Biology: Chap. 8 Mitosis and Meiosis

Mutations and Recombination

• Random mutations occur, some favorable in certain environments, driving evolution.
• However, mutations are generally bad for individuals, causing genetic diseases.
• Recombination (sex) is the primary adaptation to deal with DNA mutation.

Sexual Reproduction

• Most organisms we're used to have biological sex (male and female).
• Diploid organisms (2n) have two sets of chromosomes.
• They produce haploid gametes (n) with one set of chromosomes through meiosis. Gametes are the eggs and sperm.
• Gametes from two individuals fuse to form a diploid zygote (2n), which develops into a new organism through mitosis.

Evolutionary Question: Why Sex?

• Why not parthenogenesis: females cloning themselves to produce diploid offspring directly?
• Cloning would seem to offer a significant evolutionary advantage (more copies of oneself in the next generation).

Modeling Reproduction

• Sexual Reproduction:
  • A rat reproduces sexually, and two pups survive.
  • Each pup has 50% of the mother's DNA.
  • Grandpups have, on average, only 25% of the original mama rat's DNA.
• Parthenogenesis (Cloning):
  • A rat with a mutation reproduces parthenogenetically, cloning herself every three weeks.
  • All offspring and grand offspring are exact genetic copies of the original mama rat (100% of her DNA).
  • Cloning rat’s fitness (amount of its DNA in the next generation) is orders of magnitude higher than rat sex.

The Problem with Cloning: Mutations

• Cloning leads to a rapid accumulation of deleterious mutations.
• The reason all of this (sexual reproduction) evolved is not to make more diversity in the population.

Incest as an Analogy

• To maximize DNA similarity to offspring, one could reproduce with a sibling (incest).
• However, this is avoided due to the increased risk of passing on shared mutations.
• Reproducing with unrelated individuals allows offspring to "rescue themselves" from mutations, as each parent carries different mutations.

Hybrid Vigor

• Hybrid vigor - crossing distantly related individuals often results in healthier offspring.
• Purebred animals, due to inbreeding, often suffer from genetic problems caused by the accumulation of mutations.

Somatic Cells vs. Gametes

• Somatic Cells: Most body cells (e.g., liver, lung, brain cells).
  • Diploid (2n) - in humans, 23 pairs of chromosomes (46 total).
• Gametes: Reproductive cells (eggs and sperm).
  • Haploid (n) - in humans, 23 single chromosomes.

Meiosis: Generating Haploid Gametes

• Meiosis is a two-round division process (meiosis I and meiosis II) that produces four haploid cells from one diploid cell.

Stages of Meiosis

• Simpler case: cell with only one pair of chromosomes (one black, one white).
• More complex case: a total of four chromosomes versus two. Let's say there's the chromosomal pair one, and the chromosomal pair two.
• A cell with two pairs of chromosomes: chromosome pair one (black and red), and chromosome pair two (blue and green). Important to realize that these chromosomes will have the same genes.
• The cell is diploid because it has two of each chromosomes.

Prophase I

• Chromosomes condense, and the spindle apparatus forms.
• Homologous chromosomes (e.g., the two copies of chromosome one) find each other and attach.
• These clusters of homologous chromosomes are called tetrads (four chromatids).
• Homologous Recombination (Crossing Over):
  • Information is exchanged between homologous chromosomes.
  • Breaks occur in the DNA, and the arms of the chromosomes switch places.
  • Results in chromosomes that are a mixture of genetic material from both parents.

Metaphase I

• The nuclear membrane disappears.
• The spindle apparatus moves the tetrads to the center of the cell.

Anaphase I

• Microtubules shorten, separating the individual chromosomes (each still with two chromatids) to opposite poles of the cell.
• Crucially, the chromatids do not separate at this stage.
• Chromosome segregation is random.

Telophase I

• The cell divides into two daughter cells.
• The nuclear membrane begins to reform.
• Each daughter cell is now haploid, containing one copy of each chromosome (but each chromosome still consists of two chromatids).

Independent Assortment

• Which instance of which chromosome goes to each side of the cell is completely random.
• Genetically, this is called independent assortment.
• The only rule is every cell has to get on of every chromosome.