Question 1

Who first described the nucleus and in which year?

Accepted Answer

Robert Brown in 1831 — The nucleus was first described by Robert Brown in 1831, marking an important discovery in cell biology.

Question 2

Which of the following correctly describes the nuclear envelope?

Accepted Answer

It consists of two parallel membranes separated by a perinuclear space of 10–50 nm — The nuclear envelope is composed of two membranes separated by a perinuclear space of 10–50 nm. The outer membrane is continuous with the rough endoplasmic reticulum and contains ribosomes. Nuclear pores allow selective exchange between nucleus and cytoplasm.

Question 3

What is the function of nuclear pores in the nuclear envelope?

Accepted Answer

Nuclear pores allow selective exchange of RNA, proteins, and other molecules between the nucleus and cytoplasm. For example, mRNA exits the nucleus through nuclear pores to participate in protein synthesis in the cytoplasm. — Nuclear pores are channels formed by fusion of the two membranes of the nuclear envelope. They regulate the transport of molecules such as RNA and proteins, maintaining cellular function by controlling communication between nucleus and cytoplasm.

Question 4

Which component of the nucleus is involved in ribosomal RNA synthesis and is non-membrane bound?

Accepted Answer

Nucleolus — The nucleolus is a dense, non-membrane bound structure inside the nucleus responsible for synthesizing ribosomal RNA (rRNA), essential for ribosome assembly.

Question 5

During which phase does chromatin condense into visible chromosomes?

Accepted Answer

Cell division — Chromatin exists as a loose network during interphase but condenses into visible chromosomes during cell division (mitosis or meiosis) to ensure proper segregation of genetic material.

Question 6

What is the primary constriction on a chromosome called, and what is its function?

Accepted Answer

The primary constriction on a chromosome is called the centromere. It holds two chromatids together and serves as the attachment site for spindle fibers during cell division. — The centromere is the narrow region of a chromosome that connects sister chromatids. It plays a crucial role in chromosome movement and segregation during mitosis and meiosis by attaching to kinetochores.

Question 7

Classify chromosomes based on the position of the centromere.

Accepted Answer

Chromosomes are classified into four types based on the position of the centromere:

(a) Metacentric: Centromere is in the middle, and both arms are of equal length.
(b) Submetacentric: Centromere is slightly off-center, producing one arm longer than the other.
(c) Acrocentric: Centromere is close to one end, resulting in a very short arm and a long arm.
(d) Telocentric: Centromere is at the terminal end, effectively having only one arm.

This classification helps in identifying chromosomes during karyotyping and understanding their behavior during cell division. — Chromosome classification depends on centromere position which affects arm length ratio. Metacentric chromosomes have equal arms, submetacentric have unequal arms, acrocentric have a very short arm, and telocentric have centromere at the end. This is important for genetic studies and chromosome identification.

Question 8

Which of the following cells lack nuclei in their mature form?

Accepted Answer

Mammalian erythrocytes and sieve tube cells — Mature mammalian erythrocytes (red blood cells) and sieve tube cells in plants lack nuclei to maximize space for oxygen transport and nutrient flow, respectively.

Question 9

What are microbodies and what is their primary function in the cell?

Accepted Answer

Microbodies are small, membrane-bound vesicles present in plant and animal cells that contain enzymes catalyzing metabolic reactions. For example, peroxisomes break down fatty acids and detoxify harmful substances like hydrogen peroxide. — Microbodies help maintain cellular metabolism and homeostasis by containing enzymes such as catalase in peroxisomes that decompose toxic hydrogen peroxide into water and oxygen.

Question 10

Which enzyme in peroxisomes decomposes hydrogen peroxide, and why is this important?

Accepted Answer

Catalase — Catalase is an enzyme present in peroxisomes that decomposes hydrogen peroxide, a toxic byproduct of metabolism, into water and oxygen, thus protecting cells from oxidative damage.

Question 11

Explain the relationship between the outer membrane of the nuclear envelope and the rough endoplasmic reticulum (RER).

Accepted Answer

The outer membrane of the nuclear envelope is continuous with the rough endoplasmic reticulum and bears ribosomes. This structural continuity facilitates the transport of molecules and synthesis of proteins. — Because the outer nuclear membrane is connected to the RER, proteins synthesized on ribosomes can be easily transported into or out of the nucleus, integrating nuclear and cytoplasmic functions.

Question 12

Describe the composition of chromatin found in the nucleoplasm.

Accepted Answer

Chromatin is composed of DNA and proteins present in the nucleoplasm. For example, during interphase, chromatin exists as a loose network facilitating gene expression. — Chromatin consists mainly of DNA wrapped around histone proteins, allowing it to be compact yet accessible for transcription during interphase and condensing into chromosomes during cell division.

Question 13

What is the nuclear matrix and what role does it play in the nucleus?

Accepted Answer

The nuclear matrix is a fibrous network within the nucleoplasm that provides structural support and may organize chromatin. For example, it helps maintain the shape of the nucleus and facilitates DNA replication and transcription. — The nuclear matrix acts as a scaffold inside the nucleus, organizing chromatin and other nuclear components to ensure efficient genetic processes.

Question 14

Which of the following best describes the perinuclear space?

Accepted Answer

The space between the two membranes of the nuclear envelope, 10–50 nm wide — The perinuclear space is the gap between the inner and outer membranes of the nuclear envelope, typically 10–50 nm wide, separating the two membranes.

Question 15

Analyze why some mature cells like mammalian erythrocytes lack nuclei and discuss the advantage of this adaptation.

Accepted Answer

(a) Mammalian erythrocytes lose their nuclei during maturation to create more space for hemoglobin, the oxygen-carrying molecule.
(b) Lack of nucleus allows erythrocytes to be flexible and pass through narrow capillaries.
(c) This adaptation maximizes oxygen transport efficiency.
(d) Similarly, sieve tube cells in plants lack nuclei to facilitate unhindered flow of nutrients.

This specialization is crucial for the physiological roles of these cells, enhancing their efficiency in transport functions. — Mature mammalian erythrocytes are anucleate to maximize hemoglobin content and flexibility, facilitating efficient oxygen transport. Sieve tube cells also lack nuclei to allow smooth translocation of photosynthates. This structural adaptation exemplifies how cell structure relates to function.

CELL: STRUCTURE AND FUNCTIONS

CELL: STRUCTURE AND FUNCTIONS — Study Notes

8.1 What is a Cell?

8.2 Cell Theory

8.3 An Overview of Cell

Practice Questions — CELL: STRUCTURE AND FUNCTIONS

All 19 Chapters in Biology