Paper 1 - Dna + Extraction

Question 1

Structure of dna

Answer 1

Dna is a polymer made up of complementary base pairs joined together by weak hydrogen bonds

Question 2

Who discovered the structure of dna ?

Answer 2

Did xray crystallography showing that dna was a spiral molecule, Watson and crick used x ray 51 to deduce the double helix model of dna. Wilkins was Franklin’s advisor.

Question 3

How can we get to DNA ?

Answer 3

To access the DNA you need to get through the cell membrane AND the nuclear membrane

Question 4

How is DNA held In place ?

Answer 4

Dna surrounded by proteins that protect it and hold the shape of the chromosome - long DNA molecule in the middle of the chromosome

Question 5

What are the 4 nitrogenous DNA bases called ?

Answer 5

• Adenine
• thymine
• cystosine
• guanine

Question 6

Nucleotide structure

Answer 6

Each base is attached to a sugar (deoxyribose), each sugar is attached to a phosphate group.
Nucleotide - base + sugar + phosphate group.

Question 7

What is the backbone of DNA described as and why is it a polymer ?

Answer 7

Backbone of DNA = sugar and phosphate group. Many similar units repeated so DNA is a polymer !

Question 8

What is dna held together by x2

Answer 8

Base pairs held together by weak hydrogen bonds - weak force of attraction,
-this is because parts of DNA bases have very slight electrical charges_a negatively charged base is attracted to a slightly positively charged base

Question 9

How many hydrogen bonds do we have between bases ?

Answer 9

C and G form 3 hydrogen bonds.

A and T form 2 hydrogen bonds.

Question 10

Why are humans slightly different from one another ?

Answer 10

Different order of bases in our DNA

Question 11

DNA practical

Answer 11

• Dissolve 3g of salt into 100cm3 of water in a 250cm3 beaker - (salt makes dna clump together).
• add 10cm3 of washing up liquid and stir gently until it dissolves - (detergent breaks down cell surface and nuclei membranes).
• mash 50g of peas into pestle and mortar - (increase surface area for salt and detergent to work on).
• add mashed peas into solution made in steps 1 and 2
• stir mixtures for 1 min
• place the beaker into a water bath for 15 mins to speed up the reaction.
• place the solution through a filter. Collect the filtrate and discard the residue.
• using a measuring cylinder, measure out 10cm3 of the filtrate into a test tube
• use a pipette to add two drops of the protease solution into the test tube - (protease breaks down proteins holding together dna in chromosomes).
• tilt test tube slightly, pour ice cold ethanol down the side slowly. Stop when you have the same amount of ethanol as filtrate. Leave for 2 mins. A white precipitate (dna insoluble in ethanol) should form between the filtrate and the ethanol- DNA!!!!!!!