Genetics - A. V. Sivolob 2008

Genetics of Bacteria, Viruses, and Unicellular Eukaryotes
Unicellular Eukaryotes
Ciliate

The GENETIC APPARATUS OF Ciliates features a unique characteristic: their Cells contain not one, but Two Types of nuclei—a micronucleus and a macronucleus. The former harbors a diploid set of Chromosomes (for instance, five chromosome pairs in Tetrahymena thermophila), which serve solely as a repository of hereditary information, as micronuclear genes are not expressed. In the macronucleus, the chromosome set undergoes extensive duplication (numbering several hundred chromosomes), and it is these macronuclear genes that are actively expressed, though they are never transmitted to progeny.

During asexual reproduction, the division of the micronucleus occurs via mitosis, whereas the macronucleus divides by simple amitotic fission. Consequently, the macronucleus ages over time, leading to a decline in its activity. When this occurs, the cells undergo a sexual process known as conjugation: the macronuclei disintegrate, the micronuclei undergo Meiosis, and the cells exchange haploid nuclei—specific details of this process may vary among species, but it ultimately yields cells containing a single diploid micronucleus. Immediately thereafter, this micronucleus undergoes mitotic division, and one of the daughter nuclei differentiates into a new macronucleus.

During The formation of the macronucleus, The Genome not only undergoes massive duplication but also substantial reorganization. Initially, the precursor Nucleus of the macronucleus undergoes multiple rounds of METABOLISM/36.html">DNA Replication, a process known as chromosome polytenization. Next, numerous internal eliminated sequences (IESs) located within the micronuclear genes are excised (thus, micronuclear genes interrupted by IESs are fundamentally incapable of being expressed). Following the ligation of the coding sequences, the chromosomes fragment into specialized microchromosomes, each bearing one or several genes flanked by telomeres synthesized by telomerase. This chromosomal fragmentation also eliminates all intergenic regions, repetitive sequences, and mobile elements—stripping away all non-coding "junk" DNA. In total, up to 90% of the genome is removed. Finally, the microchromosomes are amplified through 4 to 6 rounds of replication, thereby completing the maturation of the macronucleus.

The macronuclear genome of the ciliate Tetrahymena thermophila has been fully sequenced. It contains 27,000 genes—comparable to the Gene count of higher eukaryotes (and slightly exceeding that of humans). Due to the extensive deletion of DNA, the total size of the haploid genome amounts to 105 million Base Pairs (roughly 30 times smaller than that of mammals), with repetitive sequences accounting for only ~2%. A fascinating feature of Tetrahymena's genetic apparatus is that the codons UAA and UAG (which function as stop codons in most organisms) encode The amino acid glutamine, leaving UGA as the sole stop codon.

This unusual Organization OF THE ciliate hereditary apparatus raises a fundamental question: why do Eukaryotic Genomes retain such vast quantities of non-coding DNA? Given that the overall genomic architecture of ciliates is virtually identical to that of higher eukaryotes, they nevertheless employ a mechanism to purge non-coding sequences during macronuclear development. This demonstrates that these sequences are dispensable for the expression of Genetic information. Yet, these "redundant" sequences are meticulously preserved in the micronucleus and passed on to descendants, much like in other eukaryotes. The adaptive significance of preserving such DNA remains somewhat enigmatic. Nevertheless, at least one plausible explanation stands out: non-coding DNA may act as a protective "wrapper" or physical buffer that shields functional genetic material from mutagenic damage. After all, external mutagens are statistically more likely to strike non-coding DNA simply because it constitutes the vast majority of the eukaryotic genome.

Selection/41.html">Review Questions and Exercises

1. What is the difference between bacterial strains of the F+ and Hfr types?

2. Through what mechanisms is genetic material transferred between Bacteria?

3. Into what classes can Bacteriophages be categorized based on The Nature of their genetic material?

4. Explain the difference between site-specific and Homologous Recombination. What is the Functional Significance of Site-Specific Recombination?

5. Describe The life cycle of bacteriophage λ. What factors determine the choice of developmental pathways for this phage?

6. What function does the λ phage repressor perform, and what is The Mechanism of its action?

7. Into what classes and based on what principles are Eukaryotic Viruses classified?

8. Describe the developmental cycle of Retroviruses.

9. How does reproduction occur in Saccharomyces cerevisiae?

10. What mechanism drives mating-type switching in S. cerevisiae?

11. What are the structural and Functional differences between the macro- and micronucleus in ciliates?