Mutations

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A mutation is a change in the base sequence of DNA that can affect the structure and function of proteins when they occur within a gene

Mutations can be spontaneously caused by copying errors during DNA replication or induced by exposure to external elements such as chemical mutagens, radiation or viruses

In multicellular organisms, mutations can be classed as either:

  • Somatic mutations - occur in a single body cell and cannot be inherited (can give rise to cancers)
  • Germline mutations - occur in gametes and can be passed on to offspring to either harmful, neutral or beneficial effect


Point Mutations

Point mutations are changes to one base in the DNA code and may involve either:

  • The substitution of a base (e.g. ATG becomes ACG)
  • The insertion of a base (e.g. ATG becomes ATCG)
  • The deletion of a base (e.g. ATG becomes AG)
  • The inversion of bases (e.g. ATG becomes AGT)


Overview of the Different Types of Point Mutations

Effects of Point Mutations

Base substitutions may create either silent, missense or nonsense mutations, while insertions and deletions create frameshift mutations

  • Silent mutations occur when the DNA change does not alter the amino acid sequence of the polypeptide
    • This is possible because the genetic code is degenerate and certain codons may code for the same amino acid
  • Missense mutations occur when the DNA change alters a single amino acid within a polypeptide sequence
    • Sickle cell anaemia is an example of a disease caused by a single base substitution mutation (GAG becomes GTG ; Glu becomes Val)
  • Nonsense mutations occur when the DNA change creates a premature stop codon which truncates the amino acid sequence
    • Cystic fibrosis is an example of a disease which can result from a nonsense mutation (this may not be the only cause though)
  • Frameshift mutations occur when the addition or removal of a base alters the reading frame of the gene
    • This change will affect every codon beyond the point of mutation and thus may dramatically change the amino acid sequence


Block Mutations

Block mutations are changes to segments of a chromosome, resulting in large scale changes in the DNA of an organism

Block mutations are commonly caused by transposons, mobile genetic elements that can change positions within the genome and, by doing so, alter the sequence of genes

Several types of block mutations exist, including:

  • Duplications - a part of the chromosome is copied, resulting in duplicate segments (potentially increasing gene expression)
  • Inversions - a segment of a chromosome is removed and then replaced in reverse order
  • Deletions - a portion of a chromosome is lost (along with any genes contained within this segment)
  • Insertions - a part of one chromosome is removed and added to another chromosome (may interrupt gene sequences)
  • Translocations - segments of two chromosomes are exchanged (may interrupt gene sequences)


Overview of the Different Types of Block Mutations


Chromosomal Abnormalities

Certain individuals may possess extra, or missing, chromosomes (aneuploidy) as a result of non-disjunction during meiosis

Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with an abnormal number of chromosome

The failure of the chromosomes to separate may either occur via:

  • Failure of homologues to separate during Anaphase I (resulting in four affected daughter cells)
  • Failure of sister chromatids to separate during Anaphase II (resulting in two affected daughter cells)


Non-Disjunction


When an abnormal gamete fuses with a normal gamete during fertilisation, it creates a zygote with extra or missing chromosomes

An example of a condition caused by non-disjunction is Down syndrome, whereby individuals have three copies of chromosome 21 (trisomy 21) 

  • One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
  • The other parental gamete was normal and had a single copy of chromosome 21
  • When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21, leading to Down syndrome


Karyotyping

Chromosomal abnormalities (and certain block mutations) can be detected via karyotyping

A karyotype is a visual profile of all the chromosomes in a cell

The chromosomes are arranged into homologous pairs and displayed according to their structural characteristics


Human Male Karyotype


Karyotyping involves:

  • Harvesting cells (usually from foetus or white blood cells of adults)
    • Foetal cells can be harvested from either the amniotic fluid (via amniocentesis) or the placenta (via chorionic villus sampling)
  • Chemically inducing cell division, then halting it during mitosis when chromosomes are condensed and thus visible
    • The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids
  • Staining and photographing chromosomes, before arranging them according to structure

Analysing Karyotypes