Ans. d (Acrocentric). (Ref. Cell & Molecular Biology by DeRobertis, 7th /pg. 348)CHROMOSOMES# The entire genetic information contained in the genes is packaged in the chromosomes. All the somatic cells of an individual organism contain the same number of chromosomes.# Genes are arranged linearly on the chromosomes and their positions on the normal chromosome in relation to other genes on same chromosome are fixed. This position is called locus.# Genes occupying the same locus on a pair of chromosomes are called alleles.Structure of chromosomes:# Chromosomes are long structures with a constriction at point along its length-called centromere.# The ends of the chromosomes are called telomeres. These give stability to the chromosome. In the absence of telomeres, chromosomes become unstable and tend to break until joined to a telomere. An enzyme telomerase maintains the length of the telomeric sequences. Cell aging is controlled by telomerase enzyme. (AI 2003)# It has the structure for the attachment of chromosome to the spindle fibers called the Kinetochore.Depending on the location of the centromere on the chromosome, they are classified as: Type of chromosomeComments1.MetacentricThe centromere is in the centre of the chromosome so that the chromosome has two equal arms.2.Sub-metacentricCentromeres are away from the center so that the arms are unequal in size (one arm shorter than other).3.TelocentricCentromere is at the extreme end of replicating chromosome, so that chromosome consists of only one arm.4.AcrocentricCentromere is almost at the tip (one end) of the chromosome, so that one arm is much longer than other. BANDING TECHNIQUES# Techniques using dyes such as quinacrine, or giemsa after trypsin or heat treatment are called banding techniques. The banding pattern is specific for each chromosome, and seen in both members of a pair.# Bands seen with quinacrine, a fluorescent dye, are called Q-bands.# Banding seen with giemsa stain is called G-banding (The most extensively used technique)# Mitotic cells are essential for chromosome studies. In human body, mitosis occurs in:- Bone marrow, Intestinal mucosa, Skin and Gonads.# However, biopsies are difficult to obtain and sufficient number of dividing cells are not available.# The blood lymphocytes can be stimulated to divide in culture (appropriate medium) and easy to obtain.# Small skin biopsies can be obtained and the fibroblasts grown in culture.# Lymphocyte culture: Treatment of peripheral blood lymphocytes with substances such as Phytohemagluttinin (PHA) or Concanavalin A (Con A) stimulates the T lymphocytes to transform into lymphoblasts and undergo 3-4 rounds of mitotic divisions.# Sources of human cells for cytogenetic studies (analysis of human karyotypes) are:- Fibroblasts- Amniotic cells- Cells of chorionic villi.For detailed studies on the human chromosome mitotic cells are arrested in metaphase.The metaphase chromosomes are in a highly condensed state.