Chapter 13 - Meiosis

Three events unique to meiosis I that do not occur in mitosis.

1.Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information 2.At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes 3.At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

autosome

22 pairs of chromosomes that don't determine sex

haploid

A gamete (sperm or egg) contains a single set of chromosomes (N)

homologous chromosome

Chromosomes that have the same sequence of genes and same

Describe the 3 ways for genetic variation to occur during meiosis: crossing over, independent assortment, random fertilization

Crossing over occurs during prophase I of meiosis I. It involves the switching of genes between homologues non-sister chromatids which allows the mixture of maternal and paternal genetic material with new, recombinant chromosomes. Independent assortment is the random lining up of the homologues during metaphase I of meiosis I. Between different gametes, there are 2n different possibilities of how the homologues could line up. Random Fertilization which sperm fertlizes which egg which creates even more potential variation in the offspring.

gamete

Genes are passed to the next generation through reproductive cells (sperm and eggs)

diploid

Has 2 sets of chromosomes (2N )

Compare Meiosis II to mitosis.

In Meiosis II gametes (sex cells) replicate so that each cell has HALF the genetic information of the parent cell. In Anaphase of Meiosis II (after the homologous pairs of chromosomes have been pulled apart in Meiosis I) the two sister chromatids for each chromosome are pulled apart by the spindle fibers and become a part of two daughter cells (this process happens simotaniously with the other daughter cell formed by Meiosis I). In Mitosis, the first difference is that the cell only divides once and forms tow IDENTICAL daughter cells (whereas in meiosis 4 different daughter cells are formed)

Contrast meiosis with mitosis in terms of which cells they occur in, how many daughter cells are produced, and the genetic makeup of those cells-are they the same or different from the parent cell?

Meiosis: Occur only in diploid cells; 4 each genetically different from parent cell & each other Mitosis: Occurs in both haploid & diploid cells; 2 each genetically identical to the parent with same number of chromosomes

crossing-over

Process in which homologous chromosomes exchange portions of their chromatids during meiosis

When does crossing-over occur?

Prophase 1

chiasmata

X-shaped regions where crossing over occurred

somatic cell

any cell other than a gamete, have 23 pairs of chromosomes

How does crossing-over contribute to genetic diversity in gametes?

by combining DNA from two parents into a single chromosome

sex chromosome

called X and Y. Human females have a homologous pair of X chromosomes. Human males have one X and one Y chromosomes

Tetrad

group of 4 chomatids; each tetrad usually has 1+ chiasmata x-shaped regions where crossing over occurred

synapsis

homologous chromosomes loosely pair up, aligned gene by gene

karyotype

is an ordered display of the pairs of chromosomes from a cell