A chromosome looks like an “X” at mitosis because

Correct Answer: B

Rationale: Correct Answer: B Rationale: At mitosis, a chromosome looks like an "X" because after DNA replication, each chromosome consists of two identical chromatids connected at the centromere. This structure ensures that each daughter cell receives an identical copy of the genetic material during cell division, maintaining genetic consistency. The other choices are incorrect as: A: At mitosis, chromosomes condense and are not visible as double helix strands. C: "X" does not stand for a mutation; it represents the chromatids of a replicated chromosome. D: Homologous chromosomes do not form the "X" shape; it results from sister chromatids of a replicated chromosome.

Correct Answer: A

Rationale: Step 1: Amniocentesis and chorionic villus sampling are prenatal diagnostic tests. Step 2: These tests are performed to access fetal DNA for genetic analysis. Step 3: Amniocentesis involves extracting amniotic fluid, while chorionic villus sampling involves collecting cells from the placenta. Step 4: Both procedures allow for genetic testing of the fetus. Step 5: Choice A is correct as it accurately describes the purpose of these tests. Summary: Choices B, C, and D are incorrect because they do not accurately reflect the main purpose of amniocentesis and chorionic villus sampling, which is to access fetal DNA for genetic analysis, not to result in induced miscarriage, not just when non-invasive techniques are uninformative, and not necessarily to document a genetic defect.

Correct Answer: A

Rationale: Step 1: Unique DNA sequences are low copy number regions that are present in the genome in limited copies. Step 2: These sequences anneal slowly because of their low copy number, making them distinct from high copy number sequences. Step 3: Unique sequences can include functional genes as well as non-coding regions. Step 4: By combining steps 1-3, we can conclude that all of the above statements are true for unique DNA sequences, making option A correct. Summary: Option B is incorrect because the slow annealing is specific to unique sequences. Option C is incorrect as unique sequences can include non-functional regions too. Option D is incorrect because unique sequences can represent more than 1.5% of the human genome.

Correct Answer: D

Rationale: The correct answer is D (All of the above). Fetal cells in maternal blood include lymphocytes, nucleated erythroblasts, and extra villus cytotrophoblasts. Lymphocytes can cross the placental barrier, nucleated erythroblasts are fetal blood cells, and extra villus cytotrophoblasts are from the placenta. These fetal cells can be detected in maternal blood samples through various methods, making D the correct choice. Choices A, B, and C are incorrect as they only represent specific types of fetal cells found in maternal blood, while D encompasses all potential types.

Correct Answer: A

Rationale: The correct answer is A because preimplantation diagnosis involves screening embryos for genetic abnormalities before they are transferred to the uterus, allowing for the exclusion of abnormal embryos. This ensures that only healthy embryos are selected for implantation, increasing the chances of a successful pregnancy. Choice B is incorrect because preimplantation diagnosis is not done for the creation of stem cells but rather to identify genetic abnormalities. Choice C is incorrect because the purpose of preimplantation diagnosis is not to make decisions about terminating pregnancies but to select healthy embryos for transfer. Choice D is incorrect because preimplantation diagnosis must be done before implantation to identify abnormal embryos and avoid transferring them.