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 opposite 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
• 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