Bacterial Cell Division: Binary Fission

• All species produce offspring like themselves by passing on hereditary information

Cell Division

• Necessary for reproduction, growth, wound healing, and cell replacement
• Direct in prokaryotes, but more complex in eukaryotes

Bacterial Cell Division

• Bacteria divide via binary fission
• Asexual reproduction resulting in clonal offspring
• Single, circular bacterial chromosome is replicated
• Replication initiates at the origin of replication and proceeds in two directions until termination
• Newly formed chromosomes move to opposite ends of the cell
• A septum forms, dividing the cell into two cells

Binary Fission Steps

• Prior to cell division, the bacterial DNA molecule replicates
• Replication of the double-stranded, circular DNA molecule begins at a specific site called the origin of replication
• Replication enzymes move bidirectionally from the origin, creating copies of each DNA strand
• Enzymes continue until they meet at the terminus of replication
• As DNA replicates, the cell elongates and partitions the DNA, positioning the origins at the 1/4 and 3/4 points in the cell
• Termini face toward the middle
• Septation initiates with new membrane and cell wall material forming a septum at the cell's midpoint
• The protein molecule FtsZ aids in the process
• The septum becomes complete, the cell pinches into two, forming two daughter cells, each with a bacterial DNA molecule

Septation and FtsZ

• Production of the septum separates the cell's components
• Septation begins with the formation of a ring of FtsZ proteins
• Accumulation of other proteins follows
• The structure contracts radially to divide the cell in two
• FtsZ presence is universal in prokaryotes

FtsZ Protein

• Has a structure similar to eukaryotic tubulin
• Cell division in varied organisms involves protein assemblies
• Prokaryotes employ FtsZ protein assemblies
• Eukaryotes utilize microtubule protein assemblies made from tubulin

Eukaryotic Chromosomes

• Chromosome number varies across species
• Humans possess 46 chromosomes in 23 almost identical pairs
• Additional or missing chromosomes are often fatal

Chromosome Composition

• Chromosomes are composed of chromatin, a complex of DNA and protein
• DNA within a single chromosome is a continuous, double-stranded fiber
• RNA associates with chromosomes during RNA synthesis
• A typical human chromosome in a non-dividing nucleus is 140 million nucleotides long
• Heterochromatin is not expressed
• Euchromatin is expressed

Chromosome Structure

• The nucleosome is a complex of DNA and histone proteins
• A DNA duplex coils around eight histone proteins every 200 nucleotides
• Histones are positively charged and strongly attracted to negatively charged phosphate groups of DNA
• Histones facilitate and guide DNA coiling
• Nucleosomes are further organized into higher order coils, called solenoids
• Solenoids lead to a fiber with a diameter of 30 nm
• The usual state of chromatin in nondividing cells (interphase)
• During mitosis, chromatin in the solenoid arrangement is arranged around scaffold proteins to achieve maximum compaction
• Radial looping is further aided by condensin proteins

Level of Eukaryotic Chromosomal Organization

• Mitotic chromosome
• Rosettes of chromatin loops
• Chromatin loop
• Solenoid
• Histone core
• DNA

Karyotype

• A karyotype displays a particular array of chromosomes in an organism
• Human somatic cells are diploid, containing two sets of chromosomes
• These cells contain 46 total chromosomes
• Haploid cells (ex. gametes/sex cells) contain 1 set of chromosomes (n)
• Human gametes contain 23 chromosomes
• Chromosome pairs are homologous
• Each homologue in a pair is a homologue

Chromosome Replication

• Prior to replication, each chromosome consists of a single DNA molecule
• After replication, chromosomes consist of two identical DNA molecules
• Sister chromatids are held together by cohesin proteins
• When visible, chromosomes appear as two strands held together

Eukaryotic Replication

• Eukaryotes require significant adaptations in partitioning replicated genomes into daughter cells
• The cell cycle involves: genome duplication, accurate segregation, and division of cellular components
• It is divided into five phases

Eukaryotic Cell Cycle

• (G1 phase) Primary growth phase, the longest phase
• (S phase) Replication of DNA
• (G2 phase) Organelles replicate, and microtubules organize
• (M phase) Mitosis, which is then subdivided into 5 phases
• (C phase) Cytokinesis, the separation of 2 new cells
• G1, S, and G2 are collectively called Interphase

Duration of the Cell Cycle

• Varies greatly depending on the cell
• Fruit fly embryos take 8 minutes
• Typical mammalian cells divide every 24 hours
• Some liver cells divide once a year
• Growth mainly occurs during G1, G2, and S phases
• M phase takes about one hour
• Most variation in the length of the cell cycle occurs during the resting phase

Interphase

• Cells undergo major portion of growth
• S – replicates DNA
• Chromosomes coil more tightly using motor proteins; centrioles replicate; tubulin synthesis
• Each sister chromatid has a centromere

Interphase structures

• Centrioles replicate only in animal and will form into an Aster
• Chromatin will replicate in this portion

M phase

• Mitosis is divided into 5 phases: prophase, prometaphase, metaphase, anaphase, and telophase

Prophase

• Individual chromosomes condense and become visible under a light microscope
• Condensation continues throughout prophase
• Spindle apparatus assembles
• 2 centrioles will move to opposite poles, forming spindle apparatus (no centrioles in plants)
• Asters – radial array of microtubules in animals (not plants)

Prometaphase

• Nuclear envelope disassembly occurs
• Microtubule attachment occurs
• A 2nd group grows from poles attaching to kinetochores
• Each sister is connected
• Chromosomes begin moving to the center

Metaphase

• Alignment of chromosomes happens along metaphase plate
• This is not an actual structure, but a future axis of cell division
• Chromosomes are aligned at equator of the cell, called the metaphase plate
• Chromosomes are attached to opposite poles and are under tension

Anaphase

• Begins when centromeres split
• Key events: removal of cohesin proteins from all chromosomes
• Sister chromatids are pulled to opposite pole
• Anaphase A – kinetochores are pulled to poles
• Anaphase B – poles

Telophase

• Spindle apparatus disassembles
• Nuclear envelope forms around each set of sister chromatids
• Chromosomes uncoil
• Nucleolus reappears in each new nucleus

Cytokinesis

• Also referred to as cell cleavage into equal halves
• Animal cells constrict in a process of actin filaments producing a cleavage furrow
• Plant cells – cell plate forms between the nuclei
• Fungi and some protists – nuclear membrane does not dissolve; mitosis occurs within the nucleus; division of chromosomes occurs

Cell Cycle Control

• integrates 2 concepts
• Cell cycle has 2 irreversible points
• Replication of genetic material
• Separation of sister chromatids • Cell cycle can be put on hold at specific points called checkpoints • • Processes are checked for accuracy and can be halted if there are errors • Allows cell to respond to internal and external signals

Cell Cycle Checkpoints

• G1/S checkpoint
• Cell “decides” to divide
• Primary point for external signal influence
• G2/M checkpoint
• Cell makes a commitment to mitosis
• Assesses success of DNA replication
• Late metaphase (spindle) checkpoint
• Cell ensures that all chromosomes are attached to the spindle