Nucleus and nucleolus

What is the most prominent organelle in most cells?

The nucleus.

What is the typical size of the nucleus?

About 5–8 µm in diameter.

What surrounds the nucleus?

A double membrane called the nuclear envelope.

How does the nucleus communicate with the cytosol?

Through nuclear pores that perforate the envelope.

What is the internal structure of the nucleus called?

The nuclear matrix.

What are the two membranes of the nuclear envelope?

The outer and inner nuclear membranes.

What is the outer nuclear membrane similar to?

The membrane of the endoplasmic reticulum (ER).

What are often attached to the outer nuclear membrane?

Ribosomes.

What is the function of the inner nuclear membrane?

It contains binding sites for DNA, RNA, and the nuclear lamina.

What is the nuclear lamina made of?

Intermediate filaments composed of Lamin A, B, and C.

Which lamin binds to the nuclear membrane receptor?

Lamin B.

Which lamins bind to Lamin B to link chromatin to the lamina?

Lamin A and Lamin C.

What are nuclear pores?

Gateways through which all molecules enter or leave the nucleus.

What type of traffic occurs through nuclear pores?

Bidirectional traffic: import of proteins and export of RNA and ribosomal subunits.

What proteins make up the nuclear pore complex?

About 50 different proteins called nucleoporins.

How do small molecules cross the nuclear pores?

Through water-filled channels.

What is required for large molecules to pass through nuclear pores?

A nuclear localization signal.

What is a nuclear localization signal made of?

One or two short sequences rich in lysine or arginine residues.

What provides the energy for nuclear transport?

GTP hydrolysis.

What is the nuclear matrix composed of?

Fibrillar proteins, nuclear proteins, RNA, DNA, and nuclear phospholipids.

What is the main function of the nuclear matrix?

To provide structure, organization, and possibly regulate gene expression and DNA replication.

What are the main components of the genetic apparatus?

DNA, RNA, and nuclear proteins (histones and non-histones).

What is DNA’s main function?

To store and transmit hereditary information.

What are the building blocks of DNA?

Nucleotides (phosphate, sugar, and nitrogenous base).

How is genetic information stored in DNA?

In triplets of nucleotides called codons.

How is DNA arranged in prokaryotes?

As one circular molecule.

How is DNA arranged in eukaryotes?

In linear chromosomes.

What is the genome?

The total genetic information within an organism’s chromosomes.

What are chromosomes composed of?

Long linear DNA molecules and associated proteins.

What is a replication origin?

The nucleotide sequence where DNA duplication begins.

What is a centromere?

A specialized DNA sequence allowing chromosome separation during division.

What is the kinetochore?

A protein complex that attaches duplicated chromosomes to the mitotic spindle.

What is a telomere?

Repeated DNA sequence at chromosome ends that protects DNA and allows replication.

What does the centromere divide a chromosome into?

The short (p) arm and long (q) arm.

Define metacentric chromosome.

p and q arms are nearly equal in length.

Define submetacentric chromosome.

p arm is much shorter than q arm.

Define acrocentric (subtelocentric) chromosome.

p arm is very short but visible.

Define telocentric chromosome.

p arm is extremely short or invisible.

What is a karyotype?

An organized profile of an individual’s chromosomes.

What technique is used to visualize banding patterns on chromosomes?

Giemsa staining.

What does Giemsa staining reveal?

Dark bands in regions rich in A–T nucleotide pairs.

What can changes in chromosome banding patterns indicate?

Chromosomal abnormalities, genetic defects, or cancer.

How many chromosomes are in a normal human karyotype?

46 (23 pairs).

What are autosomal chromosomes?

The 22 non-sex chromosome pairs.

What are gonosomes?

The sex chromosomes (XX in females, XY in males).

What are diploid cells?

Cells with two copies of each chromosome (somatic cells).

What are haploid cells?

Cells with one copy of each chromosome (germ cells).

Differentiate constitutional and acquired chromosomal anomalies.

Constitutional: present in all tissues from the embryo; Acquired: limited to specific organs, often in cancers.

Differentiate homogeneous and mosaic anomalies.

Homogeneous: all cells carry the anomaly; Mosaic: only some cells carry it.

Differentiate numerical and structural anomalies.

Numerical: extra or missing chromosomes (trisomy/monosomy); Structural: internal rearrangements like translocation, deletion, duplication, insertion.

What is chromatin?

The complex of DNA and proteins that form chromosomes.

What are the two types of chromatin?

Heterochromatin and euchromatin.

Describe heterochromatin.

Highly condensed, transcriptionally inactive, located at the periphery and near nucleolus.

Describe euchromatin.

Loosely packed, transcriptionally active, located in the nucleoplasm.

What are the two main types of chromosomal proteins?

Histones and non-histones.

What are non-histone proteins?

Structural, regulatory, and enzymatic proteins (e.g., HMG, Fos, Myc, polymerases).

What are histones?

Small, positively charged proteins that package DNA.

List the five types of histones.

H1, H2A, H2B, H3, and H4.

What is a nucleosome?

The basic unit of chromatin; DNA wrapped around a histone octamer.

Which histones form the nucleosome core?

Two copies each of H2A, H2B, H3, and H4.

How many base pairs wrap around a nucleosome core?

Approximately 150 base pairs.

What is the diameter of the chromatin fiber at the nucleosome level?

About 11 nm.

What is the function of histone H1?

It helps package nucleosomes into a 30 nm chromatin fiber.

What is the 30 nm fiber?

A more compact chromatin structure formed by stacked nucleosomes.

How is chromatin further condensed?

30 nm fibers form loops (~300 nm), which fold further into 700 nm and 1400 nm structures in mitotic chromosomes.

What is the nucleolus?

A dense nuclear region where rRNA genes cluster and ribosome subunits are assembled.

What is the function of the nucleolus?

rRNA synthesis and ribosomal subunit assembly.

When is the nucleolus visible?

In interphase cells under light microscopy.

What determines the size and shape of the nucleolus?

Its level of activity.