DNA & the Genome (Edexcel GCSE Biology: Combined Science)

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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

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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

DNA double helix formation, downloadable AS & A Level Biology revision notes

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, IGCSE & GCSE Biology revision notes

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

The DNA helix is made from two strands of DNA held together by hydrogen bonds, IGCSE & GCSE Biology revision notes

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 base pairs, IGCSE & GCSE Biology revision notes

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

Extracting DNA from fruit, downloadable IGCSE & GCSE Biology revision notes

A simple method for extracting DNA from fruit

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Alistair

Author: Alistair

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.