Meiosis
Reproduction
 Asexual reproduction – one parents passes all of its genetic information to its offspring (ex: hydra)
o Variation would only occur through mutations
 Sexual reproduction – two parents that produce offspring that have a combination of the two parents’ genes
o Fertilization – the fusion of gametes (sperm & egg)
o Zygote – fertilized egg
 Karyotype – a micrograph of the 46 chromosomes paired up together
 Diploid cells – contain two sets of chromosomes (1 from mother, 1 from father)
 Haploid cells – contain one set of chromosomes
 Gametes are produced by meiosis (if through mitosis they would have 2x the number of chromosomes)
 Sources of variation
o Independent assortment – chromosomes line up on the metaphase plate during metaphase I independently
of each other
o Crossing over – one chromatid from a homologous pair switches DNA with another chromatid from the
other homologous pair
o Random fertilization – the possibility of any egg being fertilized by any sperm
Chromosomes
 Homologous chromosomes
o Chromosomes that are identical in length and the type of information they code for
o Sex chromosomes X and Y are not entirely homologous
 Chromosomes vary by:
o Size
o Position of centromere
 Gene locus – the specific location of a gene along the length of a chromosome
 Autosomal chromosomes – chromosomes other than the sex chromosomes
 Sex chromosomes – either an X or Y; determines gender
 Chromosome abbreviations (2n = 46 means the diploid number of chromosomes is 46)
Meiosis I
 Interphase
o Each chromosome replicates; produces sister chromatids joined at the centromere
o Two centrosomes (with centrioles) on the outside of the nuclear membrane
 Prophase I
o Chromatin condenses into chromosomes
o Homologous chromosomes pair up
o Synapsis occurs – the homologous chromosomes are tightly attached together, forming a tetrad
o Chromosomes cross each other at the chiasmata
o Crossing over occurs
o Nuclear envelope disappears
 Metaphase I
o Tetrads line up on the metaphase plate
o Independent assortment occurs
o Microtubules attach to kinetochores of homologous pairs
 Anaphase I
o Homologous pairs separate
 Telophase I/Cytokinesis
o Cleave furrow separates one cell into two daughter cells
Meiosis II
 Prophase II
o Centrosomes replicate
o Spindle begins to form
 Metaphase II
o Sister chromatids line up on metaphase plate
o Microtubules attach to kinetochores of sister chromatids
 Anaphase II
o Sister chromatids separate
 Telophase II/Cytokinesis
o Cleavage furrow separates one cell into two daughter cells
 In the end of both meiosis I and II, there is a total of 4 daughter cells with 23 chromosomes
o Nuclear envelope reforms; chromosomes uncoil to form chromatin
Gamete Formation
 Spermatogenesis
o Production of mature sperm cells
o Sperm structure:
 Haploid nucleus
 Tipped with an acrosome
 Contains enzymes that
help the sperm penetrate
the egg
 A large number of mitochondria
provide ATP to power the
flagellum
 Oogenesis
o Development of ova
o Cytokinesis is unequal
 Cytoplasm monopolized by a
single daughter cell
 Results in only 1 viable egg
o Has long resting periods
Nondisjunction
 Failure of homologous chromosomes to separate during
anaphase I
 Failure of sister chromatids to separate during anaphase II
 Aneuploidy
o Condition in which one or more chromosomes are
either lacking or present in excess
o Trisomy - extra chromosome (3)
o Monosomy - missing a chromosome (1)
o Polyploidy – having more than two sets of
chromosomes (common in plants)
Chromosomal Disorders
 Trisomy 21: Down Syndrome (47, XX, +21)
o Symptoms:
 Impaired intelligence
 Abnormalities of the tongue, hands, eyelids
 Increased susceptibility to cardiac abnormalities
o The frequency of down syndrome correlates with the age of the mother
 Triplo-X (47, XXX)
o 2 X’s are inactivated
o Individuals nearly ‘normal’
o Some mental retardation and reduced fertility is possible
 Klinefelter Syndrome (47, XXY)
o Phenotypically male
o Small testis, enlarge breasts, long limbs, underdeveloped body hair
 Turner Syndrome (45, X)
o Only viable human Monosomy
o Phenotypically female
o Rudimentary ovaries
o Often sterile
o Short, webbed necks, hearing deficiencies, cardiovascular abnormalities
o Some individuals are somatic mosaics
 Fragile X Syndrome
o The end of the X chromosome holds on to the rest of the chromosome by a thin strand of DNA
Meiosis in Sordaria
 The frequency of crossing over is directly related to the distance of the gene from the centromere
 Genes that are further away from the centromere are more likely to be crossed-over
 Map unit – arbitrary unit of relative distance between linked genes that represents the frequency of recombinants
o 1 map unit = 1% recombination


Distance between spore color gene and centromere =
Meiosis in Sordaria

Ascus with no

Ascus with crossing over
(# 𝑜𝑓 𝑅𝑒𝑐𝑜𝑚𝑏𝑖𝑛𝑎𝑛𝑡 𝐻𝑦𝑏𝑟𝑖𝑑 𝐴𝑠𝑐𝑖) 𝑥 100
(# 𝑜𝑓 𝑇𝑜𝑡𝑎𝑙 𝐻𝑦𝑏𝑟𝑖𝑑 𝐴𝑠𝑐𝑖)
2
crossing over
Summary of Mitosis and Meiosis
 Mitosis
o Asexual reproduction
o 2 daughter cells
o Daughter cells genetically identical to
parents
o Daughter cells genetically identical to
each other
o 46 chromosomes
o 1 DNA replication
o 1 division
o Function:
 Growth
 Repair
 Replacement

Meiosis
o Sexual reproduction
o 4 daughter cells
o Daughter cells genetically different
from parents
o Daughter cells genetically different
from each other
o 23 chromosomes
o 1 DNA replication
o 2 divisions
o Function:
 Produce gametes for sexual
reproduction
Calculating the amount of time spent in each stage of mitosis
(# 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 𝑖𝑛 𝑠𝑡𝑎𝑔𝑒)

𝑥 24 ℎ𝑜𝑢𝑟𝑠
(# 𝑜𝑓 𝑇𝑜𝑡𝑎𝑙 𝐶𝑒𝑙𝑙𝑠)

General estimation of time spent in each cell division stage: