Mutations Flashcards
Gene Mutations
Can be random and / or spontaneous events that result in changes in one or more nucleotides (nitrogenous bases) in the DNA sequence.
They are usually caused during DNA replication (spontaneous mutations) or by mutagens (induced mutations) .
Mutagens
Alter the DNA nucleotide sequence and can be:
Forms of radiation (UV, X-Rays, gamma rays)
Chemicals (nicotine, pesticides, preservatives, heavy metals)
Heat (disrupts sugar- base bonding in DNA)
Viruses (disrupt DNA sequencing) & toxins produced by organisms. See Fig. 5.4.6 to the right.
Inherited mutations (that are passed on via sex cells) can produce variations in the genotype and phenotypes of offspring.
Four major types of gene mutations
Substitutions, Inversions, Insertions and Deletions.
Frame shift mutations
(eg. insertions and deletions) where every codon and therefore amino acid after the mutation point is altered. Frame shift mutations have significant effects
Point mutations
a single nucleotide (a single nitrogenous base) is altered (eg. Substitution). Point mutations may or may not have significant effects.
A non-sense mutation
Occurs when the base substitution results in an early stop codon being transcribed in the mRNA. When this occurs, the polypeptide chain translated is stopped prematurely and will often not function.
A mis-sense mutation
Occurs when the base substitution results in a different amino acid being coded for. Since there is a different amino acid in the polypeptide, it may not function correctly as the inter-molecular bonds that give the unique shape of the proteins tertiary structure may be changed and hence the shape of the final protein may be different.
A silent mutation
Occurs when the substitution does not result in a different amino acid being coded for (ie. it is neutral…..if the there is a substitution in the third base of a triplet /codon). The polypeptide will therefore contain the correct sequence of amino acids and so will still function correctly.
A substitution mutation
Where a base(s) is replaced by another base(s).
An inversion mutation
Where a sequence of a small number of bases (eg. 2 or 3) is inverted (flipped around).
An insertion or addition mutation
Where an extra base(s) is added to the base sequence.
Can cause a frameshift mutation
A deletion mutation
Where a base(s) is deleted (removed) from the base sequence.
Can cause a frameshift mutation
Number of chromosomes in normal human somatic cells
In normal human somatic cells and zygotes there are 46 chromosomes. 44 of these are autosomes (that do not have genes that help to determine sex) and 2 of these are sex chromosome
Karyotyping
A process that involves geneticists photographing (normally during prophase), classifying and arranging chromosomes in homologous pairs from largest to smallest to produce a karyogram image.
Chromosome mutations
- The addition or deletion of entire chromosomes eg. via non-disjunction during meiosis leading to aneuploidy (an extra or missing chromosome) ….or
- the addition, deletion or mixing of segments of chromosomes eg. block mutations (duplications, inversions, deletions) and translocations (sections of chromosomes break and join to other chromosomes).
Chromosome mutations affect multiple genes and thus can affect the phenotypes of individuals in a multitude of negative ways, often leading to syndromes.
