unit 4

• meiosis 


  1.dna condenses to form chromosomes, each made from two sister chromatids, which are joined in the middle by a centromere. 2. first division: the chromosomes arrange themselves into homologous pairs 3. these separate halving the chromosome number. 4. second division: pairs of sister chromatids are seperated. centromere divides. 5. four haploid cells that are genetically different are produced
• crossing over 


  chromatids cross over. means each cell has a different chromatid and therefore different sets of alleles which increases genetic variation
• independent segregation 


  each homologous chromosomes is made up of paternal and maternal chromosomes. when the separate its completely random. so the four daughter cells have different combinations of maternal and paternal chromosomes. increases genetic variation
• non-disjunction


  failure of chromosomes to separate properly. so one cell gets an extra copy of 21 and another gets none
• downs syndrome 


  caused by a person having an extra copy of chromosome 21
• genetic diversity


  number of different alleles of a gene in a species or population
• what increases genetic diversity

  mutations in DNA - forming new alleles. 2. gene flow
• genetic bottleneck 


  event that causes big reduction in a a population, this reduces the number of different alleles in the gene pool and so reduces genetic diversity. survivors reproduce (limited alleles) and new population with reduced genetic diversity is formed
• courtship behaviour can be used to help identify species

  carried out to attract a mate by releasing chemicals. is species specific, preventing interbreeding and makes reproduction more successful. the more closely related species are, the more similar their courtship behaviour
• techniques to clarify evolutionary relationships

  compare base sequence of DNA, mRNA, comparing amino acid sequence, immunological comparisons, observable characteristics
• standard deviation

  spread of data about the mean
• biodiversity 


  the variety of living organisms in an area
• habitat


  where an organism lives
• community 


  all the populations of different species in a habitat
• species richness


  measure of number of different species in a community
• index of diversity 


  describes the relationship between the no of species in a community and the number of individuals in each species
• agricultural practices that reduce biodiversity

  woodland clearance - destroys habitats, lose shelter and food source. 2. hedgerow removal - destroys habitats, loose shelter and food source. 3. pesticides - any species that feed on pests loose food source. 4. herbicides - reduce plant diversity and could reduce no, of organisms that feed on weeds. 5. monoculture - supports fewer organisms as a habitat or food source
• examples of conservation schemes 


  giving legal protection to endangered species. 2. creating protected sites such as sites of special scientific interest. 3. environmental stewardship scheme.
• structure of mRNA

  single stranded, acts as a template upon which proteins are built. codes for specific polypetides due to base sequence of codon
• structure of tRNA 


  small molecule, clover-leaf molecule. has an amino acid binding site and an anticodon
• transcription

  DNA helicase breaks hydrogen bonds between complementary base pairs, causing double helix to unwind. Only one strand acts a template Free RNA nucleotides are attracted to the exposed bases on the template and align themselves according to the base pairing rule where cytosine binds to guanine and adenine binds to uracil. RNA polymerase joins adjacent nucleotides together, by the formation of phosphodiester bonds. This creates a strand of pre-mRNA. Pre-mRNA is then spliced to remove introns to from mRNA
• translation

  mRNA attaches to ribosomes and finds start codon.

  tRNAs anticodon binds to complementary codon on mRNA

  tRNA brings a specific amino acid

  amino acids join by a peptide bond, using ATP and an enzyme

  RNA released after amino acid joined to polypeptide.

  ribosome moves along mRNA, forming a polypeptide chain