Mitosis, Binary Fission, and Cell Division

• Mitosis and binary fission involve cell division
• The notes were prepared by Dr. Diepeveen & Mr. Brownlie

Key Definitions

• Allele: A variant of a gene at the same locus, differing slightly in the DNA sequence
• Anaphase: Phase where daughter chromosomes move to opposite ends of the cell
• Cell Cycle: The complete sequence of cell divisions
• Cell Plate: Forms in dividing plant cells and becomes the new cell wall
• Centriole: A rod-shaped body outside the nuclear membrane in resting cells, doubles before mitosis and moves apart to form spindle poles, usually absent in plants.
• Centromere: The waist-like constriction in a chromosome, necessary for chromosome movement during division.
• Chromatid: One of two daughter strands of a duplicated chromosome, joined by a centromere
• Cleavage: the division of the cytoplasm in an animal cell.
• Cleavage Furrow: A shallow ring-like depression that defines where an animal cell cytoplasm will divide during cytokinesis due to contractile microfilaments.
• C Phase: Part of the cell cycle involving the division of cytoplasmic contents via cytokinesis
• Cytokinesis: The process of cell cytoplasm division.
• Diploid (2N): A cell or organism with two copies of each chromosome; represented by 2N
• Disjunction: The separation of homologous chromosomes during anaphase of meiosis
• G0 Phase: when a cell has exited the cell cycle and is not preparing to divide
• G1 Phase: Where there is a period of cell growth
• G2 Phase: The cell prepares to divide
• Haploid: A cell or organism with a genome containing one copy of each chromosome, represented by N
• Heterosome: Non-identical chromosomes that pair up at meiosis, like XY chromosomes
• Homologous Chromosomes: A pair of chromosomes with the same size, shape, and gene locations.
• Interphase: The stage between nuclear divisions.
• Metaphase: Duplicated chromosomes align along the cell's center (metaphase plate).
• Mitosis: Nuclear division to maintain the parental chromosome number for daughter cells, it is the basis for growth and asexual reproduction in eukaryotic species.
• M Phase: Where mitosis occurs in the cell cycle
• Prophase: Chromatin in the nucleus condenses into visible chromosomes
• S Phase: Where DNA replication takes place in the cell cycle
• Telophase: When separated chromosomes reach opposite poles, and two new nuclei form

Cell Division (Mitosis)

• Cells divide for growth, development, and repair
• Mitosis separates replicated chromosomes into two new nuclei
• Mitosis leads to genetically identical cells, maintaining the total chromosome number

Eukaryotic Cell Division

• Involves nuclear division (mitosis or meiosis) and cytoplasmic division (cytokinesis)
• Mitosis occurs in somatic cells, maintaining the parental diploid chromosome number in daughter cells
• Meiosis is the eukaryotic cell division for gamete production in sexually reproducing organisms, featuring two division rounds to yield four haploid daughter cells from a diploid cell

The Cell Cycle

• The cell cycle is the sequence of events from one cell division to the next
• It is continuous and ordered from parent cell formation until its own division
• The duration of the stages vary and some specialized cells don't divide.
• Understanding requires knowing the sequence, names, and purpose of each stage

Interphase Detail

• Genetic material doubles before cell division
• This doubling occurs during interphase

Stages of Mitosis

• Mitosis is a short part of the cell cycle, having four phases: prophase, metaphase, anaphase, and telophase
• Cytokinesis follows mitosis is when the cytoplasm separates
• Mitosis results in two separate, genetically identical, diploid daughter cells.

Prophase Details

• Chromatin threads condense into chromosomes
• Each chromosome consists of two sister chromatids held by a centromere
• The nuclear membrane disintegrates and the nucleolus disappears
• The mitotic spindle forms and spindle fibers attach to each chromosome at its centromere.
• Two centrosomes (with two centrioles each) move towards opposite poles of the cell

Metaphase Details

• Chromosomes move to the cell's center and align along the metaphase plate
• Centromeres of the chromosomes are aligned on the equator
• Centrioles are at opposite cell poles

Anaphase Details

• Spindle microtubules shorten and pull on the centromere to separate sister chromatids
• Spindle microtubules pull sister chromatids to opposite poles.
• The centromere is the chromosome part pulled toward poles
• Each pole should have a complete, identical set of maternal and paternal chromosomes; non-disjunction can occur
• Separated sister chromatids are now referred to as chromosomes

Telophase Details

• Chromosomes decondense to form chromatin, becoming less visible
• Two new nuclear membranes form around the separated chromosomes
• Nucleoli reappear and the spindle apparatus disappears
• The cell elongates, preparing for cytokinesis

Cell Division Summary

• Interphase includes G0, G1, S, and G2 phases and chromosomes are not condensed
• During Prophase, chromatin condenses into visible chromosomes, linked at the centromere with spindle fibers beginning to form at two centrioles
• During Metaphase, chromosomes align along the cell's equator at the metaphase plate
• During Anaphase, Spindle fibers pull apart sister chromatids towards opposite poles
• During Telophase, Separated chromatids, now chromosomes, de-condense as new nuclear membranes form
• Interphase is not considered part of Mitosis or Meiosis, nor is cytokinesis.
• Mitosis proceeds through interphase, prophase, metaphase, anaphase, and telophase.
• The specific times, pauses during each stage, and other meta information are not present.

Microscopy

• Mitosis phases are often easily visible under a microscope

Prokaryotes:

• Lack membrane-bound organelles like a nucleus.
• DNA is single circular chromosome
• The chromosome located within a nucleoid
• Plasmids are small rings of DNA located in cytoplasm

Prokaryote Cell Division

• Prokaryotes use binary fission instead of mitosis, as they lack a nucleus and have single chromosomes without centromeres
• Binary fission is asexual reproduction where a prokaryotic cell divides into two identical daughter cells
• Binary fission includes DNA replication, chromosome segregation, and cytokinesis.
• Binary fission offers limited variation, but happens quicker, resulting in higher mutation chance

Steps of Binary Fission

• Single chromosome coils tightly
• DNA copies and separates.
• Each copy attaches to the cell membrane, pulled apart as the cell elongates.
• The cell wall begins to grow and cleavage furrow develops.
• Cell wall develops fully.
• The cell divides, forming two identical daughter cells, and new copies are tightly coiled again.