Lecture I: Chromosomal Structure

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Questions and Answers

What is the primary role of DNA in chromosomes?

It provides genetic information for cell growth and development. (correct)

It serves as a storage medium for proteins.

It facilitates cell division through mitosis.

It acts as a protective barrier for the nucleus.

During which phase do chromosomes appear as cross-shaped structures?

Interphase

Metaphase (correct)

Prophase

Anaphase

What characterizes heterochromatin?

It is tightly coiled and contains inactive genes. (correct)

It is loosely arranged and promotes protein synthesis.

It represents the repetitive sequences at the chromosome ends.

It is lightly stained and associated with active genes.

What is the configuration of chromosomes during interphase in terms of DNA strands?

Single-stranded DNA in the form of chromatids. Signup and view all the answers

How many chromosomes are present in normal somatic cells?

46 chromosomes, arranged in 23 pairs. Signup and view all the answers

What is the function of telomeres in chromosomes?

To cap the ends and protect the chromosome from degradation. Signup and view all the answers

Which statement accurately describes Barr's body?

It represents the inactivated X chromosome in females. Signup and view all the answers

What technique is generally used for visualizing chromosome structure during cell division?

G banding technique. Signup and view all the answers

What characterizes metacentric chromosomes?

Centromere lies near the middle of the chromosome. Signup and view all the answers

Which chromosomes are classified as acrocentric?

Chromosomes 13, 14, and 15 Signup and view all the answers

How many pairs of sex chromosomes do humans have?

One pair Signup and view all the answers

In which Denver group are chromosomes 16, 17, and 18 classified?

Group E Signup and view all the answers

What defines a submetacentric chromosome?

P arm is shorter than the q arm. Signup and view all the answers

Which of the following options describes the structure of Group G chromosomes?

Very short acrocentric chromosomes. Signup and view all the answers

Which chromosome has secondary constriction in the Denver classification?

Chromosome 9 Signup and view all the answers

What is the characteristic of telocentric chromosomes?

Centromere is present at the terminal end. Signup and view all the answers

Which chromosome is responsible for forming a Barr body in female cells?

Inactive X chromosome Signup and view all the answers

Which of the following statements about the Y chromosome is true?

It lacks secondary constrictions. Signup and view all the answers

What is the primary method used to visualize banding patterns in chromosomes?

Chromosomal banding Signup and view all the answers

In which cells would you not expect to find a Barr body?

Turner syndrome cells Signup and view all the answers

What characteristic differentiates G-negative bands in Giemsa staining?

They are gene-poor regions. Signup and view all the answers

Which of the following accurately describes Barr bodies in an individual's cells?

They represent coiled inactive X chromosomes. Signup and view all the answers

Which chromosomal identification technique helps locate radioactive substances in tissues?

Autoradiography Signup and view all the answers

What is a key feature of the long arm of the Y chromosome?

It does not exhibit centric constriction. Signup and view all the answers

A chromosome with a very short arm and a very long arm is referred to as:

Acrocentric Signup and view all the answers

In which stage of the cell cycle are chromosomes best visualized and studied?

Metaphase Signup and view all the answers

Regarding the X chromosome, which statement is correct?

It is a submetacentric chromosome. Signup and view all the answers

According to Denver's classification of chromosomes, the acrocentric chromosomes are involved in which group?

Group A Signup and view all the answers

The number of autosomes in human somatic cells is:

23 pairs of identical chromosomes Signup and view all the answers

Which of the following statements is true regarding the ends of a chromosome?

The ends of the chromosome are called Telomeres Signup and view all the answers

What is the significance of Fluorescence In Situ Hybridization (FISH)?

It visualizes karyotypes using DNA probes. Signup and view all the answers

Which characteristic describes a metacentric chromosome?

Its arms are of equal length. Signup and view all the answers

Study Notes

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.

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Description

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.