Biology: Chap. 8

Meiosis II

• Goal: To produce haploid cells (cells with one set of every chromosome).

• Diploid Cells: Most cells in organisms (e.g., skin, liver, brain) are diploid, containing two sets of each chromosome (one from each parent).

• Meiosis: A process that produces haploid cells for reproductive purposes (gametes).

• Gametes: Eggs and sperm in humans, containing only one of every chromosome.

Meiosis I Recap

• Homologous chromosomes come together and switch information (homologous recombination).

• These pairs then separate and move to opposite poles of the cell.

After Meiosis I

• Two haploid cells are produced.

• Each cell contains one of each chromosome (e.g., black and red for chromosome one, blue and green for chromosome two).

• Different colors represent the sources of the chromosomes (e.g., black from the biological father, red from the biological mother).

• Homologous Recombination:

  • The chromosomes switch bits of information.

  • A bit of the black chromosome is on the red one, and vice versa.

  • The chromosomes are mixed and matched.

• Each cell has one of every chromosome, with each chromosome consisting of two chromatids (due to the S phase).

Meiosis II

• The chromatids separate in a manner similar to mitosis.

Prophase II

• Chromosomes condense and become visible.

• The spindle apparatus begins to grow.

Metaphase II

• The spindle apparatus pushes the chromosomes to the center of the cell.

• Important: These are individual chromosomes, not homologous pairs (as in Meiosis I).

• Chromosomes line up in a single line in the middle of the cell.

Anaphase II

• Microtubules shorten, separating the chromatids.

• The chromatids move to opposite sides of the cell.

• Each side of the cell receives one chromatid from each chromosome.

• Key Difference from Mitosis:

  • In mitosis, chromatids are genetically identical.

  • In meiosis, chromatids are genetically distinct due to homologous recombination.

Genetic Distinction

• One chromatid might have all paternal material; the other has a mix of paternal and maternal material.

• The direction in which each chromatid goes is random.

• There is no control over which side of the cell each chromatid goes to.

Telophase

• Cells begin to split.

• The nucleus begins to reform.

• Four haploid cells are produced at the end.

Gametogenesis

• Yeast/Mushroom Example: The four haploid cells would be the gametes.

• Humans: There is a biological sex difference in how this process concludes.

Biological Females

• During cell division, one cell gets the majority of the cellular material; the other three get very little.

• One cell is selected to grow, while the other three are destroyed.

• The selection of which cell grows is random.

• The cell that survives is called the oocyte or egg.

• This process occurs approximately once per menstrual cycle.

Biological Males

• All four cells get an equal amount of material.

• The cells are all the same size.

• All four cells are typically viable.

• Millions of these cells are made per day.

• These cells are very small and are called sperm.

Male vs. Female (Biologically)

• Female: Fewer, larger gametes (eggs).

• Male: Many, small gametes (sperm).

• This distinction applies to all organisms with distinct sexes.

Plant Example

• Male plants produce a lot of very small gametes (pollen).

• Female plants produce fewer, larger gametes.

Sex Determination Systems (To be discussed next lecture)

Eukaryotic Chromosomes

• Structure:

• Chromatin: DNA wrapped around histone proteins; relaxed form of genetic material.

• Chromatids: When a chromosome replicates, the two identical copies are called sister chromatids, connected at the centromere.

• Centromere: The central part that holds sister chromatids together; important during cell division.

• Telomeres: Protective ends of chromosomes; prevent deterioration or fusion with other chromosomes.

• Gene: A segment of DNA coding for a protein or function.

• Homologous Pairs: In diploid organisms, chromosomes exist in pairs—one from each parent.

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The Cell Cycle

Phases:

1. Interphase (90% of the cycle)

• G1 (Gap 1): Cell grows and performs normal functions.

• S (Synthesis): DNA is replicated; chromosomes become two sister chromatids.

• G2 (Gap 2): Cell prepares for mitosis by making proteins and organelles.

2. M Phase (Mitosis + Cytokinesis)

• Mitosis:

1. Prophase: Chromatin condenses into visible chromosomes; nuclear envelope dissolves; spindle fibers form.

2. Prometaphase: Nuclei starts to disappear and microtubles attach to centromeres of each chromosome.

3. Metaphase: Chromosomes line up at the center (metaphase plate).

4. Anaphase: Sister chromatids separate and are pulled to opposite poles.

5. Telophase: Nuclear envelope reforms; chromosomes de-condense.

• Cytokinesis: Division of cytoplasm; two daughter cells form.

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Other Key Terms

• Diploid (2n): A cell with two sets of chromosomes (e.g., human body cells).

• Restriction Point (R): A checkpoint in G1; once passed, the cell is committed to the cell cycle.

• G0 Phase: A resting/non-dividing phase. Some cells (like neurons) permanently stay in G0.

• Odd-shaped or damaged cells go to G0 and typically cannot re-enter the cycle.

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Cell Division in Different Organisms

• Unicellular Organisms: Use cell division (usually mitosis or binary fission) to reproduce.

• Multicellular Organisms: Use cell division for growth, repair, and development.

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Genetic Disorders from Chromosomal Abnormalities

• Down Syndrome: Trisomy 21; extra chromosome 21.

• Turner Syndrome: XO genotype; missing one X chromosome (females).

• Klinefelter Syndrome: XXY genotype; extra X chromosome (males).

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Cell Division and Cancer

• Cancer: Uncontrolled cell division due to failure in regulatory mechanisms.

Important Concepts:

• Contact Inhibition: Normal cells stop dividing when they touch neighbors; cancer cells ignore this.

• Tumor Suppressor Genes: Genes that slow cell division, repair DNA, or initiate apoptosis (e.g., p53).

• p53 (TP53 gene): Triggers apoptosis if DNA damage is detected; mutations in this gene are common in cancers.

• Proto-oncogenes: Normal genes that help cells grow. If mutated, they become oncogenes, causing cancer.

• Oncogenes: Mutated proto-oncogenes that promote uncontrolled growth.

• Growth Factor Receptors: Proteins on cell surfaces that receive signals to divide; can be overactive in cancer cells.

• Apoptosis: Programmed cell death; removes damaged or unnecessary cells.

• Senescence: A state where cells are alive but no longer divide; a defense against cancer.