Genetics: Cell Theory & Cell Cycle

• Genetics studies how genetic information passes from one generation to the next.

Cell Theory

• All living things consist of one or more cells.
• Cells represent the smallest units of living organisms.
• New cells arise exclusively from pre-existing cells through cell division.

Cell Cycle

• A somatic cell is a plant or animal cell within an organism's body.
• Somatic cells go through cell cycles where one cell becomes two.
• The cell cycle duration depends on cell type and organism, lasting 12-24 hours in actively dividing animal cells.
• Somatic cells divide for growth, tissue and organ repair, and maintenance to replace dead cells.
• Checkpoints monitor cell cycle growth, stopping or continuing when needed to prevent uncontrolled growth.

Stages of the Cell Cycle

• The three main stages are interphase, mitosis, and cytokinesis.
• Interphase involves growth and intense cell activity.
• Mitosis involves division of the cell nucleus and genetic material.
• Cytokinesis involves dividing the cytoplasm and creating new cells.

Interphase

• During interphase, the cell functions normally, grows, and replicates its genetic material.
• Interphase is split into Growth 1(G1): the major period of growth, Synthesis(S): DNA (chromatin) is replicated, and Growth 2(G2): further growth and molecular synthesis.
• When DNA is replicated during interphase, the double helix unwinds, and each strand of DNA serves as a template for a new strand.
• Each new double helix contains an original and a new strand.

Mitosis

• Before a cell divides, it undergoes mitosis to separate the replicated genetic material.
• A chromosome is part of mitosis.
• Sister chromatid represents part of mitosis.
• Centromere represents part of mitosis.
• The spindle fiber, a microtubule structure, facilitates chromosome movement within a cell.
• The centrosome helps form the spindle fibers.

Cytokinesis

• In animal cells, a cleavage furrow forms in the cell membrane along the cell's equator, deepening as the cytoplasm divides.
• In plant cells, a cell plate forms between the daughter nuclei because the rigid cell wall does not pinch inward.

Chromosomes

• No relationship exists between the number of chromosomes in an organism's cells and the complexity of the organism.
• Humans have two sets of 23 chromosomes in somatic cells from each parent.
• These sets are homologous, containing the same gene sequence (traits), chromosome length, centromere location, and stain banding pattern, but contain different alleles (gene forms).
• Sex chromosomes determine the sex of the individual.
• Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).
• Non-sex chromosomes (#1-22) are called autosomes, which code for everything else in the body. Each has a true homologous pair.

Karyotypes

• A karyotype is a person's set of chromosomes.
• Chromosomes are collected and stained during metaphase, appearing as sister chromatids (“X's), then subsequently arranged, and numbered by length from longest to shortest.
• Sex chromosomes are placed last in the karyotype.