3.4.3 Genetic diversity can arise as a result of mutation/during meiosis

• A mutation can lead to the production of a non-functional enzyme. Explain how. (6)

  • change in base sequence of gene
  • change in primary structure
  • change in hydrogen, ionic or disulfide bonds
  • change in tertiary structure
  • change in active site
  • substrate not complementary
• A mutation is a change in the base sequence of a gene or chromosome. This results in the formation of a new allele.
• Mutagenic agents are chemicals that can cause mutations in DNA.
• Ionising radiation is a mutagen because it can damage DNA and disrupt the structure of DNA by causing the wrong base to be inserted in a codon, or even making the wrong bases pair up.
• Carcinogens are mutagens because they can alter the structure of DNA as they can chemically change the bases so they pair up wrongly.
• An addition/insertion mutation is when an extra base is added to a base sequence. This is known as a frame shift mutation.
• A deletion mutation is when a base is deleted from a sequence of bases. This is known as a frame shift mutation.
• Frame shift mutations include insertion and deletion. This type of mutation can be very harmful because it changes the triplets further on in the DNA sequence. All the altered codons could potentially code for different amino acids and result in a very different sequence of amino acids, resulting in a non-functional protein.
• A substitution mutation is when a base is changed for a different base, but the number of bases remains the same and there is no frame shift. The result is only one codon changing, and due to the degenerate nature of the genetic code, it may still code for the same amino acid therefore having no impact on the sequence of amino acids produced.
• An inversion mutation is when a section of bases detach from the DNA sequence, but when they rejoin they are inverted so the section of code is 'backwards'. This results in different amino acids being coded for in this region, therefore inversion mutations frequently result in a non-functional protein.
• Translocation is when a section of bases on one chromosome detaches and attaches onto a different chromosome. The result is that the cut chromosome is now non-functional and the chromosome that gained the translocated section is likely to also be non-functional.
• Gene mutations produce new alleles, therefore mutation is the only source of new variation.
• Gene mutations are permanent changes in the nucleotide/base sequence.
• Gene mutations arise spontaneously during DNA replication.
• Define gene mutation and explain how a gene mutation can have:

  • no effect on individual
  • a positive effect on an individual (4)

  • change in the base sequence of chromosomes
  • results in the formation of new allele

  (Has no effect because)

  • genetic code is degenerate so amino acid sequence may not change
  • does change amino acid but no effect on tertiary structure
  • new allele is recessive so does not affect phenotype

  (Has positive effect because)

  • results in change in polypeptide that positively changes the properties of protein
  • may result in increased survival chances
• Meiosis

  A form of cell division that produces gamete cells with half the normal number of chromosomes
• Gamete cells

  • Only need half the normal number of chromosomes
  • Are said to be haploid cells as they only have one "set" of chromosomes
• Homologous chromosomes

  Pairs of chromosomes, one from each parent, that are similar in structure and function
• Somatic (body) cell

  • Contains 46 chromosomes in 23 pairs of homologous chromosomes
  • Sex cells (gametes) are haploid, containing only one copy of each chromosome
  • Somatic cells are diploid, containing two copies of each chromosome
• Meiosis

  1. Meiosis I: Two diploid cells are created

  2. Meiosis II: Four haploid daughter cells are created
• Prophase I

  1. Chromosomes condense

  2. Nuclear envelope disintegrates

  3. Spindle fibres begin to form

  4. Homologous chromosomes pair up forming bivalents

  5. Crossing over occurs
• Telophase I

  1. Nuclear envelopes reform

  2. Chromosomes uncoil

  3. Cytokinesis occurs, producing haploid cells
• Prophase II

  1. Chromosomes re-condense

  2. Nuclear envelope breaks down again

  3. Spindle fibres reform
• Anaphase II

  1. Chromatids are split apart by the spindle fibres

  2. Chromatids move to the poles of the cells
• Telophase II

  1. Chromatids uncoil

  2. Nuclear envelopes reform

  3. Cytokinesis occurs, producing 4 genetically unique haploid daughter cells
• A zygote is formed when two haploid gametes fuse.
• Metaphase I

  1. Chromosomes a line on the cell equator
  2. Spindle fibres attach to the centromeres
  3. Independent Assortment/segregation - the position of each bivalent is independent of the all the other chromosomes

  • The maternal and paternal chromosomes can be on either side of the equator
  • Random orientation of homologous pairs at the equator introduces more variation
• Anaphase I

  1. Homologous chromosomes are pulled by the spindle fibres to the poles

  • They are not pulled apart – sister chromatids stay attached

  This causes the genetic variation
• Metaphase II

  1. Chromosomes line on the equator by the spindle fibres
  2. Independent assortment occurs again

  More genetic variation is caused
• The crossing over of homologous chromosomes during meiosis I, introduces genetic variation as the chromosomes exchange parts of their genetic material (alleles).
• Independent assortment/segregation is when the chromosomes from each pair are randomly allotted to the daughter cells. It occurs in both meiosis I and II and it introduces genetic variation.
• The random fertilisation of gametes also adds variation to a species.
• Independent assortment means that each chromosome is inherited randomly and independent of other chromosomes - i.e. the inheritance of one chromosome does not affect the inheritance of another chromosome.
• A non-disjunction mutation is a chromosomal mutation in which homologous chromosomes or sister chromatids do not separate properly.
• the number of possible chromosomal combinations resulting from meiosis is equal to$2^n$

  • n is the number of homologous chromosome pairs
• The formula to calculate the number of combinations of chromosomes after the random fertilisation of two gametes is $(2^n)^2$

  • n is the haploid number and $^2$ is the number of gametes
• Chiasmata are the points of contact when pairs of homologous chromosomes lined up next to each other touch at the same loci on each.
• A bivalent is the association of two replicated chromosomes having exchanged DNA strand in at least one site called chiasmata.
• Adaptations can be anatomical, physiological or behavioural

  • behavioural: ways an organism acts that increase its chance of survival and reproduction
  • physiological: processes inside an organism's body that increase its chance of survival
  • anatomical: structural features of an organism's body that increase its chance of survival
• Stabilising and directional selection are types of natural selection that affect allele frequency.