Unit 2 Meiosis section

Prophase I

A synaptonemal protein complex attaches the homologs and they form a tetrad (or bivalent) that contains two chromosomes and four chromatids (synapsis). In the later phase DNA is exchanged between the homologs at the chismata that are formed as the chromosomes cross over each other. This process is important because it "scrambles" the genetic material resulting in recombinant chromatids and thus increases genetic kinetochores. The synaptoneal complex begins to break down.

Trisomy 21 (Down syndrome)

An example of an aneuploidy. It's a condition in which an individual has three copies of the 21st chromosome rather than two giving him/her a total of 47 chromosomes rather than 46.

Origins of genetic variation

Meiosis results in genetic variation through recombination of alleles in the genes of offspring and it takes the alleles of the gametes of both parents and shuffles them to get a new set of DNA with a recombined set of alleles. Another form of genetic variation is mutations.

Triploidy

Organism is 3n

Anaphase I

The homologous chromosomes of each tetrad separate and move along the spindle fibers toward each pole as the kinetochore tubules shorten.

Telophase I

The homologous chromosomes reach opposite poles and there is a haploid complement of chromosomes (each with two sister chromatids) at each pole. The microtubules disappear a new nuclear membrane surrounds each haploid set and the chromosomes uncoil.

Importance of apoptosis

The hypersensitive response in plants so plants that are infected by a pathogen can cut off nutrients to the pathogen by killing cells surrounding the site of infection which will prevent the pathogen from spreading to the rest of the plant.

How is apoptosis similar in other organisms?

A cell about to undergo this will detach from its neighbors, break its chromatin apart into very small pieces, and form blebs (membranous lobes that break into fragments). The remains of the dead cell are ingested by the surrounding cells by phagocytosis and the products get recycled.

Internal and external signals involved in cell death:

Controls the timing of programmed cell death. External signals (lack of a mitotic signal such as a growth factor) or internal signals (recognition of damaged DNA) can activate caspases (a class of protease enzymes that hydrolyze proteins of the plasma membrane, nuclear envelope, and nucleosomes).

Clones

Daughter cells that are genetically identical to their parent. This results from asexual reproduction.

Hexaploid

6n; example is common bread wheat. The wheat genome is thought to have undergone a number of hybridization events. First, two diploid (2n) species hybridized to form a tetraploid (4n) plant. The tetraploid plant then hybridized with another diploid species to form a 6n plant.

Example of how fungi can alternate between growth via asexual reproduction and sexual reproduction.

A bread mold that is spreading quickly on a loaf of bread is dividing by asexual reproduction which is possible and beneficial in this case because there are plenty of nutrients available in the environment. If the environment changes and nutrients are no longer available the fungi might transition to the sexual life cycle (sexual reproduction is particularly important during periods of environmental stress because it increases genetic diversity).

Polyploidy

A chromosomal abnormality in which a person has an abnormal number of chromosome sets. It can result from additional rounds of DNA duplication or because a spindle did not form properly in mitosis II. The two most common forms are triploidy and tetraploidy. This chromosomal abnormality is most common in plants though it has also been demonstrated in animals. Often these events make fruits larger and sterile (with no seeds) so fruit producers artificially manipulate plant genes to create polyploidy events (e.g The bananas from the grocery store are a triploid hybrid of two diploid species. The "parent" diploid species are not as tasty and have a lot of seeds but the hybrid triploid bananas are tasty and seedless.

Aneuploidy

A chromosomal abnormality in which a person has an abnormal number of chromosomes in his or her genome. Aneuploidies are common causes of birth defects and they are usually lethal. Approximately 10-15% of all pregnancies end in miscarriages and aneuploidies are the most common cause of miscarriages during the first trimester.

TH4: True or false, the contents of dying cells in a necrotic, gangrenous foot can be recycled and used by other cells. A. True B. False

B. False One of the key differences between necrosis and apoptosis is that the contents of dying cells are not reusable in necrosis but they are in apoptosis.

The independent assortment of chromosomes

Chromosomes line up along the metaphase plate during metaphase I of meiosis in a random fashion. A simple cell with maternal and paternal versions of just two chromosome types replicate and after they align randomly along the metaphase plate and the orientation of the chromosomes along the metaphase plate is important because it ultimately determines the genetic makeup of the gametes produced by meiosis. Chromosomes are randomly mixed producing genetic variation without any crossing over. Recombination among chromosomes will result in 2n possible combinations of chromosomes among the gametes where n is the haploid number of chromosomes. Humans (2n = 46) have 23 different chromosome types so humans can produce 8,388,608 or 2(23) different gametes.

TH3: What is the goal of meiosis? A. To create genetic diversity and double the DNA content in the daughter gametes. B. To create gametes that are clones C. To create somatic cells from gametes D. To create genetic diversity and reduce the DNA content of resulting gametes in half. E. All of these are correct

D. To create genetic diversity and reduce the DNA content of resulting gametes in half. The purpose of meiosis is to shuffle genetic information and cut the number of chromosomes in half. This is why meiosis I is sometimes called the "reduction division".

Necrosis

Damaged cells or cell starved of nutrients burst and release their contents to the extracellular environment leading to inflammation (e.g. Gangrene is a type resulting from insufficient blood supply often due to a bacterial infection). This form of cell death is not "supposed" to happen by being genetically programed to occur. It does not occur in a way that preserves organelles of broken down cells for use by other cells. The contents of the cell destroyed are not reusable by other cells.

Timing of gamete production

Differs in males and females. Males generate gametes throughout their lives in spermatogenesis and prophase I lasts for about one week. Females are born with all of the oogonia that they will ever need and prophase I begins before birth but then the meiotic process stops until puberty when the female begins her monthly ovarian cycle. Oogenesis is the production of eggs and it ends at menopause.

TH2: Consider a large fern plant. The leaf cells of this plant are... A. Haploid B. Diploid c. Triploid D. Dividing by mitosis E. More than one of these is correct

E. More than one of these is correct The leaf cells of this plant are somatic cells that are diploid and actively dividing via mitosis.

Why must genetic variation exist?

For natural selection to help organisms adapt to their environment. Mutation is the ultimate source of genetic variation; sexual reproduction allows organisms to pass variation on to future generations.

How is meiosis necessary?

For sexual reproduction and the formation of gametes. It is also important for generating diversity through crossing over and independent assortment of chromosomes whereas mitosis passes on the parental genome to daughter cells without a change in information meiosis forms haploid cells (n) from diploid cells (2n). It first reduces the number of sets of chromosomes from diploid to haploid and then reduces the number of chromosomes per homolog from two chromatids to one.

What does sexual reproduction produce?

Genetically different offspring through the combination of haploid gametes from two parents. Both the parents and their offspring are diploid but their genetic composition is different. It has an evolutionary advantage over asexual reproduction because it results in offspring genetically different from their parents and increases genetic diversity in the overall gene pool. Gametes are the haploid structures in humans and everything else is diploid. Humans produce haploid gametes in meiosis and combine their haploid gametes to produce a diploid zygote in fertilization. The zygote randomly receives half of its genes from one parent and half from the other so the genetic makeup of the zygote is different form that of either parent.

Apoptosis

Genetically programmed cell death; Occurs when a cell is no longer needed or when that cell poses a threat to the body (e.g. The fingers of a fetus are linked with connective tissue that is eventually no longer needed so those cells die out). If this form of cell death does not occur properly syndactyly can result (a condition in which the digits are fused together). Older cells may be susceptible to becoming cancerous so they are killed and replenished with new cells on a regular basis. This form of cell death is "supposed to happen" and is genetically programed to occur. It occurs in a way that preserves organelles of broken-down cells for use by other cells. The contents of the cells destroyed can be reused.

Differences between the haplontic life cycle, diplontic life cycle, and alternation of generations:

Haplontic life cycle- Dominant multicellular organism is haploid. Diplontic life cycle- Dominant multicellular organism is diploid. Alternation of generations- There are dominant haploid and diploid forms. Meiosis is paired with fertilization in each of these life cycles.

Meiosis I (Reductional Phase)

Homologous chromosomes separate and the cell divides forming two haploid cells with two sister chromatids of each homologous chromosome (e.g. A cell that contains just two homologous pairs of two types of chromosomes (a cell that is 2n = 4). After DNA duplication in the S phase of interphase there are still only two chromosomes though each chromosome now consists of a pair of sister chromatids joined at a common centromere. This cell is still 2n = 4 though its DNA content has doubled).

Difference between the haplontic and diplontic life cycles:

In the diplontic life cycle the diploid form was the dominant form and the haploid form was only present for a small portion of the life cycle. In the haplontic life cycle the haploid form is the dominant form and the diploid form is only present for a small portion of the life cycle.

Ploidy of daughters in mitosis and meiosis:

Meiosis- 1n (haploid) Mitosis- 2n (diploid)

Number of cell divisions in mitosis and meiosis:

Meiosis- 2 Mitosis- 1

Number of daughter cells in mitosis and meiosis:

Meiosis- 4 Mitosis- 2

Biological purposes of mitosis and meiosis:

Meiosis- Generating gametes Mitosis- Duplicating cells

DNA replication in mitosis and meiosis:

Meiosis- Interphase (S) Mitosis- Interphase (S)

Comparison of mitosis and meiosis:

Meiosis- Two stage type of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell. Mitosis- Process of nuclear division in eukaryotic cells that conserves the chromosome number by equally allocating replicated chromosomes to each of the daughter nuclei.

Crossing over (recombination) in mitosis and meiosis:

Meiosis- Yes (during early prophase I) Mitosis- No

Synapsis of homologs in mitosis and meiosis:

Meiosis- Yes (during early prophase I) Mitosis- No

What are the most common problems associated with DNA?

Mutations

Every cell has a lifespan and every cell dies. Two forms of cell death are:

Necrosis Apoptosis

Are sexual life cycles the same for all organisms?

No

Crossing over of chromosomes

Occurs during prophase I of meiosis I also results in genetic variation. The average chromosome crosses over two or more times per meiotic event resulting in genetic variation. If crossing over did not happen all of the genes on the same chromosome would always be inherited together. There are tens of thousands of genes in the human genome. Recombination can occur anywhere on the chromosome (multiple times) causing crossing over to lead to an unimaginable increase in the number of possible gametes and offspring that can result through meiosis and subsequent fertilization.

Nondisjunction

Occurs when homologous pairs fail to separate in anaphase I or sister chromatids fail to separate in anaphase II. It results in gametes with an abnormal number of chromosomes. If those gametes fertilize other gametes they can lead to a number of major chromosomal aberrations including aneuploidy and polyploidy.

Translocation

Occurs when there is a crossover between nonhomologous chromosomes rather than homologous chromosomes. Reciprocal translocation usually occurs due to illegitimate recombination. It can significantly impair development and the expression of other genes so it's usually a serious problem (e.g. A translocation between chromosomes 9 and 22 produces a "philadelphia chromosome" which leads to the over expression of a gene that produces a signal for cell division. This can lead to chronic myelogenous leukemia).

End results of meiosis I

One cell with two pairs of homologous chromosomes producing two daughter cells with a haploid complement of each of the chromosomes.

Tetraploidy

Organism is 4n

Telophase II and Cytokinesis

Results in the splitting of the cell into two daughter cells and the chromosomes uncoil. Two haploid daughter cells are formed for each intermediate cell formed from meiosis I so for each parent cell a total of four haploid gametes are formed.

Crossing over in meiosis:

Some of the chromosomes in haploid gametes are recombinant (they are the product of a crossing over between two chromosomes during meiosis I) whereas some chromosomes are not recombinant. Crossing over only occurs in meiosis I and not meiosis II.

Random fertilization (sexual reproduction)

The 8.3 million gamete possibilities can be combined to form more than 70 trillion possible combinations of gametes between one man and one woman. This adds significant genetic variation to the offspring.

Anaphase II

The centromeres of the sister chromatids separate and the sister chromatids move toward opposite poles. The sister chromatids are now called chromosomes.

Metaphase I

The chromosomes line up along the plate in homologous pairs and the spindles that form each pole attach to one chromosome of each pair. Homologous chromosomes line up independently of each other along the plate so either the paternal or maternal homolog may move toward a particular pole. This gives rise to independent assortment of chromosomes which is another aspect of meiosis that generates genetic variation.

Metaphase II

The chromosomes meet at the plate (just like metaphase I) and the spindle fibers of each pole attach to the kinetochores of the sister chromatids.

Three ways in which meiosis results in genetic variation in organisms through recombination:

The independent assortment of chromosomes Random fertilization (sexual reproduction) Crossing-over of chromosomes

Cytokinesis

The pinching off of the cell membrane and completes the creation of two daughter cells.

Diplontic life cycle (animals)

The sexual life cycle in animals involves the merger of haploid (1n) gametes in fertilization producing a diploid (2n) organism that exists in the free-living stage. To reproduce, the organism produces gametes through meiosis (e.g. In humans the cells in a diploid organism are 2n = 46. Sex cells undergo meiosis to form gametes (egg and sperm) that are n = 23. These gametes fuse to form a diploid zygote that is 2n = 46 and this zygote divides via mitosis). The diploid form is the dominant form or the form that exists the vast majority of the time.

Alternation of Generations (plants)

The sexual life cycle in plants involves the merger of haploid (1n) gametes in fertilization producing a diploid (2n) multicellular organism called a sporophyte. To reproduce the organism produces spores through meiosis. Those spores divide mitotically forming a haploid gametophyte that forms gametes by mitosis and the process continues. This is the only life cycle that transitions between multicellular haploid and diploid states.

Haplontic Life cycle (fungi)

The sexual life cycle in protists, fungi, and some algae involve the merger of haploid (1n) gametes in fertilization producing a diploid (2n) zygote which undergoes meiosis to form haploid cells called spores. These haploid cells grow and divide mitotically to form a haploid multicellular organism.

Meiosis II (The Equational/Restoration Phase)

The sister chromatids separate and the cells further divide into four haploid daughter cells with a single chromosome.

Prophase II

The spindle forms and the chromosomes move toward the metaphase plate.

Spores in fungi

These are produced via meiosis in specialized structures called sporangia. The spree divide via mitosis to form the haploid organism that is what you see when you look at fungus. The hyphae from different mating types will fuse to form gametangia structures that produce gametes via mitosis. These gametes then fuse to form a diploid zygospore with thick walls. These zygospores can remain in a suspended state until conditions improve when they give rise to more spores.

Two problems involving errors during meiosis:

Translocation Nondisjunction

How many times does the nucleus divide in meiosis?

Twice vs mitosis where it divides only once. DNA is only replicated once in meiosis I.

End results of meiosis II:

Two cells with each a haploid complement of chromosomes and formed four haploid daughter cells with one chromosome each.