Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It consists of two major periods: interphase (when the cell grows and carries out normal functions) and the mitotic phase (when the cell divides). Proper regulation of the cell cycle is essential for growth, development, and tissue repair.

• Interphase: The period of cell growth and DNA replication, subdivided into G1, S, and G2 phases.

• Mitotic (M) phase: The period of nuclear and cytoplasmic division, including mitosis and cytokinesis.

Phases of Interphase

• G1 phase (Gap 1): Cell grows and performs normal metabolic functions.

• S phase (Synthesis): DNA is replicated in preparation for cell division.

• G2 phase (Gap 2): Cell continues to grow and prepares for mitosis.

• G0 phase: Cells that exit the cycle and do not divide (e.g., neurons) enter this quiescent state.

DNA Replication

During the S phase, the cell duplicates its DNA to ensure that each daughter cell receives an identical set of genetic material. This process is called semiconservative replication because each new DNA molecule consists of one old strand and one new strand.

• Helicase unwinds the DNA double helix.

• DNA polymerase synthesizes new DNA strands by adding nucleotides to a primer.

• The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments (Okazaki fragments) that are later joined by DNA ligase.

Mitosis and Cytokinesis

Stages of Mitosis

Mitosis is the process of nuclear division that ensures each daughter cell receives an identical set of chromosomes. It is divided into four main stages:

• Prophase: Chromatin condenses into visible chromosomes; the mitotic spindle forms. The nuclear envelope breaks down in late prophase.

  • Each chromosome consists of two sister chromatids joined at a centromere.

  • Spindle fibers attach to kinetochores on the centromeres.

• Metaphase: Chromosomes align at the metaphase plate (cell equator).

• Anaphase: Sister chromatids separate and move toward opposite poles of the cell.

• Telophase: Chromosomes decondense, nuclear envelopes reform, and nucleoli reappear.

Cytokinesis

Cytokinesis is the division of the cytoplasm, which usually begins during late anaphase or telophase. A contractile ring of actin filaments forms a cleavage furrow, pinching the cell into two daughter cells.

Control of the Cell Cycle

Cell division is tightly regulated by internal and external signals to ensure proper growth and prevent uncontrolled proliferation (as seen in cancer).

• Checkpoints: Critical control points (e.g., G1, G2/M) where the cell assesses whether to proceed with division.

• Cyclins and Cdks: Regulatory proteins that drive the cell through the cycle by activating key enzymes.

• Contact inhibition: Normal cells stop dividing when they touch neighboring cells.

Protein Synthesis

Genetic Code and Genes

DNA contains the instructions for building proteins, which are essential for cell structure and function. A gene is a segment of DNA that codes for a specific polypeptide. The genetic code is based on sequences of three nitrogenous bases (triplets) that specify amino acids.

• Exons: Coding regions of a gene.

• Introns: Non-coding regions interspersed among exons.

Role of RNA

RNA acts as the intermediary between DNA and protein synthesis. There are three main types:

• Messenger RNA (mRNA): Carries the genetic code from DNA to ribosomes.

• Ribosomal RNA (rRNA): Structural component of ribosomes.

• Transfer RNA (tRNA): Brings amino acids to the ribosome and matches them to the mRNA code via its anticodon.

Transcription

Transcription is the process of copying the DNA code into mRNA. It occurs in three main steps:

• Initiation: RNA polymerase binds to the promoter region and unwinds the DNA.

• Elongation: RNA polymerase adds complementary RNA nucleotides to the growing mRNA strand.

• Termination: Transcription ends when RNA polymerase reaches a termination signal.

Processing of mRNA

Before mRNA leaves the nucleus, it is processed: introns are removed and exons are spliced together by spliceosomes, resulting in mature mRNA ready for translation.

Translation

Translation is the process by which ribosomes read the mRNA sequence and assemble the corresponding amino acids into a polypeptide chain. This process occurs in three phases:

• Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA (carrying methionine) binds to the start codon (AUG).

• Elongation: tRNAs bring amino acids to the ribosome, where they are joined together by peptide bonds in the order specified by the mRNA codons.

• Termination: When a stop codon is reached, the newly synthesized polypeptide is released.

The Genetic Code Table

The genetic code is universal and redundant, meaning that most amino acids are encoded by more than one codon. There are 64 possible codons (61 for amino acids, 3 for stop signals).

Cell Death, Differentiation, and Aging

Autophagy and Proteasomes

Autophagy is the process by which cells degrade and recycle their own components, including damaged organelles. Proteasomes degrade unneeded or damaged proteins tagged with ubiquitin, recycling amino acids for new protein synthesis.

Apoptosis

Apoptosis is programmed cell death, a controlled process that removes damaged or unnecessary cells without causing inflammation. It involves activation of caspases, DNA fragmentation, and phagocytosis of cell remnants.

Cell Differentiation

Although all cells contain the same DNA, they differentiate into various types by expressing different sets of genes. This process is regulated by chemical signals during development.

Cell Aging and Clinical Relevance

• Wear and tear theory: Accumulated damage from free radicals and chemicals leads to aging.

• Genetic theory: Telomere shortening limits the number of cell divisions; telomerase can extend telomeres (active in cancer cells).

• Progeria: A rare genetic disorder causing premature aging due to defective nuclear lamina protein.

Summary Table: Key Events in the Cell Cycle