Chapter 5: The mitotic cell cycle

Main stages of the mitotic cell cycle

Interphase (G1, S, G2) and M phase (mitosis + cytokinesis)

Chromosome

Long DNA molecule with associated histones, forming chromatin that condenses during mitosis

Nucleosome

DNA coiled around a histone octamer (H2A, H2B, H3, H4)

Chromatin fibre (30 nm fibre)

Coiled chain of nucleosomes stabilised by H1 histones

Supercoiled chromosome structure

Condensed chromatin loops organised by scaffold proteins and condensins

Homologous chromosomes

Pair of chromosomes with same gene loci, one maternal and one paternal

Sister chromatids

Identical DNA copies produced in S phase, joined at the centromere

Centromere

Region linking sister chromatids and binding spindle microtubules via kinetochores

Kinetochore

Protein complex on the centromere that attaches chromosomes to spindle fibres

Chromatid vs chromosome terminology

Before anaphase: 2 chromatids = 1 chromosome

Telomeres

Repetitive DNA sequences protecting chromosome ends from degradation

Telomere shortening

Gradual loss of repeats with each division, limiting cell lifespan

Telomerase

Enzyme extending telomeres using an RNA template; active in stem cells and cancer cells

Interphase

Period of preparation including growth, DNA replication and synthesis of proteins

G1 phase

Cell growth, metabolic activity, organelle production, protein synthesis

S phase

Semi-conservative DNA replication producing identical chromatids

G2 phase

Final growth, ATP accumulation, synthesis of spindle proteins, DNA error checks

Cell cycle checkpoints

Control points ensuring accuracy before progression to next stage

G1 checkpoint

Assessment of size, nutrients, growth factors and DNA condition

G2 checkpoint

Assessment of successful DNA replication and repair

Metaphase checkpoint

Confirmation of spindle attachment and chromosome alignment at equator

Cyclins

Regulatory proteins controlling cell cycle transitions by activating CDKs

Cyclin-dependent kinases (CDKs)

Enzymes phosphorylating target proteins to drive cell cycle forward

Cyclin pattern

Cyclin levels rise and fall cyclically to regulate stage transitions

Apoptosis

Programmed cell death removing damaged or abnormal cells

Mitosis

Nuclear division producing genetically identical daughter nuclei

Prophase features

Chromosome condensation, nucleolus disappearance, nuclear envelope breakdown, spindle formation

Metaphase features

Chromosomes positioned at equator, kinetochores attached to spindle fibres

Anaphase features

Centromere division and separation of chromatids to opposite poles

Telophase features

Arrival of chromatids at poles, reformation of nuclear envelopes, chromosome uncoiling

Cytokinesis (animal cells)

Cleavage furrow formation via actin-myosin ring constriction

Cytokinesis (plant cells)

Cell plate formation from Golgi vesicles, developing into a new cell wall

Spindle microtubules

Tubulin-based fibres enabling chromosome movement

Asters

Microtubule arrays radiating from centrioles in animal cells

Centrioles (animal cells)

Organisers of spindle formation; absent in plant cells

(plant cells use MTOCs)

Mitotic index

(Number of cells in mitosis ÷ total cells) × 100

Interpretation of high mitotic index

Rapid cell division indicating growth, repair, or malignancy

Cancer (cell cycle perspective)

Uncontrolled cell division due to failure of checkpoints

Proto-oncogenes

Normal genes promoting controlled division; mutation → oncogenes causing excessive division

Tumour suppressor genes

Genes inhibiting division or inducing apoptosis; inactivation removes growth control

p53 gene

Key tumour suppressor activating DNA repair or apoptosis

Benign tumour

Localised mass not invading surrounding tissues

Malignant tumour

Invasive mass capable of metastasis via blood or lymph

Metastasis

Spread of cancer cells to secondary sites

Stem cells and the cell cycle

Retention of long telomeres and active telomerase enabling repeated division

Role of ATP in mitosis

Energy supply for spindle movement, chromosome separation, cytokinesis

Nuclear envelope breakdown

Phosphorylation of nuclear lamins causing fragmentation in early mitosis

Nucleolus behaviour

Disassembly during prophase; reassembly in telophase with return of rRNA transcription

Condensins

Proteins driving chromosome supercoiling during prophase

Cohesins

Proteins holding sister chromatids together until anaphase