Lecture I: Chromosomal Structure

Lecture I Presentation: Chromosomal Structure

• The lecture presentation covered chromosomal structure, including normal morphology at interphase and metaphase, the normal chromosomal number, Barr bodies, classification of chromosomes, methods of chromosomal studying, the G-banding technique, and various chromosomal study methods.

Learning Objectives

• Identify the normal chromosomal structure at interphase and metaphase.
• Identify the normal chromosomal number.
• Describe Barr's body.
• Classify different types of chromosomes.
• Enumerate different methods of chromosomal studying.
• Explain the G-banding technique.
• Describe different methods of chromosomal study.

Chemical Structure of Chromosomes

• The nucleus houses chromosomes.
• Chromosomes consist of DNA, containing hereditary information for cell growth, development, and reproduction.
• Chromosomes are in the form of DNA and histone proteins (chromatin).
• Repeated chromatin units are called nucleosomes.
• When a cell is not dividing, chromosomes form entangled structures called chromatin.

Normal Morphology of Interphase Chromosomes

• Chromosomes are long, thread-like structures.
• Active gene portions are extended and exposed.
• This extended chromatin is not visible under a light microscope (euchromatin).
• Inactive genes are coiled in chromatin appearing denser and as electron-dense masses (heterochromatin).
• A chromosome at the G1 stage is single-threaded DNA (s-chromosome or chromatid).

Chromatin Pattern and Cell Activity

• Nucleus with euchromatin indicates an active cell with active protein synthesis and appears lightly stained.
• Nucleus with heterochromatin indicates an inactive cell with inactive protein synthesis and appears darkly stained.

Normal Morphology of Metaphase Chromosomes

• Metaphase chromosomes appear as X-shaped structures due to the fusion of two chromatids at a constricted area called the centromere.
• The centromere holds kinetochore proteins for the mitotic spindle.
• Arms extend from the centromere (long arm—q, and short arm—p).
• Telomeres are specialized repetitive sequences at chromosome ends.

Normal Number of Chromosomes

• Somatic cells contain 46 chromosomes (23 pairs).
• 22 pairs are autosomes (pairs of identical chromosomes).
• 1 pair of sex chromosomes (XX or XY).
• Gametes (sperm or ovum) contain 23 chromosomes (haploid number).

Classification of Chromosomes

• A) Based on centromere position:

  • Metacentric: Centromere is in the middle (e.g., chromosomes 1, 3, 16, 19, and 20).
  • Submetacentric: Centromere is off-center (e.g., chromosomes 2, 4–12, 17, 18, and X).
  • Acrocentric: Centromere is close to one end, resulting in a very short p arm (e.g., 13, 14, 15, 21, 22, and Y).
  • Telocentric: Centromere is at the end (not in humans).

• B) Denver Classification: Classifies chromosomes based on size, centromere position, and specific characteristics into 7 groups (A to G).

X Chromosome

• One of the longest (in group C).
• In females, one X is slightly darker than the other.

Y Chromosome

• Longer than chromosomes 21 and 22.
• Less divergent long arms.
• No secondary constrictions or satellites.
• Indistinct centromere constriction.
• Poorly defined terminal long-arm region.
• Usually found on the edge (periphery) of metaphase preparations.

Barr Body

• A darkly stained mass of chromatin (drumstick-like) located on the inner surface of the nuclear envelope.
• Seen in roughly 60% of female somatic cells.
• May be seen in buccal mucosa smears or blood leukocytes (neutrophils).
• Demonstrated using Leishman stain, and basic stains.
• Represents an inactive X chromosome.
• Present in cells with more than one X chromosome.
• Absent in males and individuals with a single X chromosome (XO).

Methods of Chromosomal Identification

• 1. Morphological approach: Distinguishing chromosomes by arm length, centromere position, and specific morphological features.
• 2. Autoradiography: Localizing radioactive substances in tissues.
• 3. Chromosomal banding: Identifying specific chromosome regions based on staining differences (e.g., Giemsa-bands).
• 4. Fluorescence in situ hybridization (FISH): Visualizing karyotypes using DNA probes that bind to specific DNA sequences.

Quiz Questions

• (1) Acrocentric Chromosome:* A chromosome with a very short arm and a very long arm is called an acrocentric chromosome.
• (2) Metaphase:* Chromosomes are best visualized and studied during metaphase.
• (3) Submetacentric Chromosome:* The X chromosome is a submetacentric chromosome.
• (4) Groups G and D:* According to the Denver classification, acrocentric chromosomes are found in groups G and D.
• (5) 22 Chromosomes:* The number of autosomes in human somatic cells is 22.
• (6) Telomeres:* The ends of a chromosome are called telomeres.

This quiz covers key concepts in chromosomal structure, including normal morphology at interphase and metaphase, chromosomal number, and the classification of chromosomes. Additionally, it explores methods for studying chromosomes, specifically the G-banding technique and Barr bodies.