DNA structure, discovery and inheritance

In a nutshell

Deoxyribonucleic acid is a large molecule found in the nucleus of almost every cell of the body. Characteristics are determined by small sections of DNA called genes. DNA is such a large molecule that it needs to be coiled around tightly to form chromosomes that can fit inside the nucleus.

Key words

KEY WORDS	DEFINITIONS
DNA	Short for deoxyribonucleic acid. It is a long molecule in almost every cell that contains all the genetic information that determines the characteristics of a living organism.
Base	The unit that makes up the genetic code.
Gene	A small section of DNA that codes for a specific characteristic.
Genome	The complete set of genes present in an organism.
Chromosome	A coiled structure of DNA that carries genetic information in the form of genes.
Allele	A different version of a gene that produces a variety of the characteristic the gene codes for.

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. DNA contains four bases: adenine, thymine, cytosine and guanine - or A, T, C and G for short. The bases of one strand bind to the bases on the other strand to join the strands together in specific base pairs. A always binds to T, and C always binds to G.

Science; Genetics and evolution; KS3 Year 7; DNA structure, discovery and inheritance

Discovery

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 code 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 - this is what introduces variety to characteristics.

Example

Specific genes will code for eye colour - however, different alleles of these genes will code for the different colours.

The complete set of genes in an organism is called its genome. Most of the genes in human DNA have been discovered and mapped out in the Human Genome Project. There are about $23,000$ genes in the human genome. However, as mentioned there can be two or more alleles of the same gene and we have identified some but not all the alleles possible yet.

Since DNA is such a large molecule, it has to be organised so it can fit into the nucleus of a cell. Therefore, DNA is coiled tightly to form structures called chromosomes. Each chromosome carries a specific set of genes.

Science; Genetics and evolution; KS3 Year 7; DNA structure, discovery and inheritance

Inheritance

DNA is inherited from parents. This is why you might share some characteristics like your hair colour or nose shape with your parents. Most cell have $46$ chromosomes except for gametes. These contain $23$ chromosomes each, because their nuclei fuse during fertilisation so that there are a total of $46$ chromosomes.

This is how genetic information is inherited during fertilisation. Most cell nuclei after fertilisation have two copies - or two alleles - of each gene. Through this process, a child inherits a mixture of both parents' characteristics.