Cell Cycle, Mitosis and Cytokinesis

• The cell cycle is the life of a cell from its formation from a dividing parent cell until it divides into two daughter cells.
• The cell cycle consists of interphase and cell division.
• Cell division (M-Phase) comprises mitosis and cytokinesis.
• Mitosis refers to nuclear division.
• Cytokinesis refers to cytoplasm division.
• Multinuclear cells are formed if cytokinesis does not follow mitosis.

Importance of Cell Division

• Crucial for reproduction in prokaryotic, unicellular eukaryotic (e.g., amoeba), and multicellular eukaryotic organisms (e.g., fertilized human egg).
• Essential for growth and development, such as in embryo development.
• Facilitates tissue renewal and repair, like bone marrow producing new blood cells.
• Most cell divisions result in genetically identical daughter cells with the exception of meiosis, that produces sperm and egg cells.

Genetic Material Organization

• Genome is a cell's DNA content, containing its genetic information.
• DNA molecules are packaged into chromosomes, common in eukaryotic cells.
• Chromatin makes up chromosomes, consisting of DNA and proteins.
• Each eukaryotic species has a specific number of chromosomes in each cell nucleus.
• Human somatic cells contain 46 chromosomes (2 sets of 23, inherited from parents).
• Germ cells (sperm and ovum) contain 23 chromosomes (one set of 23).

Interphase

• It is the stage between successive cell divisions.
• It constitutes about 90% of the cell cycle, approximately 23 hours out of 24 and is longer than the M-Phase.
• It is divided into G1-phase, S-phase, and G2-phase.
• The cell grows in size during all three phases by producing proteins and cytoplasmic organelles like mitochondria and ER.

G1-Phase (1st Gap)

• The cell grows in size.
• Formation of enzymes required for DNA synthesis occurs.
• Centrosome is duplicated.
• Non-cycling cells enter G0 phase.

S-Phase (Synthesis)

• DNA replication and chromosome duplication occur within this phase.
• Synthesis of chromosomal proteins takes place.

G2-Phase (2nd Gap)

• Protein synthesis and growth continue to prepare the cell for M-phase.

Generation Time

• It is the time taken to complete one cell cycle, which is about 24 hours in a human cell.
• Nerve cells, skeletal muscle cells, and red blood cells stop dividing and remain in G0 phase.

DNA Replication

• The semiconservative model: the two DNA strands separate and unwind, with each original strand serving as a template for building a new strand.
• Replication of the DNA double helix results in two DNA double helices, each with one new and one old strand (50%:50%).

Steps of DNA Replication

• DNA-helicase unwinds and separates the two strands of the DNA double helix at the origin of replication (replication fork).
• Single-strand binding protein prevents the rejoining of the two separated DNA strands.
• Each separated DNA strand acts as a template for building a new strand.
• RNA-primase synthesizes an RNA primer (a segment of RNA) that provides the 3-OH end for new strand extensions.
• DNA-polymerases III are the enzymes of replication and they build the new strand using nucleoside triphosphate as a source of nucleotides in the 5’ to 3’ direction.
• Two phosphates of nucleoside 3-phosphates are released, and the remaining nucleotide links to the 3’ carbon of the sugar of the last nucleotide, elongating the strand using the liberated energy.
• Topoisomerase prevents knotting and tangling during DNA replication in the unwound DNA double helix.

Origin of Replication and Replication Forks

• Prokaryotic cells have a circular chromosome with a single origin of replication.
• Eukaryotic cells initiate replication at multiple origins.
• DNA helicase acts to form the replication fork, which is the site from which DNA is synthesized.
• Two replication forks are formed on each replication origin.
• Replication grows in both directions until the newly synthesized fragments join to form the new strands.

Directions and Finishing of Replication

• Leading strand: Formed as a continuous strand in the direction toward the replication fork.
• Lagging strand: Formed as a discontinuous strand of short fragments called Okazaki fragments because it grows in a direction away from the replication fork.
• After formation of leading and lagging strands:
  • DNA polymerase-I replaces the RNA primer with a DNA fragment.
  • DNA-ligase seals the Okazaki fragments together to form a complete strand.
  • Telomerase extends the chromosomes to their original length at the end of replication.