Bio 1A Lecture 14-17: Cell Cycle and Chromosomes

Nondisjunction

chromosomes fail to seperate

Aneuploidy

zygote has abnormal # of chromosomes (2n-1)

Genomic Imprinting

expressed phenotype difference if traits are from mother vs father

Incomplete dominance

curly+straight=wavy

Codominance

Blood type A + B = AB

Transformation

cells take up external DNA, changing their genetic makeup

Cell cycle

The life of a cell from formation to its division into two progeny cells; includes G₁, S, G₂, and M phases.

G₁ phase

Metabolic activity and growth (“first gap”).

S phase

DNA synthesis plus continued growth (“synthesis”). Doubles amount of DNA

G₂ phase

Metabolic activity, growth, and preparation for cell division (“second gap”).

M phase

Cell division, consisting of mitosis (distribution of chromosomes into two identical nuclei) and cytokinesis (division of cytoplasm).

Mitosis

Distribution of chromosomes into two identical nuclei. Broken into prophase, metaphase, anaphase, and telophase/cytokinesis.

Cytokinesis

Division of the cytoplasm, producing two offspring cells.

Prophase

One of the four mitosis stages; chromosomes condense and spindle forms.

Metaphase

Chromosomes align at metaphase plate.

Anaphase

Replicated chromatids separate.

Checkpoints (Cyclin CDKs)

Internal/external signals regulate the cell cycle at G₁, G₂, and M.

G₁ checkpoint

Most important checkpoint; without a go-ahead signal, cells exit into G₀.

G₀ phase

A non-dividing state a cell enters if it does not receive the G₁ checkpoint signal.

Meiosis

Special type of cell division that reduces chromosome number from diploid (2n) to haploid (n); produces four genetically distinct gametes.

Meiosis I

Separates homologous chromosomes.

Meiosis II

Separates replicated chromatids.

Homologous chromosomes

A pair of chromosomes, one inherited from each parent.

Chiasmata

Sites where crossing over has occurred and where homologs remain attached after synapsis ends.

Crossing over

Exchange of genetic information between maternal and paternal chromatids during Prophase I, generating recombinant chromosomes.

Recombinant chromosomes

Chromosomes containing DNA from both maternal and paternal origins due to crossing over.

Law of Segregation

Maternal and paternal copies of each chromosome separate during meiosis and end up in different gametes at random.

Law of Independent Assortment

Each pair of homologous chromosomes segregates independently of all other pairs.

Random fertilization

Any sperm can fuse with any egg, producing enormous genetic variation.

Independent assortment

Random arrangement of maternal vs. paternal homologs at metaphase I creates many possible gamete combinations (2ⁿ).