Cell division

•In prokaryotic cells, reproduction is simple, because prokaryotic cells possess a single molecule. binary fission takes place in prokaryotic cells of simple single-celled organisms such as bacteria.

•In eukaryotic cells, multiple chromosomes must be copied and distributed to each of the new cells, and so cell reproduction is more complex. •Cell division in eukaryotes takes place through mitosis and meiosis

•For any cell to reproduce successfully, three fundamental events must take place: (1) its genetic information must be copied, (2) the copies of genetic information must be separated from each other, and (3) the cell must divide.

•Eukaryotic chromosomes:

Eukaryotic chromosomes are separated from the cytoplasm by the nuclear envelope. •The nucleus has a highly organized internal scaffolding called the nuclear matrix. •This matrix consists of a network of protein fibers that maintains precise spatial relations among the nuclear components and takes part in DNA replication, the expression of genes, and the modification of gene products before they leave the nucleus.

•Each eukaryotic species has a characteristic number of chromosomes per cell: potatoes have 48 chromosomes, fruit flies have 8, and humans have 46.

two sets is a consequence of sexual reproduction: one set is inherited from the male parent and the other from the female parent.

•Each chromosome in one set has a corresponding chromosome in the other set, together constituting a homologous pair

•Human cells, for example, have 46 chromosomes, constituting 23 homologous pairs.

•The two chromosomes of a homologous pair are usually alike in structure and size, and each carries genetic information for the same set of hereditary characteristics (traits). •For example, if a gene on a particular chromosome encodes a characteristic such as hair color, another copy of the gene (each copy is called an allele) at the same position on that chromosome’s homolog also encodes hair color.

•the DNA molecules in eukaryotic chromosomes are highly folded and condensed

•functional chromosome has three essential elements:

a centromere, a pair of telomeres, and origins of replication.

•The Centromere

the attachment point for spindle microtubules, which are the filaments responsible for moving chromosomes during cell division •The centromere appears as a constricted region. Before cell division, a protein complex called the kinetochore assembles on the centromere; later spindle microtubules attach to the kinetochore.

•On the basis of the location of the centromere, chromosomes are classified into four types: metacentric, submetacentric, acrocentric, and telocentric

• One of the two arms of a chromosome (the short arm of a submetacentric or acrocentric chromosome) is designated by the letter p and the other arm is designated by q.

•Telomeres

Telomeres are physical ends, the tips, of a linear chromosome; they serve to stabilize the chromosome ends. •Telomeres contain repeated nucleotide sequences that enable the ends of chromosomes to be efficiently replicated. •Telomeres also perform another function: the repeated telomere DNA sequences, together with the regions adjoining them, form structures that protect the end of the chromosome from being mistaken by the cell for a broken DNA molecule in need of repair •Telomeres act as protective caps to chromosome ends, preventing end-to-end fusion of chromosomes and DNA degradation resulting after chromosome breakage. •Telomeres provide chromosome stability. •Nonhistone proteins make complexes with telomeric DNA to protect the ends of chromosomes from nucleases located within the cell. •The telomeric region also plays a role in synapsis during meiosis. •Chromosome pairing appears to be initiated in the subtelomeric regions

•Origins of replication

Ori are the sites at which the DNA replication machinery assembles to initiate replication. •They are found some 30-40 kb apart throughout the length of each eukaryotic chromososme. •prokaryotic chromosomes typically have one origin of replication •Eukaryotic chromosomes contain many origins of replication to ensure that the entire chromosome can be replicated rapidly

Cell division:

Cell division consists of two phases— 

nuclear division followed by cytokinesis. Nuclear division divides the genetic material in the nucleus. There are two kinds of nuclear division—mitosis and meiosis. Mitosis divides the nucleus so that both daughter cells are genetically identical. In contrast, meiosis is a reduction division, producing daughter cells that contain half the genetic information of the parent cell.

cytokinesis divides the cytoplasm.

The Mitosis Cell Cycle

Mitosis is how somatic—or non-reproductive cells—divide. Somatic cells make up most of your body’s tissues and organs, including skin, muscles, lungs, gut, and hair cells. Reproductive cells (like eggs) are not somatic cells.

In mitosis, the important thing to remember is that the daughter cells each have the same chromosomes and DNA as the parent cell. The daughter cells from mitosis are called diploid cells. Diploid cells have two complete sets of chromosomes.  Since the daughter cells have exact copies of their parent cell’s DNA, no genetic diversity is created through mitosis in normal healthy cells. 

There are four phases in mitosis (adjective, mitotic): prophase, metaphase, anaphase, and telophase :

• During prophase, the nucleoli disappear, the chromatin condenses into chromosomes, the nuclear envelope breaks down, and the mitotic spindle is assembled. The development of the mitotic spindle begins as the centrosomes move apart to opposite ends (poles) of the nucleus. As they move apart, microtubules develop from each centrosome, increasing in length by the addition of tubulin units. Microtubules from each centrosome connect to specialized regions in the centromere called kinetochores. Microtubules tug on the kinetochores, moving the chromosomes back and forth toward one pole, then the other. Within the spindle, there are also microtubules that overlap at the center of the spindle and do not attach to the chromosomes.
• 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 kinetochores. 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.)
• Anaphase  begins after the chromosomes are separated into individual chromatids. During anaphase, the microtubules connected to the chromatids (now chromosomes) shorten, effectively pulling the chromosomes to opposite poles. Overlapping microtubules, originating from opposite centrosomes but not attached to chromosomes, interact to push the poles farther apart. At the end of anaphase, each pole has a complete set of chromosomes, the same number of chromosomes as the original cell. (Since it consists of only one chromatid, each chromosome contains only a single copy of the DNA molecule.)
• 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 nuclei reappear. Simultaneously, cytokinesis occurs, dividing the cytoplasm into two cells. Microfilaments form a ring inside the plasma membrane between the two newly forming nuclei. As the microfilaments shorten, they act like purse strings to pull the plasma membrane into the center, dividing the cell into two daughter cells. The groove that forms as the purse strings are tightened is called a cleavage furrow.

 Cell reproduction and the four stages of mitosis.

Once mitosis is completed and interphase begins, the cell begins a period of growth. Growth begins during the first phase, called G ₁ (gap), and continues through the S (synthesis) and G ₂ phases.

Also during the S phase the second DNA molecule for each chromosome is synthesized. As a result of this DNA replication, each chromosome gains a second chromatid. During the G ₂ period of growth, materials for the next mitotic division are prepared.

The time span from one cell division through G ₁, S, and G ₂ is called a cell cycle.

A cell that begins mitosis in the diploid state—that is, with two copies of every chromosome—will end mitosis with two copies of every chromosome. However, each of these chromosomes will consist of only one chromatid, or one DNA molecule. During interphase, the second DNA molecule is replicated from the first, so that when the next mitotic division begins, each chromosome will again consist of two chromatids.

The Meiosis Cell Cycle

Meiosis is the other main way cells divide. Meiosis is cell division that creates sex cells, like female egg cells or male sperm cells. In meiosis, each new cell contains a unique set of genetic information. After meiosis, the sperm and egg cells can join to create a new organism.    

Meiosis is why we have genetic diversity in all sexually reproducing organisms. During meiosis, a small portion of each chromosome breaks off and reattaches to another chromosome. This process is called “crossing over” or “genetic recombination.” Genetic recombination is the reason full siblings made from egg and sperm cells from the same two parents can look very different from one another.

The major distinction is that meiosis consists of two groups of divisions, meiosis I and meiosis II .

In meiosis I, homologous chromosomes pair at the metaphase plate and then migrate to opposite poles. In meiosis II, chromosomes spread across the metaphase plate, and sister chromatids separate and migrate to opposite poles. Thus, meiosis II is analogous to mitosis. 

• Prophase I begins like prophase of mitosis. The nucleolus disappears, chromatin condenses into chromosomes, the nuclear envelope breaks down, and the spindle apparatus develops. Once the chromosomes are condensed, however, their behavior differs from mitosis. During prophase I, homologous chromosomes pair, a process called synapsis. These pairs of homologous chromosomes are called tetrads (a group of four chromatids) or bivalents. During synapsis, corresponding regions form close associations called chiasmata (singular, chiasma) along nonsister chromatids. Chiasmata are sites where genetic material is exchanged between nonsister homologous chromatids, a process called crossing over. The result contributes to a mixing of genetic material from both parents, a process called genetic recombination.
• At metaphase I, homologous pairs of chromosomes are spread across the metaphase plate. Microtubules extending from one pole are attached to kinetochores of one member of each homologous pair. Microtubules from the other pole are connected to the second member of each homologous pair.
• Anaphase I begins when homologues within tetrads uncouple as they are pulled to opposite poles.
• In telophase I, the chromosomes have reached their respective poles, and a nuclear membrane develops around them. Note that each pole will form a new nucleus that will have half the number of chromosomes, but each chromosome will contain two chromatids. Since daughter nuclei will have half the number of chromosomes, cells that they eventually form will be haploid.
• Cytokinesis occurs, forming two daughter cells. A brief interphase may follow, but no replication of chromosomes occurs. Instead, part II of meiosis begins in both daughter nuclei.
• In prophase II, the nuclear envelope disappears and the spindle develops. There are no chiasmata and no crossing over of genetic material as in prophase I.
• In metaphase II, the chromosomes align singly on the metaphase plate (not in tetrads as in metaphase I). Single alignment of chromosomes is exactly what happens in mitosis—except now there is only half the number of chromosomes.
• Anaphase II begins as each chromosome is pulled apart into two chromatids by the microtubules of the spindle apparatus. The chromatids (now chromosomes) migrate to their respective poles. Again, this is exactly what happens in mitosis—except now there is only half the number of chromosomes.
• In telophase II, the nuclear envelope reappears at each pole and cytokinesis occurs. The end result of meiosis is four haploid cells. Each cell contains half the number of chromosomes and each chromosome consists of only one chromatid.

Meiosis ends with four haploid daughter cells, each with half the number of chromosomes (one chromosome from each homologous pair). These are gametes—that is, eggs and sperm. The fusing of an egg and sperm, fertilization ( syngamy), gives rise to a diploid cell, the zygote. The single‐celled zygote then divides by mitosis to produce a multicellular embryo fetus, and after nine months, a newborn infant. Note that one copy of each chromosome pair in the zygote originates from one parent, and the second copy from the other parent. Thus, a pair of homologous chromosomes in the diploid zygote represents both maternal and paternal heritage.