Cell Cycle and DNA Packaging Study Notes

DNA Packaging and Chromatin

  • Chromatin: A complex of DNA and special proteins called histones that act as spools to condense and protect the DNA molecule.

  • Euchromatin: Loose, lightly packed chromatin rich in genes; represents the active portion of the genome used for gene expression.

  • Heterochromatin: Tightly packed chromatin where genes are silenced and inactive.

  • Chromosomes: Condensed forms of chromatin that appear when a cell is ready to divide. Humans possess 23 different types (one set from each parent, totaling 46).

Genomic Terminology

  • Genome: The total of all DNA across all 46 chromosomes in a cell.

  • Gene: A specific DNA sequence on a chromosome coding for one polypeptide.

  • Allele: Alternative versions of a gene that may produce differences in appearance or behavior (e.g., hair color).

  • Homologous Chromosomes: Pairs of chromosomes that possess the same genes at the same loci but may have different alleles.

  • Autosomes: Chromosome pairs 1 through 22.

  • Sex Chromosomes: The 23rd pair (XXXX or XYXY); these are only homologous in individuals with two XX chromosomes.

Ploidy and Cell Types

  • Haploid (nn): Cells containing one set of chromosomes (n=23n = 23 in humans), specifically gametes (sperm and egg).

  • Diploid (2n2n): Cells containing two sets of chromosomes (2n=462n = 46 in humans), formed by the union of gametes into a zygote.

  • Somatic Cells: Non-reproductive body cells that divide via mitosis to produce identical cells.

  • Germ line Cells: Cells that produce reproductive gametes via meiosis, reducing the chromosome number by half.

The Cell Cycle and Interphase Stages

  • Interphase (≈90%≈ 90\% of the cell cycle):

    • G1 (Growth 1): Period of active growth, organelle synthesis, and gene expression.

    • S (Synthesis): DNA replication occurs. Before this phase, there are 46 chromosomes and 0 chromatids. After this phase, there are 46 chromosomes consisting of 92 sister chromatids (46 pairs).

    • G2 (Growth 2): Final growth and preparation for division. Centrosomes duplicate and spindle fibers (microtubules) begin to form.

  • Mitotic Phase (≈10%≈ 10\% of the cell cycle):

    • Mitosis: Distribution of genetic material.

    • Cytokinesis: Division of the cytoplasm to form two new daughter cells.