Cell Cycle and Mitosis Overview

Cell Cycle and Mitosis

• A human, like all sexually reproducing organisms, starts as a fertilized egg (zygote)
• Cell division creates complex, multicellular organisms from a single cell
• Single-celled organisms use cell division for reproduction

DNA Organization and the Cell Cycle

• The cell cycle is a series of events from a parent cell dividing to creating two identical daughter cells
• Precise regulation is key for the mechanisms of the cell cycle

Genomic DNA

• A cell's genome is its DNA, packaged as double-stranded DNA molecules
• Prokaryotes have a single circular DNA molecule in a region called the nucleoid
• Some prokaryotes have additional non-essential DNA in loops called plasmids
• Bacteria can exchange plasmids for beneficial genes, including antibiotic resistance
• Eukaryotes have multiple linear DNA molecules, with a specific number of chromosomes per species
  • Humans have 46 chromosomes in somatic cells and 23 in gametes (sperm/eggs)
  • Diploid organisms (2n) have two matched sets of chromosomes.
• Traits are variations in characteristics, determined by genes (sections of chromosomes)

Eukaryotic Chromosomal Structure and Compaction

• DNA in a human cell, if stretched out, would be two meters long, yet its diameter is only 2 nanometers
• DNA is tightly packaged into chromosomes, which are condensed
• DNA wraps around histone proteins, forming nucleosomes
• Nucleosomes coil into chromatin fibers, which further condense
• Chromosomes are duplicated in the S phase of interphase, forming linked sister chromatids
• Sister chromatids are attached at the centromere, appearing as a constricted area under a microscope

The Cell Cycle

• The cell cycle involves interphase and the M phase
• Interphase consists of G1, S, and G2 phases
  • G1: cell growth, accumulating resources
  • S: DNA synthesis (replication)
  • G2: cell growth, more resources, preparing for division
• The M phase involves mitosis and cytokinesis
  • Mitosis divides the nucleus, including prophase, prometaphase, metaphase, anaphase, and telophase
  • Cytokinesis divides the cytoplasm, forming two daughter cells

Interphase

• Normal cellular processes happen, while preparing for cell division
• G1, S, and G2 phases are all part of interphase
• G1 phase: cell growth, generating resources & proteins
• S phase: synthesis phase of DNA replication creates identical sets of sister chromatids
• G2 phase: cell growth and further preparation for mitosis

Mitotic Phase

• Mitosis (or karyokinesis): nucleus divides in the following phases
• Prophase: chromosomes condense
• Prometaphase: nuclear envelope breaks down
• Metaphase: chromosomes align at the metaphase plate
• Anaphase: sister chromatids separate
• Telophase: chromosomes reach opposite poles, new nuclear envelope forms

Cytokinesis

• Cytoplasm divides, creating two daughter cells
• Different mechanisms in different cell types.
• Animal cells: cleavage furrow forms and contracts
• Plant cells: cell plate develops and forms a new cell wall.

Go Phase

• Cells that have permanently exited the cycle and aren't actively preparing for division, like mature cardiac/nerve cells
• Cell may be in a temporary or permanent resting state.

Cell Cycle Checkpoints

• Internal control mechanisms (checkpoints) are crucial for ensuring accurate cell division
• Control points ensure conditions are favorable:
• G1 checkpoint: checks for favorable conditions and sufficient resources
• G2 checkpoint: checks for DNA replication completion and integrity
• M checkpoint: checks for proper chromosome attachment to spindle fibers
• Errors in DNA duplication risks faulty daughter cells, leading to mutations and potentially abnormal cell growth