The answer is degeneracy.
The definition "each amino acid can be coded by more than one codon" describes degeneracy (also called redundancy) of the genetic code. This means multiple different codons can encode the same amino acid - for example, leucine is encoded by 6 different codons (UUA, UUG, CUU, CUC, CUA, CUG).
Here's why the other options don't fit:
- Non-overlapping - means each nucleotide belongs to only one codon; codons are read sequentially without sharing bases
- Universality - means the same codons encode the same amino acids in virtually all organisms
- Triplet - means each codon consists of exactly 3 nucleotides
- Specificity - means each codon codes for only one specific amino acid (the opposite direction from degeneracy)
The key distinction: degeneracy = one amino acid, many codons. Specificity = one codon, one amino acid. The question's definition goes from amino acid to codon (many codons per amino acid), which is degeneracy.The correct answers are:
- ✅ Two strands of the division spindle are attached to the chromosome
- ✅ Localization of chromosomes in the plane of the cell equator
Here's what actually happens during metaphase of mitosis:
- Chromosomes align at the metaphase plate (cell equator) - this is the defining event of metaphase, also called the "equatorial plate" alignment.
- Both kinetochores of each chromosome are attached to spindle fibers from opposite poles - so two spindle fiber strands connect to each chromosome (one from each pole).
Why the other options are wrong:
-
Divergence of centrioles toward the poles - this happens in prophase, when the mitotic spindle begins to form.
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1 strand of the division spindle is attached to the chromosome - incorrect; in mitosis both sister chromatid kinetochores face opposite poles and each attaches to spindle fibers from one pole, but the chromosome as a whole has two attachments (bi-orientation).
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Division of homologous chromosomes - homologous chromosomes separate in meiosis I (anaphase I), not mitosis.
-
Spiralization of chromosomes - chromosome condensation/coiling occurs in prophase, not metaphase (though chromosomes are maximally condensed by metaphase, the spiralization process itself belongs to prophase).The correct answers are:
-
✅ First intermediate host: Freshwater mollusk
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✅ Definitive host: Human
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✅ Definitive host: Cattle
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✅ Definitive host: Dog
Life cycle of Schistosoma japonicum:
S. japonicum is unique among schistosomes because it has a very broad definitive host range - it is a zoonotic parasite.
| Host Type | Species |
|---|---|
| Intermediate host (only one) | Freshwater snail (Oncomelania spp.) |
| Definitive hosts | Humans, cattle, water buffalo, dogs, cats, rodents, pigs |
Key points:
- S. japonicum has only ONE intermediate host - a freshwater amphibious snail of the genus Oncomelania. There is no second intermediate host (unlike trematodes such as Clonorchis sinensis which use fish as a second intermediate host).
- The cercariae released from the snail penetrate skin directly to infect definitive hosts.
- It is highly zoonotic - cattle and dogs are major reservoir hosts, which makes control difficult.
Why the others are wrong:
- First intermediate host: Human - humans are definitive hosts, not intermediate hosts
- Second intermediate host: Cat / Fish - there is no second intermediate host in the Schistosoma life cycle
- First intermediate host: Crayfish - crayfish are second intermediate hosts for lung flukes (Paragonimus), not schistosomes
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