BIOLOGY - Reproduction

MEOSIS

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Mitosis Plant Cells

2 major differences: -Lack Centrioles: spindle apparatus is synthesized by invisible microtubule organizing centers -No cleavage furrow: they do it with a cell plate (expanding partition growing from middle to out

MEOSIS OVERVIEW

2 successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) progeny cells, each with half of the genetic material of the original cell. • Meiosis occurs in diploid cells. The chromosomes duplicate once, and through two successive divisions, four haploid cells are produced, each with half the chromosome number of the parental cell. • Meiosis occurs only in sexually reproducing organisms. Depending on the organism, it may produce haploid gametes, which do not divide further but instead fuse to produce a diploid zygote; or it may produce haploid spores, which divide by mitotic cell cycles and produce unicellular or multicellular organisms. • In animals, where the somatic (body) cells are diploid, the products of meiosis are the gametes. • In many fungi and some algae, meiosis occurs immediately after two haploid cells fuse, and mitosis then produces a haploid multicellular "adult" organism (e.g., filamentous fungi, algae) or haploid unicellular organisms (e.g., yeast, unicellular algae). • Plants and some algae have both haploid and diploid multicellular stages. The multicellular diploid stage is the sporophyte. Meiosis in a sporophyte produces haploid spores. These spores alone are capable of generating a haploid multicellular stage called a gametophyte. The gametophyte produces gametes by mitotic cell cycles.

Mitosis Cytokinesis

At end of telophase, cytoplasm divides into two daughter cells, each with complete nucleus and organelles. -In animal cells, Cleavage Furrow forms, indenting membrane along the equator and finally pinches them apart.

Mitosis • G1, S, and G2 are collectively called interphase. • G1 stands for gap 1, or presynthesis; S for synthesis; G2 for gap 2, or postsynthesis. • M is the mitotic division phase.

Cells that are growing and dividing go through a repeating series of events called the cell division cycle (or cell cycle). During the first phase (G1), the cell grows and prepares for DNA replication, which occurs in the subsequent S phase. Further growth takes place in the G2 phase, and finally mitosis occurs in the M phase.

Mitosis Telophase

Chromosome sets assemble at opposite poles, a nuclear envelope reforms around each set, and cytokinesis (division of the cytoplasm) usually follows. • The chromosomes assemble in sets at the two poles. • The chromosomes begin to uncoil and eventually assume the extended state characteristic of interphase. • A nuclear envelope reforms around each chromosome set, the spindle disappears, and the nucleolus reforms. Nuclear division by mitosis is complete at this point. • Cytokinesis, the division of the cytoplasm, usually is in progress before nuclear division is complete. In animal cells, cytokinesis involves the formation of a cleavage furrow resulting in the pinching of the cell into two. • In some specialized cases, such as in the development of the Drosophila embryo, nuclear division occurs without being followed by cytokinesis.

MEOSIS (Sex)

Meiosis is 2 divisions of primary sex cells = 4 haploid cells (gametes) Sex is the Fusion of 2 gametes (meiosis is how gametes are made) Similar to mitosis: cell duplicates chromosomes before undergoing process, but meiosis halves the chromosomes (haploid instead of diploid.

Mitosis http://www.phschool.com/science/biology_place/biocoach/mitosisisg/intro.html

Mitosis is the process of nuclear division of either a diploid (2N) or haploid (N) eukaryotic cell whereby two daughter nuclei are produced that are genetically identical to the parent nucleus. Cell division usually follows nuclear division. -various stages: prophase, metaphase, anaphase, and telophase. -Nuclear division (karyokinesis) is followed by cell division (cytokinesis)

Gene Segregation in Mitosis

Mitosis maintains a constant amount of genetic material from cell generation to cell generation. Consider a hypothetical diploid cell with one chromosome. Consider also that this cell is Aa; that is, it is heterozygous for a pair of alleles with A coming from one parent and a from the other parent.

Mitosis Anaphase

Sister chromatids separate, and the now-daughter chromosomes move to opposite poles of the cell. • Anaphase begins when the duplicated centromeres of each pair of sister chromatids separate, and the now-daughter chromosomes begin moving toward opposite poles of the cell due to the action of the spindle. • At the end of anaphase, a complete set of chromosomes has assembled at each pole of the cell.

Mitosis Metaphase

The chromosomes become arranged on the metaphase plate and are attached to the now fully formed spindle. -Centriole pairs now at opposite poles.

Cell division

Unicellular Organisms: it is for reproduction Multicellular: means of growth, development, and replacment Either mitosis or meiosis

Mitosis Interphase

each chromosome is replicates to 2 sisters chromatids held together by centromere. The chromosomes are not visible and the DNA appears as uncoiled chromatin. The chromosomes that were duplicated during the S phase are so extended that they cannot be seen individually.

Meiosis I: Anaphase I The two chromosomes in each bivalent separate and migrate toward opposite poles.

• Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle (disjunction). • Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate. -Disjunction accounts for a fundamental Mendelian law: Paternal and Maternal homologues can end up on daughter cells in random order.

Meiosis I: Prophase I Chromosomes become visible, crossing-over occurs, the nucleolus disappears, the meiotic spindle forms, and the nuclear envelope disappears.

• At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope. • The duplicated homologous chromosomes pair (synapsis), and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids (pair is called tetrad). The sites of crossing-over are seen as crisscrossed Nonsister chromatids and are called chiasmata (singular: chiasma). • The nucleolus disappears during prophase I. • In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. • The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. • Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.

Mitosis Prophase

• Chromosomes become more coiled and can be viewed under a light microscope. • Each duplicated chromosome is seen as a pair of sister chromatids joined by the duplicated but unseparated centromere. • The nucleolus disappears during prophase. • In the cytoplasm, the mitotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. • The nuclear envelope disappears at the end of prophase. This signals the beginning of the substage called prometaphase.

Mitosis prometaphase

• In prometaphase, the spindle enters the nuclear area. Specialized structures called kinetochores have formed on the centromeres of the chromosomes by this time; certain spindle microtubules—the kinetochore microtubules—attach to the kinetochores.

Premeiotic Interphase The chromosomes duplicate prior to meiosis.

• Prior to meiosis, all chromosomes are duplicated in a process similar to chromosome duplication prior to mitosis, resulting in 2N number of sister chomatids. • Outside the nucleus of animal cells are two centrosomes, each containing a pair of centrioles. The two centrosomes are produced by the duplication of a single centrosome during premeiotic interphase. The centrosomes serve as microtubule organizing centers (MTOCs). Microtubules extend radially from centrosomes, forming an aster. • Plant cells do not have centrosomes. Different kinds of microtubule organizing centers serve as sites of spindle formation.

Meiosis I: Metaphase I The pairs of chromosomes (bivalents) become arranged on the metaphase plate and are attached to the now fully formed meiotic spindle.

• The centrioles are at opposite poles of the cell. • The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate. • Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).

Meiosis I: Telophase I The homologous chromosome pairs reach the poles of the cell, nuclear envelopes form around them, and cytokinesis follows to produce two cells.

• The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids. • A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes. • Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Other cells do exhibit chromosome decondensation at this time; the chromosomes recondense in prophase II.

Meiosis II: Prophase II Meiosis II begins without any further replication of the chromosomes. In prophase II, the nuclear envelope breaks down and the spindle apparatus forms.

• While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II. • The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes. • The nuclear envelope breaks down, and the spindle apparatus forms.