Mitosis and Meiosis Flashcards

Mitosis vs. Meiosis

• Formiosis mentioned (likely a typo, should be "mitosis").

• Two rounds mentioned, but the second one in meiosis lacks DNA replication (no chromosome duplication).

Duplication of Chromosomes

• Refers to the number of complete interphase stages.

• One complete interphase at the beginning of mitosis and one at the beginning of meiosis.

• Intermediary interphase in meiosis doesn't involve chromosome duplication to allow for reductive division.

Number of Cells

• Mitosis: One cell to two daughter cells.

• Meiosis: One cell to two, then each of those to two again, resulting in four cells.

Chromosome Number

• Human parent cell: 46 chromosomes (regardless of mitosis or meiosis).

• Mitosis end: 46 chromosomes (identical).

• Meiosis end: 23 chromosomes (reductive division).

• Egg (23) + sperm (23) = zygote (46).

Daughter Cells

• Mitosis: Identical and diploid.

• Meiosis: Genetically different and haploid.

Function

• Mitosis (multicellular organisms): Growth, maintenance, and repair.

• Mitosis (unicellular organisms): Introduce genetic variation to increase survival chances under environmental stress.

• Goal of the second round of meiosis: To make more cells.

• Connects to Darwin's concept of overproduction.

Visual Comparison of Mitosis and Meiosis

• Diagrams often emphasize meiosis I due to synapsis, tetrad formation, and crossing over.

• Meiosis II is often depicted briefly because it resembles mitosis.

Genetic Recombination

Independent Assortment

• Occurs in prophase I.

• Homologous chromosomes pair randomly.

• Orientation of maternal and paternal chromosomes is random.

• Arrangement of recombinant and non-recombinant chromatids varies.

• Analogy: Two humans in a bed with various orientations.

• Independent assortment is necessary for proper crossing over.

• Humans have 23 pairs of homologous chromosomes.

• Number of possible arrangements: 2^{23} = 8,388,608 (over 8 million) options.

• More chromosomes lead to more variability.

Number of Chromosomes vs. Complexity

• Number of chromosomes doesn't directly equate to organism complexity.

• Fewer chromosomes mean less independent assortment.

• Example: Ants with two chromosomes (2^2 = 4) have limited genetic variation.

Crossing Over

• Occurs after independent assortment.

• Internal sister chromatids swap genetic material.

• Results in chromosomes with a mix of maternal and paternal genes.

• Leads to offspring being a combination of both parents.

Trait Dominance

• Some traits are more dominant and can mask other traits.

• Example: Height, where one parent's genes might be more influential.

• Dominant traits only require one allele to be expressed, while recessive traits require two.

• Eye color is controlled by multiple alleles and has a hierarchy of dominance.

• Polydactyly (extra digits) can be a dominant trait.

Gene Activation

• Not all genes are active; some are turned on only at certain times (e.g., hormonal changes).

• Environmental factors can influence gene activation.

• Much of the genetic code may not be actively expressed.

Nondisjunction

• Uneven split of chromosomes during meiosis (anaphase I or anaphase II), not mitosis.

• Can lead to trisomy (extra chromosome) or monosomy (missing chromosome).

• Examples: Down syndrome (trisomy 21), Klinefelter's syndrome.

Maternal Age and Nondisjunction

• Females are born with all their follicles, which age over time.

• Older follicles have stickier centromeres, leading to increased nondisjunction.

• Older women have less frequent cell replication and more nondisjunction.

• Risk of Down syndrome increases with maternal age.

• Amniocentesis can detect chromosome abnormalities.

• Freezing eggs at a younger age can preserve egg quality.

Gametogenesis (Spermatogenesis and Oogenesis)

To be covered in detail in the next unit.

• Sperm + ovum = zygote (46 chromosomes).

• After fertilization, mitosis leads to growth, maintenance, and repair.

• Zygote develops into an embryo through mitotic divisions (46 chromosomes in each cell).

Post-Fertilization Development

• Zygote undergoes cleavage, blastocyst formation, gastrulation, and neurulation.

• These stages occur before implantation in the uterus (within the first 14 days).

Chromosome Count Table

• Example table to illustrate chromosome count in mitosis and meiosis using the same parent cell to be created.