Sets of Chromosomes in Human Cells, and Meiosis

Genome

A cells endowment of DNA (Genetic information).

Zygote

Fertilized egg in single cell form. Diploid.

Anaphase 1

Homologous pairs separate, and the Kinetochore pulls the each chromosome (existing as sister chromatids) towards one of the centrosomes.

Genetic Variation

Independent assortment of chromosomes, Crossing over, and Random fertilization.

Metaphase 2

Kinetochores attach each sister chromatid to the microtubules of the spindle. Because of the crossing over, the 2 sister chromatids are NOT genetically identical. The chromosomes line up on the metaphase plate.

Metaphase 1

Kinetochores attach the centromere of each chromosome to the microtubules of the spindle. THE KINETOCHORES ATTACH TO THE CENTROMERE OF ONE CHROMOSOME, NOT TO EACH SISTER CHROMATID. The Tetrads line up on the Metaphase Plate.

Crossing over

Swapping portions of homologous pairs with each other leads to numerous different variations each chromosome can have.

Prophase 2

Spindle forms. Nucleus fragments.

Tetrad

The pair of homologous pairs that forms during Synapsis of Prophase 1.

Synapsis

The pairing of two homologous chromosomes during Prophase 1. Formation of Tetrad.

Meiosis

The process of cell division that a sexually reproducing organism uses to produce gametes. Consists of Meiosis 1 and Meiosis 2. Only diploid cells can undergo Meiosis because Meiosis transforms Diploid cells (2n) to Haploid cells (n).

Random fertilization

Any organism can reproduce with any other organism with 2^23 possible assortments of chromosomes. Ignoring crossing over, there 2^23 * 2^23 or about 70 trillion different diploid combinations.

Haploid

Cell containing one set of chromosomes. For humans, the haploid number (n) is 23: 22 autosomes, and one sex chromosome (Egg contains X, and sperm has either X or Y)

Diploid

Cell containing two sets of chromosomes.

Chromatin

Complex of DNA, proteins, and RNA that exists in eukaryotic cells. The chromatin varies in its condensation during to the process of cell division (Chromosomes).

Chromosomes

Condensed, structured "packages" of DNA that are formed during Cellular division.

Prophase 1

DNA condenses into chromosomes. Spindle forms. Nuclear envelope fragments. HOMOLOGOUS PAIRS ASSOCIATE WITH EACH OTHER, AND CROSSING OVER OCCURS (main difference between Meiosis and Mitosis).

Unequal Cytokinesis

During Oogenesis (female meiosis), Unequal Cytokinesis occurs, to allow for one cell to have enough organelles to support the Zygote because the sperm does not donate any organelles. Only 1 of the 4 cells produced during Oogenesis can become an egg.

Homologous Pairs

Each cell has 2 sets of chromosomes, and each chromosome has a complementary chromosome from the other parent which looks similar in size, shape, and length to it. These pairs are called homologous pairs. Each chromosome in a homologous pair contains the same genes.

Interphase of Meiosis 1

Much the same as Interphase of Mitosis. The cell copies all of its DNA.

Telophase 2 and Cytokinesis

Nuclei form, and cytokinesis occurs. The Meiotic division of one parent cell creates four haploid daughter cells that are ALL genetically different from each other and the parent cell.

Karyotype

Ordered display of all the chromosomes in a cell, lined up so the homologous pairs are adjacent to each other.

Crossing over

Portions of Homologous pairs swap with each other. This is a significant source of genetic variation.

Gametes

Reproductive cells in body. Contain half as many chromosomes as Somatic cells. In humans, Gametes contain 1 set of 23 chromosomes. Haploid.

Autosomes

The 22 chromosomes that are not sex chromosomes

Sex Chromosomes

The 23rd pair of chromosomes are called the sex chromosomes. Females have a homologous pair of X chromosomes (XX), whereas males have one X and one Y chromosome (XY). The Y is a tiny, chromosome, but it has counterparts not found on the X chromosome.

Interphase of Meiosis 2

The cell does not copy its DNA! Meiosis 2 is much like Mitosis (splitting of sister chromatid), but you only have 1 set of chromosomes to split instead of 2 sets

Chiasmata

The locations where crossing over has occurred.

Anaphase 2

The sister chromatids break apart, and the kinetochores pull them to one of the centrosomes.

Telophase 1 and Cytokinesis

When Telophase 1 begins, each daughter nucleus has 1 haploid set of chromosomes existing as sister chromatids. The splitting of the cell into two daughter cells occurs like Mitosis.

Independent assortment of chromosomes

When lining up on the metaphase plate, the tetrad can face towards either of the centrosomes. There are 2 directions each chromosome can line up, and there are 23 chromosomes, so 2^23 = about 8.4 million different assortments of chromosomes. 2^n.