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Question 221Question →

In eukaryotic cells, genomic DNA undergoes progressive compaction with structural proteins to form chromatin. Which of the following statements regarding the molecular organization of nucleosomes and chromatin architecture are correct?

Select all that apply

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Answer: A canonical nucleosome core particle consists of approximately 146 to 147 base pairs of DNA wrapped around an octamer composed of two molecules each of histones H2AH2A, H2BH2B, H3H3, and H4H4.; Euchromatin represents a loosely packed, transcriptionally active state of chromatin that stains lightly under standard microscopic staining during interphase.

Answer

The statements describing the nucleosome core structure (composed of DNA wrapped around a histone octamer of H2AH2A, H2BH2B, H3H3, and H4H4) and identifying euchromatin as loosely packed, transcriptionally active chromatin are correct.
The statements describing the canonical nucleosome core particle (146–147 bp146\text{--}147\text{ bp} DNA wrapped around an octamer of H2AH2A, H2BH2B, H3H3, and H4H4) and characterizing euchromatin as transcriptionally active, loosely condensed chromatin are accurate biological facts.

Step-by-Step Solution

1
Analyze the structural composition of the core nucleosome particle.
Confirm that the nucleosome core comprises ≈146–147 bp\approx 146\text{--}147\text{ bp} of DNA wrapped around a histone octamer consisting of pairs of H2AH2A, H2BH2B, H3H3, and H4H4.
This represents the fundamental repeating structural unit of eukaryotic chromatin.
2
Evaluate the functional and staining properties of euchromatin versus heterochromatin.
Identify euchromatin as lightly staining, relaxed, and transcriptionally active, whereas heterochromatin is densely packed, darkly staining, and transcriptionally repressed.
Chromatin remodeling and histone modifications regulate transcriptional accessibility across the cell cycle.
3
Assess the role and localization of linker histone H1H1.
Establish that histone H1H1 is external to the core octamer and facilitates 30 nm30\text{ nm} fiber compaction without constituting the core itself.
Only H2AH2A, H2BH2B, H3H3, and H4H4 constitute the interior octameric core.

Key Concept

Eukaryotic Chromatin Architecture and Nucleosome Organization
Estimated Time:1m 15s
Question 222Question →

Consider the following human genetic disorders:
1. Turner Syndrome
2. Haemophilia
3. Down Syndrome
4. Sickle Cell Anaemia

Which of the following pairs comprises disorders caused exclusively by chromosomal numerical aberrations (aneuploidies) rather than single-gene Mendelian mutations?

Show answer & explanation

Answer: Turner Syndrome and Down Syndrome

Answer

Turner Syndrome and Down Syndrome
The pair comprising Turner Syndrome and Down Syndrome is correct because both conditions originate from chromosomal non-disjunction during cell division, leading to whole-chromosome numerical alterations (aneuploidy). Turner Syndrome is characterized by the loss of one X chromosome (45,XO45, XO), while Down Syndrome results from an extra copy of chromosome 21 (Trisomy 21).

Step-by-Step Solution

1
Differentiate between chromosomal aneuploidies and Mendelian disorders.
Chromosomal aneuploidies result from the gain or loss of complete chromosomes due to non-disjunction during meiosis, whereas Mendelian disorders are determined by mutation in a single gene locus.
Accurate categorization requires identifying the scale and mechanism of the genetic defect.
2
Analyze the genetic basis of each listed condition.
Turner Syndrome is a sex-chromosome monosomy (45,XO45, XO). Down Syndrome is an autosomal trisomy (47,XX+2147, XX+21 or 47,XY+2147, XY+21). Haemophilia is an X-linked recessive single-gene defect. Sickle Cell Anaemia is an autosomal recessive point mutation (GAG→GTGGAG \rightarrow GTG in the β\beta-globin gene).
Evaluating each disorder reveals whether the etiology is numerical chromosomal alteration or a monogenic mutation.
3
Select the pair that contains only chromosomal numerical aberrations.
The combination of Turner Syndrome (monosomy) and Down Syndrome (trisomy) exclusively satisfies the criterion.
Both conditions involve abnormal chromosome counts rather than altered nucleotide sequences of individual genes.

Key Concept

Classification of Genetic Disorders: Chromosomal Aneuploidies versus Mendelian Monogenic Mutations
Estimated Time:1m 0s
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