin, this result is described as a false positive.

Extraction of DNA from Peas

• You will now carry out the technique.

Step 1 Step 2 Step 3

Step 4 Step 5 Step 6

Step 7 Step 5

Equipment Safety glasses50g peas10ml washing up liquid (measure with syringe)3g salt90ml distilled water10ml of very cold ethanolAccess to 60˚C water bathAccess to ice bath2-3 drops Protease enzymePestle and mortarFilter paper & funnel 2 beakersBoiling tubeStirring rodTest tube rack

Extracting Pea DNA – Class Method1. In a clean beaker dissolve 3g of salt in 90ml of distilled water.2. Add 10ml of washing up liquid and mix gently. 3. Grind 50g of peas in a pestle and mortar and add entire contents to

the beaker of salt, water and washing up liquid.4. Stand the solution in a 60˚C water bath for exactly 15 minutes.5. Cool the mixture by placing the beaker in an ice water bath for 5

minutes, stirring frequently.6. Filter this mixture into a second beaker. Ensure that any foam on top

of the liquid does not contaminate the filtrate.7. Using a syringe slowly add 10ml of your pea extract to a boiling tube,

try not to create any bubbles by running the solution down the inside of the boiling tube.

8. Add 2-3 drops of protease enzyme to the boiling tube and mix well.9. Very carefully pour ice cold ethanol (no more than 10ml) down the

inside of the boiling tube, to form a layer on top of the pea extract.10. Leave the tube, undisturbed in a test tube rack, for a few minutes.11. Nucleic acids (DNA and RNA) will precipitate into the upper layer.12. You may scoop out the DNA and RNA using a wire loop or tweezers.

MethodFilter solution through a filter funnel lined with filter paper

Results

DNA precipitate forms in the layer of ice cold ethanol