Mitosis and Meiosis

Anaphase I (Meiosis I)

the microtubules pull the homologous pairs to the opposite side of the cell

humans have 46 chromosomes, 23 homologous pairs, and a total of _____ chromatids (depending on the stage of division)

92

5. Chromosomes begin migrating to opposite poles.

A. At metaphase, the chromosomes are arranged on the metaphase plate. The end of metaphase and the beginning of anaphase is defined by the separation of the chromosomes into chromatids (which are now considered chromosomes by themselves). Once anaphase begins, the chromosomes move to one or the other pole.

Metaphase II (Meiosis II)

condensed chromosomes line up along the metaphase plate and microtubules attach to the kinetochores of the chromosomes

Cytokinesis (Meiosis II)

cytokinesis eventually splits the cell into four haploid cells each with 23 chromosomes.

Anaphase II (Meiosis II)

the sister chromatids are pulled apart to the opposite ends of the cell into identical daughter chromosomes (splitting at the centromere occurs)

a _____ is 1 of 2 identical parts of a duplicated chromosome

chromatid

Anaphase (mitosis)

•Anaphase begins after the chromosomes are separated into chromatids. During anaphase, the microtubules connected to the chromatids (now chromosomes) shorten, effectively pulling the chromosomes to opposite poles and splitting at the centromeres. •The microtubules shorten as tubulin units are uncoupled at their chromosome ends. Overlapping microtubules originating from opposite MTOCs, but not attached to chromosomes, interact to push the poles farther apart (cell elongation). •At the end of anaphase, each pole has a complete set of chromosomes, the same number of chromosomes as the original cell. (Since they consist of only one chromatid, each chromosome contains only a single copy of the DNA molecule.)

humans have _____ chromosomes, _____ homologous pairs, and a total of 92 chromatids (depending on the stage of division)

46, 23

Prophase I (Meiosis I)

•Centrosomes replicate and move to opposite sides of the cell and the chromosomes condense. •Homologous sections of the homologous chromosomes line up (Synapsis) and take part in a process called crossing over. •The nuclear envelope also starts to degrade.

Telophase I (Meiosis I)

•Early stages of cytokinesis begin and the nuclear envelopes reforms and the condensed chromosomes unravel into chromatin. •The resulting product is two haploid cells that are not identical (because of crossing over and genetic recombination).

Prophase (mitosis)

•In prophase, three activities occur simultaneously. First, the nucleoli disappear and the chromatin condenses into chromosomes. Second, the nuclear envelope breaks down. Third, the mitotic spindle is assembled. The development of the mitotic spindle begins as the MTOCs move apart to opposite ends (or poles) of the nucleus. •As they move apart, microtubules develop from each MTOC, increasing in length by the addition of tubulin units to the microtubule ends away from the MTOC. Microtubules from each MTOC connect to a specialized region in the centromere called a kinetochore. •Microtubules tug on the kinetochore, moving the chromosomes back and forth, toward one pole, then the other. In addition to these microtubules, the completed spindle also includes other microtubules from each MTOC that overlap at the center of the spindle and do not attach to the chromosomes.

Metaphase (mitosis)

•Metaphase begins when the chromosomes are distributed across the metaphase plate, a plane lying between the two poles of the spindle. •Metaphase ends when the microtubules, still attached to the kinetochores, pull each chromosome apart into two chromatids. •Each chromatid is complete with a centromere and a kinetochore. Once separated from its sister chromatid, each chromatid is called a chromosome. (To count the number of chromosomes at any one time, count the number of centromeres.)

Check Points

1. Checkpoints. At specific points during the cell cycle, the cell evaluates internal and external conditions to determine whether or not to continue through the cell cycle. The three checkpoints are as follows: • The G1 checkpoint occurs near the end of the G1 phase. If conditions are not appropriate or if the cell is genetically programmed not to divide, the cell proceeds no further through the cell cycle, remaining in an extended G1 phase (or G0 phase), never beginning the S or G2 phases (unless some internal or external cue initiates a resumption of the cell cycle). Nerve or muscle cells, for example, remain in the G0 phase, rarely dividing after they have matured. • The G2 checkpoint, occurring at the end of the G2 phase of the cell cycle, evaluates the accuracy of DNA replication and signals whether or not to begin mitosis. • The M checkpoint, occurring during metaphase, ensures that microtubules are properly attached to all kinetochores at the metaphase plate before division continues with anaphase.

usually there is _____ MTOC per cell; however, cells replicate their MTOCs during _____

1; S phase (aka 2 after s phase)

11. In typical cell divisions by mitosis and meiosis, all of the following contribute to genetic variation EXCEPT: A. anaphase of mitosis B. anaphase of meiosis I C. fertilization D. crossing over E. random union of egg and sperm

A. There are generally no events during normal mitosis that would produce genetic differences between the two daughter cells. The daughter cells are clones, genetically identical. In contrast, the independent assortment of homologues during anaphase I, the random union of egg and sperm during fertilization, and crossing over during prophase I all contribute to genetic variation.

10. Crossing over occurs during A. prophase of mitosis B. prophase I of meiosis C. prophase II of meiosis D. prophase I and II of meiosis E. prophase of mitosis and prophase I of meiosis

B. Chromosomes pair during prophase I (synapsis) forming tetrads with chiasmata. Exchanges of genetic material occur within chiasmata.

2. If a cell has 46 chromosomes at the beginning of meiosis, then at anaphase I there would be a total of A. 23 chromatids B. 23 chromosomes C. 46 chromosomes D. 46 chromatids E. 92 chromosomes

C. During metaphase I, homologous chromosomes pair at the metaphase plate. One member of each pair migrates to opposite poles during anaphase I. If the cell started with 46 chromosomes, 23 chromosomes move to each pole during anaphase II, so the total is still 46 chromosomes.

3. All of the following statements are true EXCEPT: A. Spindle fibers are composed largely of microtubules. B. Centrioles consist of nine triplets of microtubules arranged in a circle. C. All eukaryotic cells have centrioles. D. All eukaryotic cells have a spindle apparatus. E. Many of the microtubules in a spindle apparatus attach to kinetochores of chromosomes.

C. Most plants do not have centrioles

What is cancer

Cancer is characterized by uncontrolled cell growth and division. Transformed cells, cells that have become cancerous, proliferate without regard to cell cycle checkpoints, density-dependent inhibition, anchorage dependence, and other regulatory mechanisms. Thus, cancer is a disease of the cell cycle

Interphase (mitosis)

Cells spend most of their life in interphase consists of (G0, G1, S, G2 phases) during this phase chromatin is enclosed within a clearly defined nuclear envelope • G1: growth phase is the longest part of interphase, the cell is growing and preparing for DNA replication (protein/organelle production occur here) From this point cells have 3 options: Stay in G1, go to G0, or go to the S phase • G0: no more cell division (ex neuron) • S: synthesis phase is where DNA replication takes place • G2: the second growth phase, the cell is growing and preparing for mitosis (making microtubules) Now the cell is ready to enter mitosis (M phase) and replicate

centromere vs centrosome

Centromere: The centromere is the specialized DNA sequence of a chromosome that links a pair of sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore. Centromeres were first thought to be genetic loci that direct the behavior of chromosomes. Centrosome: Centrosomes are organelles which serve as the main microtubule organizing centers for animal cells. Centrosomes are made of from arrangement of two barrel-shaped clusters of microtubules, called "centrioles," and a complex of proteins that help additional microtubules to form.

Why do you have 92 Chromatid

Chromatid: each of the two threadlike strands into which a chromosome divides longitudinally during cell division. Each contains a double helix of DNA. For humans, this means that during prophase and metaphase of mitosis, a human will have 46 chromosomes, but 92 chromatids (again, remember that there are 92 chromatids because the original 46 chromosomes were duplicated during S phase of interphase)

Cytokinesis (mitosis)

Cytokinesis (divides the cytoplasm). In this phase the cytoplasm of one cell splits into two daughter cells

6. MTOCs migrate to opposite poles.

D. During prophase, the two MTOCs (and centrioles, if present) migrate to opposite poles as the spindle apparatus develops between them. Also during prophase, the nucleolus and nuclear membrane disappear, and the chromatin condenses into chromosomes.

1. If a cell has 46 chromosomes at the beginning of mitosis, then at anaphase there would be a total of A. 23 chromatids B. 23 chromosomes C. 46 chromosomes D. 46 chromatids E. 92 chromosomes

E. Metaphase ends when each chromosome separates into a pair of chromatids. During anaphase, chromatids from each pair move to opposite poles. Each chromatid is now considered a complete chromosome, since it consists of a complete DNA molecule. To count the number of chromosomes at any point during the cell cycle, count the centromeres.

Chromosome/Chromatin/Chromatid

Each chromatid consists of a single, tightly coiled molecule of DNA, the genetic material of the cell. In diploid cells, there are two copies of every chromosome, forming a pair, called homologous chromosomes. In a homologous pair of chromosomes, one homologue originated from the maternal parent, the other from the paternal parent. Humans have 46 chromosomes, 23 homologous pairs, consisting of a total of 92 chromatids.

meiosis

In contrast, meiosis is a reduction division, producing genetically variable daughter cells that contain half the genetic information of the parent cell.

Genetic Variation

In mitosis, every daughter cell is exactly like the parent cell. Meiosis and sexual reproduction, however, result in a reassortment of the genetic material. This reassortment, called genetic recombination, originates from three events during the reproductive cycle: 1. Crossing over. During prophase I, nonsister chromatids of homologous chromosomes exchange pieces of genetic material. As a result each homologue no longer entirely represents a single parent. 2. Independent assortment of homologues. During metaphase I, tetrads of homologous chromosomes separate into chromosomes that go to opposite poles. Which chromosome goes to which pole depends upon the orientation of a tetrad at the metaphase plate. This orientation and subsequent separation is random for each tetrad. For some chromosome pairs, the chromosome that is mostly maternal may go to one pole, but for another pair, the maternal chromosome may go to the other pole. 3. Random joining of gametes. Which sperm fertilizes which egg is to a large degree a random event. In many cases, however, this event may be affected by the genetic composition of a gamete. For example, some sperm may be faster swimmers and have a better chance of fertilizing the egg.

mitosis

Mitosis divides the nucleus so that both daughter cells are genetically identical.

What is synapsis in meisos and when does it occur

Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis.

Telophase (mitosis)

Telophase concludes the nuclear division. During this phase, a nuclear envelope develops around each pole, forming two nuclei. The chromosomes within each of these nuclei disperse into chromatin, and the nucleoli reappear.

Metaphase I (Meiosis I)

The homologous chromosomes line up along the metaphase plate in a random order. The microtubules attach to the kinetochores of the chromosomes.

centrosomes (animal cell MTOCs) contain a pair of _____

centrioles

microtubule organizing centers (MTOCs) are called _____ in animal cells

centrosomes (MTOCs are just called MTOCs in plants/fungi)

Why are checkpoints important in mitosis

checkpoints are important in cell replication because they make sure there is no errors in the DNA. If the DNA Passes through the checkpoints with mutations it will go into mitosis and replicate a bad genome

in _____ cells, there are 2 copies of every chromosome, forming a pair called _____ chromosomes

diploid; homologous

_____ occurs in somatic cells and _____ occurs in gametes (egg, sperm, pollen)

mitosis; meiosis

Telophase II (Meiosis II)

microtubules dissolve and the nuclear envelope starts to reform as the chromosomes unravel.

Prophase II (Meiosis II)

nuclear envelope dissolves, and chromosomes condense and the centrosomes replicate and migrate to opposite ends of the cell