3.4–3.5DNA and the genome
The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer made up of two strands forming a double helix, and it is contained in structures called chromosomes.
A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
The genome of an organism is the entire genetic material of that organism. The whole human genome has now been studied, and this will have great importance for medicine in the future.
- DNA
- A polymer made of two strands forming a double helix, carrying the genetic material.
- Gene
- A small section of DNA on a chromosome, coding for a particular sequence of amino acids to make a specific protein.
- Genome
- The entire genetic material of an organism.
Why understanding the human genome matters is limited to three things, and they make a tidy answer:
• the search for genes linked to different types of disease • understanding and treatment of inherited disorders • use in tracing human migration patterns from the past.
3.6Extracting DNA from fruit
DNA can be got out of plant cells with kitchen chemicals, and the exam asks for the reason behind each step rather than the list itself.
Mash the fruit — kiwi or strawberry works well — to break the cell walls apart. Mix it with a solution of detergent and salt: the detergent breaks down the cell membranes and the membranes around the nuclei, releasing the DNA, and the salt makes the DNA molecules stick to one another so they will be visible. Warm the mixture in a water bath at about 60 °C for around fifteen minutes, which helps break the membranes down and stops enzymes in the cell from cutting up the DNA. Cool it, then filter to remove the cell debris.
Finally, pour ice-cold ethanol gently down the side of the tube so it forms a layer on top of the filtrate. DNA is insoluble in cold ethanol, so it comes out of solution where the two layers meet and can be lifted out on a glass rod as white stringy strands.
Common mistake
The white strands are not single DNA molecules — a single molecule is far too thin to see. What you can see is many millions of them clumped together, which is what the salt was for.
3.7B–3.10BDNA structure
Biology only — not on the Combined Science specification
DNA is a polymer made from four different nucleotides. Each nucleotide consists of a common sugar and phosphate group, with one of four different bases attached to the sugar. The long strands consist of alternating sugar and phosphate sections, with a base attached to each sugar.
DNA contains four bases: A, C, G and T.
A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.
You may be asked to interpret a diagram of DNA structure, but you will not be required to reproduce it.
Higher tier only: in the complementary strands, C is always linked to G on the opposite strand, and T to A. That pairing is what lets DNA be copied accurately — each strand carries the information needed to rebuild the other.
Higher tier only: proteins are synthesised on ribosomes, according to a template. Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order. When the chain is complete it folds up to form a unique shape, and that shape is what enables the protein to do its job — as an enzyme, a hormone, or a structure in the body such as collagen.
You are not expected to know the structure of mRNA, tRNA, or the detailed structure of amino acids or proteins.
Higher tier only: mutations occur continuously. Most do not alter the protein, or only alter it slightly, so its appearance or function is not changed. A few mutations code for an altered protein with a different shape — an enzyme may no longer fit the substrate binding site, or a structural protein may lose its strength. That is the link back to enzymes and digestion: the active site's shape is everything, and a mutation is one way to change it.
Higher tier only: not all parts of DNA code for proteins. Non-coding parts can switch genes on and off, so variations in these areas may affect how genes are expressed. So a genetic variant can influence the phenotype two ways: in coding DNA by altering the activity of a protein, and in non-coding DNA by altering how genes are expressed.
Mutations come in kinds worth picturing. A substitution swaps one base for another, which changes at most one amino acid. An insertion adds a base and a deletion removes one, and either shifts every three-base group after it — so the whole rest of the sequence is read differently and the protein is usually ruined. Modelling insertions and deletions with a strip of letters shows why they matter so much more than a single swap.
Common mistake
Three bases code for one amino acid — not one base, and not one gene per protein chain of any length. And a mutation is not automatically harmful: most change nothing at all, which is why populations carry so much variation without falling apart.
- Nucleotide
- the monomer from which DNA is built, consisting of a common sugar and phosphate group with one of four bases attached to the sugar
- Genome
- the entire genetic material of an organism
Before you practise
- DNA is two strands in a double helix, held together by complementary base pairs: A with T, C with G.
- A gene is a section of DNA that codes for a protein; the genome is all of an organism's DNA.
- Three bases code for one amino acid.
- Transcription copies a gene into mRNA in the nucleus; translation builds the protein on a ribosome.
- A protein folds into a shape determined by its amino acid order, which is why a change in the order can stop an enzyme working.
- In the fruit extraction, detergent breaks the membranes, salt makes the DNA clump, and cold ethanol brings it out of solution.