Genes, Alleles, and Meiosis Review

Genes the units that determine traits; it is a segment of DNA that contains information.

Alleles Different forms of a gene that determine a specific trait

Multiple Allele Possibilities Some genes have only 2 alleles while others have dozens of different alleles

Chromosomes Chromosomes—the strands of DNA and protein inside the cell nucleus— are the carriers of genes. The genes are located in specific positions on chromosomes. Each species has a characteristic chromosome number.

Gamete the sex cells - sperm or egg

Gametes and alleles Body cells contain two alleles of each gene, one from each parent’s gamete, which can be identical or different. Therefore, if a gamete contains an allele, it will be present in the chromosomes of all body cells produced by that gamete even if it is not expressed.

Diploid Cells  Diploid cells contain 23 chromosomes from the mother’s egg and 23 chromosomes from the father’s sperm, forming sets. These sets of chromosomes are homologous, meaning that chromosomes from the male parent have a corresponding chromosome from the female parent.

Diploid Cells A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets”, represented by the symbol 2N. For a human the diploid number is 46 or 2N=46.

Haploid Cells Some cells contain only a single set of chromosomes, and therefore a single set of genes. Such cells are haploid, meaning “one set.” They have one possible allele for each gene. The gametes of sexually reproducing organisms are haploid. The haploid number in humans is 23 or N=23.

How many sets of genes do multicellular organisms inherit? Upon fertilization, the new cell now has the correct human number of 46 chromosomes. Fertilization of the egg by the sperm restores the diploid number of 46 chromosomes.

What is Meiosis? Meiosis is the production of sperm and egg cells in organisms that reproduce sexually. The number of chromosomes in a cell is reduced by meiosis starting from a cell with 1 set of 46 (23 pairs) chromosomes and ending with 4 cells each containing 1 set of 23 chromosomes.

To become diploid again— haploid gametes produced by each parent will fuse to form a zygote (during fertilization), and offspring receive one copy of each chromosome from each parent

Meiosis and variation This explains why siblings get different combinations of genes from their parents, which is why they look related but not identical.

Meiosis contains 2 separate divisions Meiosis I Meiosis II

Meiosis I Interphase Just prior to meiosis I, the cell undergoes a round of DNA replication during interphase.

Prophase I Chromatin condenses to chromosomes, the nuclear membrane breaks down, & a spindle forms. Each duplicated chromosome consists of 2 identical sister chromatids, held together by a centromere.

Prophase I The duplicated chromosomes pair up, forming a structure called a tetrad, which contains four chromatids. <2 homologous chromosomes, each of which is made up of 2 sister chromatids>

Prophase I As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over. First, the chromatids of the homologous chromosomes cross over one another.

Prophase I Then, the crossed sections of the chromatids are exchanged. Crossing-over is important because it produces new combinations of alleles in the cell, increasing genetic variation.

Metaphase I During metaphase I of meiosis, paired homologous chromosomes attach to the spindle and line up across the center of the cell.

Anaphase I During anaphase I, spindle fibers pull members of each homologous chromosome pair toward opposite ends of the cell. In mitosis, sister chromatids were separated; here they are non identical and still joined. When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

Telophase I and Cytokinesis During telophase I, a nuclear membrane forms around each cluster of chromosomes. Cytokinesis follows telophase I, forming two new cells.

Meiosis I Summary Meiosis I results in two new daughter cells. Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes that it would have in a diploid cell.

Meiosis I The two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Chromosome number has been halved.

Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

Prophase II As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible. The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I. The spindle forms.

Metaphase II During metaphase of meiosis II, chromosomes line up in the center of each cell. The spindles attach to the centromeres of the chromatids.

Anaphase II The centromere of each chromosome splits, allowing chromatids to separate and move to opposite poles of the cell

Telophase II Nuclei form around each group of chromosomes in each daughter cell, the spindles break down, and cytokinesis occurs.

The two nuclear divisions in meiosis result in four daughter cells forming from an original parent cell, each with half the chromosomes of the parent cell.

Telophase II, and Cytokinesis In the example shown here, each of the four daughter cells produced in meiosis II receives two chromatids. These four daughter cells now contain the haploid number (N)—just two chromosomes each.

OOgenesis The haploid cells produced by meiosis II are genetically unique. In female animals, generally only one of the cells produced by meiosis is involved in reproduction.

Meiosis vs. Mitosis 1. Division-In Meiosis, two cell divisions take place to produce 4 genetically different haploid daughter cells. Mitosis has one division and 2 genetically identical diploid cells produced. 2. Distribution of genes-During meiosis, chromatids undergo crossing over, so they are NOT identical when randomly distributed.

Meiosis vs. Mitosis 3. In mitosis and meiosis, chromosomes duplicate only ONCE, even though they divide twice in meiosis. 4. During meiosis, chromosomes are distributed into daughter cells randomly, not identically as in mitosis 5. Reproduction-Mitosis is a form of asexual reproduction for simple organisms, whereas meiosis is an early step in sexual reproduction for others.

Other differences when comparing Meiosis and Mitosis 6.Segregation-In meiosis, homologs move to separate daughter cells. As a result, the two alleles for each gene segregate from each other. In mitosis, the homologs remain together.