Lecture 15 - Mutations 1 Flashcards
What are mutations?
Randomly derived changes in the nucleotide sequences of the DNA of an organism, that can be passed on to daughter cells
Two types of mutations
- Somatic mutations
2. Germ line mutations
What are somatic mutations?
They are mutations that occur in the somatic (body) cells.
They are passed on by mitosis but not to sexually produced offsprings.
What are germ line mutations?
They are mutations that occur in germ line cells (the cells that give rise to gametes).
A gamete passes a mutation on at fertilization
Three different phenotypic effects of mutations
- Silent mutations
- Gain or loss of function mutations
- Conditional mutations
What are silent mutations?
Mutations that do not significantly alter the phenotype of the organism in which they occur
What are gain or loss of function mutations?
Mutations that lead to a protein with new (altered) function or to a non-functional protein
What are conditional mutations?
A mutation that has the wild-type phenotype under certain (permissive) environmental conditions and a mutant phenotype under other (restrictive) conditions.
Benefits of mutations
-Drive evolution in the form of genetic diversity.
Diversity may benefit the organism immediately - if mutation is in somatic cells or may cause an advantageous change in the offspring
Possible cost of mutations
Some germ line and somatic cell mutations are harmful or lethal
Two different nucleotide mutations
- Chromosomal mutations
2. Point mutations
What are chromosomal mutations?
They may change position or cause a DNA segment to be duplicated or lost
What is a chromosomal deletion?
It is the loss of a chromosome segment
What is a chromosomal duplication and deletion?
It results when homologous chromosomes break at different points and swap segments
What is a chromosomal inversion?
It results when a broken segment is inserted in reverse order
What is a chromosomal reciprocal translocation?
Results when non-homologous chromosomes exchange segments
What are point mutations?
They result from the gain, loss, or substitution of a single nucleotide
What is a transition mutation?
It happens when a purine is substituted for another purine (GA) or pyrimidine for another pyrimidine (CT)
What is a transversion mutation?
It happens when a purine is substituted by a pyrimidine or vice versa
What is the open reading frame?
The part of a reading frame that has the potential to be translated
Missense mutations
Point mutation in which a single nucleotide change results in a codon that codes for a different amino acid
Nonsense mutations
Point mutation in which a single nucleotide change results in a stop codon to form resulting in a shortened protein, which is usually not functional
Readthrough mutations
Involve a base substitution that causes a stop codon to be converted into an amino acid codon which results in a lengthened protein
AAs with polar but uncharged R groups
Serine, Threonine, Glutamine, Asparagine
AAs with positively charged R groups
Lysine, Arginine, Histidine
AAs with negatively charged R groups
Glutamic Acid, Aspartic Acid
Special AAs
Cysteine, Glycine, Proline
AAs with hydrophobic R groups
Isoleucine, Leucine, Methionine, Phenylalanine, Tryptophan, Valine Alanine, Tyrosine
Explain frameshift mutations
These mutations interfere with the ORF of a protein-coding gene
Phenotypic consequences of frameshift mutations
Non-functional (truncated)proteins are produced. Unless it happens at the extreme 3’ end of the gene
Two ways in which mutations can arise
- Spontaneous mutations
2. Induced mutations
What are spontaneous mutations?
Permanent changes in the genetic material that occur without any outside influence. Occur because cellular processes are imperfect
What are tautomers?
Each of the four nucleotide bases can exist in two different forms, called tautomers, one of which is common and one is rare
Mechanisms by which spontaneous mutations can arise (Lots of examples just name a few)
- The four nucleotide bases of DNA can have different structure. When a base temporarily forms its rare tautomer, it can pair with the wrong base.
- Bases in DNA may change because of a chemical reaction. Like the deamination of cytosine.
- DNA polymerase can make errors in replication.
- Meiosis is not perfect. Nondisjunction can occur, leading to one too mane or one too few chromosomes.
How do induced mutations occur?
They occur when some agent from outside the cell, a mutagen, causes a permanent change in DNA
Examples of induced mutations (Lots of examples just name a few)
- Some chemicals can alter the nucleotide bases. For example, nitrous acid (HNO2) can react with cytosine and convert it to uracil by deamination.
- Some chemicals add groups to the bases. For example, benzopyrene adds a large chemical group to guanine, making it unavailable for base pairing.
- Radiation damages the genetic material. First, ionizing radiation (X rays, gamma rats) produces highly reactive chemicals called free radicals. Free radicals an change bases in DNA to forms that are not recognized by DNA polymerase. Second, ultraviolet radiation can damage DNA in another way. It is absorbed by thymine, causing it to form covalent bonds with adjacent bases, also distorting the double helix.
