Inheritance and Variation

• Sexual reproduction involves the fusion of male and female gametes

  o in animals: sperm and egg cells

  o in flowering plants: pollen and egg cells
• Asexual reproduction involves only 1 parent and no fusion of gametes

  o there is no mixing of genetic information

  o genetically identical offspring produced (clones) – identical to the parent and other offspring

  o only mitosis involved (no meiosis) – parent cell divides by mitosis producing 2 daughter cells

  o occurs in bacteria, some plants and some animals
• Meiosis Process (involves 2 cell divisions)

  o chromosomes are duplicated (has double the amount of genetic

  information) – 92 chromosomes overall

  o cell divides in 2, each with half the number of chromosomes – 46

  chromosomes in each cell

  ▪ the chromosome pairs line up in the middle of the cell

  ▪ the pairs are pulled apart, each cell only has one copy of each

  o cells divide again producing 4 daughter cells – 23 chromosomes in each cell

  • The gametes produced are all genetically different due to the chromosomes being

  “shuffled” during meiosis
• The genetic material in the nucleus of a cell is composed of DNA

  o DNA is a polymer made up of 2 strands forming a double helix

  o DNA is contained in structures called chromosomes

  o a gene is a small section of DNA on a chromosome

  o each gene codes for a sequence of amino acids to make a specific protein
• DNA is a polymer made up of 4 nucleotides

  o each nucleotide is made of a sugar molecule (deoxyribose), a

  phosphate molecule and one base

  o the 4 bases are Guanine, Cytosine, Adenine and Thymine

  o G-C and A-T always link (complementary strands)
• A sequence of 3 bases (a codon) is the code for a particular amino acid

  o the order of the bases control which amino acids are assembled

  to produce a protein

  o there are 20 amino acids

  • there are parts of DNA that don’t code for proteins, they instead control whether a gene is expressed or not
• Protein synthesis

  • the process of producing a protein from the DNA

  o DNA is too large to move out of the nucleus

  o the 2 strands pulled apart and mRNA (messenger) matches the complementary base on the strand

  o the template of the DNA (mRNA) moves out of the nucleus and into the cytoplasm

  o tRNA (transfer) pulls the mRNA to the ribosomes where the codons on the mRNA are read and the

  corresponding amino acids are brought to the ribosome by carrier molecules

  o the amino acids connect together to form a protein, once the chain is complete the protein folds forming

  a unique 3D structure
• There is 3 ways the sequence of DNA can mutate

  o Insertion

  ▪ a new base is inserted into the DNA base sequence

  ▪ as they are read in 3s this changes the way it is read and the amino acids coded for after the

  insertion

  o Deletion

  ▪ similar effect to insertion, just a base is removed from the sequence

  o Substitutions

  ▪ a base is changed, this may change which amino acid is coded for

  ▪ this doesn’t affect the amino acids coded for before or after the affected base
• Homozygous – when both inherited alleles are the same e.g. both dominant or both recessive

  • Heterozygous – when one of the inherited alleles is dominant and the other is recessive

  • Genotype – the combination of alleles an individual has e.g. Bb

  • Phenotype – the physical characteristics that are observed e.g. eye colour
• o Polydactyly (having extra digits – fingers or toes)

  ▪ caused by a dominant allele

  o Cystic Fibrosis (disorder of cell membranes causing increased production of mucus)

  ▪ caused by a recessive allele

  ▪ means that both parents either need to be carriers (have 1 of the recessive alleles so don’t

  express it) OR one parent have CF and the other be a carrier

  o Huntington’s Chorea (affects nervous systems causing involuntary muscular movement)

  ▪ caused by a dominant allele

  ▪ age of onset is about 35-40 years old (so may already have children passing on the alleles)
• Embryonic screening allows scientists to observe whether the child will have a genetic condition or not – there

  are 2 methods:

  o amniocentesis – at 15 weeks the fluid from the sack around the foetus or foetal cells are sampled

  o chorionic villus sampling – at 10-12 weeks a small sample of tissue from the placenta is screened
• Variaition - Differences in the characteristics (phenotype) of individuals in a population
• Variation is caused by

  o inherited genes (genotype)

  ▪ genes are passed on from the parents

  ▪ the combining of genes from the mother and father creates genetic variation

  ▪ only identical twins have the same genotype

  ▪ e.g. eye colour, blood group, inherited disorders

  o conditions in which they have developed (environmental causes)

  ▪ the conditions the organism grows and developed in also affects its appearance

  ▪ e.g. scars in animals, smaller/yellow leaves in plants
• Evolution: change in the inherited characteristics of a population over time through natural selection (which may

  result in the formation of a new species)
• Theory of evolution: all species of living things today have evolved from life forms that first developed over 3 billion years ago
• speciation

  o where 2 populations of a species becoming so different in phenotype that they can no longer successfully interbreed to produce fertile offspring (reproductively isolated) they have formed 2 new species
• Selective Breeding Process

  • choosing parents with the desired characteristics from a mixed population

  • breading the chosen parents

  • from the offspring those with the desired characteristics are bred together

  • this is repeated over many generations until all the offspring show the

  desired characteristic
• Genetic Engineering Process ( bacterial cell example)

  • insulin producing gene from a chromosome isolated using a restriction enzyme

  • a plasmid is removed from a bacterial cell using lysozyme enzyme (this will be the vector to carry the gene)

  • a restriction enzyme is used to remove gene from the plasmid which is replaced with the insulin-producing gene

  from the human – the plasmid is now a recombinant plasmid

  • DNA ligase enzymes are used to join the ends of the DNA

  • as the transgenic bacterium divide the offspring will carry the same gene meaning they can all produce insulin
• Methods of plant cloning

  • Tissue culture – important to preserve rate plate species or commercial growth in plant nurseries

  o a small group of cells are put in a growth medium with nutrients and growth hormones

  o they grow into new plants (clones of the parent plant)

  • Cuttings – older, simpler method used by gardeners/horticulturists (cheaper)

  o a section of the stem (a cutting) is taken from a plant with a desirable trait

  o the cutting is planted, and the plants produced are clones of the original
• • Embryo transplants

  o sperm and egg cells from parents with desirable features are obtained (female sometimes given

  hormones to promote release of multiple eggs)

  o the egg cell is artificially fertilised in a lab to gorm an embryo

  o the embryo(es) is inserted into host mother(s), the offspring produced are all genetically identical with

  the desirable features – huge increase in the number of offspring an animal can produce
• • Adult cell cloning

  o the nucleus is removed from an unfertilised egg cell

  o the nucleus from an adult body cell e.g. a skin cell is inserted into the egg cell

  o an electric shock stimulates the egg cell to divide forming an embryo

  o the embryo cells contain the same genetic information as the adult skin cell

  o the embryo can be inserted into the womb of an adult female to continue its development
• Theory of Evolution:

  Individual organisms within a particular species show a wide range of variation for a

  characteristic due to mutations in DNA

  ▪ Individuals with characteristics most suited to the environment are more likely to survive to

  breed successfully (survival of the fittest)

  ▪ the advantageous characteristics are then passed onto future generations, over many

  generations the frequency of the advantageous characteristics increases with the population
• Speciation: process of a new species developing through genetic variation causing 2 populations to no longer

  being able to successfully interbreed to produce fertile offspring

  Species: group of similar organisms that can reproduce to give fertile offspring
• theory of speciation

  o variation exists within a population due to genetic mutations

  o alleles which provide a survival advantage become more abundant due to natural selection

  o populations of a species can become isolated e.g. due to a physical barrier

  o different alleles are advantageous in the new environment leading to more natural selection

  o overtime, the selection of different alleles increases the genetic variation between the 2 populations

  o when the 2 populations can no longer breed together to produce fertile offspring, a new species has formed
• Formation of fossils

  • mineralisation (gradual replacement by minerals)

  o teeth, shells and bones don’t easily decay so remain intact a long time after being buried

  o they are eventually replaced by minerals as they decay

  o this forms a rock-like substance which remains distinct from the remaining sedimentary rock

  • casts

  o when an organism is buried in soft material e.g. clay, the clay hardens around it

  o when the organism decays there is a cast of the organism (often plant roots or an animal’s burrow)
• More formation of fossils

  • impressions

  o footprints can be pressed into materials when they are soft, so they leave an impression when the rock hardens

  • preservation (decay of an organism may not occur due to one or more of the conditions for decay are absent)

  o in glaciers it is too cold for decay microorganism to work

  o in peat bogs it is too acidic for decay microorganisms to work

  o in amber (yellow “stone” made from fossilised resin) and tar pits there is no oxygen or moisture, so decay microorganisms die
• fossils can’t be used to tell us how live started on Earth because

  o most early life forms were soft-bodied, so they decayed completely

  o any traces left have been destroyed by geological activity e.g. crushed due to movement of tectonic plates
• Extinction - when there are no remaining individuals of a species still alive

  Causes:

  • environment changes too quickly so the species is unable to adapt fast enough

  • new predators migrate to the area (often humans)

  • new disease arises which they have no resistance too

  • competition with another species for food which had advantageous mutations for the same food source

  • a catastrophic event e.g. a volcanic eruption or collision with an asteroid

  • destruction of habitat (often deforestation)
• Process of Resistant Bacteria

  • bacteria can evolve rapidly due their rapid reproduction rate (every 20 minutes in the optimum conditions)

  • mutations of bacterial pathogens produce new strains

  • some strains may be resistant to antibiotics, so are not killed (an advantageous characteristic)

  • they survive and reproduce, so the population or resistant bacteria increases rapidly (natural selection)

  • the strain then spreads rapidly as people aren’t immune to it and there is not effective treatment
• Reducing the rate of development of antibiotic resistant strains of bacteria

  • antibiotics should not be given for viral or non-serious infections

  • specific antibiotics should be given for specific bacteria

  • patients should complete their course of antibiotics – if they don’t some bacteria may survive and mutate to

  become antibiotic resistant

  • reduced use of antibiotics in agriculture (used to prevent livestock dying of disease) – its overuse leads to

  antibiotic resistant bacteria are then transferred to humans when they consume the animals
• Carl Linnaeus in the 1700s classified living organisms into:

  Kingdom

  Phylum

  Class

  Order

  Family

  Genus

  Species
• Carol Woese developed the ‘3-domain system’, an addition to the Linnaean system

  ▪ archaea (primitive bacteria e.g. extremophiles in extreme pressure or temperature)

  ▪ bacteria (true bacteria e.g. E. coli – has different biochemical processes than archaea)

  ▪ eukaryota (broad range including animals, plants, protists and fungi – have nucleus

  enclosed in membranes)
• • Gamete – an organism’s reproductive cell which has half the number of chromosomes

  • Chromosome – a structure in the nucleus made up of a long strand of DNA

  • Gene – short section of DNA that codes for a protein

  • Allele – the different forms of the gene

  o Dominant – allele is always expressed

  o Recessive – allele is only expressed if the dominant allele isn’t present
• In meiosis why are the gametes produced all genetically different?

  Chromosomes shuffled during meiosis