M6IQ1- Mutations

Mutagens

  • mutations can occur spontaneously and randomly; errors in replication, or mutagens.
  • environmental agents that cause mutations- exposure to these substances increases their harmful effects.
  • the process of inducing a mutation is called mutagenesis.

Point and Chromosomal Mutations

Somatic and Germ-Line Mutations

Coding and Non-Coding

Effects of Mutation,
Gene Flow, and Genetic Drift

Electromagnetic Radiation

  • any radiation with a shorter wavelength than visible light can be mutagenic.
  • ionising radiation: radioactive materials from nuclear reactions, such as radiation from atomic bombs, toxic spills, and in medicine (x-rays, etc). this can remove electrons from atoms, causing mutations.
  • ultraviolet (UV) radiation: from sunlight, and also the UV lights in tanning salons.

Chemical Mutagens

  • chemicals that cause mutations to occur in DNA.
  • free radicals are chemically unstable substances in the body that have lost an electron. they try to become more stable by taking electrons from the body. antioxidants protect against this by donating electrons.
  • ingested chemicals: alcohol; tar; tobacco smoke; chemicals in diet- especially charred and fatty foods and food additives and preservatives.
  • irritants and poisons: organic solvents such as benzene, cleaning products; asbestos, coal tars; pesticides; hair dyes, certain medications

Biological Mutagens

  • organisms that cause mutagens to occur.
  • viruses reproduce by injecting their genome into a host cell and disrupting the cell's DNA. This can disturb repair mechanisms and lead to mutations.
  • the ageing process can lead to mistakes in mitosis becoming more common.
  • some chemical reactions in the body can produce free radicals which can damage DNA and cell membranes.
  • fungi produce mycotoxins that can be mutagenic to other species.

Naturally Occurring Mutagens

  • part of the environment; physical, radiation, biological or chemical.
    - radiation: cosmic rays from the sun, radiation from rocks and soil.
    - chemical: incomplete burning (bush fires, coal burning), benzene from crude oil and coal.
    - bacteria and viruses.
    - mistakes in mitosis or meiosis.

Point Mutations

  • a change in the nucleotide sequence
    of a single gene which may form a
    new allele. there are several ways
    a nucleotide sequence can change. errors in DNA replication

Deletion

  • a nucleotide is deleted from the DNA code. the effect is similar to that of an insertion. also a frameshift.

Insertion (addition)

  • one or more extra nucleotides are added to the DNA sequence, resulting in a difference in the amino acid sequence and the protein that is formed.
  • from the point of the mutation onwards, the amino acids will be completely different, causing a frameshift mutation. will usually result in a non-functioning protein.

Substitution

  • an incorrect nucleotide replaces
    one of the original ones in the DNA strand.
  • Silent mutation: a new amino acid is added with no effect on the resulting protein.
  • Missense mutation: a different protein will become incorporated into the chain during polypeptide formation and a different protein will be formed. The resulting protein may function differently to the correct protein (sickle cell anaemia).
  • Nonsense mutation: a codon is prematurely changed to a stop codon, and the resulting protein does not fully form. the protein does not function.
  • haemoglobin GAG-GTG

Chromosomal Mutations

  • can involve either a change in
    the number of chromosomes
    or a rearrangement in the
    structure of the chromosome. errors in cell division.

Inversion

  • due to a mistake in replication, a whole nucleotide triplet is back to front.
  • results in a completely different amino acid and therefore a new protein with different properties to the intended one.

Inversion

  • when the chromosome segments are turned upside down so the genes appear in reverse order

Translocation:

  • pieces are removed and inserted in another chromosome.
  • resulted in swapping bits of chromosome from one chromosome to another

Deletion

  • pieces of chromosome or a whole chromosome is lost. usually lethal.

Duplication

  • when parts of a chromosome appear twice or more

Somatic Mutations (Body Cell except Gametes)

  • mutations that occur in somatic cells can have an immediate effect on the DNA in those cells and DO NOT get passed onto the next generation.
  • can occur spontaneously or be induced by mutagens.
  • if the genes involved in DNA repair undergo mutations, serious cancers can develop.
  • lung cancer

Germ-line Mutations (Sex Cell)

  • mutations in germ line cells are inherited.
  • if the mutation is in a recessive gene, the effect may not appear for a few generations.
  • eg: royal family passed down recessive mutation causing haemophilia.
  • chemicals introduced at the embryo stage can cause birth defects
  • trisomy 21

Causes of
Genetic
Variation

Coding DNA and Mutation

  • DNA is transcribed into mRNA and this code is translated to make proteins. this is often known as gene expression.
  • if DNA is altered by a mutagen, and that DNA is part of a gene that could code for a protein, then changing the code could alter the activity of the cell.
  • this coding DNA is the exons that are kept after transcription. any change in an exon is carried over into errors in protein production.

Non-Coding DNA and Mutation

  • non-coding DNA is not involved in gene expression. this DNA is not translated to make proteins. it can be the introns that are removed after transcription or junk DNA that does not code for a protein.
  • some non-coding DNA triggers the action of genes that do code for proteins. if this is changed, then genes that are important for polypeptide synthesis may be switched off or on too often.
  • mutations in non-coding DNA often have little effect unless the introns get converted into exons.

Meiosis and Independent Assortment

  • during meiosis, the pairs of homologous chromosome are divided in half to form haploid cells and this separation, or assortment, of homologous chromosomes is random.
  • this means that all of the maternal chromosomes will not be separated into one cell, and all of the paternal chromosomes will be separated into one cell.
  • as a result, alleles for different traits are unrelatedly sorted into gametes.

Meiosis and Random Segregation

  • the independent assortment of chromosomes at stage 1 of meiosis leads to random separation of each pair of chromosomes (random segregation) in the 4 possible sex cells that form.
  • as a result during gamete formation, alleles responsible for the same trait separate from one another.

Fertilisation

  • the combination of ova and sperm at fertilisation randomly selects one sperm and one egg.

Effects of Mutation

  • Meiosis and fertilisation cause much variation but only from the available gene pool.
  • mutations introduce new genes to the gene pool, making variations available for the development of new species.
  • when the environment changes, some variations will be more successful and new species can arise.

Effects of Gene Flow

  • gene flow occurs when new alleles are added or removed from the gene pool, due to the movement of fertile individuals or gametes.
  • this can be caused by migration of individuals from one population to another, and can be influenced by natural selection or random chance.
  • gene flow reduces the difference between populations by altering gene frequencies.

Effect of Genetic Drift

  • genetic drift occurs when chance events (random- no natural selection) cause unpredictable changes in allele frequencies from one generation to the next.
  • examples of random events: natural disasters, accidental death, extreme weather, etc.
  • founder effect: few individuals become isolated from larger population and form a new population.
  • bottleneck effect: large population suddenly in size.

Non-Disjunction

  • chromosomes don't separate properly. trisomy 21