Chapter 12- Meiosis
Ultimate explanation: Evolutionary. Why something happens
Proximate explanation: Mechanistic. How something happens
Meiosis: gamete formation
12.1 How Does Meiosis Occur?
Homologous chromosomes (homologs): two chromosomes of each type. Similar in size, shape, content
(genes). Carry same genes but may carry different alleles.
Karyotype: number and types of chromosomes present
Diploid: two versions of each type of chromosome. Two alleles of each gene; one on each chromosome.
Haploid: one allele of each gene. One chromosome of each kind. n
Haploid number (n): The number of distinct types of chromosomes in a given cell. Human’s haploid
number is 23
Diploid number (2n): Number of chromosomes present in a diploid cell ex humans have 46
Combination of a number and n is termed the cell’s ploidy.
Polyploid: 3 or more of each type of chromosome
-Unreplicated and replicated chromosomes are each considered single chromosomes—even though the
replicated chromosome comprises two sister chromatids
Unreplicated chromosome: Achromosome that consists of a single copy
Replicated chromosome: has been copied; consists of two linear structures joined at the centromere
Sister chromatids: the chromosome copies in a replicated chromosome
Non-sister chromatids: the chromosome copies in homologous chromosomes
Tetrad: homologous chromosomes that are joined together
-When two gametes fuse during fertilization, a full complement of chromosomes is restored. Resulting
cell is diploid and called zygote. Each diploid receives both a haploid chromosome set from its mother
Life cycle: The sequence of events that occurs over the life span of an individual from fertilization to the
production of offspring.
Chiasma: each crossover forms an x-shaped structure PRIOR TO MEIOSIS: Chromosomes replicate, forming
1.Interphase: Chromosomes replicate in parent cell, in
MEIOSIS 1: Homologs separate
2.Early Prophase 1: Chromosomes condense, 3.Late Prophase 1: Crossing over of non sister
nuclear envelope breaks up, spindle apparatus forms. chromatids (often multiple cross-overs between the
Synapsis of homologous chromosomes. Spindle same chromatids). Mix of chromosome segments
fibers attach to kinetochores at centromeres from maternal and paternal chromosomes
4.Metaphase 1: tetrads migrate to metaphase plate 5.Anaphase I: Homologs separate and begin
and line up moving to opposite sides of cell MEIOSIS 2:
6. Telophase I and cytokinesis: Chromosomes move to opposite sides of cell, then cell
7. Prophase II: Spindle apparatus forms. Nuclear 8. Metaphase II: Chromosomes line up at the middle of
envelope (if formed in Telophase I) breaks apart cell (metaphase plate).
9.Anaphase II: Sister chromatids separate, begin 10. Telophase II and Cytokinesis: Chromosomes
moving to opposite sides of cell move to opposite sides of cell; nuclear envelope forms.
then cell divides. Sister chromatids separate -Each point where crossing over occurs, the non-sister chromatids from ach homolog get physically brken
at the same point and attached to each other,As a result, segments of maternal and paternal chromosomes
Feature Mitosis Meiosis
Number of cell divisions One Two
Number of chromosomes in Same Half
daughter cells compared with
Synapsis of homologs No Yes
Number of crossing over events None One of more per pair of