DNA structure and purpose

DNA is found in the nucleus of any cell [except red blood cells which have none]. They must be able to do two things:

1. Contain instructions which control protein production hence which control the characteristics of an organism.
2. Be able to replicate in a way that preserves all the genetic information required. It should be able to be transferred from a parent to a child.

DNA is in the form of a double helix, with two strands forming the shape of a twisted ladder. Each 'rung' of the ladder consists of a base, Adenine, Thymine, Cytosine and Guanine. Bases are held together by intermolecular forces.

A sequence of bases contains a code that informs the cell of what protein to make. There are millions of sequences and a vast number of proteins that can be produced. A section of DNA that contains the instructions to create on complete protein is called a Gene.

DNA = sentence
Gene= Paragraph
Chromosome= Chapter

We have a pair of each chromosome so that there we have two copies of each gene which results in more variation within our species. A gene can come in different forms such as alleles. Alleles can be either dominant or recessive. We may have a chromosome from our mother who has the gene for brown hair and a chromosome from our father who has a gene for blue eyes. We have characteristics from both parents. A child who looks very much like their parent means their alleles are most likely dominant.

Source: BBC Bitesize

A pair of chromosomes carry the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called alleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them.

A dominant allele is the one that will be made.
A recessive allele is one that will be masked.
A homozygous genotype is one that contains two of the same allele.
A heterozygous genotype is one that contains two different alleles, one dominant and one recessive.
Phenotype: A set of characteristics in an organism that originate from a combination of a genotype and interaction with the environment.
Genotype: Set of genes in DNA responsible for a particular trait.