The Cell Cycle Overview

The Cell Cycle Overview

  • The cell cycle is a series of events that cells go through as they grow and divide.

  • Key Points:

    • The cell cycle is coordinated and controlled at multiple levels: time, position, environment, and damage.

    • Phases of the eukaryotic cell cycle include G1, S, G2, and M phases.

  • Learning Objectives:

    • Understand the heterodimer of cyclin and cyclin-dependent kinase (CDK) as the driver of the eukaryotic cell cycle.

    • Understand the prokaryotic cell cycle.

Purpose of the Cell Cycle

  • Functions of the cell cycle include:

    • Replacing lost or damaged cells. (maintain organ and tissue function)

    • Enabling a multicellular organism to grow to adult size.

    • Maintaining the total cell number of an adult organism. (cells undergo wear and tear)

    • Copying the genome and partitioning the copies equally between daughter cells, applicable to unicellular and multicellular organisms.

Prokaryotic Cell Division

  • Binary Fission Process:

    • Contains a circular DNA 

    • Prokaryotes divide by binary fission:

    • Cell enlarges.

    • DNA duplicates. preserving the sequence of the genome, copies of genome is identical 

    • A septum forms.

    • Cell divides into two, partitioning DNA into each new cell's nucleoid.

    • Pathways to Coordinate:

    • 1. Replication of DNA (partitioning two copies).

    • 2. Cytokinesis (cell separation).

DNA Replication in Prokaryotes

  • Prokaryotic cells have a circular chromosome with one origin of replication (ori). 

  • helicase is used to split the dna strands 

  • Key Features:

    • Two identical copies of the circular chromosome are formed.

    • Bidirectional replication occurs from the origin; two replication forks (RF) form at the origin.

    • hybrids, original strand and new strand 

Cytokinesis in Prokaryotes

  • FtsZ Ring Formation:

    • Early bacterial cytokinesis involves FtsZ protein formation on the inner surface of the cytoplasmic membrane at the future division site. forms a contractile ring that leads to the constriction of the cell membrane, ultimately facilitating the separation of the two daughter cells.

    • FtsZ protein is distributed randomly throughout the cytoplasm. which leads to continued seperation of cells 

Cell Cycle Coordination

  • Two pathways must be coordinated. replication of DNA (and the partition of the two copies) and cytokinesis (cell seperation)

  • The cycle of rapidly growing bacteria is shorter than the time needed to copy DNA, leading to potential scenarios where some cells will lack DNA.

    • Example timing:

    • Cell division takes 20 minutes.

    • DNA replication takes 40 minutes.

    • The mismatch is resolved by initiating DNA replication before finishing the previous round, known as multifork replication. bacteria found a way to make up for the time lag - multifork replication

    • 4 replication forks as compared to 2

Eukaryotic Cell Cycle

  • Eukaryotes face additional complexities:

    • Their genome consists of multiple linear chromosomes necessitating coordinated replication and faithful segregation.

    • linear vs bacteria circular = implication paradox(?) 

    • Multicellularity requires consideration of cells within organs and tissues. prokaryotes dont have intracellular organelles, in eukaryotes organelles also must be divided and duplicated into the two daughter cells 

    • must be coordinated so that it doesnt give you too much ——-

    • details of the cell cycle vary from organism to organism and at different times in an organisms life 

    • universal characterists:

      • dna must be faithfully replicated, otherwise mutations can occur(?)  

      • replicated chromosomes must be accurately segregated 

Phases of Eukaryotic Cell Cycle

  1. G1 Phase (Gap 1):

    • Growth phase where mass of organelles and proteins doubles.

    • Enzymes required for DNA replication are synthesized.

    • has to commit to the cell cyle, or stops the cell cycle depending on what its told to do, control from the outside  

  2. S Phase (Synthesis):

    • DNA replication occurs, resulting in pairs of identical sister chromatids.

    • sister chromatids must not be allowed to sperate from eachother, otherwise bipolar attatchment to the miotic spindle would be difficult to achieve 

    • Cohesin proteins prevent sister chromatids from drifting apart.

    • chromasomes within SMC3, SMC1 (structual maintianence of chromosomes), Kleisin

    • ends up with 2 sisters form each chromosome 

  3. G2 Phase (Gap 2):

    • Preparation for mitosis with chromosomal organization.

    • Key Events:

      • 1st event: Chromosome condensation begins. condensin encircles loops of DNA and compresses the sister chromatids to give a compact structure 

      • successive levels of packaging 

      • 2nd event: Formation of the mitotic spindle occurs, which is a bipolar array of microtubules.

      • mesh of proteins which stretched from one end of the cell to the other 

      • compressed complex of proteins areas of dna attatched to the centromere - kinetochore, allows the chromosome to be bound to the spindle apparatus. 

      • once the spindle binds to the spindle it exerts negative pressure that seperates the copies of chromosomes. sister chromatids are still held together by cohesin  

  4. M Phase (Mitosis):

    • Nuclear division takes place, followed by cytokinesis.

    • During cytokinesis, cytoplasm divides via a contractile ring made of actin and myosin II.

  • Visual Summary of Phases: G1 → S → G2 → M

  • Cytokeniesis ————— animal cells divide from the outside in 

  • Cytokensis in plants divides from inside out 

Events During Mitosis

  • 1st Event: Chromosome Condensation

    • Chromosomes become visible and compact due to condensin protein assisting in organizing DNA loops.

  • 2nd Event: Formation of the Mitotic Spindle

    • Role of Checkpoints:

    • Ensure all processes are complete before moving to the next phase.

    • Checkpoint examples:

      • G1: Restriction point where division signals must be present.

      • G2/M checkpoint ensuring DNA synthesis is complete.

      • Spindle checkpoint assessing chromosome attachment to spindle.

Consequences of Checkpoint Failure

  • Failure at checkpoints can lead to anomalies like:

    • Human aneuploidies (e.g., Down’s Syndrome).

    • Cancer due to unchecked cell cycle progression and ignored genome protection mechanisms.

Summary of Cell Cycle Regulation

  • Critical Features:

    • The eukaryotic cell cycle is divided into distinct phases, driven by the cyclin-dependent kinase (CDK) and cyclin complexes.

    • Regulation includes checkpoints to prevent mutation accumulation and chromosome mis-segregation.

    • Prokaryotic cell cycles utilize multifork replication for synchronization between DNA replication and cell division.