Chapter three pt. 4

Chapter 3: Genetics and Cell Cycle

Learning Outcomes

• Describe the steps involved in transcription and translation.

• Explain post-translational modification and its necessity.

• Describe the cell cycle and stages of mitosis.

• Describe the connection between cell cycle regulation and cancer.

Genetic Information Flow: Transcription and Translation

Transcription: Synthesis of mRNA from DNA

Transcription is the process by which a cell makes an mRNA copy of a gene's DNA sequence. This mRNA transcript then exits the nucleus and is used for protein synthesis in the cytoplasm.

• RNA polymerase: The enzyme responsible for building the mRNA transcript. It binds to the gene and brings in complementary nucleotides, linking them together to form mRNA.

• Transcription occurs in three main stages:

  • Initiation: Transcription factors bind to the promoter region near the gene on the template DNA. RNA polymerase also binds to the promoter with the help of the enzyme helicase, which unwinds the DNA.

  • Elongation: RNA polymerase covalently bonds complementary nucleotides to the growing mRNA molecule, using the DNA template.

  • Termination: When the last triplet of the gene is reached, the newly formed pre-mRNA molecule is released for modification.

Example: The gene for hemoglobin is transcribed in red blood cell precursors to produce mRNA, which is then translated into the hemoglobin protein.

Post-Transcriptional Modification

Before mRNA can be translated, it must be modified. Sections of the gene that do not code for amino acids (introns) are removed, and coding regions (exons) are spliced together. The mature mRNA then exits the nucleus through a nuclear pore and enters the cytosol for translation.

• Introns: Non-coding regions removed from pre-mRNA.

• Exons: Coding regions joined together to form mature mRNA.

Translation: Protein Synthesis from mRNA

Translation is the process by which the nucleotide sequence of mRNA is converted into an amino acid sequence, forming a protein. This occurs at the ribosome with the help of transfer RNA (tRNA).

• tRNA: Made in the nucleus, tRNA carries specific amino acids to the ribosome. Each tRNA has an anticodon that is complementary to the mRNA codon.

• The ribosome has three binding sites for tRNA:

  • A site (aminoacyl site): Binds incoming tRNA carrying an amino acid.

  • P site (peptidyl site): Holds the tRNA with the growing peptide chain.

  • E site (exit site): Where empty tRNA exits the ribosome.

• Translation occurs in three stages:

  • Initiation: Initiator tRNA binds to the start codon in the P site.

  • Elongation: Next tRNA binds to the A site; peptide bonds form between amino acids; tRNAs shift through the sites as the chain grows.

  • Termination: When the ribosome reaches a stop codon, the new peptide is released.

Example: The mRNA for insulin is translated at the ribosome to produce the insulin protein.

Post-Translational Modification

Newly formed polypeptides must be modified after translation to become fully functional proteins. This process is called post-translational modification.

• Polypeptides may fold on their own or with the help of chaperone proteins.

• Polypeptides destined for secretion or for insertion into organelles/membranes are often modified in the rough endoplasmic reticulum (RER) and Golgi apparatus.

Example: Digestive enzymes are synthesized in the RER, processed in the Golgi, and then secreted from the cell.

The Cell Cycle and Mitosis

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase and M phase (mitosis and cytokinesis).

• Interphase: Period of growth and preparation for cell division, divided into:

  • G1 phase (first gap): Cell performs normal functions and prepares for DNA synthesis.

  • S phase (synthesis): DNA replication occurs.

  • G2 phase (second gap): Cell grows and prepares for division; proteins and centrioles are produced.

• M phase: Cell division, including mitosis (division of genetic material) and cytokinesis (division of cytoplasm).

DNA Replication (S Phase)

During the S phase, DNA is replicated so that each daughter cell receives an identical set of chromosomes.

• Helicase: Enzyme that separates DNA strands.

• Primase: Builds RNA primer on exposed DNA strands.

• DNA polymerase: Adds nucleotides to the new DNA strand.

• Replication is semiconservative: Each new DNA molecule has one old and one new strand.

$\text{DNA Replication:}~\text{Parent DNA} \rightarrow \text{2 Daughter DNA (each: 1 old + 1 new strand)}$

Mitosis: Division of Genetic Material

Mitosis is the process by which a cell divides its genetic material between two daughter cells. It consists of four stages:

• Prophase: Chromatin condenses into chromosomes; nuclear envelope breaks down; spindle fibers form.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids are pulled apart to opposite poles; each is now called a daughter chromosome.

• Telophase: Nuclear envelope re-forms; chromosomes uncoil into chromatin; nucleoli reappear.

Cytokinesis

Cytokinesis is the division of the cytoplasm and organelles, resulting in two genetically identical daughter cells. The cell membrane pinches in at the equator, forming a cleavage furrow that eventually separates the cells.

Cell Cycle Regulation and Cancer

Checkpoints and Regulation

The cell cycle is tightly regulated by checkpoints that ensure proper division. The most important checkpoint occurs in G1, where the cell responds to extracellular signals and may halt division if conditions are unfavorable.

• Requirements for division:

  • Sufficient nutrients in the extracellular fluid (ECF).

  • Growth factors must be present.

  • Cell density and anchorage to neighboring cells are monitored.

• Cells that cannot pass checkpoints undergo apoptosis (programmed cell death).

Example: During fetal development, apoptosis separates fingers and toes by removing webbing.

Cancer: Loss of Cell Cycle Control

Changes in DNA can disrupt cell cycle regulation, leading to uncontrolled cell division and tumor formation.

• Benign tumor: Confined to original location; does not invade surrounding tissues; less likely to cause death.

• Malignant tumor: Made up of cancer cells; invades other tissues; can metastasize and cause widespread tissue destruction.

• Malignant cells are not inhibited by high cell density or loss of anchorage; they divide indefinitely if nutrients are available.

Comparison of Benign and Malignant Tumors

Feature	Benign Tumor	Malignant Tumor
Location	Confined to original site	Invades surrounding tissues
Growth Rate	May grow slowly	May grow rapidly
Metastasis	No	Yes
Cell Cycle Control	Partially retained	Lost

Additional info: The notes and images are consistent with standard college-level Anatomy & Physiology curriculum, focusing on molecular genetics, cell cycle, and cancer biology.