Q.39 The sequences essential for centromere function are:

1. About 200 bp long and very rich in G = C pairs.
2. About 1000 bp long and very rich in A = U pairs.
3. About 130 bp long and very rich in A = T pairs.
4. About 20,000 bp and very rich in lysine and arginine.

   Centromere Function Sequences: ~130 bp A=T Rich Regions

   Centromere function relies on ~130 bp α-satellite DNA sequences that are very rich in A=T pairs (AT-rich), making option (3) correct. These point centromeres or minimal functional units recruit kinetochore proteins like CENP-B.

   Introduction

   Sequences essential for centromere function consist of short ~130 bp AT-rich repeats that serve as nucleation sites for kinetochore assembly. These minimal centromere elements (point centromeres) contrast with large regional centromeres, providing precise binding for spindle microtubules during chromosome segregation.

   Option Analysis

   Centromere DNA architecture: Point centromeres (yeast) vs regional centromeres (higher eukaryotes).

   • About 200 bp long and very rich in G=C pairs: Incorrect. GC-rich = CpG islands/promoters. Centromeres AT-rich (~60-70% AT); 178 bp Arabidopsis satellite mentioned but not functional minimum.

   • About 1000 bp long and very rich in A=U pairs: Incorrect. A=U = RNA base pairs, not DNA (A=T). 1000 bp too long for point centromere; human α-satellite monomers ~171 bp.

   • About 130 bp long and very rich in A=T pairs: Correct. Yeast CDEIII (~125 bp) and human α-satellite monomers (~171 bp) are AT-rich. CENP-B box (17 bp) within 171 bp monomer binds centromere-specific protein.

   • About 20,000 bp and very rich in lysine and arginine: Incorrect. 20 kb = histone tail length, not DNA sequence. Lysine/arginine = basic aa in histone tails (H3K4me, H3K9me); centromeres use CENP-A histone H3 variant.

   Centromere DNA Composition

   Functional units:

   Organism	Sequence Length	Composition	Key Feature
   Yeast	~125 bp CDEIII	AT-rich	Point centromere
   Human	171 bp α-satellite	AT-rich	CENP-B box
   Arabidopsis	178 bp satellite	AT-rich	Regional centromere
   Drosophila	~420 kb complex	AT-rich	Retrotransposons

   Human α-satellite:

   text

   Monomer (171 bp): 60-70% AT + CENP-B box (17 bp)
HOR (higher-order repeat): 2-5 monomers
Array: 0.3-5 Mb per centromere [web:228]


   Kinetochore Recruitment

   text

   AT-rich DNA → CENP-A nucleosomes → CCAN → KMN network → Microtubules


   Exam distinction: Point centromeres = short specific sequence vs regional centromeres = large repetitive arrays.

   Exam Relevance

   GATE Life Sciences: “CENTromere = 130 bp AT (not GC).” Mnemonic: “A=T = ATTACH microtubules.” Classic trap: GC-rich (promoters) vs AT-rich (centromeres/telomeres).

   Key numbers: 125 bp yeast, 171 bp human α-satellite, AT-rich composition.