Cell Division and Protein Synthesis: Study Notes for Anatomy & Physiology

Cell Division and Protein Synthesis

Overview of the Cell Cycle

The cell cycle describes the sequence of events a cell undergoes from its formation until it reproduces. It is essential for growth, repair, and maintenance in multicellular organisms.

• Interphase: The cell carries out its normal functions and prepares for division.

• Mitotic (M) Phase: The cell divides into two daughter cells.

Diagram: The cell cycle is typically divided into G1, S, G2 (all part of interphase), and M phase (mitosis and cytokinesis).

Interphase

Interphase is the period between cell divisions, during which the cell grows, performs its normal functions, and prepares for division. The nuclear material is in the form of chromatin.

• G1 Phase (Gap 1): Vigorous growth and metabolic activity. Some cells permanently cease dividing and are said to be in the G0 phase.

• S Phase (Synthesis): DNA replication occurs, resulting in the duplication of chromosomes.

• G2 Phase (Gap 2): Preparation for cell division, including synthesis of proteins and organelles.

DNA Replication

DNA replication is the process by which a cell makes an exact copy of its DNA, ensuring that each daughter cell receives a complete set of genetic information.

• Initiation: DNA helices unwind and unzip at the replication fork, where the two strands separate.

• Each old strand acts as a template for a new complementary strand.

• Primase lays down a short RNA primer to initiate synthesis.

• DNA polymerase attaches to the primer and adds complementary nucleotides, synthesizing both new strands simultaneously.

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

After replication, each DNA molecule consists of one old (parental) strand and one new (daughter) strand. This is called semiconservative replication.

Equation:

$\text{Parental DNA} \rightarrow \text{Parental Strand} + \text{New Strand}$

Cell Division (M Phase)

Cell division is necessary for growth, repair, and maintenance. Most cells divide regularly, but some (e.g., cardiac and nerve cells) do not divide efficiently.

• M Phase: The phase of the cell cycle in which division occurs, consisting of mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis

Mitosis is divided into several stages:

• Prophase: Chromatin condenses into visible chromosomes. Each chromosome consists of two sister chromatids joined at a centromere. The mitotic spindle forms, and microtubules called asters radiate from the centrosome.

• Metaphase: The nuclear envelope breaks up. Chromosomes align at the cell's equator (metaphase plate), and spindle fibers attach to kinetochores on the centromeres.

• Anaphase: Sister chromatids separate and are pulled toward opposite poles of the cell by spindle fibers.

• Telophase: Chromosomes uncoil back into chromatin, nuclear envelopes reform, and the spindle breaks down.

• Cytokinesis: The cytoplasm divides, resulting in two genetically identical daughter cells.

Control of Cell Division

Cell division is tightly regulated by internal and external signals to ensure proper growth and tissue maintenance.

• Surface-to-volume ratio: When a cell grows too large, its surface area becomes insufficient for exchange, triggering division.

• Growth factors and hormones: External signals that stimulate cell division.

• Contact inhibition: Normal cells stop dividing when they come into contact with other cells.

• Cyclins and cyclin-dependent kinases (Cdks): Regulatory proteins that control progression through the cell cycle.

• Checkpoints: Critical control points where the cell cycle can be stopped if errors are detected (e.g., G1 restriction point).

Protein Synthesis

Protein synthesis is the process by which cells build proteins based on genetic instructions encoded in DNA. It involves two main steps: transcription and translation.

Genetic Code

• The genetic code is the sequence of nitrogenous bases (Adenine, Guanine, Cytosine, Thymine) in DNA that determines the order of amino acids in a protein.

• A gene is a segment of DNA that codes for a specific polypeptide.

• The code is read in triplets (three-base sequences called codons), each specifying a particular amino acid.

• There are 64 possible codons ($4^3 = 64$), but only 20 amino acids, so some amino acids are specified by more than one codon (redundancy).

Types of RNA

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

• Ribosomal RNA (rRNA): Forms the core of ribosomes, the site of protein synthesis.

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

Transcription

Transcription is the process of copying the DNA sequence of a gene into mRNA.

• Initiation: Transcription factors bind to the promoter region of the gene, allowing RNA polymerase to attach and begin synthesis.

• Elongation: RNA polymerase moves along the DNA, adding complementary RNA nucleotides to build the mRNA strand.

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

• The initial mRNA transcript (pre-mRNA) is processed by removing introns (non-coding regions) and joining exons (coding regions) via spliceosomes.

Translation

Translation is the process by which the sequence of codons in mRNA is used to assemble amino acids into a polypeptide chain at the ribosome.

• Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA, then the large subunit joins to form a functional ribosome.

• Elongation: tRNAs bring amino acids to the ribosome, matching their anticodons to the mRNA codons. The ribosome joins the amino acids together, forming a growing polypeptide chain.

• Termination: When a stop codon is reached, the polypeptide is released, and the ribosome disassembles.

Summary Table: Types of RNA and Their Functions

RNA Type	Function
mRNA (Messenger RNA)	Carries genetic code from DNA to ribosome
rRNA (Ribosomal RNA)	Forms ribosomes, the site of protein synthesis
tRNA (Transfer RNA)	Brings amino acids to ribosome; matches codon with anticodon

Cell Aging and Death

Cells have mechanisms for removing damaged or obsolete components and for programmed cell death (apoptosis).

• Autophagy: The process of degrading and recycling cellular components via autophagosomes.

• Proteasomes: Protein complexes that degrade ubiquitin-tagged proteins.

• Apoptosis: Programmed cell death, important for removing damaged or dangerous cells.

Cellular Aging

• Telomeres: Protective nucleotide sequences at the ends of chromosomes that shorten with each cell division, limiting the number of times a cell can divide.

• Telomerase: An enzyme that lengthens telomeres, present in germ cells and cancer cells.

• Theories of Aging: Include the free radical theory (damage from reactive oxygen species), genetic theory (programmed cell death), and autoimmune theory (immune system decline).

Homeostatic Imbalance Example: Progeria

• Progeria: A rare genetic disorder causing accelerated aging due to defective proteins affecting nuclear stability.

• Symptoms include slow growth, fragile bones, arthritis, and cardiovascular disease.

Additional info: These notes expand on the provided slides by clarifying terminology, adding definitions, and summarizing key regulatory mechanisms and disease relevance for Anatomy & Physiology students.