DNA: structure, genes and chromosomes

In a nutshell

Deoxyribonucleic acid contains all the instructions needed for an organism to function. It is vital for us to understand the structure of DNA so that we can improve our understanding of the human body and our health, along with the biology of other organisms.

Structure

DNA is a long molecule made of two strands that are attached to each other and twisted in a spiral shape. This structure is called a double helix.

Each strand of DNA contains bases. These are the units that make up the genetic code. DNA contains four bases: adenine, thymine, cytosine and guanine; or A, T, C and G for short. Bases are a part of a larger molecule called a nucleotide. A nucleotide consists of the base, a sugar molecule (in the case of DNA, the sugar molecule is called deoxyribose) and a phosphate group.

The bases of one strand bind to the bases on the other strand to join the strands together in specific base pairs. This pairing is also known as complementary base pairing. A always binds to T and C always binds to G. The bases are bonded by hydrogen bonds.

Biology; Genetics; KS4 Year 10; DNA: structure, genes and chromosomes

The structure of DNA was only recently discovered. James Watson and Francis Crick are credited with building the first model of DNA in $1953$ . They did this by using data from other scientists' experiments, such as the X-ray diffraction images of DNA produced by Rosalind Franklin and Maurice Wilkins.

Watson, Crick and Wilkins received a Nobel Prize for their work in $1962$ , but unfortunately Rosalind Franklin did not. This is because she passed away before the prize was awarded, and the Nobel Prize is not given posthumously.

Genes and chromosomes

Small sections of DNA are responsible for specific characteristics such as hair colour. These small sections are called genes. Genes can vary slightly between people so that there are different versions of the same gene called alleles. Alleles introduce variety to characteristics.

Example

Specific genes will be responsible for hair colour, however, different alleles of this gene will code for the different colours.

Genes code for different sequences of amino acids, which are the small molecules that make up proteins. These different amino acid sequences form different types of proteins. The type and combination of proteins is what ultimately makes up a characteristic.

Example

A gene may code for a sequence of amino acids that makes up a structural protein in hair. This might determine the quality or colour of your hair. When a mutation occurs to change the sequence of bases on a gene, the sequence of amino acids also changes. This will result in a change in the proteins formed and characteristics expressed.

DNA is a large molecule so it has to be organised to fit into the nucleus of a cell. It is coiled tightly to form structures called chromosomes. Each chromosome carries a specific set of genes. Most cells have $46$ chromosomes except for the sex gametes. These contain $23$ chromosomes each, because they fuse during fertilisation and share their DNA so that there are a total of $46$ chromosomes.

Body cells are called diploid cells because they contain two copies of chromosomes; gametes are called haploid cells because they contain only one copy of the chromosome.

Biology; Genetics; KS4 Year 10; DNA: structure, genes and chromosomes

Investigating DNA from strawberries

Even you can have a go at investigating DNA. You can isolate DNA from strawberries. You will need the following:

Strawberries
Plastic bag
Laundry detergent
Salt
Water
Plastic cup
Coffee filter paper or fine sieve
Rubbing alcohol (cooled before the experiment)
Coffee stirrer or wooden spoon

Biology; Genetics; KS4 Year 10; DNA: structure, genes and chromosomes

procedure

1.	Remove any greens from the strawberries.
2.	Put the strawberries in a plastic bag and crush gently until it is completely pureed. This will begin to break the strawberry cells to release DNA.
3.	Mix together laundry detergent, salt and some water and add this to the plastic bag. This will break up the strawberry cells more.
4.	Continue to crush the strawberries in the bag to properly mix in the detergent mixture.
5.	Place coffee filter paper or a fine sieve on top of a plastic cup and pour the mixture into the cup. This will get rid of any large bits of strawberry.
6.	Pour cold rubbing alcohol into the cup in equal amounts to your strawberry mix. DO NOT stir.
7.	Wait $5\ minutes$ . After this, you should see a layer of white substance over the strawberry mix. This is your isolated DNA.
8.	Use a coffee stirrer or wooden spoon to lift the white substance off the mix and into another container.