Cell Nucleus: Structure and Function

• The nucleus is the largest structure in the cell and serves as its control center.
• It houses the genetic material, DNA, and is typically singular within a cell, apart from erythrocytes (anucleate) and skeletal muscle cells (multinucleate).

Nuclear Envelope

• The nuclear envelope is a double-membrane of phospholipids enclosing the nucleus and separating the cytoplasm from the nucleoplasm.
• It is continuous with the rough endoplasmic reticulum.
• Nuclear pores are protein-formed open passageways in the envelope that facilitate the movement of large molecules, ions, and water-soluble molecules.

Nucleolus

• The nucleolus is a dark-staining, non-membrane-bound spherical body composed of protein and RNA.
• It produces small and large ribosome subunits, with the number of nucleoli varying depending on the cell type.

Deoxyribonucleic Acid (DNA)

• DNA, housed in the nucleus, consists of nucleotide monomers, each with deoxyribose, a phosphate group, and one of four nitrogenous bases: adenine, cytosine, guanine, or thymine.
• DNA has two complementary nucleotide strands in a spiral ladder structure.
• Sugar and phosphates create the backbone of the ladder, while nitrogenous base pairs form the rungs linked by hydrogen bonds: Adenine - Thymine; Cytosine - Guanine
• DNA double helixes are wound around histones, forming nucleosomes.
• When not dividing, DNA exists as a finely filamented chromatin; during division, it condenses into tightly coiled chromosomes.

Genes

• Genes are DNA segments that provide instructions for synthesizing specific proteins.

Protein Synthesis

• Protein synthesis, essential for all cellular activities, is directed by DNA in two stages: transcription and translation.
• Transcription involves forming an RNA copy of a gene from DNA in the nucleus.
• Translation is synthesizing a protein from the transcript's instructions using ribosomes.

Transcription: Synthesizing RNA from DNA

• Initiation begins with DNA unwinding, catalyzed by specific enzymes, to allow RNA polymerase access.
• RNA polymerase attaches to the unwound DNA at the promoter region, marking the start point for transcription.
• The template strand is the DNA strand copied, while the other strand is the coding strand.
• During elongation, free ribonucleotides pair with exposed bases on the DNA template strain, held by hydrogen bonds.
• Phosphodiester bonds link the growing RNA strand as RNA polymerase moves along the template until the entire gene is transcribed.
• Termination occurs when RNA polymerase reaches the gene's terminal region and releases.
• Hydrogen bonds break, DNA recoils, and a pre-mRNA strand is formed.

Modification to mRNA

• Pre-mRNA is modified into mature mRNA by splicing, capping, and poly A tail addition.
• Splicing involves removing noncoding regions (introns) and joining coding regions (exons).
• Capping adds a guanine nucleotide to the mRNA's front end for stability and protection.
• Poly A tail addition adds approximately 200 adenine nucleotides to the mRNA tail for nuclear export, translation, and stability.

Types of RNA

• Messenger RNA (mRNA) carries instructions for synthesizing a protein.
• Transfer RNA (tRNA) functions in translation.
• Ribosomal RNA (rRNA) also functions in translation and comprises about 60% of ribosome weight.

Translation

• Translation is the synthesis of a new protein at ribosomes in the cytoplasm.
• mRNA is threaded through the ribosome, and its nucleotide sequence is translated into an amino acid sequence.
• Ribosomes are composed of large and small subunits, with rRNA acting as a catalyst.
• mRNA carries instructions via codons (3-base units).
• The start codon (AUG) signals the beginning of protein synthesis, while the stop codon signals the end.
• tRNA acts as an adapter, bringing specific amino acids to specific mRNA codons, with an amino acid acceptor and an anticodon region.
• The nucleotide sequence of the anticodon determines the amino acid attached.

Translation Process

• Initiation requires small and large ribosome subunits, the new mRNA, and tRNA.
• tRNA base pairs its anticodon (UAC) to the start codon (AUG) in the mRNA.
• Elongation involves the delivery of amino acids to form the protein, where a new tRNA attaches to the subsequent mRNA codon.
• Two adjacent amino acids connect with a peptide bond while the ribosome moves downstream on the mRNA.
• Termination happens when a stop codon enters the ribosome.
• A release factor enters the ribosome, separating the ribosome subunits from the mRNA and releasing the new protein.
• A single mRNA can be read by multiple ribosomes simultaneously forming polyribosomes.

Cell Division

• Cell division produces daughter cells which is essential for development, tissue growth, cell replacement, tissue repair, and sex cell production.
• Mitosis occurs in somatic cells (non-sex cells), producing two daughter cells.
• Meiosis occurs in sex cells (sperm or oocytes), producing four daughter cells.

Cell Cycle

• The cell cycle consists of interphase (~95% of the cell's existence) and the mitotic (M) phase (~5% of the cell's existence).

Interphase

• Interphase is the preparation phase, which consists of three phases: G1, S, and G2.
• DNA exists in loosely coiled chromatin form.
• G1 (1st gap) is the cell's growth phase, which produces new organelles and proteins for DNA replication, and centrioles begin to replicate.
• S (synthesis) is the phase where DNA replicates using deoxyribonucleotides and DNA polymerase in the nucleoplasm of the nucleus.
• G2 (2nd gap) is a brief phase where the cell prepares for division, completing centriole replication and synthesizing proteins for cell division.

DNA Replication

• DNA strands unwind and hydrogen bonds break.
• Both strands are read by DNA polymerase enzymes.
• The leading strand is synthesized continuously, whereas the lagging strand is synthesized discontinuously in segments.
• DNA ligase splices together short segments of the discontinuous strand.
• DNA double strands return to their coiled helix structure, resulting in two DNA molecules.
• Replicated DNA strands, called sister chromatids, remain attached at the centromere.

M phase (mitotic phase)

• Mitosis is the division of the nucleus, and cytokinessis is the division of the cytoplasm.
• Mitosis comprises four phases: prophase, metaphase, anaphase, and telophase.

Prophase

• Chromatin condenses into chromosomes, and the nucleolus and nuclear envelope break down.
• Microtubules (spindle fibers) grow from centrioles, and centriole pairs migrate to opposite poles, forming a mitotic spindle.
• Kinetochore proteins attach to centromeres.

Metaphase

• Kinetochore microtubules extend from centrioles and attach to kinetochores at the centromere of each chromosome.
• Chromosomes align on the metaphase plate.
• Sister chromatids detach at the centromere.

Anaphase

• Centromeres of sister chromatids separate.
• Kinetochore microtubules pull sister chromatids toward opposite poles of the cell.
• Each sister chromatid transforms into a chromosome.

Telophase

• Chromosomes arrive at each pole.
• Chromosomes uncoil and return to chromatin form.
• A new nucleolus forms in each cell.
• A new nuclear envelope forms around chromosomes.
• A cleavage furrow forms.

Cytokinesis

• Cytokinesis divides the cytoplasm, and occurs alongside of anaphase or telophase.
• A ring of microfilament proteins contracts at the cell's equator, resulting in a cleavage furrow.
• Cytokinesis completes when the cleavage furrow fully separates the mother cell into two daughter cells.

Explore the nucleus, the cell control center. Learn about the nuclear envelope, nucleolus, and DNA. Understand the role of each component within the cell.