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.

• Discuss the connection between cell cycle regulation and cancer.

Transcription and Translation

Transcription: Synthesis of mRNA

Transcription is the process by which a messenger RNA (mRNA) copy of a gene is made from DNA. This mRNA transcript carries genetic information from the nucleus to the cytoplasm, where it can be translated into protein.

• RNA polymerase: The enzyme responsible for building the mRNA transcript. It binds to a specific region of DNA called the promoter and brings in complementary nucleotides to form the mRNA strand.

• Transcription proceeds in three main stages:

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

  2. Elongation: RNA polymerase covalently bonds complementary nucleotides to the growing mRNA molecule, using the DNA template strand as a guide.

  3. Termination: When the end of the gene is reached, the newly formed pre-mRNA is released and ready for modification.

• Pre-mRNA Processing: The initial mRNA transcript (pre-mRNA) contains non-coding sequences called introns and coding sequences called exons. Introns are removed and exons are spliced together to form mature mRNA, which then exits the nucleus through a nuclear pore.

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

Translation: Protein Synthesis

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 and involves transfer RNA (tRNA).

• tRNA: Made in the nucleus, tRNA molecules pick up specific amino acids and transfer them to the ribosome. Each tRNA has an anticodon (a sequence of three nucleotides) that is complementary to a codon on the mRNA.

• The ribosome has three binding sites for tRNA:

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

  • P site (peptidyl site): The amino acid is removed from its tRNA and added to the growing peptide chain.

  • E site (exit site): tRNA exits the ribosome to pick up another amino acid.

• Translation occurs in three stages:

  1. Initiation: The initiator tRNA binds to the start codon in the ribosome's P site.

  2. Elongation: The next tRNA binds to the A site, allowing two amino acids to be linked by a peptide bond. The first tRNA exits from the E site, and the second tRNA moves into the P site.

  3. Termination: When the ribosome reaches a stop codon on the mRNA, the peptide is released.

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

Post-Translational Modification

After translation, newly formed polypeptides must be modified to become fully functional proteins. This process is known as post-translational modification.

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

• Some polypeptides are combined with others to form multi-subunit proteins.

• Polypeptides destined for secretion or membrane insertion are modified in the rough endoplasmic reticulum (RER) and further processed in the Golgi apparatus.

Example: Collagen polypeptides undergo extensive post-translational modifications before becoming functional collagen fibers.

The Cell Cycle

Main Phases of the Cell Cycle

The cell cycle consists of two main phases: interphase and M phase (cell division).

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

  • G1 phase (first gap): Cell performs normal metabolic activities, produces new organelles, cytoskeleton, and proteins, and prepares for the next phase.

  • S phase (synthesis): DNA replication occurs, ensuring each daughter cell receives a complete set of genetic material.

  • G2 phase (second gap): Cellular growth continues, proteins required for cell division are produced, and centrioles are duplicated.

• M phase: Includes mitosis (division of genetic material) and cytokinesis (division of cytoplasm and organelles).

DNA Replication

DNA replication occurs during the S phase of interphase and ensures that each daughter cell receives an identical copy of the genome.

• Helicase: Separates the two DNA strands.

• Primase: Builds RNA primers on exposed DNA strands.

• DNA polymerase: Adds nucleotides to the RNA primer, synthesizing new DNA strands in opposite directions.

• RNA primers are eventually replaced with DNA nucleotides.

• End result: Two identical double helices, each with one old and one new strand (semiconservative replication).

Equation:

Mitosis and Cytokinesis

Mitosis is the division of the cell's genetic material into two identical daughter cells. It consists of four stages:

• Prophase: Chromatin condenses into visible chromosomes, each with two sister chromatids joined at the centromere. The nucleolus disintegrates, and the mitotic spindle forms as centrioles migrate to opposite poles.

• Metaphase: Spindle fibers attach to each sister chromatid at the centromere and align chromosomes along the cell's equator.

• Anaphase: Sister chromatids are pulled apart toward opposite poles, becoming daughter chromosomes. Cytokinesis may begin at the end of this stage.

• Telophase: Nuclear envelope reassembles around each set of chromosomes, and nucleoli reappear.

Cytokinesis: The cytoplasm and organelles are divided between the two daughter cells. The cell splits apart as actin and myosin proteins tighten around the equator, creating a cleavage furrow.

Example: Skin cells undergo mitosis to replace cells lost due to injury.

Cell Cycle Regulation and Cancer

Cell Cycle Checkpoints

The cell cycle is tightly regulated by checkpoints that ensure proper cell division and prevent uncontrolled growth.

• G1 checkpoint: Occurs about three-fourths of the way through G1. The cell responds to extracellular signals and may not proceed if conditions are unfavorable.

• Requirements for cell division:

  • Sufficient nutrients in the extracellular fluid (ECF).

  • Presence of growth factors secreted by other cells.

  • Cell density and anchorage to neighboring cells and the extracellular environment.

• Cells that cannot pass through checkpoints and cannot be repaired undergo programmed cell death (apoptosis).

Example: During embryonic development, apoptosis removes cells between developing fingers and toes.

Cancer and Loss of Cell Cycle Control

Cancer results from changes in DNA that cause loss of cell cycle control, leading to uncontrolled cell division and tumor formation.

• Benign tumor: Confined to its original location, does not invade surrounding tissues, and is less likely to cause death.

• Malignant tumor: Made up of cancer cells that are not limited by cell density or anchorage. These cells grow and divide indefinitely, invade other tissues (metastasis), and can cause widespread tissue destruction and death.

Example: Renal cell carcinoma is a malignant tumor of kidney cells.

Summary Table: Cell Cycle Phases and Key Events

Additional info: The notes have been expanded with academic context and examples to ensure completeness and clarity for college-level Anatomy & Physiology students.