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Biology Unit 2 Notes
Variation The differences that exist between individuals, there are two types:
Interspecific – variation that exists between different species
Intraspecific – differences that occur within a species, caused by genetic and environmental factors.
Individuals of the same species may seem similar but no two are exactly alike.
Genetic Factors:
All the members of a species have the same genes which makes them come from the same species, but individuals within a species can have different alleles (different versions of those genes)
The alleles an organism has make up its genotype - different genotypes result in variation in phenotype (the characteristics displayed by an organism)
Examples of variation in humans caused by genetic factors include eye colour and blood type
Genes are inherited from parents thus genetic variation is inherited.
Environmental Factors:
Phenotype is also affected by the environment
Plant growth is affected by the amount of minerals, such as nitrate and phosphate, available in the soil
Fur colour of the Himalayan rabbit is affected by temperature – most of its fur is white except the ear, feet and tail which are black, the black only develops in temperatures below 25 deg. C.
Identical twins are genetically identical – same alleles thus any differences are due to the environment.
Variation is often a combination of genetic and environmental factors. An individual may have the genetic information for a particular characteristic, but environmental factors may affect the expression of this characteristic.
In any group of individuals, there is a lot of variation however it’s not always clear if this variation is caused by the genes, the environment, or both.
Overeating – thought to be caused only by environmental factors, later discovered that food consumption increases brain dopamine levels in animals. Once enough dopamine was released, people would stop eating. Researchers discovered that people with one particular allele had 30% fewer dopamine receptors. They found that people with this particular allele were more likely to overeat. Therefore based on this evidence, scientists now think that overeating has both genetic and environmental causes.
Antioxidants – many foods contain antioxidants- compounds that are thought to play a role in preventing chronic diseases, e.g. berries. Scientists thought that the berries produced by different species of plant contained different levels of antioxidants because of genetic factors. Experiments were carried out to see if environmental conditions affected antioxidant levels found that the environmental conditions caused a great deal of variation. Scientists now believe that antioxidant levels in berries are due to both genetic and environmental factors.
GeneticsDNA is a polynucleotide – made up of lots of nucleotides joined together. Each nucleotide is made from a pentose sugar, a phosphate group and a nitrogenous base. The sugar in DNA nucleotides is a deoxyribose sugar. Each nucleotide has the same sugar and phosphate. The base on each nucleotide can vary though. There are four possible bases…
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Two polynucleotide strands join together to form a double-helix by hydrogen bonds between the bases. Each base can only join with one particular partner – specific base pairing.
Adenine always pairs with Thymine (A----T)
Guanine always pairs with Cytosine (G----C)
The two strands wind up to form the DNA double-helix
Contains your genetic information – all the instructions needed to grow and develop from a fertilised egg to a fully grown adult.
Molecules are v. long and are coiled up very tightly thus a lot of genetic information can fit into a small space in the cell nucleus.
Molecules have a paired structure, making it much easier to copy itself – self replication. Important for cell