Chapter 7.3: Sexual Reproduction by Meiosis Halves the Number of Chromosomes and Generates Genetic Diversity

Flashcards from Chapter 7.3 of Principles of Life, 3rd Edition.

Meiosis

The type of cell division occurring through two nuclear divisions labeled I and II, together producing four genetically distinguishable haploid gamete cells

• Can take from weeks to years in humans

Meiosis I

The first nuclear division of meiosis, where:

• Entire replicated homologous chromosomes pair with each other along their entire lengths for exchanges (unlike mitosis, where no pairing takes place)

• Homologous chromosome pairs separate and move to opposite poles (unlike mitosis, where only sister chromatids separate)

• Chromosome numbers are halved, as Anaphase I divides genetic material into two nuclei without corresponding homologs

Meiosis II

The second nuclear division of meiosis, where:

• Chromosomes are not replicated, but condense and attach to kinetochores

• Sister chromatids align and separate

• The nuclear envelope forms, and chromosomes uncondense

• Cytokinesis occurs, forming four distinguishable cells

Independent assortment

The independent movement of each of the paternal and maternal chromosomes in a cell for diversity among chromosomes, resulting in:

• A 50% chance that a chromosome will face a particular pole

• A very high chance of new combinations of genes in gametes

• The chance of a gamete having the same chromosomal makeup as one of the parental gametes being (1/2)^(n-1), where n is the number in a haploid set

Crossing over

The exchange of genetic information between non-sister homologous chromatids from prophase I to anaphase I

• Occurs at X-shaped attachment points called chiasmata

Chiasmata

The points on two non-sister homologous chromatids where crossing over occurs; these can appear in different numbers at any point on a chromosome

Recombinant chromatids

Chromatids that are genetically different from the chromosomes present in the parental gametes, formed as a result of crossing over