Meiosis

Learning targets One diploid nucleus divides by meiosis to produce four haploid nuclei The halving of the chromosome number allows a sexual life cycle with fusion of gametes DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation Orientation of pairs of homologous chromosomes prior to separation is random Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number Crossing over and random orientation promotes genetic variation Fusion of gametes from different parents promotes genetic variation Non-disjunction events can cause Down syndrome and other chromosome abnormalities Studies show age of parents influences chances of non-disjunction Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed Gametes are haploid so contain only one allele of each gene The two alleles of each gene separate into different haploid daughter nuclei during meiosis Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles

Meiosis and Sexual Reproduction Meiosis is the process by which an individual’s genome is reduced from two sets of chromosomes (2n, the diploid number) to one set of chromosomes (n, the haploid number). The chromosomes are isolated in special cells called gametes. These are the sex cells in the body. The non-sex cells are known as somatic cells. Gametes are produced in specialized reproductive structures or organs. In animals, gametes are produced by gonadal tissue. In meiosis, specialized cells called gametes are made which have only one of each type of chromosome (therefore half of the chromosomes that a diploid cell would have). We call this a haploid (n) number of chromosomes. In sexual reproduction, two haploid cells fuse together to form a diploid cell. The new combination of genes from mother and father are what we call genetic recombinations. Genetic recombination leads to variation within the traits of a population and as we’ve learned before this, variation is necessary in order for natural selection to occur.

Meiosis in General The goals of meiosis and sexual reproduction are to reduce the number of chromosomes in two parents from diploid to haploid, place them in gametes (sex cells/ sperm and egg), then fuse the sex cells and restore the diploid number in a new genetic recombination. Meiosis and sexual reproduction work together. Meiosis reduces the chromosome number to half – so the gametes are haploid. Then those haploid cells fuse together in sexual reproduction to restore the diploid number. When two gametes fuse together, the resulting cell is called a zygote. The zygote will go on to divide into multiple cells, which will eventually form the new organism.

Two Divisions, Not One Meiosis I Meiosis II During Prophase I, the two homologs pair up in a process called synapsis. In Meiosis, there are two divisions. The first division is called Meiosis I and the second division is called Meiosis II. In Meiosis I, the homologous chromosomes are separated from each other. In Meiosis II, the sister chromatids are separated from each other (just like what occurred in mitosis)

Synapsis Meiosis I: Just as our body cells go through the S phase of interphase before cell division occurs, our germ cells do as well. This means that before Meiosis occurs, each chromosome is in its duplicated form at the beginning of meiosis. In prophase I, the homologous chromosomes pair up. You can see them touching each other in the image above. This is called synapsis. This is when crossing over occurs. The spindle fibers attach to the homologous chromosomes (not sister chromatids), and pull them to line up together in Metaphase I. (This looks different than metaphase in mitosis, because pairs of chromosomes are lined up – not individual chromosomes). In Anaphase I, the homologous chromosomes are separated from each other. In Telophase I, chromosomes are bundled into two new nuclei. Cytokinesis occurs during Telophase I as well. Notice that in the picture, this particular cell has 4 chromosomes to begin with. This hypothetical organism, therefore, has a diploid number of 4. At the end of Meiosis I, both new cells have 2 chromosomes – this means they are already haploid cells. These chromosomes are in their duplicated state, therefore, so there will be another round of nuclear and cell divisions – called Meiosis II.

Meiosis II: The two new cells, both already with a haploid number of chromosomes, now go through one more round of nuclear and cell division. The steps of Meiosis II are the same as the steps of Mitosis. Spindles attach to the chromosomes and begin to pull them into position (Prophase I). The chromosomes line up in the middle of the cell (Metaphase I), the sister chromatids are pulled apart at the centromeres (Anaphase I), and the new chromosomes form new nuclei (Telophase I) in each new cell that divides from cytokinesis.

Nondisjunction- The failure of homologous chromosomes or sister chromatids to separate

Crossing Over Between Homologous Chromosomes Crossing over isthe process by which a chromosome and its homologous partner exchange heritable information in corresponding segments. Occurs during prophase I when the homologous pairs are in their tetrad arrangement. Crossing Over – When the homologous chromosomes are in synapsis (during Prophase I), small pieces of the chromosomes are snipped by enzymes, exchanged with each other, and resealed to form new combinations of genes on the chromosomes. This means that some genes from the maternal chromosome will end up on the paternal chromosome, and vice versa. When the homologous chromosomes are in synapsis, it is called a tetrad. Crossing over is depicted in the image, where you see different colors swapped on the chromosomes. It’s important to note that after crossing over occurs, each chromosome still has the same types of genes, but now it may have a different version of the genes that it started with. This is because genes are located at specific places on chromosomes and crossing over occurs between the same locations on both homologous chromosomes.

Independent Assortment

Independent Assortment of Chromosomes into Gametes Homologous chromosomes can be attached to either spindle pole in prophase I, so each homologue can be packaged into either one of the two new nuclei. In this example, there are three pairs of chromosomes, each with two possible ways to line up: 23 = 8 possible different nuclei. In humans, random assortment produces 223 (8,388,608) possible combinations of homologous chromosomes. Independent Assortment is a term that was first used by Mendel, who we will study in Chapter 11. It refers to the fact that the homologous chromosomes are grouped together in new gametes randomly. For example, in the hypothetical cell shown in the image, the germ cells begin with 6 chromosomes – 3 pairs of homologous chromosomes. The image shows all the possible ways that these chromosomes could line up during Metaphase I of meiosis. When you look at the four new nuclei in each possible case, you can see that there are many different ways to combine the maternal and paternal chromosomes of each of the three types of chromosomes. Since there are only three pairs of chromosomes in this cell, this means there are 23, or 8, possible combinations of chromosomes in the new nuclei. Imagine the different combinations in human cells, of which there are 23 pairs. (223 possible combinations). That is a lot of possible variety.

Crossing Over and Independent Assortment Crossovers and the random sorting of chromosomes during Prophase I and Metaphase I in meiosis introduce novel combinations of alleles into gametes, resulting in new combinations of traits among offspring. Crossing Over and Independent Assortment, therefore, are both two processes that lead to an increase in genetic variety in sexually reproducing offspring. Why is this beneficial???

Gamete Formation in Animals Males End product: 4 sperm Females End product: 1 ovum and 3 polar bodies We know that meiosis is necessary to form haploid nuclei, which are necessary in gametes. However, gamete formation is slightly different in male and female animals. The description in the slide summarizes the differences.

A quick animation or two… McGraw Hill Meiosis Video

Asexual vs. Sexual Reproduction Variation is introduced through mutations. Reproduction is fast. Requires only one parent. Sexual Variation is introduced through mutation and genetic recombination Reproduction is slower. Requires two parents (and extra energy expended on mating)

Goal of Sexual Reproduction The goal of sexual reproduction is to produce one or more offspring from different parents that are genetic recombinations of half of each parent’s two sets of chromosomes (genome). Offspring are genetically unique from their parents or siblings, and this provides the raw material for Natural Selection. In meiosis, specialized cells called gametes are made which have only one of each type of chromosome (therefore half of the chromosomes that a diploid cell would have). We call this a haploid (n) number of chromosomes. In sexual reproduction, two haploid cells fuse together to form a diploid cell. The new combination of genes from mother and father are what we call genetic recombinations. Genetic recombination leads to variation within the traits of a population and as we’ve learned before this, variation is necessary in order for natural selection to occur.