- Deoxyribose nucleic acid - Found in the nucleus - All DNA has a similar structure
Major components of DNA
- Sugars - Phosphates - Bases - These make up the ‘back bone’ of the DNA structure
4 types of bases (an their pairs)
- Adenine always pairs with thymine - Guanine always pairs with cytosine
What is a nucleotide?
- Sub-unit of the DNA molecule - Consists of one sugar, one phosphate and one base molecule
What is a chromosome?
DNA wrapped into a double helix structure around proteins called histones
What is a gene?
- Each section of DNA that controls a trait - Humans have around 25,000 genes in their chromosomes - Genes for a particular trait are controlled at a particular point in the chromosome
46 chromosomes (23 pairs)
23 single chromosomes
a pair of chromosomes containing the pair of genes for a particular trait
What are alleles?
- The alternative forms of a gene that are available - The pairs of alleles that each person inherits from their parents control and determine the characteristics of the individual - Many genes have two alleles (dominant and recessive)
What is a genotype?
- The genetic makeup of an individual - It is inherited - E.g. BB/bb/Bb
What is a phenotype
- The observed expression of the alleles for the characteristics - Determined by the genotype and the environment
What is monogenic inheritance?
- Exists when an individual receives just one pair of alleles for a trait - E.g. blood groupings - The traits are discrete (either there or not there) - Only two alleles for the trait - Only two phenotype
What is polygenic inheritance
- Many pairs of genes control the trait - Continuous variation of the trait - Alleles will have an additive effect - E.g. skin colour height and weight - Continuous traits = many phenotypes
- Is a polygenic trait - Many genes contribute - Continuous variation - Depends on the amount of melanin in the skin
What do melanocytes produce?
Melanocytes produce structures called melanosomes that contain melanin and are found in the bottom of the epidermis of the skin
How is melanin produced?
It is made from an amino acid called tyrosine
Amounts of melanocytes and melanosomes in different people
Most people have similar amounts of melanocytes and varying amounts of melanosomes
Darker skinned people
- Can convert more tyrosine into melanin because they have more tyrosinase - More melanosomes containing more melanin - There are also more evenly distributed throughout the skin - This prevents burning
- Less tyrosinase and therefore can’t convert as much tyrosine into melanin - Melanin is not evenly distributed and form clumps - Burns when exposed to UV light
- Brings about the production of melanin - Stimulated by UV rays - People with the enzyme will become darker skinned in the sun - Those with less tyrosinase will tan less well - Therefore, the environment affects the phenotype - Darker skin will prevent sun burn
What is a mutation?
- A permanent change to a gene or chromosome leading to a new characteristic in an organism - Due to changes in the base sequences in a section of DNA which alters the types of proteins that will be synthesised by the body - Can be advantageous or disadvantageous
What is a mutant?
An organism with a characteristic resulting from a mutation
What are mutagens?
- Agents that are known to increase the rate at which mutations occur - E.g. UV light, X-rays, radiation, chemicals such as sulphur dioxide or antibiotics
- Occur in a body cell - The reproductive cells are not affected and once the organism dies, the mutation will be lost
- Mutation occurring in the gametes - The individual is not usually affected - The individual will produce gametes with changed DNA
Main types of mutations
- Gene mutations - Chromosomal mutations
- Change in a single gene - They occur during the replication of the DNA molecule before cell division - If the DNA copies incorrectly, the changed DNA will continue to be copied in all future divisions - May code for a new amino acid and a completely different protein
What is a point mutation?
a change in one base
- The result of a missing protein - The protein responsible for the pigment (melanin) of hair, skin and eyes is missing - Patients have whitish blond hair, extremely pale skin and pinkish eyes