Genetics, Biodiversity and Classification

• Transcription in eukaryotes

  • In the nucleus, DNA helicase breaks hydrogen bonds between bases and unwinds the DNA strand
  • A single strand acts as a template and is transcripted
  • Free floating complementary RNA nucleotides are attracted to the strand
  • RNA polymerase joins adjacent bases together using phosphodiester bonds
  • Hydrogen bonds reform in DNA
  • Pre-mRNA’s spliced to remove introns and becomes a true mRNA strand that leaves via the nuclear pore
• tRNA structure

  • a small single strand of RNA
  • It is folded into a clover leaf shape due to hydrogen bonds
  • It has an anti codon at one end and the specific amino acid binding site at the other
• Translation

  • Occurs in the cytoplasm
  • mRNA attaches to a ribosome - it covers 2 codons
  • tRNA with complementary anti-codon attaches to codon (complementary base pairing) of mRNA
  • tRNA carries specific amino acid for that codon
  • Sequence of codons/bases on mRNA determines order of amino acids
  • Formation of peptide bonds/amino acids joined by condensation reactions
  • Attachment of amino acid to tRNA facilitated by hydrolysis of ATP
  • Process continues until polypeptide has been made as ribosome move down 1 codon at a time
• Mutations


  Mutations are the changes in the DNA base sequences of chromosomes
• Causes of mutation


  Mutations occur spontaneously - especially during transcription and DNA replication

  The rate of mutation increases on exposure to mutagenic radiation these include : ionising radiation, phenolic compounds and viruses
• Types of mutation


  Substitution - This is where one base is replaced by another. There are degenerates so it doesn't always change the amino acid sequence

  Deletion - This causes a 'frameshift downstream' of the mutation - ie after the mutation, all of the triplets will code for different amino acids

  Addition - This also causes a 'frameshift downstream' and therefore a change in the primary structure
• Purposes of meiosis

  1. Increases variation in a population
  2. Halves the number of chromosomes so that the original diploid number can be restored at fertilisation
• Meiosis 1 - Metaphase


  Crossing over is when chromatids from adjacent homologous chromosomes wrap around each other, snap off and alleles are exchanges. These give new allelic combinations

  Chromosomes form a bivalent and chiasmata

  Independent segregation is the random lining up of the homologous chromosomes in their pairs. This increases the variation in the daughter cells produced. It can be calculated 2^the haploid number

  Spindle fibres join the centriole to the centromeres of the homologous chromosomes
• Meiosis 1 - Anaphase


  The spindle fibres contract and homologous chromosomes are drawn to either pole
• Meiosis 1 - Telophase


  Each one of the homologous pairs has reached thee poles and cytokinesis occurs, resulting in 2 daughter cells.

  There is only 1 copy of each chromosome in the daughter cell so the cells are now haploid
• Meiosis 2 - Metaphase


  The chromosomes line up on the equator in a single file

  Spindle fibres join the centrioles to the centromeres
• Meiosis 2 - Anaphase


  The spindle fibres join contract and chromatids are drawn to either pole as the centromeres are pulled apart
• Meiosis 2 - Telophase


  The chromatids have reached the poles and are referred to as chromosomes

  Cytokinesis results in the formation of 4 non identical daughter cells - gametes
• Selection


  Variation

  Mutations

  Advantageous alleles

  Selected for

  Survival

  Reproductive success

  Passed on

  Allele frequency
• Causes of natural selection


  Predation, disease and competition
• Stabilising selection


  This occurs in an unchanging environment

  The medium/mean value is selected for and the extreme values are selected against
• Examples of stabilising selection - birth weights


  Very large babies’ alleles are selected against due to the difficulties in childbirth and hence high mortality

  Very small babies’ alleles are selected against as their large sa : vol causes too much heat loss to maintain a constant temperature (enzymes) and they die

  Alleles for median/mean birth weights are therefore selected for
• Directional selection


  This occurs on a changing environment

  One of the extreme values is selected for
• Examples of directional selection - antibiotic resistance


  When a mutation causes a bacterium to become resistant to an antibiotic, these alleles will give a huge advantage where the antibiotic is in use

  The allele will cause the bacterium to survive and pass on the allele so it increases in frequency in the population rapidly
• Classification


  Involves putting species in groups based on their evolutionary relationships (phylogyny)
• Classification - comparing


  Frequency of observable or measurable characteristics

  DNA base sequencing

  mRNA base sequencing

  Amino acid sequencing
• Courtship


  Attracts same species

  Attracts members of same sex

  Gives and indication of sexual maturity

  Stimulates release of gametes

  Form pair bond - male and female mate for life for successful mating
• Pioneer species


  The first species to arrive and survive in a sterile environment
• Climax community


  The final stage of succession
• Colonisers


  Species which arrive and survive during succession
• Primary succession


  This occurs where there is no soil, seeds or any life
• Secondary succession 


  This occurs where there is soil, usually with seeds and spores present
• Abiotic conditions


  Non living/environmental conditions
• Biotic conditions 


  Conditions to do with life
• Conservation of habitats


  Involves management of succession

  Increases biodiversity for : tourism, medicines and culture
• Succession - characteristics of pioneer species


  Large number of wind blown seeds and spores

  Be able to photosynthesise

  Nitrogen fixers

  Roots enable them to anchor to bare rock
• Succession

  Pioneer species change the abiotic conditions e.g they die and for humus

  Conditions are less hostile

  New colonisers arrive and outcompete previous species

  The next serial stage occurs

  This continues - the environment becomes progressively more hostile as new species outcompete previous ones until a climax community is reached
• Succession - characteristics of a climax community

  Stable population Stable population

  Stable community

  High biodiversity

  High biomass

  Many niches

  Many food sources

  Many habitats
• Species richness


  A measure of the number of different species in a community
• Index of diversity


  The relationship between the number of species in a community and the number of individuals in each species