unit 1: dna and the genome organisation of dna in prokaryotes and eukaryotes

Unit 1: DNA and the Genome

Organisation of DNA in prokaryotes and

eukaryotes

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Prior knowledge

• DNA is the genetic material of living things.

• DNA structure.• Difference between a prokaryote and

eukaryote.

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Prokaryotes v EukaryotesProkaryotes 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 non-essential 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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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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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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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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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 moon and back!

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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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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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Level 2: Thick chromatin fibre

The chain of nucleosomes then folds into a thicker chromatin fibre. Seen during interphase.

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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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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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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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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.