AQA Unit 2 Biology

Unit 2: The variety of living organisms.Variation: Interspecific variation = if one species differs from another Intraspecific variation = if members of the same species differ from each othercontinuous variation = variation in which organisms don't fall into distinct categories but show graduations from one extreme to another, represented by a normal distribution curve and can be effected by both genetic and environmental influences discontinuous variation = variation shown when the characters of organisms fall into distinct categories, represented by a bar graph can only be effected by genetic influencesgenetic differences are due to the different genes that each individual organism possesses. genetic variation arises as a result of:- mutations, the sudden changes in the base sequence of DNA, these may or may not be passed on to the next generation-meiosis, form of nuclear division which forms the gametes-fusion of gametes during fertilisation which is a random processDNA and Meiosis:DNA is a polynucleotide, double helix structure.Universal = the triplet code codes for the same amino acid in all living organisms Degenerate = more than one triplet code codes for one amino acida nucleotide is made up of three components- a deoxyribose sugar- a phosphate group - an organic base (Adenine, Thymine, Cytosine and Guanine)A is complementary to T C is complementary to GDNA is very stable and can pass from generation to generation without change, the two strands are joined by hydrogen bonds therefore can separate for replication and protein synthesis, back bone and inward shape protects from outside chemicals and processesThe triplet code: genes are sections of DNA that contain the coded information for making polypeptides. a polypeptide is a sequence of amino acids. there are 64 possible codes and only 20 amino acidsDNA and chromosomes: Genetic diversity: The variety of life:The cell cycle:Cellular organisation:Exchange and transport:Classification: Evidence for relationships between organisms:Adaptation and selection:- Genetic variation in bacteria: one of the most diverse and adaptable groups of organisms is bacteria. - Bacteria's adaptation to a changing environment can be seen in their ability to develop a resistance to antibiotics .-Diversity in organisms arises from changes to its DNA which can occur in two ways: Changing the quantity or structure of the DNA of an organism. this is known as a mutation. Recombining the existing DNA of two individuals. This occurs during sexual reproduction. Bacteria also increase diversity by changing there DNA in a similar manner to other organisms, namely by mutations and conjugation. Mutations - are changes in DNA that results in different characteristics. Mutations arise in many ways. Eg. one or more bases in DNA sequence may be added, deleted or replaced by others during replication. A triplet of bases on a DNA molecule codes for a single amino acid. A change in base sequence may result in a different sequence of amino acids being coded for: this different amino acid can result in a different polypeptide chain therefore a different protein (or even no protein) being produced. If this protein is an enzyme, it is likely to disrupt metabolic pathways, leading to production of other substances, including proteins which can alter the organisms characteristics. Conjugation - occurs when one bacterial cell transfers DNA to another bacterial cell. It takes place as follows: One cell produces a thin projection that meets another cell and forms a thin conjugation tube between the two cells.The donor cell replicates one of its smaller pieces of DNA (plasmid). The circular DNA is broken to make it linear before it passes along the tube into the recipient cell. Contact between the cells is brief, leaving only time for a portion of the donor's DNA to be transferred. In this way the recipient cell acquires new characteristics from the donor cell. In conjugation, DNA in the form of genes can be passed from one species to another species. This is known as horizontal gene transmission. Where genes are passed down from one generation of a species to the next generation of the same species, the process is known as vertical gene transmission. Antibiotics: are substances produced by living organisms that can destroy or inhibit the growth or microorganisms. one way in which antibiotics work is by preventing bacteria from making normal cell walls. They inhibit the formation of important peptide cross linkages in bacterial cell walls. This weakens the walls so can no longer withstand high pressures. As a result, they are unable to prevent water entering the cell, so osmotic lysis occurs, killing the bacterium. Antibiotic resistance: resistance is not due to the build up of tolerance to the antibiotic, but to a chance mutation within the bacteria. The mutation leads to the bacteria being able to produce a protein/enzyme that can break down the antibiotic. It is only occasionally that a mutation will be advantageous. If the bacteria becomes resistant, it can survive and divide, increasing the allele frequency of this mutant gene. Due to conjugation this means this gene can be passed through both horizontal and vertical gene transmission. Antibiotic resistance:TB: medication for TB needs to be taken for 6-9 months, the antibiotics initially destroy the least resistant strain of the TB causing bacterium, however this makes that patient feel better, so many will stop taking the medication. Leaving the more resistant strain behind, which multiply and spread to others, there is an environmental pressure that leads to the development of strains that do not respond to the antibiotic. These strains can then interchange genes with other strains by conjugation. Biodiversity:Species diversity: biodiversity is the number and variety of living organisms in a particular area and has three components: Species diversity = refers to the number of different species and the number of individuals of each species within any one community. Genetic diversity = refers to the variety of genes possessed by the individuals that make up one species. Ecosystem diversity = refers to the range of different habitats within a particular area. One measure of Biodiversity is species diversity, It has two components: The number of different species in a given area The proportion of the community that is made up of an individual species. d = Species diversity index N ( N - 1 ) N = Total number of organisms of all species d = ------------------n = Total number of organisms of each species Zn ( n - 1 ) Z = The sum of Species diversity and Human activities: In our efforts to provide enough food for the human population at a low cost, mankind has had a considerable impact on the natural world. This impact has led to a reduction in biodiversity. Agriculture: farmers select species for particular qualities that make them more productive. as a result the number of species, and the genetic variety of alleles they possess is reduced, in comparison to the complex food webs that make up the natural ecosystems. To be economic, the numbers of these desirable species needs to be large. Any particular area can only support a certain amount of biomass. The species diversity index of and agricultural ecosystem in therefore low. Deforestation: forests form many layers between the ground and the tops of the trees therefore a range of habitats. Deforestation is the permanent clearing of forests and the conversion of land for other uses. Deforestation causes a drastic loss in biodiversity.