Overview
This lecture explains human genetics focusing on chromosome number (ploidy), how it changes during the cell cycle, and the genetic contributions from each parent.
Chromosomes and Ploidy in Humans
- Humans have 23 different chromosomes, numbered from largest (1) to smallest (23).
- Humans are diploid (2n), meaning they have two sets of 23 chromosomes, or 46 total.
- Each parent contributes one set of 23 chromosomes via sperm (father) and egg (mother).
- Fertilization combines both sets, resulting in a diploid zygote (fertilized embryo).
The Cell Cycle and Changes in DNA Content
- After mitosis (M phase), cells start in G1 phase with 2n (two copies of each chromosome).
- G1 is a growth phase with no change in chromosome number.
- The S (synthesis) phase is when each of the 46 chromosomes is replicated, doubling the DNA content to 4n (tetraploid) temporarily.
- G2 is a second growth phase where the DNA content remains at 4n.
- During mitosis, the cell divides, splitting 4n into two 2n daughter cells, each with two copies of every chromosome.
- Ploidy refers to the number of sets of chromosomes and is reset to 2n after mitosis in diploid organisms.
Summary of Ploidy Across the Cell Cycle
- G1 phase: ploidy is set (2n for humans).
- S phase: DNA content doubles (2n to 4n).
- G2 phase: DNA content is maintained at 4n.
- Mitosis: daughter cells return to 2n.
- In meiosis (not detailed here), two divisions reduce DNA content so daughter cells end up with 1n (haploid).
Key Terms & Definitions
- Diploid (2n) — Two sets of chromosomes (one from each parent); 46 total in humans.
- Ploidy — The number of complete sets of chromosomes in a cell.
- Tetraploid (4n) — Four sets of chromosomes; temporary state after DNA replication.
- Haploid (1n) — One set of chromosomes, as in eggs or sperm.
- Cell cycle — Sequence of growth (G1), DNA replication (S), growth (G2), and division (M) phases in a cell.
Action Items / Next Steps
- Draw and bring to class a ploidy chart across the cell cycle for a tetraploid organism.