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Biology · DNA and Protein Synthesis

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Biology · Edexcel Paper 1

DNA and Protein Synthesis

The Structure of DNA

DNA carries the instructions for building every living thing.

  • DNA Deoxyribonucleic acid: the molecule that carries the genetic code, made of two strands in a double helix.
  • Polymer A long molecule made of many small repeating units joined together.
  • Nucleotide The repeating unit of DNA, made of a sugar, a phosphate group and a base.
Three joined nucleotides, each made of a phosphate group, a sugar and a base.
Each nucleotide has a phosphate, a sugar and one of four bases.

The Genome and the Human Genome Project

A genome is an organism's complete set of DNA. This lesson explains what the human genome contains, how the Human Genome Project mapped it.

  • Genome The entire DNA of an organism.
  • Gene A section of a DNA molecule that codes for a specific protein.
  • Human Genome Project An international project, completed in 2003, that found the order of all of the bases in human DNA.
Three panels showing the human genome has 23 pairs of chromosomes, about 20 000 genes and 3 billion base pairs.
The human genome is stored in 23 pairs of chromosomes in the nucleus of every body cell.

Benefits and concerns about knowing your genome

  • Benefits Early diagnosis, and prevention of some diseases; Treatment matched to the person; Faster development of new drugs
  • Concerns Privacy: who should have access to the information?; Insurance and employers could discriminate against people; Cost, and an unequal chance of benefiting

The Human Genome Project

  • 1990 Scientists from many countries start work, with the aim of…
  • Cut up the DNA Samples of DNA from volunteers are broken into small pieces that…
  • Read the pieces The order of bases in each piece is found by a sequencing machine.
  • Put it together Computers match overlapping pieces and assemble them into the…

Extracting DNA from Fruit

You can see DNA with your own eyes. This lesson explains a simple method to extract DNA from fruit using household chemicals, why each step is needed.

  • Extraction Separating a substance from the material it is in.
  • Detergent A chemical that breaks down cell membranes by dissolving the lipids in them.
  • Filtering Separating solid pieces from a liquid, using filter paper.
Five steps showing fruit being mashed, mixed with detergent and salt, filtered, covered with cold ethanol, and white strands of DNA appearing.
The white strands that appear between the layers are made of millions of DNA molecules.

Qualitative and quantitative results

  • Qualitative results Describe what you see, such as stringy, white and cloudy; Quick and easy to do; Different observers may disagree
  • Quantitative results Measure the length of the strands, or their mass; Give numbers that can be compared and graphed; Less affected by opinion

The method

  • Mash the fruit Crush a small amount of fruit in a beaker or bag, to break open the cells.
  • Add the extraction solution Add a solution of detergent and salt, and mix gently for several minutes.
  • Filter the mixture Pour it through a filter paper or a coffee filter into a clean tube.
  • Add cold ethanol Tilt the tube and slowly pour ice-cold ethanol down the side.
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Biology · Edexcel Paper 1

DNA and Protein Synthesis continued

From Genes to Proteins (Higher tier) (Separate only)

A gene is a code for a protein. This lesson is for separate Biology only.

  • Protein A molecule made of one or more chains of amino acids, folded into a specific shape.
  • Amino acid The small unit that proteins are made from. There are 20 different ones.
  • Polypeptide A long chain of amino acids.
A gene is read in triplets of bases, each triplet gives an amino acid, and the chain of amino acids folds into a protein with an active site.
The order of bases decides the order of amino acids, and the folded chain makes the protein.

DNA and proteins

  • DNA Made of nucleotides; Four different bases in the code; A double helix, with the same structure in all genes
  • Proteins Made of amino acids; 20 different amino acids; Many different folded shapes

From gene to working protein

  • The gene A section of DNA contains a sequence of bases.
  • The triplets The bases are read in groups of three.
  • The polypeptide Amino acids are joined in the order specified by the triplets.
  • The protein The polypeptide folds into a specific shape, which lets it do its job.

Protein Synthesis (Higher tier) (Separate only)

A protein is made in two stages: transcription in the nucleus and translation at a ribosome.

  • Transcription Making a complementary mRNA copy of a gene from DNA.
  • Translation Reading the code in mRNA at a ribosome to build a polypeptide.
  • RNA polymerase The enzyme that makes mRNA by joining RNA nucleotides complementary to the DNA.
Transcription of a gene into mRNA in the nucleus, and translation of the mRNA into a polypeptide at a ribosome using tRNA.
The code is copied into mRNA, then read at the ribosome to build the polypeptide.

DNA and RNA

  • DNA Two strands in a double helix; Contains the sugar deoxyribose; Bases A, T, C and G
  • RNA A single strand; Contains the sugar ribose; Bases A, U, C and G, with uracil (U) instead of thymine

Transcription in the nucleus

  • RNA polymerase binds The enzyme RNA polymerase attaches to the non-coding DNA that…
  • The DNA unwinds The hydrogen bonds between the bases break and the two strands…
  • mRNA is built RNA polymerase moves along the coding DNA of the gene and joins RNA nucleotides.
  • mRNA leaves The complementary mRNA strand is released, the DNA closes up.

Genetic Variants and Their Effects (Higher tier) (Separate only)

A change in the DNA can change a protein. This lesson is for separate Biology only.

  • Genetic variant A difference in the base sequence of DNA between individuals.
  • Mutation A change in the base sequence of DNA.
  • Substitution A mutation in which one base is replaced by another.
Three rows showing a normal gene, a variant in non-coding DNA that reduces the amount of protein, and a variant in coding DNA that changes the protein.
A variant in front of the gene changes how much protein is made. A variant in the gene changes what protein is made.

Non-coding and coding variants

  • Variant in non-coding DNA In the region in front of the gene; Changes how well RNA polymerase binds; Changes the quantity of protein produced
  • Variant in coding DNA Inside the gene itself; Can change the amino acid sequence; Can change the shape and activity of the protein