AQA AS Biology Unit 2

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  • Created on: 01-06-13 20:16

Variation

There are different type of variation

  • Intraspecific - Variation within a species (genetic and environmental factors)
  • Interspecific - Variation between different species (a group of organisms which can interbreed to produce fertile offspring)

Genetic factors - organisms of the same species have the same genes, but each individual has different versions of these genes - causing intraspecific variation

Variation is often a combinations of genetic and environmental factors

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Investigating Variation

  • To study variation - you have to obtain a sample
  • The sample has to be random - so each individual has an equal chance of being picked - this eliminates any bias
  • It's important to analyse the results statistically to make sure any variation in the sample was due to chance - letting you be confident that the results are a true reflection of the whole population
  • Mean - tells you the average of the values collected in a sample
  • Standard deviation - shows how much the values in a sample vary - it's a measure of spread of the values around the mean
    • Large standard deviation - values in sample vary a lot
    • Small standard deviation - values in sample vary little
  • If standard deviations overlap for the different variables then there may not be a significant difference between the means - meaning there may have been a difference of means due to chance
  • When collecting data, the more data you have, the more representative your mean becomes, NOT your results
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DNA

  • DNA is a polynucleotide - it is made up of lots of nucleotides joined together with strong phosphodiester bonds
  • Each nucleotide is made up of a pentose sugar (deoxyribose), a phosphante group and a nitrogenous base (A,G,T,C)
    • A=T - double hydrogen bond
    • GC - triple hydrogen bond
  • The phosphate group and the pentose sugar create a sugar-phosphate backbone - creating a polynucleotide strand
  • The bases join to complementary bases (AT,GC) on another strand of DNA - the strands are antiparallel
  • The strands join together and create a DNA double helix - the hydrogen bonds keep it coiled together
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DNA - Structure

The Structure Makes It Good At It's Job:

  • DNA is coiled up tightly, so it takes up less space in the nucleus of a cell, allowing more space for more information to be kept there
  • DNA has a paired structure which makes replication easy
  • The double helix structure also makes the DNA a very stable structure and can pass from generation to generation without change or damage
  • The two separate strands are joined y hydrogen bonds which allows the to separate for replication and protein synthesis
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DNA in Different Organisms

Eukaryotic DNA (Animal and Plant Cells):

  • Eukaryotic cells contain linear DNA molecules which exist as chromosomes
  • The DNA molecule is long, so it is wound up to fit into the nucleus - it is wound around proteins called histones - they also help to support the DNA
  • The DNA and protein are coiled up very tightly to make a compact chromosome

Prokaryotic (Bacterial Cells):

  • These also carry DNA as chromosomes - but these are shorter and more circular
  • This DNA isn't wound around proteins - it condenses to fit into the cell through supercoiling
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Genes

  • Genes are found in sections of DNA - they are found on chromosomes and are in a specific sequence
  • Genes code for proteins (polypeptides) - they contain the instructions needed to make them
  • Proteins are made of amino acids
  • Three bases, a triplet, code for each amino acid in a protein
  • The order of the bases determines the sequence of amino acids in a protein
  • Different sequences of bases code for different amino acids
  • In eukaryotes, not all the DNA codes for amino acids
  • These are called introns
  • The sections of DNA that DO code for amino acids are called exons
  • Intons are removed during protein synthesis
  • There are also sections of multiple repeats in the genes - these don't code for amino acids either
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Genes

The Nature and Development of Organisms is Determined by Genes:

  • Enzymes speed up our metabolic pathways - resulting in the ways in which we grow and develop
  • Enzymes are proteins - meaning they are made of amino acids (just like we saw before)
  • The order of bases code for the sequence of amino acids, which code for the protein (enzyme), which then affects our nature and development
  • This means our genes affect our nature and development because they contain all the information that codes for all of our proteins and enzymes
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Genes

  • Genes can exist in different forms called alleles 
  • The order of bases in alleles can be different, meaning they code for different types of a certain characteristic
  • Genes are found in chromosomes, which all have a homologous pair - both chromosomes in a homologous pair can have the same genes, but different alleles
  • Alleles coding for the same characteristic will be found at the same point (locus) on each chromosome in a homologous pair
  • Gene mutations can result in non-functioning proteins
  • Mutations are changes in the base sequence in DNA
  • The bases code for the sequence of amino acids, which code for the protein
    • The base sequence changes, the sequence of amino acids and therefore protein will also change - making a new protein or making the protein non-functional
  • E.g. for an enzyme, if there's a mutation in the gene, the active site may change shape, so the substrate can't bind to it, making it a non-functional enzyme
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Meiosis

  • Meiosis is the formation of gametes - these are sperm and egg cells - they are haploid cells (have one copy of a chromosome) - at fertilisation they make a diploid cell (two chromosomes)
  • Meiosis is where a diploid cell becomes a haploid cell
    • 1. DNA unravels and replicates - two copies of each chromosome - chromatids
    • 2. DNA condenses and the chromatids join to make a chromosome
    • 3. Meiosis 1 - chromosomes line up in homologous pairs in the cell
    • 4. The homolous pairs are separated into two cells - halving the chromosome number
    • 5. Meiosis 2 - Pairs of sister chromatids spearated into another pair of cells - each cell has     half a chromosome
    • 6. Four haploid cells are produced which are genetically different from each other
  • Chromatids cross over in meiosis 1 - bits of thw chromatids cross over, they contain the same genes but different alleles
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Meiosis

                                         (http://i2.squidoocdn.com/resize/squidoo_images/250/draft_lens18485053module165689025photo_1365424093-aa.png)

  • Meiosis produces cells which are genetically different
  • This is done through the crossing over of chromatids
  • This is also done through how the cells are split, and how the homolgous pairs were organised, so different combinations of the maternal and paternal chromosomes go into each daughter cell with different alleles - independent segregation
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Genetic Diversity

  • Variation in DNA can lead to genetic diversity
    • The differences within a species is genetic diversity
  • Genetic diversity within a species is caused by different alleles
  • Old genes don't disappear, and new genes don't just appear
    • It's just that different forms of genes appear, because of different alleles
    • The more alleles present, the more genetically diverse a population is
    • Genetic diversity is increased through mutations - new alleles are introduced
  • Genetic bottlenecks - this is an event which causes a big reduction in population, and the populations must build up again
    • This reduces the number of diffent alleles in the gene pool, decreasing the genetic diversity.
    • The new grown population has little genetic diversity 
  • Founder effect - This is a type of genetic bottleneck, where there are only a few left in the population and they start a new colony - this can lead to a lot of inbreeding, which can lead to a higher incidence of genetic disease
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Genetic Diversity

Selective Breeding

  • This is where you choose which plants and animals should reproduce depending on their characteristics in order to produce high-yielding breeds
  • Selective breeding leads to a reduction in genetic diversity - as the desired organisms are bred, they will continue to breed together - so only similar organisms with similar characteristics i.e. similar alleles are bred together
  • This reduces the number of different alleles in the gene pool
  • Advantages 
    • High yielding animals and plants
    • Can produce organisms with high resistance to disease - fewer drugs needed
    • Organisms bred to withstand bad conditions
  • Disadvantages
    • Causes health problems - short life expectancy 
    • Reduces genetic diversity - higher incidence of genetic disease and increased susceptibility to new disease as there is little variation in alleles
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Haemoglobin

  • Haemoglobin is a large protein with a quaternary structure - it's made up of more than one polypeptide chain
  • It's purpose is to carry and transport oxygen
  • Haemoglobin has a high affinity for oxygen - each molecule can carry four oxygen molecules
  • It is a reversible reaction - in the lungs, oxygen joins to the haemoglobin to make oxyhaemoglobin
  • The oxygen can then leave the oxyhaemoglobin (dissociate from it) near cells which need it and turn back to haemoglobin
  • The ppO2 is a measure of oxygen concentration - the greater the conc of O2 in the cells, the higher the partial pressure - where pp02 is low, haemoglobin will readily give away 02, where pp02 is high, haemoglobin will readily take 02 
  • The ppCO2 is a measure of carbon dioxide concentration - where ppCO2 is higher, haemoglobin gives away oxygen more readily
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Variation in Carbohydrates

  • Carbohydrates are made from monosaccharides - these join together through condensation reactions to make polysaccharides joined by glycosidic bonds
  • Starch - the main energy storage material in plants
    • Plants store excess glucose as starch - if it needs more glucose it'll break it down and use it
    • Amylose is stored as long, unbranched chains - it is in coils, so it is compact and easy to store
    • Amylopectin is stored as long, branched chains - the branches allow enzymes to break the glycosidic bonds easily so the glucose can be released quickly
  • Starch is insoluble in water so it doesn't make water enter cells thorugh osmosis, making it good for storage
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Variation in Carbohydrates

  • Glycogen - Main energy storage for animals
    • It's structure is similar to amylopectin, but it has a lot more branches coming off it - meaning it can be released quickly - this is important for energy release in animals
    • It is alse very compact, so is good for storage
  • Cellulose - This is made up of long, unbranches molecule of beta-glucose
    • The bonds between the sugars are straight, so the whole molecule is straight
    • The straight chain molecules are held together by hydrogen bonds,to form strong fibres called microfibrils - these provide strong structural support for cells e.g. in plant cell walls
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Variation in Cell Structure

  • Animal cells
    • Plasma membrane - holds cell, controls what goes in and out
    • Cytoplasm - most chemical reactions happen here, contains enzymes which control reactions
    • Nucleus - contains all genetic material, controls activites in cell
    • Mitochondria - where most reactions for respiration take place (respiration needs energy)
    • Ribosomes - all proteins are made
  • Plant cells
    • Plant cells have all those features and some more
    • Cell wall - rigid, supports and strengthens cell
    • Permanent vacuole - contains cell sap
    • Chloroplasts - where photosynthesis occurs
      • They have a double membrane on the outside, and inner membranes - thylakoid membranes - they stack up to form grana - these are linked by lamellae
      • Some photosynthesis happens in the grana,some in the stroma (fluid in chloroplasts)
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Variation in Cell Structure

  • Animal cells
    • Plasma membrane - holds cell, controls what goes in and out
    • Cytoplasm - most chemical reactions happen here, contains enzymes which control reactions
    • Nucleus - contains all genetic material, controls activites in cell
    • Mitochondria - where most reactions for respiration take place (respiration needs energy)
    • Ribosomes - all proteins are made
  • Plant cells
    • Plant cells have all those features and some more
    • Cell wall - rigid, supports and strengthens cell
    • Permanent vacuole - contains cell sap
    • Chloroplasts - where photosynthesis occurs
      • They have a double membrane on the outside, and inner membranes - thylakoid membranes - they stack up to form grana - these are linked by lamellae
      • Some photosynthesis happens in the grana,some in the stroma (fluid in chloroplasts)
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Cell Cycle

                              (http://daneshnameh.roshd.ir/mavara/img/daneshnameh_up/1/1c/cellcycle7.JPG)

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Cell Cycle

(http://bio-education.weebly.com/uploads/9/4/9/5/949532/4638164.jpg?349x288)

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DNA Replication

  • 1. DNA helicase breaks hydrogen bonds between the polynucleotide DNA strands
    • The helix unzips to form two separate strands
  • 2. Each parent strand acts as a template for a new strand
    • free floating nucleotides join to the exposed bases on each parent strand with specific base pairing
  • 3. Nucleotides on new strand are joined to parent strand through enzyme DNA polymerase
    • Hydrogen bonds form between bases on original and new strand
  • 4. New DNA formed made up of one strand of original DNA molecule and one new strand 
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Cell Division - Mitosis

  • Mitosis cell division produces genetically identical cells
    • It divides to create daughter cells with the exact same copy of the parent DNA
  • Mitosis is needed for growth of multicellular organisms - for growth and repair
  • Prophase - Chromosomes condense, centrioles move to poles of cell, nuclear envelope disappears
  • Metaphase - Chromosomes line up at middle of cell, spidle fibres from centrioles attach to centromeres on chromosome
  • Anaphase - Chromosomes pulled apart by spindle fibres, sister chromatids separated, to opposite ends of cell to centrioles
  • Telophase - Chromatids reach each end of cell. Chromatids grow longer into chromosomes. Nuclear envelope forms again, two nuclei. Cytoplasm divides, two daughter cells which are genetically identical
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Cell Division - Mitosis

  • Cancer is the result of uncontrolled cell division
    • Uncontrolled cell division happens when there is a mutation in a gene which controls cell division
    • The cells keep in dividing, forming a tumour
    • Tumours are dangerous because they invade tissues and damage them
  • Treatments - you can treat cancer by trying to target certain stages in the cell cycle
    • G1 (cell growth and production) - chemotherapy - drugs which prevents synthesis of enzymes needed for DNA replication. Without the enzymes the cell can't go to S1, the cell kills itself
    • S phase (Replication) - Radiation and some drugs damage DNA - if damaged DNA is detected in the S phase, the cell kills itself, so radiation puposely damages the DNA, making the cell kill itself
  • Removal of part of the tumour through surgery - gives remaining cells access to nutrients so they enter cell cycle - making them more susceptible to treatments
  • Repeated treatments in order to get rid of ALL the cancerous cells
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Cell Differentiation

  • Multicellular organisms have different types of cells - these cells are specialised - they are designed for specific functions
  • Epithelial cells - these are thin, with not much cytoplasm - they line the alveoli and the thin walls allow gases to pass through easily
  • Palisade mesophyll cells - in leaves - many chloroplasts - absorb as much light as possible - walls thin - CO2 can enter easily
  • These cells are organised into tissues
    • Epithelial cells - created a single layer of flat cells lining a surface
    • Phloem tissue - transports sugars around plant - arranged in tubes - has sieve cells - have holes at the ends of them - lets cell sap move through them easily
    • Xylem tissue - transports water and supports plant - has xylem vessel cells - have thickened walls - support - also have parenchyma cells - fill gaps between vessels - support
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Cell Differentiation and Organisation

An organ is a group of tissues that work together to perform a particular function

  • The leaf
    • Lower epidermis - Stomata - allow air in and out of leaf for gas exchange
    • Spongy mesophyll - lots of air spaces - lets air circulate - larger surface area for gas exchane
    • Palisade mesophyll - most photosynthesis occurs here - cells packed together - more light absorbed
    • Upper epidermis - waxy cuticle - reduces water loss
  • The lungs 
    • Epithelium tissue - surrounds alveoli - allows efficent exchange of gases
    • Fibrous connective tissue - forms a mesh around lungs - helps force air back out of lungs
    • Blood vessels - capillaries - surround alveoli - constant blood supply - maintains a concentration gradient
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Gas Exchange

  • Single celled organisms
    • These absorb and release gases through diffusion through their outer surface
    • Large surface area, thin surface, short diffusion pathway 
    • No need for gas exchange system
  • Fish
    • Water, with oxygen, enters fish's mouth and passes through gills
    • Gills have filaments - increase surface area for exchange of gases
    • Filaments have lamellae - increase surface area more
    • Lamallae have lots of blood capillaries and thin surface - short diffusion pathway
    • Counter current flow
      • Blood flows in one direction and water flows in opposite direction - maintains a large concentration gradient between water and blood - maximises amount of oxygen diffusing into blood
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Gas Exchange

  • Insects
    • Tracheae - microscopic air filled pipes 
    • Air moves into tracheae though pores on the surface - spiracles
    • Oxygen travels down conc gradient towards cells - Carbon dioxide moves down conc gradient towards spiracles
    • Tracheae branch off into smaller tracheoles - with thin, permeable walls - which go to each individual cell - so they don't need a transport system
    • They use rhythmical abdominal movements to move air in and out of spiracles
  • Dicotyledonous plants
    • Mesophyll cells - main gas exchange surface - large surface area
    • Air moves to mesophyll cells through stomata in the epidermis
    • Stomata can control whether gases are allowed in or not, if there is too much water loss they close
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Controlling Water Loss

  • Insects
    • close their spiracles using muscles
    • have waterproof cuticle all over their body
    • small hairs on body to reduce evaporation
  • Plants
    • Open during the day for gas exchange
    • Water enters guard cell - keeps them turgid - leave stomata open
    • If plant loses water - water moves out of guard cell - flaccid - stomata close
  • Xerophytes
    • Stomata in sunken pits - trap moist air - reduce evaporation
    • Curled leaves - stomata inside - protect from wind
    • Hairs on epidermis - trap moist air - reduces evaporation - reduces conc gradient
    • Reduced number of stomata - less places for water to escape from
    • Waxy, waterproof cuticles on leaves and stems - reduce evaporation
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Circulatory System

  • Multicellular organisms - like mammals - have a low surface area to volume ratio - need a good specialised transport system to carry nutrients for cells
  • Blood transports respiratory gases, products of digestion, metabolic waste and hormones around the body
  • Arteries 
    • From heart to body
    • Walls - thick, muscular, elastic tissue for high pressure
    • Inner lining (endothelium) folded - can expand - cope with high pressure
    • Carry oxygenated blood around body
    • Except pulmonary artery - carriess deoxygenated blood away from heart to lungs
  • Arterioles
    • Smaller vessels from arteries - form a network around body
    • Blood directed to areas of body blood is needed - contracts to restrict blood flow - relaxes to allow full blood flow
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Circulatory System

  • Capillaries
    • Always near cells of exchange tissues - ensures short diffusion pathway
    • Walls one cell thick - short diffusion pathway
    • Many capillaries - increased surface area
  • Tissue fluid
    • Start of capillary bed (nearest to arteries) - high pressure in capillaries - forces fluid out of capillaries to tissues - tissue fluid
    • As fluid leaves - pressure in capillaries decreases - lower pressure at end of capillary bed (nearest to veins)
    • From fluid loss - water potential lower at end - water from tissue flows into capillaries through osmosis
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Water Transport in Plants

  • Water enters plant throught roots
  • Root cells have root hairs - increase surface area - speeds up water uptake
  • Water goes through cortex and endodermis to xylem
  • Different routes through the root
    • Symplast pathway - through living parts of root - cytoplasm - cytoplasm of cells connect through plasmodesmata (gaps in cell wall)
    • Apoplast pathway - Non-living parts of root - cell walls - walls are absorbent - water diffuses through - as well as using the plasmodesmata
  • Cohesion tension theory
    • Water evaporates from top of xylem at leaves - creates tension (suction) - pull water into leaf - water molecules are cohesive - they stick together - all water moves up xylem - water enter from roots
  • Root pressure theory
    • Water goes into xylem from roots - pushes water up roots 
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Transpiration

  • Transpiration is loss of water from a plant's surface
    • Water evaporated form cell walls and accumulate in air space
    • when stomata open, water evaporates out
  • Factors affecting transpiration rate
    • Light - more light - faster transpiration - stomata open when its light - for exchange of gases - so water is lost
    • Temperature - higher - faster transpiration - warmer water molecules have more energy - evaporate faster - increases conc gradient between inside and out of leaf - water diffuses out faster 
    • Humidity - lower humidity - faster tranpiration - greater conc gradient between dry outside and moist inside of leaf - increased transpiration
    • Wind - more wind - faster transpiration - air movement - blows water molecules away from stomata - increases conc gradient - increased transpiration
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Classification

  • Taxonomy - science of classification
  • Classification - a hierarchy in which groups are contained in larger groups with no overlap
  • Species - a group of similar organisms able to reproduce to give fertile offspring
  • Phylogenetics - based on evolutionary patterns in the history of organisms
  • Hierarchy - Kingdom, Phylum, Class, Order, Family, Genus, Species
  • Defining an organism as a distinct species is hard:
    • they could be extinct
    • you may not be able to study whether certain organisms reproduce successfully in the wild, and you can't study that in a lab - unethical
  • Because of these problems scientists compare DNA and see how related they are
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Classification

  • Taxonomy - science of classification
  • Classification - a hierarchy in which groups are contained in larger groups with no overlap
  • Species - a group of similar organisms able to reproduce to give fertile offspring
  • Phylogenetics - based on evolutionary patterns in the history of organisms
  • Hierarchy - Kingdom, Phylum, Class, Order, Family, Genus, Species
  • Defining an organism as a distinct species is hard:
    • they could be extinct
    • you may not be able to study whether certain organisms reproduce successfully in the wild, and you can't study that in a lab - unethical
  • Because of these problems scientists compare DNA and see how related they are
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Classifying Species

  • Species can be classified into different groups based on similarities and differences in their genes
    • This can be done by looking at DNA sequences - or proteins - the more similar they ae, the more related they are
  • DNA sequences 
    • Looking at the order of the bases between pieces of DNA in different organisms - a higher percentage of similarity means the organisms are closely related
  • DNA hybridisation
    • DNA from two different organisms are collected - they are separated into single strands and mixed together
    • Where the base sequences are the same- the base pairings will make hydrogen bonds
    • The DNA is then heated to separate the strands again
    • The higher the temperature needed to separate the strands - similar DNA will have more hydrogen bonds - needing a higher temperature to separate the strands
  • Protein comparison - comparing amino acid seuqences
  • Immunology - Similar proteins will bind to the same antibodies - if a ceetain antibody joins to two different proteins from an organism - they are closely related
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Antibiotic Action

  • Antibiotics are used to treat bacterial diseases - they kill or inhibit the growth of bacteria
  • Different types of bacteria kill or inhibit the growth of bacteria in different ways
    • Some prevent the growth of a bacterial cell wall which can lead to osmotic lysis:
      • Anitbiotics inhibit enzymes that makes bonds within a cell wall
      • The prevents the cell from growing properly and weakens the cell wall
      • Water moves into cell by osmosis
      • Weakened cell wall can't stand pressure and bursts - lysis
  • Mutations in bacterial DNA can make them resistant to antibiotics
    • Mutations are changes in the base sequence of DNA
    • Some mutations result in the bacteria not being affected by anitibiotics - antibiotic resistance
  • Mutations can be passed on vertically - through asexual reproduction - same mutation passed on
    • or horizontally - two bacteria join together - they conjugate - and a copy of a plasmid is passed to the other cell - the plasmid contains the antibiotic resistance gene
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Antibiotic Resistance and Natural Selection

  • Individuals with better adaptations are more likely to survive and reproduce - passing on the alleles which cause their adaptations to their offspring
    • Some bacteria have a alleles which give them resistance to an antibiotic
    • The population is exposed to this antibiotic, killing the bacteria without the resistant allele
    • The resistant bacteria survive and reproduce, passing the resistant allele on
    • Most of the new population is then resistant to that certain antibiotic
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