Meiosis Reduces Chromosome Number From Diploid to Haploid
Meiosis and mitosis look similar-both are preceded by the replication of cell's DNA, for instance, but in meiosis this replication is followed by two stages of cell division, meiosis I, and meiosis II.
The result of meiosis is four daughter cells, each of which has half as many chromosomes as the parent cell.
The Stages of Meiosis are as follows
The chromosomes replicate resulting in two sister chromatids attached at the centromere and the centrosomes are replicated.
The chromosomes begin to condense, and homologs loosely pair along their lengths, aligned gene by gene. Crossing over (the exchange of corresponding segments of DNA molecules by nonsister chromatids) is completed while homologs are in synapsis, (joining of two homologous chromosomes along their length) held tightly together by proteins along their lenghts. The new structure is a tetrad. Synapsis ends in mid-prophase, and the chromosomes in each pair move apart slightly. Each homologous pair has one or more chiasmata, points where crossing over has occurred and the homologs are still associated due to cohesion between sister chromatids. Centrosome movement, spindle formation, and nuclear envelope breakdown also all occurs in Prophase I. In late Prophase I, microtubules from one pole or the other attach to the two kinetochores. The homologous pairs then move toward the metaphase plate.
Homologous pairs of chromosomes are lined up at the metaphase plate and microtubules from each pole attach to each of the members of homologous pairs in representation for pulling them to opposite ends of the cell.
The spindle apparatus helps to move the chromosomes toward the opposite ends of the cell; sister chromatids stay connected and move together toward the poles. The breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allow homologs to separate.Telophase I and Cytokinesis
At the beginning of telophase I, each half of the cell has a complete haploid set of replicated chromosomes. Each chromosome is composed of two sister chromatids; one or both chromatids include regions of nonsister chromatid DNA. Homologous chromosomes move until they reach opposite poles, so that each pole contains a haploid set of chromosomes with each chromosome still made up of two sister chromatids. Cytokinesis occurs at the same time as telophasae-a cleavage furrow occurs in animal cells and cell plates occur in plant cells. Both result in the formation of two daughter cells. No replication occurs between meiosis I and meiosis II.
A spindle apparatus forms and sister chromatids move towards the metaphase plate.Metaphase II
The chromosomes are lined up on the metaphase plate, and the kinetochores of each sister chromatid prepare to move to opposite poles of the cell. Because of crossing over in meiosis I, the two sister chromatids of each centromere are not genetically identical.
The centromeres of the sister chromatids separate, and individual chromosomes move to opposite ends of the cell.Telophase II and Cystokinesis
The chromatids have moved all the way to opposite ends of the cell; nuclei reappear, and cytokinesis occurs. Each daughter cell (there are a total of 4) has the haploid number of chromosomes. Each of the four daughter cells is genetically distinct from the other daughter cells and from the parent cells.Origins of Genetic Variation
Here are some of the processes that contribute to variation in offspring of sexully reproducing organisms:
1. Independent Assortment
2. Crossing Over
3. Random Fertilization
1. Independent Assortment of Chromosomes
In metaphase I, when the homologous chromosomes are lined up on the metaphase plate, they can pair up in any combination, with any of the two homologous pairs facing either pole. This means that there is a 50-50 chance that a particular daughter cell will get a maternal chromosome or a paternal chromosome from the homologous pair.2. Crossing Over
After prophase I, homologous chromosomes synapse nd the homolous chromosomes exchange homologous parts of two on-sister chromatids. Then, during metaphase II, chromosomes that now have recombinant chromatids can be facing either of the two poles with respect to each other, which further increases variation in reproduction.3. Random Fertilization
This refers to the fact that fertilization (in which an egg meets with a sperm) is random. Since each egg and sperm is different, as a result of independent assortment and crossing over, each combination of egg and sperm is unique.
Chapter 14 Vocabulary
norm of reaction
The range of phenotypes produced by a single genotype, due to enviornmental influences.
The genetic makeup, or sets of alleles of an organism.
Law of Segregation
Mendel's first law statting that the two alleles in a pair segregate (separate) into different gametes during gamete formation.
An additive effect of two or more genes on a single phenotypic character.
An organism that is heterozygous with respect to two genes of interest. All offspring form a cross between parents doubly homozygous for different alleles are dihybrids.
An allele whose phenotypic effect is not observed in a heterozygote.
Any of the alternative versions of a gene that produce distinguishable phenotypic affects.
The physical and physiological traits of an organism, which are determined by its genetic makeup.
A heritable feature that varies continuously over a range rather than in an either-or.
Breeeding an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype. The ratio in the offspring reveals the unknown genotype.
An allele that is fully expressed in the phenotype of a heterozygous.
An organism that is heterozygous with respect to a single geneof interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids.
The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways.
The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.
The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.
The ability of a single gene to have multiple effects.
A type of gene interaction in which one gene alters the phenotypic effects of anothr gene that is independently inherited.
In genetics, the mating or crossing, of two true-breeding varieties.
Referring to plants that produce offspring of the same variety when they self-pollinate.
law of independent assortment
Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes.
1. All occurs in Telophase I and Cytokinesis except
a. Homologous chromsomes move until they reach opposite poles
b. Each pole contains a haploid set of chromosomes
c. Spindle apparatus helps move chromosomes towards opposite ends of the cell
d. The formation of two dadughter cells
2. True/False Meiosis II separates homologous chromosomes.
3. ________ occurs after prophase I when homologous chromosomes synapse and the homologous chromosomes exchange homologous parts two non-sister chromatids.
4. Meiosis and mitosis both are preceded by
a. Condensing of chromosomes
b. Replication of cell's DNA
c. The pairing of homologs
d. Chromosomes moving towards the metaphase plate
5. What are the origins of genetic variation
a. Random Fertilization
b. Independent Assortment
c. Crossing Over
d. all of the above
answers: 1)c 2) false 3) Crossing Over 4) b 5) d