Chapter 7 Meiosis

Meiosis is a form of cell division that Halves the number of chromosomes When forming specialized Reproductive cells.

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Meiosis involves 2 divisions of the Nucleus - meiosis I and meiosis II - And each division is subdivided Into prophase, metaphase, Anaphase and telophase stages. Before meiosis begins, the DNA in the Original cell is replicated.

There are 8 stages to Meiosis… Step 1 - Prophase I Step 2 - Metaphase I Step 3 - Anaphase I Step 4 - Telophase I Step 5 - Prophase II Step 6 - Metaphase II Step 7 - Anaphase II Step 8 - Telophase II

In prophase I, the chromosomes Condense, and the nuclear envelope Breaks down. Homologous chromosomes Pair all along their length and then Cross-over. (crossing over occurs when Portions of a chromatid on 1 Chromosome are broken and exchanged With the corresponding portions On one of the chromotids of the Other chromosome.

In metaphase I, the pairs of Homologous chromosomes are moved By the spindle to the equator Of the cell.

In anaphase I, the homologous Chromosomes separate. (just like mitosis) But the chromatids do not separate At their centromere! Each chromosome Is still composed to 2 chromatids.

In telophase I, individual chromosomes Gather at each of the poles. Then Cytokinesis occurs forming 2 cells.

Now we get into mitosis II. The Chromosomes do not replicate Between mitosis I and II!! In prophase II, a new spindle forms Around the chromosomes.

In metaphase II, the chromosomes Line up along the equator and are Attached at their centromeres To the spindle fibers.

In anaphase II, the centromeres Divide, and the chromatids (now called chromosomes) move to Opposite poles of the cell.

And lastly in telophase II, a nuclear Envelope forms around each set of Chromosomes. The spindle breaks Down, and the cell undergoes Cytokinesis.

The result of meiosis is 4 The 4 resulting cells are the Haploid cells. The 4 resulting cells are the Reproductive cells.

The chromosome pairs separate Independently. The chromosomes that an offspring Receives from each pair is A matter of chance. The random distribution of homologous Chromosomes during meiosis is called Independent assortment.

Meiosis and the joining of gametes Are essential to evolution. The DNA exchange that occurs at Crossing over adds even more Recombination to the independent Assortment of chromosomes that Occurs later in meiosis. Thus the number of genetic Combination that can occur among Gametes it practically unlimited.

The process by which sperm are Produced in male animals is Called spermatogenesis. The process by which gametes are Produced in female animals is Called oogenesis. Meiosis for males creates 4 cells, But only 1 cell in females.

Some organisms look exactly like Their parents and siblings. 7-2 Sexual Reproduction Some organisms look exactly like Their parents and siblings. These traits are passed down In 2 ways, asexual and sexual Reproduction.

In asexual reproduction, a single Parent passes copies of all of Its genes to each offspring, Basically creating a clone. In sexual reproduction, 2 parents Each form haploid cells, which join To form offspring.

Asexual reproduction is the simplest And most primitive Method of reproduction. But it allows organisms to produce Many offspring in a short period Of time. But the DNA does not vary much Between individuals.

The entire span in the life of an Organism from one generation To the next is called a life cycle. Eukaryotes that undergo sexual Reproduction can have one of three Types of sexual life cycles: Haploid Diploid Alternation of Generations

The haploid life cycle is the simplest. Haploid cells occupy the major Portion of the life cycle, the zygote Is the only diploid cell and it Undergoes meiosis immediately After creation. This type is usually found in protists, Or fungus.

The diploid life cycle is the one That we live in. Most of the life cycle is diploid, With haploid cells only for Reproduction. The haploid gametes join in a Process called fertilization, which Results in a diploid zygote.

Plants, algae and some protists Have a life cycle that regularly Alternate between haploid And diploid. The diploid phase of a plant is Called a sporophyte, while a spore Is haploid. The gametophyte is the haploid Phase of a plant.

LET'S SUM IT UP!

Steps of Meiosis I Reductional In humans, n=23, so 2n=46. Division ! n=23 in humans

Steps of Meiosis II

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