Cell Biology: Nucleus Structure

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Questions and Answers

What is the role of linker DNA in nucleosomes?

It synthesizes ribosomal RNA.

It connects chromatin to the nuclear membrane.

It attaches nucleosomes to each other. (correct)

It organizes inactive chromatin.

Which structures are found within the nucleolus?

Endoplasmic reticulum components.

Nucleolar organizer DNA and ribosomal subunits. (correct)

Chromatin and linker DNA.

Ribosomal proteins only.

What does karyorrhexis indicate in a dying cell?

The nucleus breaks into smaller fragments. (correct)

The nucleus remains unchanged.

The nucleus becomes deeply stained.

The nucleus restores its size.

What process occurs during the S-phase of interphase?

DNA duplication and centrioles formation. Signup and view all the answers

What is the primary function of the nucleolus?

To synthesize rRNA and assemble ribosomes. Signup and view all the answers

Which protein is NOT a component of the fibrous lamina?

Histone. Signup and view all the answers

What denotes the physical appearance of pyknosis in a cell's nucleus?

A smaller and deeply stained nucleus. Signup and view all the answers

What does a dark-stained ring of marginal chromatin typically indicate?

Organization of inactive chromatin. Signup and view all the answers

What occurs during the G2 phase of the cell cycle?

Energy production for the next cell division takes place. Signup and view all the answers

Which type of cells are unable to divide but can be replaced by stem cells?

Cells of the small intestine Signup and view all the answers

During which stage of mitosis do chromosomes align at the equatorial plane?

Metaphase Signup and view all the answers

What marks the end of prophase in mitosis?

The disappearance of the nuclear membrane Signup and view all the answers

In which phase of mitosis do chromatids separate and move towards opposite poles?

Anaphase Signup and view all the answers

Which of the following accurately describes meiosis?

Involves two divisions with only one DNA replication. Signup and view all the answers

What is produced at the end of mitosis?

Two daughter cells with the same number of chromosomes as the parent cell Signup and view all the answers

What occurs at the end of telophase during mitosis?

The nuclear envelope and nucleoli reappear Signup and view all the answers

What determines the site of the nucleus within a cell?

The type of cell it is Signup and view all the answers

Which structure is formed by the fusion of the nuclear envelope layers?

Nuclear pore complex Signup and view all the answers

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

To allow the exchange of molecules Signup and view all the answers

What characteristic distinguishes mononucleated cells from multinucleated cells?

Number of nuclei present Signup and view all the answers

What type of chromatin is typically found in lymphocytes?

Condensed chromatin Signup and view all the answers

Which of the following structures is present in the center of the nuclear pore complex?

Central granule Signup and view all the answers

What is the typical diameter range of nuclear pores?

30-100 nm Signup and view all the answers

What type of cell is commonly binucleated?

Osteoclasts Signup and view all the answers

What is the outcome of the first meiotic division in terms of daughter cells?

Four haploid cells Signup and view all the answers

During which stage of Prophase I do homologous chromosomes form bivalents?

Zygotene stage Signup and view all the answers

What significant event occurs during the Diplotene stage of Prophase I?

Homologous chromatids become more obvious and chiasmata form Signup and view all the answers

What happens to the chromosomes during Anaphase I of meiosis?

Homologous chromosomes are separated and move to poles Signup and view all the answers

What is the main distinction between Metaphase I of meiosis and metaphase of mitosis?

Pairs of chromosomes are aligned in Metaphase I Signup and view all the answers

What is the result of the second meiotic division immediately after Telophase II?

Formation of four haploid cells Signup and view all the answers

In Prophase II, what is notable about this phase compared to earlier stages of meiosis?

It resembles mitosis without DNA replication Signup and view all the answers

How many polar bodies are produced by the ovary during meiosis?

Three polar bodies Signup and view all the answers

What is the primary function of the central granule and radial spokes in nuclear pore complexes (NPCs)?

Forming a barrier to movement through nuclear pores Signup and view all the answers

How does the nuclear pore complex facilitate the transport of macromolecules larger than 10 nm?

By widening the channel to 20 nm or more using energy-dependent mechanisms Signup and view all the answers

Which type of chromatin is characterized as the most active and involved in protein synthesis?

Euchromatin Signup and view all the answers

What distinguishes heterochromatin from euchromatin in terms of structure?

It appears as aggregations of electron-dense granules Signup and view all the answers

What roles do nucleosomes play in the structure of chromatin?

They coil the DNA double helix around histones Signup and view all the answers

Which component is NOT part of the core histones in a nucleosome?

H1 Signup and view all the answers

Where is heterochromatin predominantly located within the nucleus?

Close to the inner nuclear membrane and around the nucleolus Signup and view all the answers

What does the term 'basophilic' refer to in the context of chromatin observed in light microscopy?

An affinity for basic stains Signup and view all the answers

Study Notes

Nucleus Structure

The nucleus of an interphase cell, or non-dividing cell, consists of the nuclear envelope, chromatin, nucleolus, and nuclear sap.
The nuclear envelope surrounds the nucleus and is visible as a singel basophilic line under the light microscope due to peripheral chromatin on its inner side.
Under the electron microscope, the nuclear envelope consists of two membranes, each 8nm thick, separated by a 25nm-wide perinuclear space.
The outer membrane is continuous with the membranes of the endoplasmic reticulum in the cytoplasm.
The inner membrane has chromatin granules, known as peripheral chromatin.
Nuclear pores, approximately 30-100nm in diameter, interrupt the nuclear membrane at intervals.
The nuclear pore complex (NPC), formed at the pores, facilitates the exchange of chemical molecules between the nucleus and the cytoplasm.
Each NPC is a tubular structure anchored to the nuclear envelope, with a ring-shaped structure on each end.
The outer ring is attached to the outer envelope membrane, and the inner ring is attached to the inner envelope membrane.
The NPC has an octagonal shape, with its wall composed of eight subunits.
A central granule, or plug, projects inwards from the subunits of the NPC, with eight radial spokes converging at the central granule.
The central granule and radial spokes act as the primary barrier to movement through the NPC.

Nuclear Pore Complex Barrier

The NPC barrier contains a 10nm central channel, allowing passive diffusion of molecules less than 10nm in diameter, facilitating two-way traffic between the cytoplasm and nucleoplasm.
Macromolecules larger than 10nm, such as ribosomes (leaving the nucleus) and cytoplasmic proteins (entering the nucleus), require energy-dependent mechanisms (active transport).
These mechanisms widen the opening of the channels from 10nm to 20nm or more to permit the transport of large molecules.

Chromatin

Under the light microscope (LM), chromatin appears as basophilic granules, either coarse or fine depending on their size.
Under the electron microscope (EM), there are two types of chromatin: euchromatin and heterochromatin.
Euchromatin, also known as extended chromatin or active chromatin, represents the extended (uncoiled) portions of the chromosomal thread.
Euchromatin is the most active chromatin and controls protein synthesis.
Condensed chromatin, also known as heterochromatin or inactive chromatin, appears as aggregations of electron-dense granules (coiled parts).
Condensed chromatin can be found in different locations within the nucleus:
- Peripheral chromatin clustered near the inner nuclear membrane.
- Nucleolus-associated chromatin aggregated around the nucleolus.
- Chromatin islands clumped together within the nuclear sap.

Function of Chromatin

Chromatin directs and guides the synthesis of proteins necessary for a cell's specialized function.
Chromatin stores the genetic material, directing cellular activities and regulating cellular structure.

Nucleosomes

When chromatin threads are fully extended (each thread becomes several centimeters long), examinations using EM, biochemical, and X-ray analysis reveal that each chromatin thread consists of a string of beads.
Each bead-like particle is called a nucleosome.
Each nucleosome core is composed of two molecules, each containing four histones (H2A, H2B, H3, and H4), resulting in eight molecules (an octomer).
The DNA double helix makes two and a half turns around each nucleosome.
Nucleosomes are linked together by segments of the DNA molecule, known as linker DNA.

Nucleolus

The nucleolus is a round, dense, and well-defined structure in the nucleus, lacking a limiting membrane.
Typically, there are 1-4 nucleoli per cell nucleus, with more nucleoli found in more active cells.
Under the electron microscope (EM), the nucleolus comprises the following:
- Nucleolar organizer DNA: DNA that codes for rRNA.
- Pars fibrosa: Sites of rRNA synthesis.
- Pars granulose: Maturing ribosomal subunits.

Functions of Nucleolus

The nucleolus is the site of rRNA synthesis and ribosome assembly.

Nuclear Sap

Also known as the nuclear matrix, karyoplasms, or nucleoplasm, nuclear sap is a semi-fluid substance filling the space between chromatin particles and the nucleolus.
The fibrous lamina, composed of the proteins Lamins A, B, and C, lies immediately beneath and is connected to the inner nuclear membrane.

Degenerative Changes in Nucleus Indicating Cell Death

Three signs can indicate cell death:
- Pyknosis: The nucleus shrinks in size and becomes deeply stained.
- Karyorrhexis: The nucleus breaks down into smaller fragments.
- Karyolysis: The nucleus dissolves and eventually disappears.

Life Cycle of the Cell

The cell cycle includes an interphase and a complete cell division.
Interphase, the period between two consecutive cell divisions, is divided into three phases:
- G1-Phase (Gap or Presynthesis): The cell restores its normal size after mitosis, synthesizes RNA and proteins for growth.
- S-Phase (DNA Synthesis): DNA and centrioles replicate.
- G2-Phase (Post DNA Duplication): The cell generates energy for the upcoming cell division.

Cell Cycle

Cells vary in their ability to divide and renew:
- Non-dividing cells: Nerve cells.
- Cells unable to divide but replaced by stem cells after degeneration: Cells of the small intestine.
- Cells dividing only when necessary: Cells of the liver and endocrine glands.

Mitosis

Mitosis is cell division that occurs in somatic cells, resulting in two daughter cells with the same number of chromosomes as the parent cell (46 chromosomes).
Mitosis involves four stages: prophase, metaphase, anaphase, and telophase.

Stages of Mitosis

1. Prophase:
- Chromatin transforms into chromosomes, becoming thick and rod-like.
- Each chromosome consists of two chromatids joined at the centromere, totaling 92 chromatids.
- Centrioles, duplicated during the S-phase, move to opposite poles of the cell.
- Microtubules start to form between the centrioles, constructing the mitotic spindle.
- The nuclear membrane and nucleoli disappear, marking the transition from prophase to metaphase.
2. Metaphase:
- Chromosomes align on the equatorial plane of the cell.
- The mitotic spindle fully develops, with its microtubules attaching to the centromeres of the chromosomes (kinetochore).
- At the end of metaphase, centromere DNA replicates, and the centromere splits.
- The two chromatids separate and begin to move towards the corresponding centrioles at the opposite poles of the cell.
3. Anaphase:
- Chromatids separate and move towards their respective centrioles.
- 46 chromatids migrate towards each pole.
4. Telophase:
- Chromatids elongate and form chromatin in the new nuclei.
- The nuclear membrane and nucleoli reappear.
- A constriction forms at the equatorial plane of the parent cell progressing until the cytoplasm and its organelles are divided into two.
- Two daughter cells are produced, each with a diploid number of chromosomes (46 chromosomes).

Mitotic Spindle

The mitotic spindle is a structure composed of microtubules that originates from the centrioles and extends to the chromosomes during mitosis.
The spindle fibers attach to the centromeres of the chromosomes, pulling the chromatids apart during anaphase.

Meiosis

Meiosis is a reductional cell division occurring in germ cells of the testes and ovary.
Meiosis involves two divisions but only one DNA replication (duplication of DNA).
The parent cell gives rise to four daughter cells with a haploid number of chromosomes (n-chromosome = 23 chromosomes)
In the testes, four sperm cells are produced. In the ovary, one ovum and three polar bodies (which degenerate) are formed.

First Meiotic Division

1. Prophase I:
- The longest phase of meiosis, with several stages:
  - Leptotene stage: Chromosomes appear as thin threads, as in early prophase of mitosis.
  - Zygotene stage: Chromosomes pair up in homologous pairs. Each pair forms a bivalent (one chromosome is from the father, the other is its homologous counterpart from the mother).
  - Pachytene stage: Each chromosome splits lengthwise into two chromatids connected at their centromeres. The bivalent now consists of four chromatids, forming a tetrad structure.
  - Diplotene stage: Homologous chromatids partially separate, making the tetrad formation more evident. Crossing over of genetic material occurs at points along their length, forming chiasmata (X-shaped structures).
  - Diakinesis: The nuclear membrane and nucleolus disappear, and the tetrads migrate to the equator of the cell.
2. Metaphase I:
- Similar to metaphase of mitotic division, except homologous pairs of chromosomes (bivalents), not individual chromosomes, are arranged on the equatorial plate and attach to the microtubules of the spindle.
3. Anaphase I:
- Homologous chromosomes, each with two chromatids, separate (centromeres remain undivided) and move towards opposite poles of the cell.
- 23 chromosomes (n-chromosome) move toward each pole.
- Each chromosome carries a double amount of DNA, maintaining a diploid (2n) total amount of DNA within the daughter cells.
4. Telophase I:
- Chromosomes form the chromatin of the new nuclei, and the cytoplasm divides into two parts.

Second Meiotic Division

Prophase II:
- Resembles prophase of mitosis with no DNA replication.
Metaphase II:
- Chromosomes align at the equatorial plate.
Anaphase II:
- The centromeres split, separating the chromatids.
Telophase II:
- Nuclear membranes and nucleoli reappear.
- Cytoplasm divides, producing four daughter cells, each with a haploid number of chromosomes.

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Description

Explore the intricate structure of the cell nucleus in this quiz. Learn about its components, including the nuclear envelope, chromatin, and nuclear pore complex. Gain an understanding of the nucleus's role in interfacing with the cytoplasm and its functional significance.