cell cycle

Cell Cycle

Repeated pattern of cell growth, DNA replication, and division in eukaryotic cells

Purpose of Cell Cycle

Growth, tissue repair, development, and production of new cells

Interphase

Longest stage of the cell cycle where growth and DNA replication occur

Interphase Stages

G1, S, and G2

G1 Phase

Cell growth and normal cellular functions

S Phase

DNA replication occurs creating duplicated chromosomes

G2 Phase

Additional growth and preparation for mitosis

M Phase

Includes mitosis and cytokinesis

Mitosis

Division of the nucleus producing genetically identical nuclei

Cytokinesis

Division of cytoplasm producing two daughter cells

G0 Phase

Non-dividing state where cells perform normal functions

Cells Commonly in G0

Mature nerve and muscle cells

Cells That Can Re-enter Cell Cycle

Liver cells

Purpose of Mitosis

Produce two genetically identical daughter cells

Chromatin

DNA associated with proteins in uncondensed form

Chromosome

Condensed structure containing DNA

Chromatid

One copy of a duplicated chromosome

Sister Chromatids

Identical chromosome copies joined at the centromere

Centromere

Region where sister chromatids attach and spindle fibers connect

Telomeres

Protective chromosome ends that do not contain genes

Four Phases of Mitosis

Prophase, metaphase, anaphase, telophase

Prophase

Chromosomes condense, nuclear envelope disappears, spindle forms

Metaphase

Chromosomes align at center of cell

Anaphase

Sister chromatids separate and move to opposite poles

Telophase

Nuclear envelopes reform and chromosomes uncoil

Order of Mitosis

PMAT (Prophase → Metaphase → Anaphase → Telophase)

Animal Cell Cytokinesis

Cell membrane pinches inward forming cleavage furrow

Plant Cell Cytokinesis

Cell plate forms between daughter cells

Checkpoint

Control point where cell cycle progression is assessed

Three Major Cell Cycle Checkpoints

G1/S, G2/M, and spindle checkpoint

G1/S Checkpoint

Determines whether cell enters DNA replication

Most Important Checkpoint

G1/S checkpoint (restriction point)

G2/M Checkpoint

Verifies DNA replication is complete and accurate

Spindle Checkpoint

Confirms chromosomes are attached before separation

Internal Signals for Cell Division

Cell size and nutritional status

External Signals for Cell Division

Growth factors

Growth Factors

Protein signals that stimulate cell division

Density-Dependent Inhibition

Cells stop dividing when crowded

Anchorage Dependence

Cells divide only when attached to a surface

Cyclins

Regulatory proteins whose levels rise and fall during cell cycle

CDKs (Cyclin-Dependent Kinases)

Enzymes activated by cyclins that phosphorylate proteins

Cyclin-CDK Complex

Drives progression through stages of cell cycle

Phosphorylation in Cell Cycle

Activates or inactivates proteins controlling division

MPF (Mitosis Promoting Factor)

Cyclin-CDK complex that triggers mitosis

APC (Anaphase Promoting Complex)

Protein complex that promotes progression through anaphase

Cancer Connection to Cell Cycle

Loss of checkpoint regulation causes uncontrolled division

Karyotype

Display of chromosome number and appearance

Somatic Cells

Body cells that are diploid

Diploid (2n)

Cell containing two sets of chromosomes

Human Diploid Number

46 chromosomes (23 pairs)

Autosomes

Non-sex chromosomes

Gametes

Sex cells (egg and sperm)

Haploid (n)

Cell containing one set of chromosomes

Human Haploid Number

23 chromosomes

Meiosis

Cell division producing haploid gametes

Purpose of Meiosis

Reduce chromosome number and increase genetic variation

Meiosis I

Homologous chromosomes separate

Meiosis II

Sister chromatids separate

Meiosis I vs Mitosis

Meiosis I separates homologous chromosomes while mitosis separates sister chromatids

Prophase I

Homologous chromosomes pair and crossing over occurs

Synapsis

Pairing of homologous chromosomes during meiosis I

Tetrad

Group of homologous chromosomes containing four chromatids

Crossing Over

Exchange of DNA between homologous chromosomes creating variation

Independent Assortment

Random orientation of homologous chromosomes producing variation

Metaphase I

Tetrads align in middle of cell

Anaphase I

Homologous chromosomes separate

Telophase I

Two haploid cells form

Meiosis II

Similar to mitosis; separates sister chromatids

Final Product of Meiosis

Four genetically unique haploid cells

Spermatogenesis

Formation of sperm producing four functional gametes

Oogenesis

Formation of eggs producing one ovum and polar bodies

Fertilization

Fusion of two haploid gametes producing diploid zygote

Main Sources of Genetic Variation

Crossing over and independent assortment