Genetic alteration in fragile X syndrome?. Effect on behavioral development.
Expansion trinucleotide repeats (CGG) disorder in long arm of X chromosome - FMR1 (fragile X mental retardation) gene.
Most common inherited cause of intellectual disability (2nd genetic most common cause, 1st Down syndrome).
Which mechanism repair the accelerated deamination of cytosine in DNA? Sequence of reparation. What other changes are repaired by that mechanism?
- Base excision repair ▶️ glycosylases (cleave the altered DNA base - leave an AP [apurinic/apirymidinic] site) ▶️ endonucleases ▶️ lyase (both extract AP site) ▶️ DNA polymerase ▶️ ligase
- Depurination, alkylation, oxidation
Causes and consequences of the deamination of the DNA
- Spontaneously
- Exogenous chemicals ▶️ ⬆️ dietary nitrites
- if not corrected ▶️ mutations ▶️ carcinogenesis
How is the genomic imprinting done and what is it?
- Offspring’s genes are expressed in a parent specific manner→ex; allele from father don’t express, the allele from mother is the only expressed.
- Methylation→DNA methyltransferase (uses S-adenosyl-methionine [SAM]→methyl group donor) in cytosine, guanine, adenine.
Which is the mutation in fragile X syndrome? What type of epigenetic change does it cause in the DNA? How do you test it?
- “Full mutation” in FMR1 gene (normally 5-55 CGG repeats) ▶️ >200 CGG repeats ▶️ hypermethylation ▶️ 🚫 transcription ▶️ impair neural development
- Southern blot ▶️ degree of methylation and number of CGG repeats
How do you know when two loci are in linkage disequilibrium?
Frequency or probability of 2 alleles to be inherited together > than expected (multiply both independent frequencies)
What is a dominant negative mutation? Example.
- Abnormal gene negatively affects the production of the wild-type gene in the same cell
- p53 mutation→p53 abnormal protein prevents wild-type p53 from binding to the promoter of its target genes
What would be the phenotype of newborn with karyotype 46XX t(14;21)? what is the disease and the mechanism that cause it?
- Female with Down syndrome by unbalanced Robertsonian translocation→46 chromosomes but 3 copies of chromosome 21.
- Congenital cardiac defects (VSD), epicanthal folds, upslanting palpebral fissures, protruding tongue, increase skin at nape of neck, low birth weight and length, hypotonia, weak startle reflex (moro).
What genetic diseases are associated with meiotic nondisjunction?
Turner syndrome, klinefelter syndrome, trisomies 13, 18, 21.
Why do the turner syndrome patients have short stature?
Loss X chromosome→miss SHOX gene→long bone growth
What is the heteroplasmy and what does it explain?
- Random distribution of mitochondria during mitosis between daughter cells▶️some cells with mostly damaged mtDNA and other cells with mostly normal mtDNA
- Clinical variability of mitochondrial diseases
What paths for processing and handling of mRNA is realized within cytoplasm? And for what?
Interaction with P bodies
- mRNA translation regulation▶️translation repression (content micro RNA→mRNA silencing), storage of mRNA (later release for translation)
- mRNA degradation, quality control▶️content RNA exonucleases, mRNA decapping enzymes
What proteins can bind to DNA? What allow that property?
- Transcription factors, steroids, thyroid hormones, vitamin D receptors, retinoic acid receptors, DNA transcription and replication proteins. Ex, N-myc (MYC are transcription factors)
- Can be detected by DNA probes
Function of RNA interference. Types and clinical application of them.
- Posttranscriptional gene silencing
- small interfering RNA (siRNA), microRNA (miRNA)
- Silence specific pathogenic genes
What is the DNA laddering? What does it indicate?
- DNA fragmentation in multiples of 180 base pairs in gel electrophoresis▶️action of specific endonucleases during karyorrhexis (may occur when withdraw stimulant factors - IL)
- Apoptosis
*Fragments absent in malignant cells
What is the germline mosaicism and when do you suspect it? Of what depends the likelihood to be affected?
- Mutation in oocyst or spermatocyte
- Genetic mutation identified in the offspring but no in the parents
- Proportion of mosaic to wild-type germ cells in the mosaic parent
Differences between DNA and DNA replication of eukaryotes and prokaryotes.
- Prokaryotes→DNA polymerases (I, II, III), circular DNA, single origin of replication
- Eukaryotes→DNA polymerases (α [primase], β, γ, δ, ε), linear DNA, larger, more complex (>introns), multiple origins of replication
Which proteins binds to the stop codons and what is their function?
Releasing factors→bind to the ribosome→release of the formed polypeptide chain and dissolution of the ribosome-mRNA complex
Most typical mutation of cystic fibrosis, type of mutation.
*Mode of inheritance and frequency.
70% of cases (northern europe decent)→F508 mutation→deletion of 3 nucleotides (in frame) of CFTR gene on chromosome 7→loss Phe at position 508 of sequence
Autosomal recessive. Most common lethal genetic disease in Caucasian population.
How is regulated the lac operon sequence in a bacteria?
- Negatively→binding of the repressor protein (Lac I gene product) to the operator locus
- Positively→cAMP-CAP binding upstream from the promoter region
Why do you need to know the flanking sequences of the target region to make a polymerase chain reaction (PCR)?
To make the primers to star the PCR
What is a telomerase?
Ribonucleoprotein→add TTAGGG repeats to the 3’ end chromosomes (telomere region)►reverse transcriptase (RNA dependent DNA polymerase)
Which cells have abundant telomerase?
Cells with long telomeres→embryonic or adult stem cell
- Adult stem cells→Epidermal basal cells, bone marrow stem cells
In prokaryotes which is the unique enzyme that has 5’→3’ exonuclease activity and for what?
DNA polymerase I→remove RNA primer and repair damaged DNA sequences
*DNA polymerase III→has 5’→3’ polymerase and 3’→5’ exonuclease activity
