Question

What statements describes the eukaryotic chromosome? ________

a) The number of genes on each chromosome differs in different cell types of an organism

b) It contains a single linear molecule of double stranded DNA

c) The nucleosome is its most basic func. subunit

d) Active transcription occurs on heterochromatin

e) It is composed only of DNA

Why is letter B the correct answer and why are E,D,C,A incorrect. Define the terms chromosome, nucleosome, transcription, and heterochromatin in the answer.

Answer 1

Option (B) is the correct answer since eukaryotic chromosome contains a long linear dsDNA molecule which is associated with proeins and compactly folded and packed to form the chromosome structure. Prokaryotic chriomosome on te contrary is a circular DNA molecule.

Option (A) is incorrect as the number of genes on each chromosome remains the same in different cell types beacause they have originated from the same initial cell (zygote). However, cell differentiation into different cell types is achieved by spatio-temporal regulation of different genes in the cell, where different sets of genes are turn on at different times to give rise to the cell specificity and cell type. The regulation of gene transcription ( process where genetic information is transcribed to form a mRNA by RNA Polymerase) is responsible for cellular differentiatin and development.

Option (C) is incorrect since nucleosome is the most basic structural subunit of the eukaryotic chromosome and not the functional unit. Nucleosome consists of a small stretch of negatively charged DNA ( ~146 bp) wrapped around a positively charged ( lysine and arginine rich) core of eight histone proteins ( 2 copies of H2A, H2B, H3, H4) and H1 with the linker DNA. The basic function unit of chromosome is gene.Option (D) is incorrect as the heterochromatin consists of transcritionally inactive DNA. The heterochromatin consists of tightly packed genomic regions which are inaccessible to polymerases and are therefore not transcribed. Methylated DNa and CpG islands are characteristic features of heterochromatin. Heterochromatin can be classified as constitutive and facultative. Constituitive heterochromatin can have positional effect on the expression of nearby genes and is present in centromeres and telomeres (repititive sequences).

Facultative heterochromatin is a result of silencing of gene via histone deacetylation and RNai mechanisms. It is not permanently inactive like constituitive but can become active by losing its condensed structure.

Finally option (E) is also incorrect sinc eeukaryotic chromosome does not contain only DNA but is also packed with proteins known as histones.