DNA & the Genome (Edexcel GCSE Biology)

The Genome

• The entire set of the genetic material of an organism is known as its genome

• In 2003, scientists completed a 13-year project in which they sequenced the genes that make up the whole human genome

  • This project was named the human genome project

• A gene is a section of DNA that codes for a particular sequence of amino acids

• These sequences of amino acids form proteins

• Genes control our characteristics as they code for proteins that play important roles in what our cells do

The Structure of DNA

• DNA, or deoxyribonucleic acid, is the genetic material found in the nucleus of a cell

• DNA is a polymer made up of two strands coiled around to make a double helix

The structure of DNA and complementary base pairs

Nucleotides

• DNA is a polymer (a molecule made from many repeating subunits)

• These individual subunits of DNA are called nucleotides

• Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar

A nucleotide

• The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) and the base pairs of each strand connect to form the rungs of the ladder

DNA is a polynucleotide - this means it is a polymer made up of many repeating subunits (monomers) known as nucleotides

Base pairing

• There are four different nucleotides

• These four nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached

• There are four different bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G)

• The bases on each strand pair up with each other, holding the two strands of DNA in the double helix together

• The bases always pair up in the same way:

  • Adenine always pairs with Thymine (A-T)

  • Cytosine always pairs with Guanine (C-G)

• This is known as ‘complementary base pairing’

• The complementary base pairs (A-T and C-G) pair up by forming weak hydrogen bonds with each other

DNA complementary base pairs

Extracting DNA

• DNA can actually be extracted from fruit using some basic classroom equipment

• Fruits that have relatively large amounts of DNA in their cells, such as strawberries, bananas and kiwis, can be used

Method

• Mash the fruit and mix this into a beaker containing a solution of detergent (e.g. washing up liquid) and salt

  • The detergent breaks down the cell membranes (and the nuclear membranes), causing the fruit cells to release their DNA

  • The salt causes the DNA to stick together

• Filter the mixture into a test tube

  • This removes any debris (big, insoluble bits of cell) from the mixture

• Gently add some ice-cold ethanol to the filtrate (the filtered mixture) by pouring the ethanol slowly down the side of the test tube

  • This causes the DNA to precipitate (i.e. to come out of solution) as DNA is not soluble in cold alcohol

• The DNA will appear as a stringy white precipitate (a solid)

• If needed, this precipitate can be carefully extracted using a glass rod

A simple method for extracting DNA from fruit