Gene Mutation and DNA Repair Quiz

Gene Mutation and DNA Repair

• Mutations are variations in DNA, arising from errors during DNA copying, such as inserting the wrong base or skipping a base.
• Mutations originate from the Latin word mutare, meaning "to change."
• Mutations in DNA can cause heritable changes in genetic information.
• Organisms have developed repair mechanisms to fix damaged DNA.
• Mutations can occur at either the chromosomal or gene level.
• Gene mutations affect single genes.
• Chromosomal mutations affect whole chromosomes.
• Point mutations are changes in one or a few nucleotides, occurring at a specific point in the DNA sequence.
• Point mutations typically arise during DNA replication.
• A point mutation involves a base substitution, resulting in either a transition or a transversion.
• A transition is a change from one pyrimidine to another, or one purine to another.
• A transversion is a change from a pyrimidine to a purine, or vice versa.
• Mutations can also involve additions or deletions of short sequences of DNA.
• Mutations in the coding sequence may lead to different types of mutations.
• Silent mutations do not alter the amino acid sequence due to the degeneracy of the genetic code.
• Missense mutations alter the amino acid sequence, some may not affect function.
• Nonsense mutations change a codon to a stop codon, resulting in a truncated polypeptide.
• Frameshift mutations involve the addition or deletion of nucleotides in multiples of one or two, but not three (the size of a codon), altering the reading frame downstream from the mutation.
• New mutations are more likely to lead to reduced function than enhanced function, except for silent mutations.
• Occasionally, a mutation can lead to an enhanced ability of a polypeptide to function, raising the likelihood of survival and reproduction.
• Mutations can occur in germ-line cells (producing gametes) or somatic cells (all other cells).
• The earlier the mutation, the larger the affected area of the body.
• Mutations can be spontaneous or induced.
• Spontaneous mutations arise due to abnormalities in cellular processes, such as during DNA replication.
• Induced mutations are caused by environmental factors, such as chemical or physical mutagens.
• Types of spontaneous mutations include depurination, deamination, and tautomeric shifts.
• Depurination is the removal of a purine (guanine or adenine) from the DNA.
• Deamination is the removal of an amino group from a cytosine base, changing it to uracil.
• Tautomeric shifts are temporary changes in base structure which can lead to mismatches.
• Chemical mutagens, such as nitrous acid, alkylating agents, intercalating agents, and base analogs, can alter the structure of DNA, contributing to mutations.
• Physical mutagens, such as ionizing radiation (X-rays, gamma rays) and non-ionizing radiation (UV light) can cause DNA damage potentially leading to mutations.
• DNA repair mechanisms, such as direct repair, nucleotide excision repair, and mismatch repair, are essential for fixing various types of DNA damage.
• Several human diseases are associated with defects in genes involved in DNA repair.
• Diseases like xeroderma pigmentosum, and Cockayne syndrome are examples of such diseases arising from defects in DNA repair.
• The correct recognition and removal of abnormal DNA is critical in DNA repair processes.

DNA Repair

• Living cells contain several mechanisms for DNA repair.
• DNA repair is mostly a multi-step process.
• DNA repair starts with detecting an irregularity in DNA structure.
• The abnormal DNA is then removed.
• Normal DNA is synthesized to repair the damage.
• Types of DNA repair mechanisms include direct repair, base excision repair, nucleotide excision repair, mismatch repair, homologous recombination repair, and non-homologous end joining.
• Photolyase repairs thymine dimers, restoring the DNA.
• Nucleotide excision repair fixes a wide range of DNA damages including thymine dimers and chemically modified bases.

Mismatch Repair

• DNA replication can occasionally lead to base pair mismatches.
• DNA polymerases have 3' to 5' proofreading abilities that correct these errors.
• A mismatch repair system is activated if proofreading fails, coming to the rescue, by directing the removal of the error in the DNA strand
• These systems are found in all species.
• In humans, defects can lead to specific types of cancer.
• MutL, MutH, and MutS proteins are involved in mismatch repair in E. coli and recognize/remove the error in the newly synthesized DNA strand.

Recombination Repair

• DNA double-strand breaks are very dangerous and can lead to chromosomal rearrangements and deletions.
• Homologous recombination repair (HRR) and non-homologous end joining (NHEJ) repair these breaks in DNA structure by pairing with a homologous chromosome.

Position Effects

• Chromosome rearrangements may affect the expression of a gene due to the breakpoint in the gene itself or its new location.
• This is called position effect; gene expression can be affected because the gene is moved to a position near regulatory sequences for a different gene or to a heterochromatic region.
• The location of regulatory sequences is often bidirectional.