Edexcel GCSE Biology | 1) DNA

DNAIt’s structure and practicalEdexcell GCSE

gcsepresentations

*BASICS* !

DNA is fundamental for all life - it contains genes (short sections of DNA), which code for making different proteins, the building

blocks of life. !

DNA is what you find, tightly coiled up, inside all 46 chromosomes in every nucleus in every single human diploid body cell.

!It consists of two strands coiled together with a double helix

structure and just 4 base pairs:!!

A (adenine) <—> T (thymine)!

C (cytosine) <—> G (guanine)

*FRANKLIN & WILKINS* !

Franklin and Wilkins were the brains behind DNA and it’s structure:!

They emitted X-Rays at the crystallised DNA and recorded the patterns of the reflected rays onto a screen.

!From this they discovered the DNA’s helical structure

!

*WATSON & CRICK* !

Watson and Crick used information of the double helix !

and that the amount of A + G was equal to the amount of C + T to make a model of DNA with the paired bases.

*PRACTICAL EXPERIMENT* !

This practical shows DNA as a stringy white substance, which proves that there is DNA in cells of living organisms.

*STEP 1* !

Chop a big juicy fresh onion into tiny pieces.!

Put it into a beaker.

*STEP 2* !

Add salt and detergent to the onions and stir.!!!!!!!!!!!

The detergent will help to break down the cell membranes;!

The salt helps to make the DNA stick together.

*STEP 3* !

Put the beaker into a water bath.!!!!!!!!!!!

Set the water to 60 celsius for 15 minutes and then remove the solution.

*STEP 4* !

Cool the solution with ice.!!!!!!!!!!!

This stops the DNA from breaking down.

*STEP 5* !

Blend the mixture.!!!!!!!!!!!

To open the cell walls and reelase the DNA.

*STEP 6* !

Cool the solution with ice.!!!!!!!!!!!

This stops the DNA from breaking down.

*STEP 6* !

Filter the mixture.!!!!!!!!!!!

Removing any big parts of cell and stop froth.

*STEP 6* !

Add ice cold alcohol to the solution.!!!!!!!!!!!

Do this slowly, and you should see the DNA separating from the cells as a white stringy substance, which can be removed with a glass rod.

DNA