10 Questions
What is the primary function of the centromere in a chromosome?
To hold two chromatids of a chromosome
The centrioles in a centrosome lie parallel to each other.
False
What are the disc-shaped structures present on the sides of the centromere?
Kinetochores
The chromatin contains DNA and some basic proteins called ______________.
histones
Match the types of chromosomes with their characteristic centromere position:
Metacentric = Middle centromere Sub-metacentric = Centromere slightly away from the middle Acrocentric = Centromere close to the end Telocentric = Terminal centromere
Who first described the nucleus as a cell organelle in 1831?
Robert Brown
The nuclear envelope is a single membrane that separates the nucleus from the cytoplasm.
False
What is the function of nucleolus in the nucleus?
Site for active ribosomal RNA synthesis
The outer membrane of the nuclear envelope usually remains continuous with the ________________________.
endoplasmic reticulum
Match the following components of the nucleus with their descriptions:
I. Chromatin = B) Highly extended and elaborate nucleoprotein fibers II. Nucleolus = A) Site for active ribosomal RNA synthesis III. Nuclear matrix = C) Loose and indistinct network of nucleoprotein fibers
Study Notes
Nucleus
- First described by Robert Brown in 1831
- Chromatin, a material in the nucleus, was named by Flemming
- Interphase nucleus has:
- Chromatin (nucleoprotein fibers)
- Nuclear matrix
- One or more nucleolus (spherical bodies)
- Nuclear envelope:
- Consists of two parallel membranes with a perinuclear space (10-50 nm)
- Forms a barrier between nucleus and cytoplasm
- Outer membrane is continuous with endoplasmic reticulum and bears ribosomes
- Has minute pores for movement of RNA and protein molecules between nucleus and cytoplasm
Nuclear Pores and Variations
- Nuclear pores are passages for RNA and protein molecules
- Normally, one nucleus per cell, but variations in number of nuclei are observed
- Some mature cells lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants
Nucleolus and Chromatin
- Nucleolus:
- Spherical structure in nucleoplasm
- Site for active ribosomal RNA synthesis
- Larger and more numerous in cells actively carrying out protein synthesis
- Chromatin:
- Contains DNA, histones, non-histone proteins, and RNA
- Forms structured chromosomes during cell division
- A single human cell has approximately 2-meter long thread of DNA distributed among 46 chromosomes
Chromosomes
- Chromosomes:
- Visible only in dividing cells
- Have a primary constriction or centromere
- Centromere holds two chromatids of a chromosome
- Classification of chromosomes based on centromere position:
- Metacentric (middle centromere)
- Sub-metacentric (centromere slightly away from middle)
- Acrocentric (centromere close to end)
- Telocentric (terminal centromere)
Centrosome and Microbodies
- Centrosome:
- Organelle containing two cylindrical structures (centrioles)
- Surrounded by amorphous pericentriolar materials
- Each centriole has a cartwheel organization
- Microbodies:
- Membrane-bound minute vesicles
- Contain various enzymes
- Present in both plant and animal cells
Learn about the discovery and structure of the nucleus, including chromatin, nuclear matrix, and nucleolus. Understand the composition of the nuclear envelope and its functions.
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