Cell Nucleus and Cell Division

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Questions and Answers

What role does rRNA play in the process of translation?

Providing a structural and functional role in the protein-rRNA complex (correct)

Replicating the DNA in the nucleus

Migrating to the opposite poles of the cell

Bringing amino acids to the growing protein chain

What occurs during the metaphase stage of mitosis?

The chromosomes condense and become discernible by microscopy

The chromosomes align on the equator of the cell (correct)

The sister chromatids separate and are pulled to different poles of the cell

The nuclear membrane reforms and microtubule apparatus dismantles

What is the process of copying the DNA in the nucleus called?

Cytokinesis

Replication (correct)

Mitosis

Translation

What is the function of cyclins in the cell cycle?

To regulate the stages of the cell cycle Signup and view all the answers

What is the term for the division of the cytoplasm into two daughter cells each containing one nucleus?

Cytokinesis Signup and view all the answers

What is the process of mitosis and cytokinesis collectively known as?

Cell replication Signup and view all the answers

During which stage of the cell cycle do the centrosomes migrate to the opposite poles of the cell?

Prophase Signup and view all the answers

What is the term for the replication origin in DNA?

ARS Signup and view all the answers

What is the primary function of the nucleolus?

To transcribe rRNA Signup and view all the answers

What is the function of importins in nuclear localisation signals?

To import proteins into the nucleus Signup and view all the answers

What is the function of histones in the nucleus?

To compact DNA Signup and view all the answers

What is the difference between heterochromatin and euchromatin?

Heterochromatin is tightly packed while euchromatin is loosely packed Signup and view all the answers

What is the function of the nuclear envelope?

To separate the nucleus from the cytoplasm Signup and view all the answers

What is the function of the nuclear lamina?

To support the nuclear envelope Signup and view all the answers

What is the difference between prokaryotes and eukaryotes in terms of transcription and translation?

Prokaryotes have coupled transcription and translation while eukaryotes have decoupled transcription and translation Signup and view all the answers

What is the function of the perinuclear space?

To communicate between the nucleus and the cytoplasm Signup and view all the answers

What is the function of the nuclear pores?

To transport molecules between the nucleus and the cytoplasm Signup and view all the answers

What is the difference between DNA and RNA?

DNA has thymine while RNA has uracil Signup and view all the answers

Study Notes

The Nucleus

Contains DNA compacted into chromosomes
DNA is transcribed into RNA in the nucleus
In eukaryotes, transcription and translation are decoupled, with transcription occurring in the nucleus and translation occurring in the cytoplasm

Cell Division

Mitosis is nuclear division, while cytokinesis is cellular division
Mitosis can occur without cytokinesis, resulting in a cell with multiple nuclei (syncytium)
In muscle cells, mitosis occurs without cytokinesis, producing multiple nuclei in one cell

Structure of the Nucleus

The nucleus is a membrane-bound sac held together by an intermediate filament called the nuclear lamina
The nucleus has a double membrane, with a perinuclear space between the membranes
The nucleoplasm communicates with the cytoplasm through nuclear pores
Dark patches in the nucleus are heterochromatin, while light patches are euchromatin

Chromatin

Found during interphase, chromatin is the material that chromosomes are composed of (containing DNA, RNA, and protein)
Can be highly compacted (heterochromatin) or loosely compacted with an open structure (euchromatin)
Chromatin can have two states: when stored, it is compacted as heterochromatin, and when used, it is turned into euchromatin

The Nuclear Envelope

The nucleus is 5-20µm in diameter and is demarcated by the nuclear envelope
Inside the envelope is the nucleoplasm
The perinuclear space is between the inner and outer membranes of the nuclear envelope and is continuous with the inside of the ER

Endoplasmic Reticulum

Smooth ER is where membrane synthesis occurs
Rough ER is one of the places where protein synthesis takes place

Nuclear Lamina

The nuclear lamina is a mesh of fibers that supports the nuclear envelope
Lamins are a type of intermediate filament composed of polymerized sub-units
During mitosis, the nuclear lamina network must break down, but reforms at the end of mitosis
Phosphorylation and dephosphorylation control the assembly and disassembly of lamin fibers

Nuclear Pores

The interior of the nucleus (nucleoplasm) communicates with the cytosol via protein-bound nuclear pores
Nuclear pores allow the free diffusion of small molecules but larger molecules must be targeted and actively transported
Each pore has 8 or 9 separate channels that allow the diffusion of small molecules between the nucleoplasm and the cytoplasm

Transport through the Nuclear Pores

RNA must be exported out of the nucleus, and proteins must be imported into the nucleus
Proteins to be imported into the nucleus have Nuclear Localization Signals (NLS)
NLS are the first 20 bases of a protein that provides a signal telling a protein to be taken into or out of the nucleus

The Nucleolus

Site of transcription of rRNA
A round structure present in the nucleus, with several nucleoli per nucleus
The primary function of the nucleolus is the transcription of rRNA
rRNA is ribosomal RNA and is an important part of the ribosome, which translates mRNA into protein

DNA in the Nucleus

DNA is compacted with proteins, wrapped around histone proteins into chromosomes
Inactive heterochromatin is tightly packed, while active euchromatin is loosely packed
Histones help compact DNA, representing the lowest level of DNA compaction
Histone modification has been implicated in the control of gene expression

Chromosomes

Can become visible to light microscopy during the mitotic process
Chromosomes can be counted during metaphase
Humans have 22 pairs of chromosomes and 1 pair of non-homologous sex-determining chromosomes

Function

Many organisms have more than one set of chromosomes
Many single-celled organisms carry a single set of chromosomes (haploid), while most multi-cellular organisms carry two sets of chromosomes (diploid)
The sets of chromosomes can have different alleles
A gene can be the same on the different copies of the chromosome (homozygous), or different versions on different copies of a chromosome (heterozygous)

Transcription in the Nucleus

The nucleus holds the genetic material (DNA)
Active DNA is transcribed (copied into RNA)
RNA is processed in the nucleus prior to export to the cytoplasm, where it is translated into protein

RNA and DNA

Both are nucleic acids
DNA is double-stranded and holds the genetic information, while RNA is a single-stranded copy of parts of the DNA
RNA has a 2' OH on the ribose ring, which DNA lacks, and instead of thymine, RNA has the base uracil

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Description

Learn about the structure and function of the cell nucleus, including DNA transcription and translation, as well as the processes of mitosis and cytokinesis in cell division.