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What is a mutation?
Any change in the base sequence of DNA
What’s so special about abnormal DNA mutations
Mutations are heritable. Once the mutation arises, it stays and is passed on.
What can a mutation do
It can lead to a faulty or missing protein.
Are mutations important only for inherited diseases?
No, mutations in somatic cells can do a lot of damage as well.
Two types of Single-base substitution (“pointmutation”)
- Transition - Pyr for a Pyr (T-C; A-G)
- Transversion Pyr for a Pur or vice a vs
Somatic mutations are important for
Aging and Cancer
In gametes or the precursors of gametes. Can lead to genetic diseases
Duplication is most common when
During crossing over in Meiosis
How many hydrogen bonds are between the base pairs
What is the weakest bond between bases in DNA
A Purine base, and a deoxyribose
90% of disease causing mutations are found in what
The coding sequence of the gene
What is a somatic cell
Any cell but those in the germline
What is neoplasia
An abnormal growth
Each child is born with about
- 100 new mutations.
- − Most of these mutations are harmless.
- − A few are slightly unfavorable. They contribute to polygenic diseases.
- − Rarely, a single mutation is bad enough to cause a disease.
- − Favorable mutations are very rare
Why are most serious genetic diseases rare?
Those carrying the genes usually die or can't reproduce
Is purifying selection more effective for recessive or for dominant disease mutations?
- Recessive can be passed without ever manifesting itself
- Dominant always express themselves, but remain low causing disease. This is when natural selection kicks in, therefore dominant
Causes of mutations can be
- Spontaneous tautomeric shifts
- Ionizing radiation (X-rays, radioactivity)
- UV radiation (sunlight)
Spontaneous tautomeric shifts
A change in a bases in nucleic acid, shift between keto and enol forms or between amino and imino forms
The risk of new mutations depends on parental
- Increased maternal age is a risk factor for aberrations in chromosome number.
- Advanced paternal age is a risk factor for point mutations.
Nucleotide excision repair
- This is for lesions that are large enough to distort the geometry of the DNA double helix
- The removed gap is filled by DNA polymerase and DNA ligase
Two defects of nucleotide excision repair
- Xeroderma pigmentosu
- Cockayne syndrome
- − Defect of genome-wide nucleotide excision repair
- − Sunburn, skin cancer
- − Autosomal recessive
- − 7 different types
- − Defect of transcription-coupled nucleotide excision
- − Poor growth, neurological problems, early senility
- − Autosomal recessive
- − 2 different types
LYNCH SYNDROME (HEREDITARY NON-POLYPOSIS COLON CANCER)
- Happens most in quickly dividing cells
- − Defect of post-replication mismatch repair
- − ≥50% risk of colon cancer
- − Also increased risk of other cancers
- − Autosomal dominant inheritance
Post-replication mismatch repair happens how
When mismatched DNA is found it is removed (while still attached to the original strand) and then repaired by DNA polymerase and ligase
Why does a defect of post-replication mismatch repair cause cancer
All cells have a heterozygous defect in a mismatch repair protein. When a somatic mutation disrupts the single intact copy of the gene, the cell becomes a mutator. Some of these mutator cells become cancerous.
- Caused by a mutation in a signaling protein required for the repair of DNA double-strand breaks.
- Cerebellar ataxia (loss of body movement) starting in early childhood
- Dilation of small blood vessels
- Sensitivity to ionizing radiation
Radiation ionizing has what effect on the body
- 1. Can knock off electrons off orbitals
- 2. Can hit DNA directly
- 3. Makes hydroxyl radicals
What is the only base with a methyl group
Free radicals are present in the body where
Wherever oxydation takes place
What can a Thymine base be replaced with
Nitrite is partially
Mutagenic and is used as a preservative in meat
A base is deleted
AP endonuclease, Polymerase B
- AP endonuclease, polymerase beta remove wrong material
- Polymerase Beta replaces with correct nucleotide
- Ligase fills the small gap leftover
Why is the advantage of having Thymine instead of Uricil in the DNA
The deamination of uricil to cytosine is not detected and therefore won't be repaired
Methylcytosine has high mutation rate why
- Its deamnination produces thymine that won't be detected to be repaired by incision repair
- This is what makes CG sequences a hot spot for mutations
Nucleotide Excision repair
Is one of the most important repair systems!!
Removal of an amine. Because base pair excision can't recognize this it is considered a hot spot for mutations
What are intercalating agents
Aromatic structures that place themselves between base pairs pushing them apart. These can contribute to small insertions and deletions