alternation_of_generations.html: 02_12-alternation_of_generations.jpg
Life cycle of an angiosperm (flowering plant).

A plant alternates between a multicellular diploid (2n) sporophyte and a multicellular haploid (n) gametophyte generation.

1. A mature plant is a multicellular diploid sporophyte with reproductive structures.
   
2. Anthers contain microsporangia in which germ cells divide by meiosis to produce microspores.
   
3. Ovaries contain megasporangia in which germ cells divide by meiosis to yield 4 megaspores each.
   
4. Microspores divide by mitosis to form multicellular male gametophytes (pollen grains), which contain sperm cells.
   
5. One of the 4 megaspores divides by mitosis to form a multicellular female gametophyte (embryo sac), which contains an egg cell.
   
6. Fertilization (pollination) occurs when a sperm fuses with an egg, producing a diploid single-celled zygote.
   
7. The zygote develops by mitosis to produce the sporophyte.

animal-cell.html: 02_01-animal-cell.jpg
An eukaryotes cell such as an animal cell has several membrane-bound organelles.

The genetic material DNA resides mainly in the nucleus, which is surrounded by a double membrane with numerous openings called nuclear pores.

When the cell is not dividing, DNA is complexed with proteins in dispersed fibers called chromatin, which condense into visible chromosomes during cell division.

The nucleolus is the region where ribosomal RNA (rRNA) is made and ribosomes are initially assembled.

Other organelles reside in the cytoplasm.

Centrioles, located in a region called the centrosome and made of bundles of microtubules, organize spindle fibers to move chromosomes during cell division.

Surrounding the cell is a plasma membrane. In plant cells, a cell wall made of cellulose surrounds the membrane.

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Under ideal conditions, an eukaryotic cell can complete a cell cycle in about 16 hours, of which about 1 hour is spent in mitosis.

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Centromere locations and designations of chromosomes based on centromere location. Note that the shape of the chromosome during anaphase is determined by the position of the centromere.

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Humans (Homo sapiens have a haploid number of 23: each somatic cell contains 23 homologous pairs of chromosomes.

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In a typical somatic cell cycle, the period of active cell division is mitosis and the interval between cell divisions is interphase. After mitosis, the cells enters the first gap phase, G1, and may then become nondividing (G0) or continue to S (DNA synthesis), G2, and undergo mitosis again.

karyotype.html: 02_04-human_karyotype.jpg
This is a human male karyotype, prepared from a dividing cell in metaphase. All but the X and Y chromosomes are present in homologous pairs. Each chromosome is a double structure, constituting a pair of sister chromatids joined by a common centromere.

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DNA synthesis occurs during interphase before the beginning of meiosis I but does not occur again before meiosis II.

In prophase I, homologous chromosomes pair up into tetrads in a process called synapsis, and crossing over occurs, where genetic information is exchanged between nonsister chromatids of the homologues.

Crossing over contributes to genetic variation in sexual reproduction.

metaphase I

meiosis_II.html: 02_10-meiosis_II.jpg
Meiosis II is similar to mitosis, except the cell undergoing division is haploid rather than diploid.

In prophase II, each dyad has two chromatids attached to a common centromere.

In metaphase II, chromosomes move to the metaphase plate, and centromeres start to divide.

In anaphase II, the divided centromeres pull sister chromatids to opposite poles (disjunction).

After telophase II and cytokinesis II, cell division is complete, producing monads.

Each haploid daughter cell is a potential gamete and has one member of each pair of homologous chromosomes.

Note that if disjunction fails (nondisjunction) in either meiosis I or II, the gametes will have abnormal numbers of chromosomes.

meiosis_anaphase_I.html: 02_10-meiosis_anaphase_I.jpg
In anaphase I, one half of each tetrad (one pair of sister chromatids), now called a dyad is pulled, attached to one kinetochore, toward one pole of the dividing cell (disjunction). Each dyad is potentially a mosaic of maternal and paternal genetic material due to crossing over.

telephase I

meiosis_metaphase_I.html: 02_10-meiosis_metaphase_I.jpg
In metaphase I, the paired tetrads line up on the metaphase plate, the sister chromatids of each chromosome held together by a single centromere, which does not divide. The terminal chiasmata reach the ends of the chromatids and separate.

The alignment of maternal and paternal chromosomes with respect to the poles is random.

anaphase I

meiosis_prophase_Ia.html: 02_09-meiosis_prophase_Ia.jpg
Prophase I of meiosis is divided into five substages.

• In Leptonema (Lepto: thin), the replicated chromosomes begin to condense at areas called chromomeres. Also, homologous chromosomes begin to find each other in a process called homology search.
  
• In Zygonema (Zygo: pair), the chromosomes continue to condense, and homologous chromosomes undergo loose alignments with each other, forming bivalents, each of which contains four chromatids, though the individual chromatid are yet visible at this substage.

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meiosis_prophase_Ib.html: 02_09-meiosis_prophase_Ib.jpg
Prophase I continued.

• In Pachynema (Pachy: thick), the aligned homologous chromosomes (bivalents) continue to condense and 'zipper' together in a process called synapsis, forming tetrads which are held together by a synaptonemal complex. Non-sister chromatids may start exchanging genetic material in a process called crossing over.
  
• In Diplonema (Diplo: double), the homologues begin to separate, and chiasmata, regions where non-sister chromatids are intertwined and crossing over occurs, become visible.
  
• In Diakinesis (Dia: across), the chiasmata terminalize (move to the end of the tetrads). The homologues separate further, and centromeres attach the chromosomes to spindle fibers. The nucleolus and nuclear envelope break down.

meiosis_telophase_I.html: 02_10-meiosis_telophase_I.jpg
At the end of telophase I and cytokinesis I, two haploid cells are produced, with two chromatids still attached to the chromosomes.

mitosis-meiosis.html: 02_08-mitosis_meiosis.jpg
Comparison of mitosis and meiosis.

Note that meiosis has a reduction division followed by an equational division.

As a result of the two divisions, the tetrads are reduced to monads.

mitosis_anaphase.html: 02_07-mitosis_anaphase.jpg
In anaphase, sister chromatids separate from each other (disjoin) and migrate to opposite poles.

Each centromere divides, resulting in two kinetochores.

Each chromatid is attached to one kinetochore, and is pulled to a pole by shortening the spindle fibers to which the chromatid is attached.

The separated sister chromatids are called daughter chromosomes.

mitosis_interphase.html: 02_07-mitosis_interphase.jpg
From the G2 phase of interphase, where chromosomes are uncoiled and dispersed in a diffuse chromatin, cells enter prophase, the first stage of mitosis.

mitosis_metaphase.html: 02_07-mitosis_metaphase.jpg
In metaphase, the chromosomes are aligned at the equatorial plane, bound to spindle fibers at kinetochores.

mitosis_prometaphase.html: 02_07-mitosis_prometaphase.jpg
In prometaphase, the chromosomes move to the equatorial plane, or metaphase plate, of the cell.

mitosis_prophase.html: 02_07-mitosis_prophase.jpg
In prophase, the centrioles divide and move apart, the nuclear envelope breaks down, and the chromatin condense and become visible as chromosomes.

The centrioles in animal cells organize microtubules into a series of spindle fibers.

Sister chromatids are connected by a protein structure called the kinetochore at the centromere.

mitosis_telophase.html: 02_07-mitosis_telophase.jpg
In telophase, The separated daughter chromosomes complete migration to opposite poles of the cell and the chromosomes uncoil to their interphase structure.

The nuclear envelope reforms to complete karyokinesis.

Cytokinesis begins, dividing the cytoplasm and produce two daughter cells.

• In animal cells, the plasma membrane forms a cleavage furrow that pinches the cell in two.
• In plant cells, a cell plate grows at the equator, then a new cell wall form between the daughter cells.

oogenesis_I.html: 02_11-oogenesis_I.jpg
Oogenesis in humans occurs in ovaries and starts when germ cells called oogonia grow and mature into primary oocytes enclosed in small follicles.
The primary oocytes start meiosis I, but stop at prophase I.
Beginning at puberty, periodic hormone secretions induce a few primary oocytes to complete meiosis I, each producing a non-functional polar body and a haploid secondary oocyte where most of the cytoplasm from the primary oocyte is concentrated.

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oogenesis_II.html: 02_11-oogenesis_II.jpg
Oogenesis (continued)

One of the secondary oocytes completes growth in the ovary and begins meiosis II, which is arrested at metaphase II.
The mature follicle ruptures and releases the oocyte from the ovary in a process called ovulation.
Completion of meiosis II does not occur until the oocyte is penetrated by a sperm, producing a second polar body and the ovum.

prokaryote.html: 02_02-prokaryote.jpg
A prokaryote cell is surrounded by a plasma membrane and a cell wall made of peptidoglycan.
This E. coli cell is undergoing cell division in a process called binary fission. The DNA (red) has been duplicated and partitioned into the daughter cells in their nucleoid areas.

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Spermatogenesis in animals occurs in testes and starts when a germ cell called spermatogonium grows and differentiates into a primary spermatocyte.
Each primary spermatocyte undergoes meiosis I, producing two haploid secondary spermatocytes.

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spermatogenesis_II.html: 02_11-spermatogenesis_II.jpg
Spermatogenesis (continued)
The secondary spermatocytes undergo meiosis II.
The final products of Spermatogenesis from one primary spermatocyte are four equal-sized spermatids, which then differentiate into motile spermatozoa, or sperm cells.

synaptonemal_complex.html: 02_14-synaptonemal_complex.jpg
The synaptonemal complex holds together synapsed homologues during Meiosis I and plays a role in crossing over. A protein central element is surrounded by lateral elements which form associations with chromatin fiber, thus keeping the homologous chromosomes in close proximity.