SB 2: Chromosomes & Inheritance

Normal human chromosomes

• Each human cell carries 23 pairs of chromosomes in the nuclei (46 total)

• Within each pair, chromosomes match

• Exception of male sex chromosomes (XY)

Scientists studying fertilization

• Found that maternal and paternal gamete (cells) equally contribute egg and sperm nuclei

• Implies something in the nucleus contains hereditary material

• More studies revealed smaller, discrete, colored bodies called chromosomes

Mitosis vs. Meiosis

• Mitosis results in two daughter cells that are identical to the parent, somatic

• Meiosis allows for the chromosomes composing each pair to segregate, so the resulting gamete receives only one chromosome from each pair

• Generates egg/sperm cells containing half the number of chromosomes (haploid)

Fertilization

• Chromosomes in a fertilized egg consist of two matching sets, one by maternal gamete, one by paternal

• Maternal and paternal chromosomes form pairs, alike in size and shape

• These cells are diploid (two matching sets)

Chromatin

• A mass of extremely fine tangled string that resembles the chromosome

• Surrounded by the nuclear envelope

• Each chromatin thread is composed of DNA (genetic info) and protein (package and manage info)

Interphase

• Gap 1 (G1) - cell achieves most of its growth, period from birth of a new cell to onset of chromosome replication

• Synthesis (S) - DNA synthesis and chromosome duplication

• Gap 2 (G2) - Cell grows, synthesizes proteins needed for mitosis

Prophase

• Mass of chromatin condenses into discrete, individual chromosomes

• Each condensed chromosome has been duplicated during interphase, so it consists of sister chromatids attached at the centromere (X shape)

• Dark-stained nucleoli breaks down, ceasing the manufacture of ribosomes to focus energy on cell division

Prophase outside the nucleus, in the cytoplasm

• Microtubule network from interphase gets replaced with new set of microtubules that are more dynamic (grow outward and shrink back from centrosomes)

• Duplicated (2) centrosomes from earlier travel outside the nucleus to opposite ends of the cell

• Separation driven by the microtubules within each centrosome interacting and generating force to push apart

Prometaphase

• Breakdown of nuclear envelope, allowing microtubules extending from centrosomes to invade nucleus

• Chromosomes attach to microtubules using kinetochore (structure in centromere region of chromatid)

Metaphase & Anaphase

• Metaphase plate

• Anaphase, separation

• Telophase, prophase in reverse

• Cytokinesis

Regulatory checkpoints

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