Mitosis

I. Organization of Genetic Material

A. Introduction

• Cell theory - all cellular life is made of one or more cells

• Cells are the smallest unit of life 

• All cells come from other cells 

• Some cells are terminally differentiated 

  • Do not divide

  • Ex. Erythrocytes, neurons 

  • Most cells are going to divide at some point 

Cell Division = Cell reproduction

• Unicellular organisms - just reproduction 

• Multicellular organisms

  • Growth and development 

  • Renewal and repair

• During cell division 

  • You must allocate all parts of the existing cell into those two daughter cells

  • Each daughter cell must get a FULL copy 

B. Genetic Material

• Genome - a cells total genetic material 

  • Most of the genetic material is going to be in the nucleus 

  • Prokaryotes - usually 1 circular DNA molecule 

  • eukaryotes - usually >1 linear DNA molecules 

• Chromosome - molecule of DNA in a cell

  • Can be in eukaryotic and prokaryotic cells 

DNA Arrangement

• Nucleosomes: 8 histones with DNA wrapped around

  • Histones - DNA wrapped proteins, proteins that associate with DNA

• DNA is very precisely packaged because the amount of DNA is worth 2 meters of DNA in each cell 

• Adjacent nucleosomes linked by linker DNA

Chromatin

• DNA/protein complex in a dispersed state

• DNA in its lowest density of packaging

• DNA is usually in this state 

• When cell division is done, this is condensed and wrapped around itself to make structures called chromosomes 

• “Colored body” 

• DNA wrapped around proteins, highly organized

• Densely packed chromosomes are only present during cell division 

Genes

• Informational unit of DNA, parts that contain the information to make proteins 

• 100s or 1000s of genes per chromosome

• Each gene is at a specific place on a chromosome - Locus

• Arranged linearly on chromosomes

C. Chromosome Number

• Haploid (n) - having one complete set of chromosomes 

• Haploid cells have 1 of each chromosome 

• Gametes are haploid, some organisms are haploid for most of their lives 

• Diploid (2n) - when you have 2 complete sets of chromosomes in your cells 

  • 2 of same chromosomes - homologous chromosomes/pair 

  • Same length, centromere location, genes 

  • Somatic cells in humans are diploid 

Diploid Numbers 

• Humans 46

• More complex does not mean more chromosomes

• Haploid and diploid cells can undergo mitosis 

II. Phases of the Cell Cycle

A. Introduction

• Cells do not divide continuously 

  • Ex. For eukaryotes - interphase = non-dividing

  • M phase = dividing 

All Domains Divide

• Prokaryotes - binary fission 

B. Events of Interphase

• The time in between cell division where the cells are doing growth and synthesis 

  • Not a resting stage - metabolically active 

• Long - 90% of cell cycle 

• DNA is going to exist as chromatin, more diffused form of DNA 

• 3 phases of interphase

  • 1 G1 (gap 1)

    • Growth and normal development and functions

    • Preparation for S phase

    • No DNA synthesis 

    • Many cells spend most of their lives here 

  • S phase

    • DNA “synthesis” phase

    • Chromosomes duplicated - DNA and chromosomal protein synthesis 

      • Does not change ploidy 

  • G2 (Gap 2)

    • Usually shorter than G1 or S

    • DNA is still chromatin

    • High metabolic activities, preparation for mitosis 

    • Centrosome duplicate

      • Fibers that give the cell the shape get rearranged to help the cell divide 

Chromosomes After Replication

• 2 sister chromatids

  • Exact copies of DNA

  • Connected at the centromere 

    • Proteins attach to the centromere - kinetochore

      • Attach to microtubules

      • Move chromosomes during division 

• DNA is still chromatin at this stage 

C. Events of M Phase

• Shortest part of the cell cycle 

• Mitosis and cytokinesis

Mitosis

• Nuclear division of somatic cells 

  • Nonreproductive cells

• Continuous process

• Divides into 4 stages

  • Prophase, metaphase, anaphase, telophase 

1. Prophase

• 3 big things happen

  • Chromosomes condense

  • Mitotic spindle forms

    • Rearranging the cytoskeleton to make the spindle that sets the orientation of the cell and helps chromosomes move 

    • Made of microtubules 

  • Nucleus breaks down 

    • Allocate the DNA at the front of the process 

    • Disassemble to put them together later 

Mitotic Spindle

• Microtubules responsible for separation of chromosomes

• Microtubules - hollow rods of protein tubulin 

• Mitotic spindle oriented from pole to pole of cell 

2. Metaphase

• Chromosomes align at metaphase plate

  • Not a structure but a region

• Chromosomes get pushed to the center of the cell

• Kinetochores attach to spindle

  • As the spindle fibers orient themselves across the cell it pushes the chromosomes all into line 

3. Anaphase

• Sister chromatids separate, move to opposite poled

• Pulled nu kinetochores

• Separate - enzyme that separates sister chromatids

• After separation, each chromatid considered to be a chromosome 

4. Telophase

• Opposite of prophase

• Chromosomes start to de-cocndense

• Nuclear envelope reforms 

• New nuclei are identical to the parent nucleus of the original cell that divided 

Cytokinesis

• Part of M phase

• Cytoplasmic division -. 2 cells, each with 1 nucleus

• Generally overlaps with telophase 

• Nucleus reverts to interphase condition during/after cytokinesis 

• Animal cells have a cleavage furrow - cell pinches

• Plant cells have a cell late - new cell wall in between