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Mutations

by | Feb 18, 2024

  • A change in the sequence of nucleotides in an original DNA molecule
    • Mutations are irreversible and the main cause of genetic variation (alleles)
  • Mutations can cause changes in phenotype (ex. cystic fibrosis) or disorders
    • Alterations in DNA can lead to changes in type or amount of protein produced
  • DNA mutations can be positive, negative, or have no consequence; effect is determined by environmental conditions (usefulness of a mutated phenotype determined by natural selection)

 

Causes for Mutations

  • Mutagens: radiation or chemicals that cause mutations → damage DNA (ex: deamination, double strand breaks)
    • Carcinogens: mutagens that activate uncontrolled cell growth (cancer)

Environmental:

  • UV light → forms dimers; radiation (X-rays); chemical mutagens

 

Internal:

  • Errors in DNA replication, proofreading, or repair

 

Point Mutation: single or few nucleotide errors that include…

  1. Substitution: when DNA sequence contains an incorrect nucleotide instead of the correct one
  • An amino acid substitution would always alter the primary structure of the protein, sometimes alter the tertiary structure of the protein and its biological activity
  1. Deletion: when a nucleotide is omitted from the nucleotide sequence → missing amino acid
  • Deletions closer to start point of coding are more harmful
  • Can occur when homologs don’t align properly
  • An amino acid deletion will alter the primary, secondary, and tertiary structure → can’t fold properly → nonfunctional
  1. Insertion: when a nucleotide is added to nucleotide sequence
  2. Frameshift: result of deletion or insertion & occurs when the number of nucleotides is not divisible by three → alters the way that the genetic messages (mRNA codons) is read
  • All other codons and amino acids (proteins) following mutation will be wrong

Effects of Point Mutations

  • Point mutations may or may not have a significant phenotypic effect.
    • Depends on whether amino acid had important chemical properties or part in proteins structure
      • Might change the lvl of protein activity bcuz might substitute an amino acid in the active site.
  1. Silent Mutation: when new codon still codes for the same amino acid
  • More common when nucleotide substitution results in a change of last of 3 in a codon nucleotide
  • Wobble Pairing: relaxed requirement for in the third position nucleotide
  1. Missense Mutation: when new codon codes for new amino acid
  • Effect can be minor or result in the production of a protein that can’t fold into 3D shape and carry out function specific to shape
    • Ex: hemoglobin protein that causes sickle-cell disease
  1. Nonsense Mutation: when new codon codes for stop codon
  • Causes translation to be terminated early → resulting polypeptide is shorter → usually nonfunctional

Alterations to Chromosome Structure:

  • Chromosomal aberrations occur when chromosome segments are changed, often result of crossing over errors
  • Duplications: broken DNA fragment reattached as an extra to chromatid resulting in repeating segment
    • Ex: Huntington’s disease is caused by insertion of multiple repeats of 3 nucleotides → codes for defective enzyme → death
  • Deletions: during crossing over, one chromosome takes the WHOLE part of the other that crossed over and the other one is left shorter. Usually involve lots of genes
    • Cri du chat: cry of the cat, caused by deletion in chromosome 5
  • Inversion: piece of chromosome that is crossed over goes on backwards,
    • So order of the genes matters (usually don’t express abnormalities as long as does not introduce duplications or deletions)
  • Translocation: two nonhomologous chromosomes cross over which shouldn’t
    • A parent with balanced translocation has chance to produce offspring with deletion or duplication

Notes

  • Random mutations = source of new alleles = new phenotypes
  • Deletion and duplication more common during crossing over when non-sister chromatids exchange unequal amounts of DNA
  • Translocations during mitosis can cause some cancers
  • Deletions and translocations more lethal
    • Syndrome = not as lethal, group of traits caused by gene mutation or chromosome alteration
  • Translocations and inversions don’t usually alter phenotype

 

Human Genetic Disorders

  1. Point Mutations:
  • Sickle-cell disease: caused by nucleotide substitution → defective hemoglobin → blood cell to become sickle shaped in low-oxygen → cell does not flow through capillaries freely and oxygen not delivered throughout body
    • Heterozygote generally without symptoms
  • Tay-Sachs Disease: nucleotide insertion → lysosomes lack functional enzyme to break down glycolipids → fat accumulate → death
  1. Aneuploidy:
  • Down Syndrome: when egg or sperm with with extra number 21 chromosome fuses with normal gamete → results in gamete with 3 copies of chromosome 21 (trisomy 21)
  • Turner Syndrome: nondisjunction of the sex chromoosome (monosomy)
    • Sperm will have either both sex chromosomes (XY) or none (O)
    • When normal gamete fuses with (O) gamete will have zygote that is XO → sterile female (45 chromosomes) with abnormalities
    • Although absence of single chromosome usually fatal, Y chromosome has such few genes that not too harmful
  • Klinefelter Syndrome: occurs when XY or XX, produced as result of nondisjunction, combines with normal gamete to form XXY zygote (male with extra X)
    • Only affects male → still male as long as you have Y chromosome with the SRY gene
      • Explains how there can be male calico cats
      • Might express female secondary sex characteristics

Transfer and Sharing of DNA: Prokaryotes Vs. Eukaryotes

Bacteria: 3 Methods of Transferring DNA

  1. Transduction                                                                                             
  • Viruses that infect bacteria (bacteriophages) move DNA from one bacterium to another “by accident.”   
  1. Transformation
  • Bacteria takes in DNA from its environment,
  • DNA often from other bacteria as a plasmid

              Which can be copied and passed onto descendants

Conjugation

  • DNA is transferred from one bacterium to another.

  • Donor cell (F+) uses pilus to transfer DNA from its cell to recipient cell (F-) → recipient becomes donor
    • Usually DNA is in the form of a plasmid
  • Donor cells typically act as donors because they have a chunk of DNA called the fertility factor (or F factor)

    • F Factor: codes for the proteins that make up the sex pilus.

    • also contains a special site where DNA transfer during conjugation begins

Horizontal Gene Transfer:

  • Horizontal Gene Transfer: transmission of DNA between different genomes
    • Source of genetic variation in bacteria: horizontal gene transfer & mutations
      • Bacteria use horizontal gene transfer used to spread virulent genes to harmless stains                                       
    • Vertical Gene Transfer transmission of genetic material from parents to offspring during reproduction

Occurs in: eukaryotes, prokaryotes; chloroplasts, nucleus, & mitochondrion