Genetic Diversity & Mutations

• MUTATIONS:

  • MUTATION: A change in the quantity or structure of the DNA of an organism. Mutations that occur during the formation of gametes may be inherited. 
  • GENE MUTATION: Any changes to 1 or more nucleotide bases/ change in the sequence of bases in DNA. Gene mutations can occur during DNA replication (base substitution and base deletion)
• MUTATIONS:

  • ADVANTAGES & DISADVANTAGES: Can produce natural diversity necessary for natural selection & speciation but mostly always harmful and produces an organism less suited to its environment
  • Mutation in body cells rather than gametes -> can lead to the disruption of normal cellular activities (eg cell division) & cause cancer
• SUBSTITUTION OF BASES:

  • DEFINITION: Replaces a base in the nucleotide sequence with another.
  • EFFECT: Substitution causes sickle cell anaemia
• SUBSTITUTION OF BASES:

  • DIFFERENT AMINO ACID CODED: final polypeptide chain differs by 1 amino acid -> could affect tertiary structure of the protein (if that amino acid was important in forming bonds that determine the tertiary structure) -> different shape -> does not function properly (eg protein: active site changed)
  • SAME AMINO ACID CODED: If the mutation codes for the same amino acid nothing will change (eg if the 3rd base of GTC is replaced to become GTT there is no difference as they both code for glutamine)
• DELETION OF BASES:

  • DEFINITION: Occurs when 1 or more nucleotide bases is lost from the normal DNA sequence
  • EFFECT: Deletion of 1 nucleotide-> amino acid sequence completely changes (the DNA sequence is read in 3s so 1 deleted nucleotide -> triplets are read differently/ shifted to the left by 1) -> completely different protein coded for, high chance of it being non functional
• ADDITION OF BASES:

  • DEFINITION: Occurs when 1 or more  nucleotides is added to the normal DNA sequence
  • EFFECT: Addition of 1 nucleotide-> amino acid sequence completely changes (the DNA sequence is read in 3s so 1 added nucleotide -> triplets are read differently/ shifted to the right by 1) -> completely different protein coded for, high chance of it being non functional
• INVERSION:

  • DEFINITION: A segment of the DNA sequence is read in reverse (a group of bases become separated from the DNA sequence and then rejoin at the same position but in the reverse order)
  • EFFECT: The reversed portion of the sequence has completely different amino acids coded for. If short section reversed -> minimal damage
• TRANSLOCATION:

  • DEFINITION: a group of bases become separated from the DNA sequence on one chromosome and are inserted into the DNA sequence on another chromosome
  • EFFECT: Directly affects both chromosomes (and the DNA sequences). Can cause cancer or reduced fertility
• SPONTANEOUSLY/ DURING DNA REPLICATION:

  • When DNA is replicated: the bases of the DNA sequence are read and copied. There can be permanent, random errors in the replication process-> causes mutations. They occur without any outside influence, but mutation rates can be increased by outside factors (mutagenic agents)
• MUTAGENIC AGENTS:

  • EFFECT: Increases the rate at which mutations occur
  • EXAMPLES: Chemicals (eg bromine and benzene), exposure to radiation (eg ionising radiation and ultraviolet radiation)

  -HIGH ENERGY IONISING RADIATION: e.g. alpha and beta particles, short wavelength radiation (x-rays and UV). Can disrupt the structure of DNA

  -CHEMICALS: e.g. nitrogen dioxide can affect DNA structure & transcription. Benzopyrene (in tobacco smoke) is a mutagen that inactivates a tumour-suppressing gene -> cancer
• CHROMOSOME MUTATIONS:

  • DEFINITION: Changes in the structure or quantity of whole chromosomes
  • Chromosome mutations can affect the number of chromosomes in a developing zygote (aneuploidy)
• CHROMOSOME MUTATIONS:

  • CHANGES IN WHOLE SETS OF CHROMOSOMES: When organisms have 3 or more sets rather than 2 (called polyploidy, occurs mostly in plants)
  • CHANGES IN NUMBER OF INDIVIDUAL CHROMOSOMES: Sometimes individual homologous pairs fail to separate during meiosis (=non-disjunction, usually results in a gamete having 1 or more fewer chromosomes)
  • HYBRIDISATION: Combining the genes of different varieties/ species of organisms to produce a hybrid. Some hybrids can be formed by combining sets of chromosomes from 2 different species
• CHROMOSOME MUTATIONS- NONDISJUNCTION:

  • What is supposed to separate fails during anaphase (eg homologous chromosomes during meiosis 1 or sister chromatids  during meiosis 2)
  • Gametes with an incorrect number of chromosomes are produced
• OTHER MUTATIONS: (changes of alleles in depth)

  • DIFFERENT ALLELES: Each individual inherits one allele per parent. Different alleles have different base sequences so produce different polypeptides. Changes to base sequence of a gene -> new allele -> new sequence of amino acids being coded for -> new polypeptide produced-> new protein produced (mutation). The new protein might not function properly/ at all (eg if enzyme might have different shape that might not fit the enzyme’s substrate-> enzyme cannot function -> consequences for the organism
• OTHER MUTATIONS: (changes of alleles in depth)

  DEGRADATION OF NUCLEOTIDES: The structure of nucleotides (specifically the organic bases) helps against mutation (which can be from radiation) as nucleotides can degrade into different nucleotides. One example includes thymine degrading into adenine, which is why uracil is used in RNA (less susceptible to degrading which would change the DNA & protein structure)
• IMPORTANCE OF MEIOSIS:

  • DEFINITION: produces (haploid) gametes/ 4 (genetically different) daughter cells, each with ½ the number of chromosomes as the parent cell
• IMPORTANCE OF MEIOSIS:

  • CONSTANT NUMBER OF CHROMOSOMES EACH GENERATION: In sexual reproduction: 2 haploid gametes fuse to produce diploid offspring, if each gamete had a full set of chromosomes (diploid instead of haploid) then the cell they produce would double this number. This would happen each generation so to have a constant one number of chromosomes they are halved at a stage of the life cycle (which is in meiosis). Most animals: meiosis occurs in formation of gametes. In some plants: gametes produced by mitosis.
• IMPORTANCE OF MEIOSIS:

  • HOW: Every diploid cell has 2 complete sets of chromosomes (1 set per parent). During meiosis homologous pairs of chromosomes separate (-> only 1 chromosome enters 1 daughter cell each) = haploid number of chromosomes (23 in humans). The haploid gametes fuse in fertilisation which restores the diploid number of chromosomes
• IMPORTANCE OF MEIOSIS:

  • BRINGS GENETIC VARIATION: Which may result in adaptations (that improve survival chances). Meiosis does this in 2 ways: independent segregation of homologous chromosomes & new combinations of maternal and paternal alleles by crossing over
• IMPORTANCE OF MEIOSIS:

  • HOW: INDEPENDENT SEGREGATION: when the homologous pairs drift to opposite poles, the combination of chromosomes in each daughter cell is random as the alleles of the chromosomes might differ.
• IMPORTANCE OF MEIOSIS:

  • CROSSING OVER: chromosomes line up -> chromatids of each pair twist around each other (the attached pair of chromosomes= a bivalent)  -> portions of the chromatids break off (the points where the chromosomes are joined= chiasmata) -> the broken portions might rejoin with the chromatids of the homologous partner (usually equivalent portions are exchanged) -> new combinations of maternal & paternal alleles (the new chromosomes= recombinant chromosomes). Chromosomes cross over many times.
• IMPORTANCE OF MEIOSIS:

  • Recombination: when the broken off portions of the chromatid recombine with another chromatid
• 1)PROPHASE 1 (pro= 3 letters, 3 steps)


  1)Each DNA strand shortens & becomes fatter/ condenses and becomes a chromosome (now it is visible).

  2) Each chromosome makes a copy of itself joined at the centromere.

  3) Homologous chromosomes pair up, crossing over occurs
• 2)METAPHASE 1 (M= middle)

  The pairs of chromosomes line up along the middle of the cell
• 3)ANAPHASE 1 (pulled apart chromosomes look like 2 As)

  The homologous pairs of chromosomes separate to opposite sides of the cell
• 4)TELOPHASE 1 (telo=end)

  The cell splits down the middle
• 5)PROPHASE 2

  1)There are now two cells but they are haploid (no pairs).

  2) Chromatids condense

  3) New set of spindle fibres form
• 6)METAPHASE 2

  All chromosomes become arranged in a line down the middle of the cell
• 7)ANAPHASE 2

  The chromatids separate at the centromere and move to opposite sides of the cell. 
• 8)TELOPHASE 2

  The cell splits down the middle.
• 8)TELOPHASE 2

  The cell splits down the middle.

  Two haploid cells have been made from one of the first division cells, this occurs with the other cell simultaneously. 4 genetically different haploid gametes have been produced
• Meiosis vs Mitosis

  • NUMBER OF CELLS: Meiosis produces 4 cells. Mitosis produces 2 cells.
  • NUMBER OF CHROMOSOMES: Meiosis produces haploid cells, with the chromosomes in the daughter cells being half the parent cells. Mitosis produces diploid cells, chromosomes in the daughter cells being the same as the parent cell
  • GENETIC VARIATION: Meiosis produces genetically different daughter cells. Mitosis produces genetically identical daughter cells.
  • PRODUCTS: Meiosis only produces gametes (sex cells). Mitosis allows the replication of every other body cell.
• CHROMOSOME MUTATIONS:

  
• CROSSING OVER:

  
• MEIOSIS: