Mitosis and Meiosis

     The pachytene. During the pachytene does the pairing stabilize. The number of synaptic complexes corresponds to the number of chromosomes in a haploid set of the respective species. The pairs are also called bivalents. The

Exercise 3A is a study of mitosis. You will simulate the stages of mitosis by using chromosome models.You will use prepared slides of onion root tips to study plant mitosis and to calculate the relative duration of the phases of mitosis in the meristem of root tissue. Prepared slides of the whitefish blastula will be used to study mitosis in animal cells and to compare animal mitosis and plant mitosis.

Meiosis consists of one DNA replication and two nuclear divisions resulting in 4 daughter cells. The process which provides for genetic variation is crossing over. Crossing over occurs in the early stages when homologous chromosomes move together so that their chromatids form a tetrad. This is called synapsis and allows for the exchange of chromosome sections.

These bivalents line up along the equator during metaphase I, the arrangement of the bivalent is completely random and relative to the orientation of the other bivalents, this is known as the independent assortment of chromosomes. This is followed by anaphase I where the homologous chromosomes separate and move to the opposite poles of the cell. At telophase I the cell divides into two, each cell contains one chromosome from each homologous pair. The second stage of meiosis is similar to mitosis.

Mitosis and meiosis are similar in several ways and different in others. The similarities include that both processes involve IPMAT. IPMAT is interphase, prophase, metaphase, anaphase and telophase. The parent cells are diploid. They both end with cytokinesis. In Metaphase and Metaphase II, the sister chromatids line up along the center. Then in Anaphase and Anaphase II, these chromatids are split and pulled towards the centrosomes. The differences are that mitosis consists of 1 division while meiosis consists of 2. Four genetically different, haploid sex cells are the products of meiosis and 2 identical, diploid somatic cells from mitosis. Mitosis occurs in all organisms except viruses and meiosis only occurs in plants, animals, and fungi.

1 by crossing over in prophase I , 2 , by indpendenent assortment and three by mutations in S phase

A) The genetic material in one eukaryotic cell is copied and distributed to two identical daughter cells.

A) Meiosis consists of two cell divisions and is broken up into Meiosis I and Meiosis II. At the beginning of the Cell Cycle, in this case there are four chromatids each from the homologous pairs being A, a, B, b. This is the Diploid number (4) meaning it is 2 times the haploid number that will be seen at the end of meiosis II. During the S phase of interphase, the chromatids replicate and reach the end of G2 phase. Now starting meiosis, during the first stage of prophase I the chromosomes condense and pair up through synapsis with their sister chromatids creating AA, aa, BB, bb. After they pair up they go through a process called crossing over, where the homologous chromosomes share a piece of their genetic material with each other. Crossing over allows for the genetic diversity of chromosomes. Now there are four homologous chromosomes Aa, Aa, Bb, Bb, each containing heterozygous alleles because the sister chromatids exchanged genetic information with their homologous pair. During late prophase I, spindle fibers being to form where they will later attach to a homologous chromosomes centromere. The next stage is Metaphase I. During metaphase I, the homologous pairs line up at the metaphase plate, also known to be the center of the cell. The homologous pairs form a tetrad which is considered a group of four homologous chromosomes. These homologous chromosomes orient themselves randomly, which is know as the process of independent

Anaphase – the chromosomes are divided into single from pair and the chromosomes move to opposite poles

For a diploid cell, this new species would have 26 chromosomes in it. A pair of each autosome (24) as well as two sex chromosomes (2).

During interphase (G1 + S + G2), chromosomes are fully or partially decondensed, in the form of chromatin, which consists of DNA wound around histone proteins (nucleosomes).In G1, each chromosome is a single chromatid. More organelles are produced and the volume of the cytoplasm increases with more protein synthesis. If the cell is not to divide again, it will enter G0. In synthesis (S), the cell duplicates its DNA. In G2, each chromosome consists of a pair of identical sister chromatids, where each chromatid contains a linear DNA molecule that is identical to the joined sister. The sister chromatids are joined at their centromeres. A pair of sister chromatids is a single replicated chromosome, a single package of hereditary information. The cell is now ready for the mitosis

The interphases of meiosis are similar to the interphase of mitosis. G phases are times of cell growth and preparation for division. The first division, meiosis 1. Consist of the following. Prophase 1, metaphase 1, anaphase 1, and telophase 1. The nuclear envelope starts to break down during prophase 1 of meiosis and microtubules are assembled. Previously replicate chromosomes are condensed to help them move around the cell without being tangled. These condensed chromosomes can only be seen under a microscope. The homologous pairs of chromosomes exchange genetic information. This is called crossing over. The homologous pairs line up in the center of the cell during metaphase 1. Near the

Compare: Both diploid and haploid cells contain genetic information within their structures and have at least one sex chromosomes within their makeup. Both haploid and diploid cells undergo cell division such as mitosis or meiosis in order to produce these haploid and diploid cells.

Chromosomes line up in different orders (random assortment) so when they go to either new cell on cell might have dominant for one characteristic (e.g. brown eyes), recessive for another (e.g. blond hair), and dominant for a third one (e.g. can roll tongue) whereas the other created cell might be dominant, dominant dominant (brown eyes, brown hair and can roll tongue.

Heredity – the transmission of traits from one generation to another, from parents to offspring; the protoplasmic continuity between parents and offspring