Genetics

Question 1

Genetic alteration in fragile X syndrome?. Effect on behavioral development.

Answer 1

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).

Question 2

Which mechanism repair the accelerated deamination of cytosine in DNA? Sequence of reparation. What other changes are repaired by that mechanism?

Answer 2

• 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

Question 3

Causes and consequences of the deamination of the DNA

Answer 3

• Spontaneously
• Exogenous chemicals ▶️ ⬆️ dietary nitrites
• if not corrected ▶️ mutations ▶️ carcinogenesis

Question 4

How is the genomic imprinting done and what is it?

Answer 4

• 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.

Question 5

Which is the mutation in fragile X syndrome? What type of epigenetic change does it cause in the DNA? How do you test it?

Answer 5

• “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

Question 6

How do you know when two loci are in linkage disequilibrium?

Answer 6

Frequency or probability of 2 alleles to be inherited together > than expected (multiply both independent frequencies)

Question 7

What is a dominant negative mutation? Example.

Answer 7

• 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

Question 8

What would be the phenotype of newborn with karyotype 46XX t(14;21)? what is the disease and the mechanism that cause it?

Answer 8

• 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).

Question 9

What genetic diseases are associated with meiotic nondisjunction?

Answer 9

Turner syndrome, klinefelter syndrome, trisomies 13, 18, 21.

Question 10

Why do the turner syndrome patients have short stature?

Answer 10

Loss X chromosome→miss SHOX gene→long bone growth

Question 11

What is the heteroplasmy and what does it explain?

Answer 11

• 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

Question 12

What paths for processing and handling of mRNA is realized within cytoplasm? And for what?

Answer 12

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

Question 13

What proteins can bind to DNA? What allow that property?

Answer 13

• 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

Question 14

Function of RNA interference. Types and clinical application of them.

Answer 14

• Posttranscriptional gene silencing
• small interfering RNA (siRNA), microRNA (miRNA)
• Silence specific pathogenic genes

Question 15

What is the DNA laddering? What does it indicate?

Answer 15

• 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

Question 16

What is the germline mosaicism and when do you suspect it? Of what depends the likelihood to be affected?

Answer 16

• 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

Question 17

Differences between DNA and DNA replication of eukaryotes and prokaryotes.

Answer 17

• 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

Question 18

Which proteins binds to the stop codons and what is their function?

Answer 18

Releasing factors→bind to the ribosome→release of the formed polypeptide chain and dissolution of the ribosome-mRNA complex

Question 19

Most typical mutation of cystic fibrosis, type of mutation.

*Mode of inheritance and frequency.

Answer 19

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.

Question 20

How is regulated the lac operon sequence in a bacteria?

Answer 20

• Negatively→binding of the repressor protein (Lac I gene product) to the operator locus
• Positively→cAMP-CAP binding upstream from the promoter region

Question 21

Why do you need to know the flanking sequences of the target region to make a polymerase chain reaction (PCR)?

Answer 21

To make the primers to star the PCR

Question 22

What is a telomerase?

Answer 22

Ribonucleoprotein→add TTAGGG repeats to the 3’ end chromosomes (telomere region)►reverse transcriptase (RNA dependent DNA polymerase)

Question 23

Which cells have abundant telomerase?

Answer 23

Cells with long telomeres→embryonic or adult stem cell

- Adult stem cells→Epidermal basal cells, bone marrow stem cells

Question 24

In prokaryotes which is the unique enzyme that has 5’→3’ exonuclease activity and for what?

Answer 24

DNA polymerase I→remove RNA primer and repair damaged DNA sequences

*DNA polymerase III→has 5’→3’ polymerase and 3’→5’ exonuclease activity

Question 25

What explain the phenomenom that allow tRNA recognize multiple different codons coding for the same amino acid?

Answer 25

Wobble hypothesis→first 2 nucleotide positions on the mRNA codon require traditional (watson-crick) base pairing with their complementary tRNA. 3rd nucleotide “wobble” may undergo nontraditional (less stringent) base pairing

Question 26

Which pairs of base or sequence do the spliceosomes splice? Consequence if mutations at those sites?

Answer 26

• Remove introns containing GU at the 5’ splice site and AG at the 3’ splice site
• Mutations can lead in inappropriate remove of exons, retention of introns→dysfunctional proteins

Question 27

Examples of Y-linked disorders

Answer 27

Retinitis pigmentosa

Question 28

What is the lac-operon and in which type of organism is present?

Answer 28

Genome in E. coli (procaryote)→polycistronic gene (feature of bacterial mRNA)→mRNA for a lot of proteins►lactose metabolism

• Composition:
• Regulatory gene (i), promoter region (p), operator region (o), three structural genes→z (B-galactosidase), y (permease), a (B-galactoside transacetylase)

Question 29

Pattern of inheritance of androgenic alopecia

Answer 29

Polygenic with variable expresivity→short arm of chromosome 20, chromosome X and possibly Y►some genetic factors are X-linked recessive (gene variations in androgen receptor) and others autosomal dominant

*Most common type of hair loss in male and females

Question 30

What is the slipped-strand mispairing and its consequence?

Answer 30

• Mutation process which occurs during DNA replication
• Since there are some repeated nuleotide sequences widespread in the DNA, a loop may be formed in one strand→mispairing those repeated complementary bases→excision of the loop by repair enzymes►deletion or frameshift mutation seen in subsequent cell progeny

*Some chemicals stabilizes the loop→↑time vulnerable to excision