Prep Guide: Structure of Chromosome

Key Concepts (Recall & Understanding)

• Chromosome: A highly condensed structure of DNA and proteins (chromatin) that carries genetic information.
• Chromatin: The complex of DNA and histone proteins. Exists as less condensed euchromatin (active genes) and more condensed heterochromatin (inactive genes).
• DNA: Deoxyribonucleic acid, a double helix molecule composed of a sugar-phosphate backbone and nitrogenous bases (Adenine, Thymine, Guanine, Cytosine). A-T and G-C pairing.
• Gene: A specific segment of DNA that codes for a protein or functional RNA molecule, determining a trait.
• Metaphase Chromosome Structure: Consists of two identical sister chromatids joined at a centromere. The ends of the chromosome are protected by telomeres.
• Types of Chromosomes (by centromere position): Metacentric (middle), Submetacentric (off-center), Acrocentric (near end), Telocentric (at end).
• Autosomes vs. Sex Chromosomes: Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females, XY for males).

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Application Corner

1. During which phase of the cell cycle would you expect to see chromosomes in their most condensed state? Why is this condensation necessary?

   • Answer: Metaphase of mitosis. This condensation is necessary to allow the chromosomes to be easily segregated and moved to opposite poles of the cell without tangling or breaking.

2. A scientist discovers a new species. They find that its DNA contains 20% Guanine. What are the percentages of the other three bases (Adenine, Thymine, Cytosine)?

   • Answer: If Guanine is 20%, then Cytosine must also be 20% (since G pairs with C). This makes a total of 40%. The remaining 60% must be Adenine and Thymine. Since A pairs with T, they must be in equal amounts. Therefore, Adenine is 30% and Thymine is 30%.

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Analytical Thinking

1. Odd One Out: Autosome, Sex Chromosome, Chromatid, Homologous Chromosome.

   • Odd One: Chromatid.
   • Category: The rest are types or classifications of whole chromosomes.

2. Scenario: If the telomeres of a chromosome were to be damaged or shortened, what would be the likely consequence for the cell over several generations of division?

   • Answer: Damaged or shortened telomeres would lead to the loss of important genetic information from the ends of the chromosomes with each cell division. This can lead to cellular aging (senescence) and eventually cell death, as the cell loses essential genes.

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Key Case Study

Down Syndrome: A Chromosomal Abnormality

Down syndrome, also known as Trisomy 21, is a genetic condition caused by the presence of a full or partial extra copy of chromosome 21. This is an example of an aneuploidy, where there is an abnormal number of chromosomes. The extra genetic material disrupts the normal course of development, leading to the characteristic physical features and intellectual disabilities associated with Down syndrome. This case highlights the critical importance of having the correct number of chromosomes for normal human development and shows that even a single extra autosome can have profound effects.

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Assertion-Reason Practice

Assertion (A): In humans, a male has an XY pair of sex chromosomes. Reason (R): The Y chromosome carries the gene that determines maleness.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

• Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). The presence of the Y chromosome, specifically the SRY gene on it, triggers the development of male characteristics.

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HOTS (Higher-Order Thinking Skills) Question

Euchromatin is described as "transcriptionally active" while heterochromatin is "transcriptionally inactive." Based on your understanding of chromosome compaction, provide a structural explanation for this difference in activity.

• Answer: Euchromatin is a loosely packed form of chromatin. This open structure allows transcription factors and RNA polymerase to access the DNA and initiate the process of transcription (reading the gene to make RNA). In contrast, heterochromatin is highly condensed and tightly packed. This dense structure physically blocks the transcription machinery from accessing the DNA, thus keeping the genes within it "silent" or inactive.