Biology Meiosis – Flashcards

Diploid cells have a full set of chromosomes - 23 pairs of two chromosomes. (46) Haploid cells have only 23 chromosomes - half of a set. If haploid cells have N chromosomes, then diploid cells have 2N.

Gametes must be haploid because two gametes fuse together to make a diploid cell, a zygote. Since we are fusing two cells together, they must each have half as many chromosomes as the final cell must have.

-*Chromatin condenses into chromosomes* and the nuclear membrane dissolves. -*Homologous chromosomes pair up*; these pairs are referred to as tetrads. -*Crossing-over* occurs between the homologous chromosomes, causing them to swap parts of their DNA. -*Spindle fibers* attach to the tetrads.

*The tetrads line up along the center of the cell* . Each tetrad is *randomly oriented*, so which chromosomes end up in which daughter cells is random.

*The tetrads are pulled apart,* with each chromosome in the tetrad being pulled by the spindle fibers to one or the other end of the cell.

Cytokinesis is occurring, leading to cell division. The nuclear membrane does not reform.

In each daughter cell, the chromosomes - now unpaired - are attached to by the spindle fibers again.

The chromosomes line up in the middle of the cell.

Each of the two chromatids which make up the chromosomes are pulled to one or the other end of the cell, causing the separation of the chromatids.

The nuclear membrane reforms. Cytokinesis is occurring, leading to another cell division.

Two chromosomes are homologous if they have the exact same genes on them. They may have different alleles of those genes, but not different genes. This is important because only homologous chromosomes (or the X and Y chromosomes, with each other) can pair up during prophase I.

A tetrad is a pair of homologous chromosomes. It forms during prophase I.

Crossing over is when two chromosomes "overlap", resulting in the two chromosomes swapping two sections of DNA with each other. These sections always contain the same genes as each other.

Only homologous chromosomes can cross over with each other; thus, in males, the X and Y chromosomes do not cross over as they are not homologous, even though they will pair for the purpose of being split apart evenly.

They are both processes of cell division in which genetic material is split between the two cells being produced. In both, the centrioles extend spindle fibers which attach to the centromeres of the chromosomes, line them up in the middle of the cell, and then split them apart; after this, cytokinesis occurs and the cell divides. However, meiosis is a more complicated process; it has two stages, meiosis I and meiosis II, each of which is a cell division. While meiosis II is the same as mitosis, meiosis I is significantly different. It involves the splitting of homologous pairs of chromosomes, not just sister chromatids, as well as crossing over between the homologous pairs. Finally, the end goal of the two processes are very different: mitosis produces two genetically identical cells, while meiosis produces four genetically different cells.

In the testes for males and the ovaries for females.

a. Spermatogenesis occurs constantly throughout a mature male's lifespan, while oogenesis begins with a set number of eggs at birth, though it is not completed until after the onset of puberty. b. Oogenesis only results in one ovum (egg) and three mostly-useless polar bodies that cannot be fertilized, while spermatogenesis forms four fully functional spermatozoa. c. Spermatogenesis takes only a few hours. Oogenesis is completed once every 28 days.

A karyotype is an organized image of an organism's chromosomes, arranged with the homologous chromosomes in pairs and from the largest to the smallest chromosomes. We can identify some genetic disorders that are caused by an individual having more or fewer chromosomes than normal by producing a karyotype from one of their cells.

An autosome is a chromosome that is not a sex chromosome. In humans, pairs 1-22 are autosomes.

Nondisjunction is when chromosomes or chromatids do not separate properly in anaphase I or II. This can result in gametes containing more or fewer chromosomes than they should, and if those gametes participate in fertilization, the result will be a zygote with more or fewer chromosomes than normal.

An individual with Turner Syndrome has only one X and no Y chromosomes.

An individual with Down Syndrome has three copies of chromosome 21, rather than just two.

An individual with Klinefelter Syndrome is a male with more than one X chromosome - usually just one extra, but rarely, more.

As the name implies, an individual with XYY Syndrome has two Y chromosomes rather than just one.