DNA and the Genome Unit 1: CFE Higher Biology June – October Unit Assessment – before October break

Key area 1: The structure of DNA

Take a moment…think… DNA What have you learned about DNA so far? In groups, collect an sheet of poster paper. Nominate a scribe. Write down as many things you can remember about DNA from your school/life so far. DNA

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.

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”.

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.

The structure of DNA

Prior knowledge DNA is the genetic material.

Deoxyribonucleic acid The DNA molecule is comprised of two chains of nucleotides. The nucleotides are comprised of a sugar, a phosphate and a base.

Nucleotides 5’ end Base Phosphate 3’ end 5’ pronounced “5 prime” Sugar

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…

Carbon 5 Carbon 1 Carbon 4 Carbon 2 Carbon 3

Nucleotides in a chain are joined together by sugar-phosphate bond to make long chains of nucleotides – called polynucleotides. (poly = many)

The bases In DNA there are four different bases: Adenine Thymine Cytosine Guanine or A, T, C and G

Base pairing These bases are described as being complementary to each other. This means their shapes match up.

DNA base pair rules Adenine always joins to Thymine Cytosine always joins with Guanine The bases are joined by hydrogen bonds.

The double helix The chains of polynucleotides are joined together by the bases, by hydrogen bonds, for form a double helix structure.

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…

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’.

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.

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  

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.

Read the instruction sheet for more details.

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.

Organisation of DNA in prokaryotes and eukaryotes

Prior knowledge DNA is the genetic material of living things. DNA structure. Difference between a prokaryote and eukaryote.

Prokaryotes v Eukaryotes Prokaryotes are organisms lacking a nucleus. Think back to National 5…which organisms did not have a nucleus? e.g. bacteria

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.

The DNA is tightly packaged with proteins to form a nucleoid The DNA is tightly packaged with proteins to form a nucleoid. How long is the DNA?

An Escherichia coli (E. coli) cell is 1 μm wide by 2 μm long 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?

Supercoiling Take a large elastic band. Hold both ends and begin to twist it…what happens? Write a description in your jotter.

Eukaryotes Eukaryotes are organisms with a nucleus containing several linear chromosomes. Eukaryotes also have extra DNA out with the nucleus – mitochondrial and chloroplast DNA.

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.

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.

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.

Endosymbiont theory

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!

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…

The stages of the mitosis have different names.

Level 1: Nucleosomes DNA double helix is wrapped around histone proteins forming nucleosomes (beads on a string)

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.

Level 2: Thick chromatin fibre The chain of nucleosomes then folds into a thicker chromatin fibre. Seen during interphase.

Level 3: Looped fibres The thick chromatin fibre then folds again, on a non-histone protein scaffold, to form looped fibres. Seen in prophase.

Level 4: More folds to make chromosome The folded chromatin then folds further. To produce a condensed chromosome – seen in metaphase.

Let’s package some DNA… Need: 4 m string (to represent 4 cm DNA in each cell) 80 milk bottle tops

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.

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.