GATE Biotechnology Genetics, Cellular and Molecular Biology Flashcards

24 sample cards from the Genetics, Cellular and Molecular Biology deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What is the difference between an F+ and an Hfr cell?

   In F+ cells the F plasmid is free in the cytoplasm and transfers only the plasmid (recipient becomes F+). In Hfr (High frequency recombination) cells the F factor is integrated into the chromosome, allowing transfer of chromosomal genes; recipients usually remain F-.

2. What is an F' (F-prime) plasmid?

   An F plasmid that has excised imperfectly from an Hfr chromosome, carrying a small piece of adjacent chromosomal DNA. It transfers these host genes to recipients (sexduction/F-duction), making them partial diploids (merodiploids).

3. Define horizontal gene transfer (HGT).

   The transfer of genetic material between organisms other than by vertical (parent-to-offspring) descent. In bacteria its three main mechanisms are transformation, transduction, and conjugation; it is a major driver of antibiotic resistance spread.

4. What are transposable elements (transposons)?

   DNA segments capable of moving (transposing) from one genomic location to another, discovered by Barbara McClintock in maize. They can cause mutations, chromosomal rearrangements, and changes in gene expression.

5. Distinguish conservative (cut-and-paste) from replicative (copy-and-paste) transposition.

   Conservative: the element is excised from the donor site and inserted into a new site, with no increase in copy number. Replicative: the element is copied; one copy stays at the donor site and a new copy inserts elsewhere, increasing copy number.

6. What distinguishes a retrotransposon from a DNA transposon?

   Retrotransposons (class I) move via an RNA intermediate that is reverse-transcribed into DNA before insertion (copy-and-paste). DNA transposons (class II) move directly as DNA, usually via a transposase-mediated cut-and-paste mechanism.

7. What is a bacterial insertion sequence (IS element)?

   The simplest transposable element in bacteria, carrying only the transposase gene needed for its own movement, flanked by short inverted terminal repeats. It carries no other (e.g., antibiotic resistance) genes.

8. List the four main types of chromosomal structural aberrations.

   Deletion (loss of a segment), duplication (extra copy of a segment), inversion (segment reversed in orientation), and translocation (segment moved to a non-homologous chromosome).

9. Differentiate paracentric from pericentric inversion.

   Paracentric inversion does not include the centromere (both breakpoints on the same arm). Pericentric inversion includes the centromere (one breakpoint on each arm) and can change the chromosome's arm-length ratio.

10. Define aneuploidy and give the general formula notation.

    Aneuploidy is the gain or loss of individual chromosomes from the normal set, e.g., monosomy ($2n-1$), trisomy ($2n+1$), nullisomy ($2n-2$). It usually results from nondisjunction during meiosis.

11. Distinguish euploidy from aneuploidy.

    Euploidy: variation in complete chromosome sets ($n$, $2n$, $3n$ triploid, $4n$ tetraploid, etc.). Aneuploidy: variation in the number of individual chromosomes rather than whole sets ($2n\pm1$).

12. What chromosomal abnormality causes Down syndrome?

    Trisomy 21 ($2n+1$, i.e., three copies of chromosome 21), karyotype 47,XX,+21 or 47,XY,+21, usually due to meiotic nondisjunction.

13. What are the karyotypes of Turner and Klinefelter syndromes?

    Turner syndrome: 45,X (monosomy X, sterile female). Klinefelter syndrome: 47,XXY (male with reduced fertility, sometimes 48,XXXY etc.).

14. What type of mutation causes sickle cell anemia?

    A point (missense) mutation in the $\beta$-globin gene: GAG $\to$ GTG, replacing glutamic acid with valine at position 6 of the $\beta$-globin chain (Glu6Val). It is autosomal recessive.

15. State the Hardy-Weinberg equilibrium equations.

    For a two-allele locus with allele frequencies $p$ and $q$: $$p + q = 1 \quad\text{and}\quad p^{2} + 2pq + q^{2} = 1$$ where $p^{2}$ = homozygous dominant, $2pq$ = heterozygous, $q^{2}$ = homozygous recessive frequencies.

16. List the assumptions required for Hardy-Weinberg equilibrium.

    No mutation, no migration (gene flow), no natural selection, random mating, and an infinitely large population (no genetic drift). When all hold, allele and genotype frequencies remain constant across generations.

17. If the frequency of a recessive disease ($q^{2}$) is $\frac{1}{10000}$, what is the carrier frequency?

    $q = \sqrt{\frac{1}{10000}} = 0.01$, so $p = 0.99$. Carrier frequency $2pq = 2(0.99)(0.01) \approx 0.0198$, i.e., about $1.98\%$ (roughly 1 in 50).

18. Define epigenetics.

    The study of heritable changes in gene expression that occur without alterations to the underlying DNA sequence. Key mechanisms include DNA methylation, histone modifications, and non-coding RNAs.

19. How does DNA methylation typically affect gene expression?

    Methylation of cytosine residues at CpG islands (forming 5-methylcytosine) in promoter regions usually represses/silences transcription by blocking transcription factor binding and recruiting repressive proteins.

20. What is genomic imprinting?

    An epigenetic phenomenon in which a gene is expressed in a parent-of-origin-specific manner — only the maternal or only the paternal allele is active (the other is silenced by methylation). Examples: Prader-Willi (paternal) and Angelman (maternal) syndromes.

21. Distinguish natural selection from genetic drift as evolutionary forces.

    Natural selection is the non-random, directional change in allele frequencies driven by differential reproductive fitness. Genetic drift is the random change in allele frequencies due to chance sampling effects, strongest in small populations and independent of fitness.

22. Compare directional, stabilizing, and disruptive selection.

    Directional selection favors one extreme phenotype, shifting the mean. Stabilizing selection favors intermediate phenotypes, reducing variance. Disruptive (diversifying) selection favors both extremes against the intermediate, increasing variance and potentially leading to bimodality.

23. What is the neutral theory of molecular evolution (Kimura)?

    It proposes that most molecular-level evolutionary changes and polymorphisms are caused by random genetic drift of selectively neutral (or nearly neutral) mutations rather than by natural selection. Neutral mutations have no effect on fitness.

24. What are the founder effect and bottleneck effect?

    Both are forms of genetic drift. The founder effect occurs when a small group establishes a new population, carrying only a subset of the source gene pool. The bottleneck effect occurs when a population is drastically reduced by a random event, randomly altering allele frequencies and reducing genetic diversity.

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